CHAPTER 43: STAPHYLOCOCCAL INFECTIONS

Staphylococcus aureus, the most virulent of the many staphylococcal species, has demonstrated its versatility by remaining a major cause of morbidity and mortality worldwide despite the availability of numerous effective antistaphylococcal antibiotics. S. aureus is a pluripotent pathogen, causing disease through both toxin- and non-toxin-mediated mechanisms. This organism is responsible for numerous nosocomial and community-based infections that range from relatively minor skin and soft tissue infections to life-threatening systemic infections.

The “other” staphylococci, collectively designated coagulase-negative staphylococci (CoNS), are considerably less virulent than S. aureus but remain important pathogens in infections that are primarily associated with prosthetic devices.

Staphylococci, gram-positive cocci in the family Micrococcaceae, form grapelike clusters on Gram’s stain (Fig. 43-1). These organisms (~1 μm in diameter) are catalase-positive (unlike streptococcal species), nonmotile, aerobic, and facultatively anaerobic. They are capable of prolonged survival on environmental surfaces under varying conditions. Some species have a relatively broad host range, including mammals and birds, whereas for others the host range is quite narrow—i.e., limited to one or two closely related animals.

More than 30 staphylococcal species are pathogenic. Identification of the more clinically important species has generally relied on a series of biochemical tests. Automated diagnostic systems, kits for biochemical characterization, and DNA-based assays are available for species identification. With few exceptions, S. aureus is distinguished from other staphylococcal species by its production of coagulase, a surface enzyme that converts fibrinogen to fibrin. Latex kits that detect both protein A and clumping factor also distinguish S. aureus from most other staphylococcal species. S. aureus ferments mannitol, is positive for protein A, and produces DNAse. On blood agar plates, S. aureus tends to form golden β-hemolytic colonies; in contrast, CoNS produce small white nonhemolytic colonies. Increasingly, sequence-based methods (e.g., 16S rRNA) are being used to identify different staphylococcal species.

Determining whether multiple staphylococcal isolates from different patients are the same or different is often relevant when there is concern that a nosocomial outbreak is due to a common point source (e.g., a contaminated medical instrument). Molecular typing methods, such as pulsed-field gel electrophoresis and sequence-based techniques (e.g., staphylococcal protein A [SpA] typing), have increasingly been used for this purpose. More recently, whole-genome sequencing has enhanced the ability to discriminate among clinical isolates.

EPIDEMIOLOGY

S. aureus is both a commensal and an opportunistic pathogen. Approximately 30% of healthy persons are colonized with S. aureus, with a smaller percentage (~10%) persistently colonized. The rate of colonization is elevated among insulin-dependent diabetics, HIV-infected patients, patients undergoing hemodialysis, injection drug users, and individuals with skin damage. The anterior nares and oropharynx are frequent sites of human colonization, although the skin (especially when damaged), vagina, axilla, and perineum may also be colonized. These colonization sites serve as a reservoir for future infections.

Transmission of S. aureus most frequently results from direct personal contact. Colonization of different body sites allows transfer from one person to another during contact. Spread of staphylococci in aerosols of respiratory or nasal secretions from heavily colonized individuals has also been reported. Most individuals who develop S. aureus infections become infected with a strain that is already a part of their own commensal flora. Breaches of the skin or mucosal membrane allow S. aureus to initiate infection.

Some diseases increase the risk of S. aureus infection; diabetes, for example, combines an increased rate of S. aureus colonization and the use of injectable insulin with the possibility of impaired leukocyte function. Individuals with congenital or acquired qualitative or quantitative defects of polymorphonuclear leukocytes (PMNs) are at increased risk of S. aureus infections; this group includes neutropenic patients (e.g., those receiving chemotherapeutic agents), those with chronic granulomatous disease, and those with Job’s or Chédiak-Higashi syndrome. Other groups at risk include individuals with end-stage renal disease, HIV infection, skin abnormalities, or prosthetic devices.

S. aureus is a leading cause of health care–associated infections (Chap. 17). It is the most common cause of surgical wound infections and is second only to CoNS as a cause of primary bacteremia. These isolates are generally resistant to multiple antibiotics; thus available therapeutic options are limited. In the community, S. aureus remains an important cause of skin and soft tissue infections, respiratory infections, and (among injection drug users) infective endocarditis. The increasing use of home infusion therapy is another cause of community-acquired staphylococcal infections.

In the past two decades, there has been a dramatic change in the epidemiology of infections due to methicillin-resistant S. aureus (MRSA). In addition to its major role as a nosocomial pathogen, MRSA has become an established community-based pathogen. Numerous outbreaks of community-associated MRSA (CA-MRSA) infections have been reported in both rural and urban settings in widely separated regions throughout the world. The outbreaks have occurred among such diverse groups as children, prisoners, athletes, Native Americans, and drug users. Risk factors common to these outbreaks include poor hygienic conditions, close contact, contaminated material, and damaged skin. These infections have been caused by a limited number of MRSA strains. In the United States, strain USA300 (defined by pulsed-field gel electrophoresis) has been the predominant clone. In other geographic regions of the world, different strains of CA-MRSA have been responsible for these community-based outbreaks. Although the majority of infections caused by these strains have involved the skin and soft tissue, 5–10% have been invasive and potentially life-threatening. CA-MRSA strains have also been responsible for an increasing number of nosocomial infections. Of concern has been the apparent capacity of CA-MRSA to cause disease in immunocompetent individuals.

PATHOGENESIS

General concepts

S. aureus is a pyogenic pathogen known for its capacity to induce abscess formation at sites of both local and metastatic infections. This classic pathologic response to S. aureus defines the framework within which the infection will progress. The bacteria elicit an inflammatory response characterized by an initial intense infiltration of PMNs and a subsequent infiltration of macrophages and fibroblasts. Either the host cellular response (including the deposition of fibrin and collagen) contains the infection, or infection spreads to the adjoining tissue or the bloodstream.

In toxin-mediated staphylococcal disease, infection is not invariably present. For example, once toxin has been elaborated into food, staphylococcal food poisoning can develop in the absence of viable bacteria. In staphylococcal toxic shock syndrome (TSS), conditions allowing toxin elaboration at colonization sites (e.g., the presence of a superabsorbent tampon) suffice for initiation of clinical illness.

The S. aureus genome

The complete genomes of numerous strains of S. aureus have now been fully sequenced. Among the interesting revelations are (1) the high degree of nucleotide sequence similarity of the core genomes of different strains; (2) acquisition of a relatively large amount of genetic information by horizontal transfer from other bacterial species; and (3) the presence of unique “pathogenicity” or “genomic” islands—mobile genetic elements that contain clusters of enterotoxin and exotoxin genes and/or antimicrobial resistance determinants. Among the genes in these islands are those carrying mecA, the gene responsible for methicillin resistance. Methicillin resistance–containing islands have been designated staphylococcal cassette chromosome mec (SCCmec) types and range in size from ~20 to 60 kb. To date, 11 SCCmec types have been identified. Among the more common types, types 1–3 are traditionally associated with nosocomial MRSA isolates, whereas types 4–6 have been associated with the epidemic CA-MRSA strains.

A limited number of MRSA clones have been responsible for most community- and hospital-associated infections worldwide. A comparison of these strains with those from earlier outbreaks (e.g., the phage 80/81 strains from the 1950s) has revealed preservation of the nucleotide sequence over time. This observation suggests that these strains possess determinants that facilitate survival and spread.

Regulation of virulence gene expression

In both toxin-mediated and non-toxin-mediated diseases due to S. aureus, the expression of virulence determinants associated with infection depends on a series of regulatory genes (e.g., accessory gene regulator [agr] and staphylococcal accessory regulator [sar]) that coordinately control the expression of many virulence genes. The regulatory gene agr is part of a quorum-sensing signal transduction pathway that senses and responds to bacterial density. Staphylococcal surface proteins are synthesized during the bacterial exponential growth phase in vitro. In contrast, many secreted proteins, such as α toxin, the enterotoxins, and assorted enzymes, are released during the postexponential growth phase in response to transcription of the effector molecule of agr, RNAIII.

It has been hypothesized that these regulatory genes serve a similar function in vivo. Successful invasion requires the sequential expression of these different bacterial elements. Bacterial adhesins are needed to initiate colonization of host tissue surfaces. The subsequent release of various enzymes enables the colony to obtain nutritional support and permits bacteria to spread to adjacent tissues. Studies with strains in which these regulatory genes are inactivated show reduced virulence in several animal models of S. aureus infection.

Pathogenesis of invasive S. aureus infection

Staphylococci are opportunists. For these organisms to invade the host and cause infection, some or all of the following steps are necessary: contamination and colonization of host tissue surfaces, breach of cutaneous or mucosal barriers, establishment of a localized infection, invasion, evasion of the host response, and metastatic spread. Colonizing strains or strains transferred from other individuals are introduced into damaged skin, a wound, or the bloodstream. Recurrences of S. aureus infections are common, apparently because of the capacity of these pathogens to survive, to persist in a quiescent state in various tissues, and then to cause recrudescent infections when suitable conditions arise.

S. aureus colonization of body surfaces

The anterior nares is one of the primary sites of staphylococcal colonization in humans. Colonization appears to involve the attachment of S. aureus to keratinized epithelial cells of the anterior nares. Other factors that may contribute to colonization include the influence of other resident nasal flora and their bacterial density, host factors, and nasal mucosal damage (e.g., that resulting from inhalational drug use). Other colonized body sites, such as damaged skin, the groin, and the oropharynx, may be particularly important reservoirs for CA-MRSA strains.

Inoculation and colonization of tissue surfaces

Staphylococci may be introduced into tissue as a result of minor abrasions, administration of medications such as insulin, or establishment of IV access with catheters. After their introduction into a tissue site, bacteria replicate and colonize the host tissue surface. A family of structurally related S. aureus surface proteins referred to as MSCRAMMs (microbial surface components recognizing adhesive matrix molecules) plays an important role in mediating adherence to these sites. By adhering to exposed matrix molecules (e.g., fibrinogen, fibronectin), MSCRAMMs such as clumping factor and collagen-binding protein enable the bacteria to colonize different tissue surfaces; these proteins contribute to the pathogenesis of invasive infections such as endocarditis and septic arthritis by facilitating the adherence of S. aureus to surfaces with exposed fibrinogen or collagen.

Although CoNS are classically known for their ability to elaborate biofilms and to colonize prosthetic devices, S. aureus also possesses the genes responsible for biofilm formation, such as the intercellular adhesion (ica) locus. Binding to these devices occurs in a stepwise fashion, involving staphylococcal adherence to serum constituents that have coated the device surface and subsequent biofilm elaboration. S. aureus is thus a frequent cause of biomedical-device infections.

Invasion

After colonization, staphylococci replicate at the initial site of infection, elaborating enzymes that include serine proteases, hyaluronidases, thermonucleases, and lipases. These enzymes facilitate bacterial survival and local spread across tissue surfaces, although their precise role in infections is not well defined. The lipases may facilitate survival in lipid-rich areas such as the hair follicles, where S. aureus infections are often initiated. The S. aureus toxin Panton-Valentine leukocidin is cytolytic to PMNs, macrophages, and monocytes. Strains elaborating this toxin have been epidemiologically linked with cutaneous and more serious infections caused by strains of CA-MRSA. MSCRAMMs also appear to play an important role in the ability of S. aureus to spread and cause disease at other tissue sites.

Constitutional findings may result from either localized or systemic infections. The staphylococcal cell wall—consisting of alternating N-acetyl muramic acid and N-acetyl glucosamine units in combination with an additional cell wall component, lipoteichoic acid—can initiate an inflammatory response that includes the sepsis syndrome. Staphylococcal α toxin, which causes pore formation in various eukaryotic cells, can also initiate an inflammatory response with findings suggestive of sepsis.

Evasion of host defense mechanisms

Staphylococci have a multitude of immune evasion strategies that are critical to their success as invasive pathogens. They possess an antiphagocytic polysaccharide microcapsule. Most human S. aureus infections are due to capsular types 5 and 8. The zwitterionic (both negatively and positively charged) S. aureus capsule plays a critical role in the induction of abscess formation. Protein A, an MSCRAMM unique to S. aureus, acts as an Fc receptor, binding the Fc portion of IgG subclasses 1, 2, and 4 and preventing opsonophagocytosis by PMNs. Both chemotaxis inhibitory protein of staphylococci (CHIPS, a secreted protein) and extracellular adherence protein (EAP, a surface protein) interfere with PMN migration to sites of infection.

An additional potential mechanism of S. aureus evasion is its capacity for intracellular survival. Both professional and nonprofessional phagocytes internalize staphylococci. Internalization by these cells may provide a sanctuary that protects bacteria against the host’s defenses. The intracellular environment favors the phenotypic expression of S. aureus small-colony variants. Small-colony variants are found in patients receiving antimicrobial therapy (e.g., with aminoglycosides) and in those with cystic fibrosis or osteomyelitis. These variants, whether intra- or extracellular, may facilitate prolonged staphylococcal survival in different tissue sites and enhance the likelihood of recurrences. Finally, S. aureus can survive within PMNs and may use these cells to spread and to seed other tissue sites.

Pathogenesis of community-acquired MRSA infections

A number of virulence determinants have been identified as contributing to the pathogenesis of CA-MRSA infections. There is a strong epidemiologic association linking the presence of the gene for the Panton-Valentine leukocidin with skin and soft tissue infections as well as with necrotizing postinfluenza pneumonia. Other determinants that play a role in the pathogenesis of these infections include the arginine catabolic mobile element (ACME), a cluster of unique genes that may facilitate evasion of host defense mechanisms; phenol-soluble modulins, a family of cytolytic peptides; and α toxin.

Host response to S. aureus infection

The primary host response to S. aureus infection is the recruitment of PMNs. These cells are attracted to infection sites by bacterial components such as formylated peptides or peptidoglycan as well as by the cytokines tumor necrosis factor (TNF) and interleukins (ILs) 1 and 6, which are released by activated macrophages and endothelial cells.

Although most individuals have antibodies to staphylococci, it is not clear that antibody levels are qualitatively or quantitatively sufficient to protect against infection. Although anticapsular and anti-MSCRAMM antibodies facilitate opsonization in vitro and have been protective against infection in several animal models, they have not yet successfully prevented staphylococcal infections in clinical trials.

Pathogenesis of toxin-mediated disease

S. aureus produces three types of toxin: cytotoxins, pyrogenic toxin superantigens, and exfoliative toxins. Both epidemiologic data and studies in animals suggest that antitoxin antibodies are protective against illness in TSS, staphylococcal food poisoning, and staphylococcal scalded-skin syndrome (SSSS). Illness develops after toxin synthesis and absorption and the subsequent toxin-initiated host response.

Enterotoxin and toxic shock syndrome toxin 1 (TSST-1)

The pyrogenic toxin superantigens are a family of small-molecular-size, structurally similar proteins that are responsible for two diseases: TSS and food poisoning. TSS results from the ability of enterotoxins and TSST-1 to function as T cell mitogens. In the normal process of antigen presentation, the antigen is first processed within the cell, and peptides are then presented in the major histocompatibility complex (MHC) class II groove, initiating a measured T cell response. In contrast, enterotoxins bind directly to the invariant region of MHC—outside the MHC class II groove. The enterotoxins can then bind T cell receptors via the vβ chain; this binding results in a dramatic overexpansion of T cell clones (up to 20% of the total T cell population). The consequence of this T cell expansion is a “cytokine storm,” with the release of inflammatory mediators that include interferon γ, IL-1, IL-6, TNF-α, and TNF-β. The resulting multisystem disease produces a constellation of findings that mimic those in endotoxin shock; however, the pathogenic mechanisms differ. The release of endotoxin from the gastrointestinal tract may synergistically enhance the toxin’s effects.

A different region of the enterotoxin molecule is responsible for the symptoms of food poisoning. The enterotoxins are heat stable and can survive conditions that kill the bacteria. Illness results from the ingestion of preformed toxin. As a result, the incubation period is short (1–6 h). The toxin stimulates the vagus nerve and the vomiting center of the brain. It also appears to stimulate intestinal peristaltic activity.

Exfoliative toxins and SSSS

The exfoliative toxins are responsible for SSSS. The toxins that produce disease in humans are of two serotypes: ETA and ETB. These toxins are serine proteases, which cleave desmosomal cadherins in the superficial layer of the skin, triggering exfoliation. The result is a split in the epidermis at the granular level, which is responsible for the superficial desquamation of the skin that typifies this illness.

DIAGNOSIS

Staphylococcal infections are readily diagnosed by Gram’s stain (Fig. 43-1) and microscopic examination of abscess contents or of infected tissue. Routine culture of infected material usually yields positive results, and blood cultures are sometimes positive even when infections are localized to extravascular sites. S. aureus is rarely a blood culture contaminant. Polymerase chain reaction (PCR)–based assays have been applied to the rapid diagnosis of S. aureus infection and are increasingly used in clinical microbiology laboratories. A number of point-of-care tests are now available to screen patients for colonization with MRSA. Determining whether patients with documented S. aureus bacteremia also have infective endocarditis or a metastatic focus of infection remains a diagnostic challenge. Uniformly positive blood cultures suggest an endovascular infection such as endocarditis (see “Bacteremia, Sepsis, and Infective Endocarditis,” below).

CLINICAL SYNDROMES

(Table 43-1)

Skin and soft tissue infections

S. aureus causes a variety of cutaneous infections, many of which can also be caused by group A streptococci or (less commonly) other streptococcal species. Common factors predisposing to S. aureus cutaneous infection include chronic skin conditions (e.g., eczema), skin damage (e.g., insect bites, minor trauma), injections (e.g., in diabetes, injection drug use), and poor personal hygiene. These infections are characterized by the formation of pus-containing blisters, which often begin in hair follicles and spread to adjoining tissues. Folliculitis is a superficial infection that involves the hair follicle, with a central area of purulence (pus) surrounded by induration and erythema. Furuncles (boils) are more extensive, painful lesions that tend to occur in hairy, moist regions of the body and extend from the hair follicle to become a true abscess with an area of central purulence. Carbuncles are most often located in the lower neck and are even more severe and painful, resulting from the coalescence of other lesions that extend to a deeper layer of the subcutaneous tissue. In general, furuncles and carbuncles are readily apparent, with pus often expressible or discharging from the abscess. Other cutaneous S. aureus infections include impetigo and cellulitis. S. aureus is one of the most common causes of surgical wound infection.

Mastitis develops in 1–3% of nursing mothers. This infection of the breast, which generally presents within 2–3 weeks after delivery, is characterized by findings that range from cellulitis to abscess formation. Systemic signs, such as fever and chills, are often present in more severe cases.

Musculoskeletal infections

S. aureus is among the most common causes of bone infections—both those resulting from hematogenous dissemination and those arising from contiguous spread from a soft tissue site. Hematogenous osteomyelitis in children most often involves the long bones. Infections present as fever and bone pain or with a child’s reluctance to bear weight. The white blood cell count and erythrocyte sedimentation rate are often elevated. Blood cultures are positive in ~50% of cases. When necessary, bone biopsies for culture and histopathologic examination are usually diagnostic. Routine x-rays may be normal for up to 14 days after the onset of symptoms. ^99mTc-phosphonate scanning often detects early evidence of infection. MRI is more sensitive than other techniques in establishing a radiologic diagnosis.

In adults, hematogenous osteomyelitis involving the long bones is less common. However, vertebral osteomyelitis is among the more common clinical presentations. Vertebral bone infections are most often seen in patients with endocarditis, those undergoing hemodialysis, diabetics, and injection drug users. These infections may present as intense back pain and fever but may also be clinically occult, presenting as chronic back pain and low-grade fever. S. aureus is the most common cause of epidural abscess, a complication that can result in neurologic compromise. Patients complain of difficulty voiding or walking and of radicular pain in addition to the symptoms associated with their osteomyelitis. Surgical intervention in this setting often constitutes a medical emergency. MRI most reliably establishes the diagnosis (Fig. 43-2).

Bone infections that result from contiguous spread tend to develop from soft tissue infections, such as those associated with diabetic or vascular ulcers, surgery, or trauma. Exposure of bone, a draining fistulous tract, failure to heal, or continued drainage suggests involvement of underlying bone. Bone involvement is established by bone culture and histopathologic examination (revealing evidence of PMN infiltration). Contamination of culture material from adjacent tissue can make the diagnosis of osteomyelitis difficult in the absence of pathologic confirmation. In addition, it is sometimes hard to distinguish radiologically between osteomyelitis and overlying soft tissue infection with underlying osteitis.

In both children and adults, S. aureus is the most common cause of septic arthritis in native joints. This infection is rapidly progressive and may be associated with extensive joint destruction if left untreated. It presents as intense pain on motion of the affected joint, swelling, and fever. Aspiration of the joint reveals turbid fluid, with >50,000 PMNs/μL and gram-positive cocci in clusters on Gram’s stain (Fig. 43-1). In adults, septic arthritis may result from trauma, surgery, or hematogenous dissemination. The most commonly involved joints include the knees, shoulders, hips, and phalanges. Infection frequently develops in joints previously damaged by osteoarthritis or rheumatoid arthritis. Iatrogenic infections resulting from aspiration or injection of agents into the joint also occur. In these settings, the patient experiences increased pain and swelling in the involved joint in association with fever.

Pyomyositis is an unusual infection of skeletal muscles that is seen primarily in tropical climates but also occurs in immunocompromised and HIV-infected patients. It is believed to arise from occult bacteremia. Pyomyositis presents as fever, swelling, and pain overlying the involved muscle. Aspiration of fluid from the involved tissue yields pus. Although a history of trauma may be associated with the infection, its pathogenesis is poorly understood.

Respiratory tract infections

Respiratory tract infections caused by S. aureus occur in selected clinical settings. S. aureus is a cause of serious respiratory tract infections in newborns and infants; these infections present with shortness of breath, fever, and respiratory failure. Chest x-ray may reveal pneumatoceles (shaggy, thin-walled cavities). Pneumothorax and empyema are recognized complications.

In adults, nosocomial S. aureus pulmonary infections are common among intubated patients in intensive care units. Nasally colonized patients are at increased risk of these infections. The clinical presentation is no different from that encountered in pulmonary infections of other bacterial etiologies. Patients produce increased volumes of purulent sputum and develop respiratory distress, fever, and new pulmonary infiltrates. Distinguishing bacterial pneumonia from respiratory failure or other causes of new pulmonary infiltrates in critically ill patients is often difficult and relies on a constellation of clinical, radiologic, and laboratory findings.

Community-acquired respiratory tract infections due to S. aureus usually follow viral infections—most commonly influenza. Patients may present with fever, bloody sputum production, and midlung-field pneumatoceles or multiple, patchy pulmonary infiltrates. Diagnosis is made by sputum Gram’s stain and culture. Blood cultures, although useful, are usually negative.

Bacteremia, sepsis, and infective endocarditis

S. aureus bacteremia may be complicated by sepsis, endocarditis, vasculitis, or metastatic seeding (establishment of suppurative collections at other tissue sites). The frequency of metastatic seeding during bacteremia has been estimated to be as high as 31%. Among the more commonly seeded tissue sites are bones, joints, kidneys, and lungs.

Recognition of these complications by clinical and laboratory diagnostic methods alone is often difficult. Comorbid conditions that are frequently seen in association with S. aureus bacteremia and that increase the risk of complications include diabetes, HIV infection, and renal insufficiency. Other host factors associated with an increased risk of complications include presentation with community-acquired S. aureus bacteremia (except in injection drug users), lack of an identifiable primary focus of infection, and the presence of prosthetic devices or material.

Clinically, S. aureus sepsis presents in a manner similar to that documented for sepsis due to other bacteria. The well-described progression of hemodynamic changes—beginning with respiratory alkalosis and clinical findings of hypotension and fever—is commonly seen. The microbiologic diagnosis is established by positive blood cultures.

The overall incidence of S. aureus endocarditis has increased over the past 20 years. S. aureus is now the leading cause of endocarditis worldwide, accounting for 25–35% of cases. This increase is due, at least in part, to the increased use of intravascular devices. Studies of patients with S. aureus bacteremia and intravascular catheters that used transesophageal echocardiography found an infective endocarditis incidence of ~25%. Other factors associated with an increased risk of endocarditis are injection drug use, hemodialysis, the presence of intravascular prosthetic devices at the time of bacteremia, and immunosuppression. Patients with implantable cardiac devices (e.g., permanent pacemakers) are at increased risk of endocarditis or device-related infections. Despite the availability of effective antibiotics, mortality rates from these infections continue to range from 20% to 40%, depending on both the host and the nature of the infection. Complications of S. aureus endocarditis include cardiac valvular insufficiency, peripheral emboli, metastatic seeding, and central nervous system (CNS) involvement (e.g., mycotic aneurysms, embolic strokes).

S. aureus endocarditis is encountered in four clinical settings: (1) right-sided endocarditis in association with injection drug use, (2) left-sided native-valve endocarditis, (3) prosthetic-valve endocarditis, and (4) nosocomial endocarditis. In each of these settings, the diagnosis is suspected by recognition of clinical stigmata suggestive of endocarditis. These findings include cardiac manifestations, such as new or changing cardiac valvular murmurs; cutaneous evidence, such as vasculitic lesions, Osler’s nodes, or Janeway lesions; evidence of right- or left-sided embolic disease; and a history suggesting a risk for S. aureus bacteremia. In the absence of antecedent antibiotic therapy, blood cultures are almost uniformly positive. Transthoracic echocardiography, while less sensitive than transesophageal echocardiography, is less invasive and may establish the presence of valvular vegetations. The Duke criteria (see Table 24-3) are now commonly used to help establish the likelihood of this diagnosis.

Acute right-sided tricuspid valvular S. aureus endocarditis is most often seen in injection drug users. The classic presentation includes a high fever, a toxic clinical appearance, pleuritic chest pain, and the production of purulent (sometimes bloody) sputum. Chest x-rays or CT scans reveal evidence of septic pulmonary emboli (small, peripheral, circular lesions that may cavitate with time) (Fig. 43-3). A high percentage of affected patients have no history of antecedent valvular damage. At the outset of their illness, patients may present with fever alone, without cardiac or other localizing findings. As a result, a high index of clinical suspicion is essential for diagnosis.

Individuals with antecedent cardiac valvular damage more commonly present with left-sided native-valve endocarditis involving the damaged valve. These patients tend to be older than those with right-sided endocarditis, their prognosis is worse, and their incidence of complications (including peripheral emboli, cardiac decompensation, and metastatic seeding) is higher.

S. aureus is one of the more common causes of prosthetic-valve endocarditis. This infection is especially fulminant in the early postoperative period and is associated with a high mortality rate. In most instances, medical therapy alone is not sufficient and urgent valve replacement is necessary. Patients are prone to develop valvular insufficiency or myocardial abscesses originating from the region of valve implantation.

The increased frequency of nosocomial endocarditis (15–30% of cases, depending on the series) reflects in part the increased use of intravascular devices. This form of endocarditis is most commonly caused by S. aureus. Because patients often are critically ill, are receiving antibiotics for various other indications, and have comorbid conditions, the diagnosis is often missed.

Urinary tract infections

Urinary tract infections (UTIs) are infrequently caused by S. aureus. The presence of S. aureus in the urine generally suggests hematogenous dissemination. Ascending S. aureus infections occasionally result from instrumentation of the genitourinary tract.

Prosthetic device–related infections

S. aureus accounts for a large proportion of prosthetic device–related infections. These infections often involve intravascular catheters, prosthetic valves, orthopedic devices, peritoneal catheters, pacemakers, left-ventricular-assist devices, and vascular grafts. In contrast with the more indolent presentation of CoNS infections, S. aureus device-related infections are often more acute, with both localized and systemic manifestations. The latter infections also tend to progress more rapidly. It is relatively common for a pyogenic collection to be present at the device site. Aspiration of these collections and performance of blood cultures are important components in establishing a diagnosis. S. aureus infections tend to occur more commonly soon after implantation unless the device is used for access (e.g., intravascular or hemodialysis catheters). In the latter instance, infections can occur at any time. As in most prosthetic-device infections, successful therapy usually involves removal of the device. Left in place, the device is a potential nidus for either persistent or recurrent infections.

Infections associated with community-acquired MRSA

Although the skin and soft tissues are the most common sites of infection associated with CA-MRSA, 5–10% of these infections are invasive and can even be life-threatening. The latter unique infections, including necrotizing fasciitis, necrotizing pneumonia, and sepsis with Waterhouse-Friderichsen syndrome or purpura fulminans, were rarely associated with S. aureus prior to the emergence of CA-MRSA. These life-threatening infections reflect the increased virulence of CA-MRSA strains.

Toxin-mediated diseases

Food poisoning

S. aureus is among the most common causes of foodborne outbreaks of infection in the United States. Staphylococcal food poisoning results from the inoculation of toxin-producing S. aureus into food by colonized food handlers. Toxin is then elaborated in such growth-promoting food as custards, potato salad, or processed meats. Even if the bacteria are killed by warming, the heat-stable toxin is not destroyed. The onset of illness is rapid, occurring within 1–6 h of ingestion. The illness is characterized by nausea and vomiting, although diarrhea, hypotension, and dehydration may also occur. The differential diagnosis includes diarrhea of other etiologies, especially that caused by similar toxins (e.g., the toxins elaborated by Bacillus cereus). The rapidity of onset, the absence of fever, and the epidemic nature of the presentation (without second-degree spread) arouse suspicion of staphylococcal food poisoning. Symptoms generally resolve within 8–10 h. The diagnosis can be established by the demonstration of bacteria or the documentation of enterotoxin in the implicated food. Treatment is entirely supportive.

Toxic shock syndrome

TSS gained attention in the early 1980s, when a nationwide outbreak occurred in the United States among young, otherwise healthy, menstruating women. Epidemiologic investigation demonstrated that these cases were associated with the use of a highly absorbent tampon that had recently been introduced to the market. Subsequent studies established the role of TSST-1 in these illnesses. Withdrawal of the tampon from the market resulted in a rapid decline in the incidence of this disease. However, menstrual and nonmenstrual cases continue to be reported. Nonmenstrual cases are frequently seen in patients with surgical or postpartum wound infections.

The clinical presentation is similar in menstrual and nonmenstrual TSS. Evidence of clinical S. aureus infection is not a prerequisite. TSS results from the elaboration of an enterotoxin or the structurally related enterotoxin-like TSST-1. More than 90% of menstrual cases are caused by TSST-1, whereas a high percentage of nonmenstrual cases are caused by enterotoxins. TSS begins with relatively nonspecific flulike symptoms. In menstrual cases, the onset usually comes 2 or 3 days after the start of menstruation. Patients present with fever, hypotension, and erythroderma of variable intensity. Mucosal involvement is common (e.g., conjunctival hyperemia). The illness can rapidly progress to symptoms that include vomiting, diarrhea, confusion, myalgias, and abdominal pain. These symptoms reflect the multisystemic nature of the disease, with involvement of the liver, kidneys, gastrointestinal tract, and/or CNS. Desquamation of the skin occurs during convalescence, usually 1–2 weeks after the onset of illness. Laboratory findings may include azotemia, leukocytosis, hypoalbuminemia, thrombocytopenia, and liver function abnormalities.

Diagnosis of TSS still depends on a constellation of findings rather than one specific finding and on a lack of evidence of other possible infections (Table 43-2). Other diagnoses to be considered are drug toxicities, viral exanthems, Rocky Mountain spotted fever, sepsis, and Kawasaki disease. Illness occurs only in persons who lack antibody to TSST-1. Recurrences are possible if antibody fails to develop after the illness.

Staphylococcal scalded-skin syndrome

SSSS primarily affects newborns and children. The illness may vary from a localized blister to exfoliation of much of the skin surface. The skin is usually fragile and often tender, with thin-walled, fluid-filled bullae. Gentle pressure results in rupture of the lesions, leaving denuded underlying skin (Nikolsky’s sign; Fig. 43-4). The mucous membranes are usually spared. In more generalized infection, there are often constitutional symptoms, including fever, lethargy, and irritability with poor feeding. Significant amounts of fluid can be lost in more extensive cases. Illness usually follows localized infection at one of a number of possible sites. SSSS is much less common among adults but can follow infections caused by exfoliative toxin–producing strains.

PREVENTION

Primary prevention of S. aureus infections in the hospital setting involves hand washing and careful attention to appropriate isolation procedures. Through careful screening for MRSA carriage and strict isolation practices, several Scandinavian countries have been remarkably successful at preventing the introduction and dissemination of MRSA in hospitals.

Decolonization strategies, using both universal and targeted approaches with topical agents (e.g., mupirocin) to eliminate nasal colonization and/or chlorhexidine to eliminate cutaneous colonization with S. aureus, have been successful in some clinical settings (e.g., intensive care units) where the risk of infection is high. An analysis of clinical trials suggests that there may also be a reduction in the incidence of postsurgical infections among persons who are nasally colonized with S. aureus.

“Bundling” (the application of selected medical interventions in a sequence of prescribed steps) has reduced rates of nosocomial infections related to such procedures as the insertion of intravenous catheters, in which staphylococci are among the most common pathogens (see Table 17-4). A number of immunization strategies to prevent S. aureus infections—both active (e.g., capsular polysaccharide–protein conjugate vaccine) and passive (e.g., clumping factor antibody)—have been investigated. However, none has been successful for either prophylaxis or therapy in clinical trials.

Although considerably less virulent than S. aureus, CoNS are among the most common causes of prosthetic-device infections. Approximately half of the identified CoNS species have been associated with human infections. Of these species, Staphylococcus epidermidis is the most common human pathogen. This component of the normal human flora is found on the skin (where it is the most abundant bacterial species) as well as in the oropharynx and vagina. Staphylococcus saprophyticus, a novobiocin-resistant species, is a common pathogen in UTIs.

PATHOGENESIS

S. epidermidis is the CoNS species most often associated with prosthetic-device infections. Infection is a two-step process, with initial adhesion to the device followed by colonization. S. epidermidis is uniquely adapted to colonize these devices by its capacity to elaborate the extracellular polysaccharide (glycocalyx or slime) that facilitates formation of a protective biofilm on the device surface.

Implanted prosthetic material is rapidly coated with host serum or tissue constituents such as fibrinogen or fibronectin. These ­molecules serve as potential bridging ligands, facilitating initial bacterial attachment to the device surface. A number of staphylococcal surface-­associated proteins, such as autolysin (AtlE), fibrinogen-binding ­protein, and accumulation-associated protein (AAP), may play a role in attachment to either modified or unmodified prosthetic surfaces. The polysaccharide intercellular adhesin facilitates subsequent ­staphylococcal colonization and accumulation on the device surface. In S. epidermidis, intercellular adhesin (ica) genes are more commonly found in strains associated with device infections than in strains associated with colonization of mucosal surfaces. Biofilm appears to act as a barrier protecting bacteria from host defense mechanisms as well as from antibiotics, while providing a suitable environment for bacterial survival. Poly-γ-DL-glutamic acid is secreted by S. epidermidis and provides protection against neutrophil phagocytosis.

Two additional staphylococcal species, Staphylococcus lugdunensis and Staphylococcus schleiferi, produce more serious infections (native-valve endocarditis and osteomyelitis) than do other CoNS. The basis for this enhanced virulence is not known, although both species appear to share more virulence determinants with S. aureus (e.g., clumping factor and lipase) than do other CoNS.

The capacity of S. saprophyticus to cause UTIs in young women appears to be related to its enhanced capacity to adhere to uroepithelial cells. A 160-kDa hemagglutinin/adhesin may contribute to this affinity.

DIAGNOSIS

Although the detection of CoNS at sites of infection or in the bloodstream is not difficult by standard microbiologic culture methods, interpretation of these results is frequently problematic. Because these organisms are present in large numbers on the skin, they often contaminate cultures. It has been estimated that only 10–25% of blood cultures positive for CoNS reflect true bacteremia. Similar problems arise with cultures obtained from other sites. Among the clinical findings suggestive of true bacteremia are fever, evidence of local infection (e.g., erythema or purulent drainage at the IV catheter site), leukocytosis, and systemic signs of sepsis. Laboratory findings suggestive of true bacteremia include multiple isolations of the same strain (i.e., the same species with the same antibiogram or a closely related DNA fingerprint) from separate cultures, growth of the strain within 48 h, and bacterial growth in both aerobic and anaerobic bottles.

CLINICAL SYNDROMES

CoNS cause a diverse array of prosthetic device–related infections, including those that involve prosthetic cardiac valves and joints, vascular grafts, intravascular devices, and CNS shunts. In all of these settings, the clinical presentation is similar. The signs of localized infection are often subtle, the rate of disease progression is slow, and the systemic findings are often limited. Signs of infection, such as purulent drainage, pain at the site, or loosening of prosthetic implants, are sometimes evident. Fever is frequently but not always present, and there may be mild leukocytosis. Acute-phase reactant levels, the erythrocyte sedimentation rate, and the C-reactive protein concentration may be elevated.

Infections that are not associated with prosthetic devices are infrequent, although native-valve endocarditis due to CoNS has accounted for ~5% of cases in some reviews. S. lugdunensis appears to be a more aggressive pathogen in this setting, causing greater mortality and rapid valvular destruction with abscess formation.

TREATMENT