CHAPTER 48: LISTERIA MONOCYTOGENES INFECTIONS

Listeria monocytogenes is a food-borne pathogen that can cause serious infections, particularly in pregnant women and immunocompromised individuals. A ubiquitous saprophytic environmental bacterium, L. monocytogenes is also a facultative intracellular pathogen with a broad host range. Humans are probably accidental hosts for this microorganism. L. monocytogenes is of interest not only to clinicians but also to basic scientists as a model intracellular pathogen that is used to study basic mechanisms of microbial pathogenesis and host immunity.

L. monocytogenes is a facultatively anaerobic, nonsporulating, gram-positive rod that grows over a broad temperature range, including refrigeration temperatures. This organism is motile during growth at low temperatures but much less so at 37°C. The vast majority of cases of human listerial disease can be traced to serotypes 1/2a, 1/2b, and 4. L. monocytogenes is weakly β-hemolytic on blood agar, and (as detailed below) its β-hemolysin is an essential determinant of its pathogenicity.

Infections with L. monocytogenes follow ingestion of contaminated food that contains the bacteria at high concentrations. The conversion from environmental saprophyte to pathogen involves the coordinate regulation of bacterial determinants of pathogenesis that mediate entry into cells, intracellular growth, and cell-to-cell spread. Many of the organism’s pathogenic strategies can be examined experimentally in tissue culture models of infection (Fig. 48-1). Like other enteric pathogens, L. monocytogenes induces its own internalization by cells that are not normally phagocytic. Its entry into cells is mediated by host surface proteins classified as internalins. Internalin-mediated entry is important in the crossing of intestinal, blood-brain, and fetoplacental barriers, although how L. monocytogenes traffics from the intestine to the brain or fetus is only beginning to be investigated. In a pregnant guinea pig model of infection, L. monocytogenes was shown to traffic from maternal organs to the placenta; surprisingly, however, it also trafficked from the placenta back to maternal organs. These data are consistent with a model in which miscarriage can be viewed as a host defense strategy to eliminate a nidus of infection.

An essential determinant of the pathogenesis of L. monocytogenes is its β-hemolysin, listeriolysin O (LLO). LLO is a pore-forming, cholesterol-dependent cytolysin. (Related cytolysins include streptolysin O, pneumolysin, and perfringolysin O, all of which are produced by extracellular pathogens.) LLO is largely responsible for mediating the rupture of the phagosomal membrane that forms after phagocytosis of L. monocytogenes. LLO probably acts by insertion into an acidifying phagosome, which prevents the vesicle’s maturation. In addition, LLO acts as a translocation pore for one or both of the L. monocytogenes phospholipases that also contribute to vacuolar lysis. LLO synthesis and activity are controlled at multiple levels to ensure that its lytic activity is limited to acidic vacuoles and does not affect the cytosol. Mutations in LLO that influence its synthesis, cytosolic half-life, or pH optimum cause premature toxicity to infected cells. There is an inverse relationship between toxicity and virulence—i.e., the more cytotoxic the strain, the less virulent it is in animals. This relationship may seem paradoxical, but, as an intracellular pathogen, L. monocytogenes benefits from leaving its host cell unharmed.

Shortly after exposure to the mammalian-cell cytosol, L. monocytogenes expresses a surface protein, ActA, that mediates the nucleation of host actin filaments to propel the bacteria intra- and intercellularly. ActA mimics host proteins of the Wiskott-Aldrich syndrome protein (WASP) family by promoting the actin nucleation properties of the Arp2/3 complex. Thus, L. monocytogenes can enter the cytosol of almost any eukaryotic cell or cell extract and can exploit a conserved and essential actin-based motility system. Other pathogens as diverse as certain Shigella, Mycobacterium, Rickettsia, and Burkholderia species use a related pathogenic strategy that allows cell-to-cell spread without exposure to the extracellular milieu.

The innate and acquired immune responses to L. monocytogenes have been studied extensively in mice. Shortly after IV injection, most bacteria are found in Kupffer cells in the liver, with some organisms in splenic dendritic cells and macrophages. Listeriae that survive the bactericidal activity of initially infected macrophages grow in the cytosol and spread from cell to cell. L. monocytogenes triggers three innate immune pathways: a MyD88-dependent pathway leading to inflammatory cytokines, a STING/IRF3 pathway leading to a type I interferon response; and low-level inflammasome activation. Neutrophils are crucial to host defense during the first 24 h of infection, whereas an influx of activated macrophages from the bone marrow is critical subsequently. Mice that survive sublethal infection clear the organisms within a week, with consequent sterile immunity. Studies with knockout mice have been instrumental in dissecting the roles played by chemokines and cytokines during infection. For example, interferon γ, tumor necrosis factor, and CCR2 are essential in controlling infection. While innate immunity is sufficient to control infection, the acquired immune response is required for sterile immunity. Immunity is cell mediated; antibody plays no measurable role. The critical effector cells are cytotoxic (CD8+) T cells that recognize and lyse infected cells; the resulting extracellular bacteria are killed by circulating activated phagocytes.

A hallmark of the L. monocytogenes model is that killed vaccines do not provide protective immunity. The explanation for this fundamental observation is multifactorial, involving the generation of appropriate cytokines and the compartmentalization of bacterial proteins for antigen processing and presentation. Because the organism has the capacity to induce a robust cell-mediated immune response, attenuated strains have been engineered to express foreign antigens and are undergoing clinical studies as therapeutic vaccines for cancer.

L. monocytogenes usually enters the body via the gastrointestinal tract in foods. Listeriosis is most often sporadic, although outbreaks do occur. No epidemiologic or clinical evidence supports person-to-person transmission (other than vertical transmission from mother to fetus) or waterborne infection. In line with its survival and multiplication at refrigeration temperatures, L. monocytogenes is commonly found in processed and unprocessed foods of animal and plant origin, especially soft cheeses, delicatessen meats, hot dogs, milk, and cold salads; fresh fruits and vegetables can also transmit the organism. Because food supplies are increasingly centralized and normal hosts tolerate the organism well, outbreaks may not be immediately apparent. The U.S. Food and Drug Administration has a zero-tolerance policy for L. monocytogenes in ready-to-eat foods.

Symptoms of listerial infection overlap greatly with those of other infectious diseases. Timely diagnosis requires that the illness be considered in groups at risk: pregnant women; elderly persons; neonates; individuals immunocompromised by organ transplantation, cancer, or treatment with tumor necrosis factor antagonists or glucocorticoids; and patients with a variety of chronic medical conditions, including alcoholism, diabetes, renal disease, and rheumatologic and hepatic illnesses. Meningitis in older adults (especially with parenchymal brain involvement or subcortical brain abscess) should trigger consideration of L. monocytogenes infection and treatment. Listeriosis occasionally affects healthy, young, nonpregnant individuals. HIV-infected patients are at risk; however, listeriosis seems to be prevented by trimethoprim-sulfamethoxazole (TMP-SMX) prophylaxis targeting other AIDS-related infections. The diagnosis is typically made by culture of blood, cerebrospinal fluid (CSF), or amniotic fluid. L. monocytogenes may be confused with “diphtheroids” or pneumococci in Gram-stained CSF or may be gram-variable and confused with Haemophilus species. Polymerase chain reaction diagnostics have been described but are not widely available, and serology is not ­clinically useful.

Listerial infections present as several clinical syndromes, of which meningitis and septicemia are most common. Monocytosis is seen in infected rabbits but is not a hallmark of human infection.

Gastroenteritis

Appreciated only since the outbreaks of the late 1980s, listerial gastroenteritis typically develops within 48 h of ingestion of a large inoculum of bacteria in contaminated foods. Attack rates are high (50–100%). L. monocytogenes is neither sought nor found in routine fecal cultures, but its involvement should be considered in outbreaks when cultures for other likely pathogens are negative. Sporadic intestinal illness appears to be uncommon. Manifestations include fever, diarrhea, headache, and constitutional symptoms. The largest reported outbreak occurred in an Italian school system and included 1566 individuals; ~20% of patients were hospitalized, but only one person had a positive blood culture. Isolated gastrointestinal illness does not require antibiotic treatment. Surveillance studies show that 0.1–5% of healthy asymptomatic adults may have stool cultures positive for the organism.

Bacteremia

L. monocytogenes septicemia presents with fever, chills, and myalgias/arthralgias and cannot be differentiated from septicemia involving other organisms. Meningeal symptoms, focal neurologic findings, or mental status changes may suggest the diagnosis. Bacteremia is documented in 70–90% of cancer patients with listeriosis. A nonspecific flulike illness with fever is a common presentation in pregnant women. Endocarditis of prosthetic and native valves is an uncommon complication, with reported fatality rates of 35–50% in case series. A lumbar puncture is often prudent, although not necessary, in pregnant women without central nervous system (CNS) symptoms.

Meningitis

L. monocytogenes causes ~5–10% of all cases of community-acquired bacterial meningitis in adults in the United States. Case-fatality rates are reported to be 15–26% and do not appear to have changed over time. This diagnosis should be considered in all older or chronically ill adults with “aseptic” meningitis. The presentation is more frequently subacute (with illness developing over several days) than in meningitis of other bacterial etiologies, and nuchal rigidity and meningeal signs are less common. Photophobia is infrequent. Focal findings and seizures are common in some but not all series. The CSF profile in listerial meningitis most often shows white blood cell counts in the range of 100–5000/μL (rarely higher); 75% of patients have counts below 1000/μL, usually with a neutrophil predominance more modest than that in other bacterial meningitides. Low glucose levels and positive results on Gram’s staining are found ~30–40% of the time. Hydrocephalus can occur.

Meningoencephalitis and focal cns infection

L. monocytogenes can directly invade the brain parenchyma, producing either cerebritis or focal abscess. Approximately 10% of cases of CNS infection are macroscopic abscesses resulting from bacteremic seeding; the affected patients often have positive blood cultures. Concurrent meningitis can exist, but the CSF may appear normal. Abscesses can be misdiagnosed as metastatic or primary tumors and, in rare instances, occur in the cerebellum and the spinal cord. Invasion of the brainstem results in a characteristic severe rhombencephalitis, usually in otherwise healthy older adults (although there are numerous other infectious and noninfectious causes of this syndrome). The presentation may be biphasic, with a prodrome of fever and headache followed by asymmetric cranial nerve deficits, cerebellar signs, and hemiparetic and hemisensory deficits. Respiratory failure can occur. The subacute course and the often minimally abnormal CSF findings may delay the diagnosis, which may be suggested by MRI showing ring-enhancing lesions after gadolinium contrast and hyperintense lesions on diffusion-weighted imaging. MRI is superior to CT for the diagnosis of these infections.

Infection in pregnant women and neonates

Listeriosis in pregnancy is a severe and important infection. The usual presentation is a nonspecific acute or subacute febrile illness with myalgias, arthralgias, backache, and headache. Pregnant women with listeriosis are usually bacteremic. This syndrome should prompt blood cultures, especially if there is no other reasonable explanation. Involvement of the CNS is rare in the absence of other risk factors. Preterm delivery is a common complication, and the diagnosis may be made only post-partum. As many as 70–90% of fetuses from infected women can become infected. Prepartum treatment of bacteremic women enhances the chances of delivery of a healthy infant. Women usually do well after delivery: maternal deaths are very rare, even when the diagnosis is made late in pregnancy or post-partum. Overall mortality rates for fetuses infected in utero approach 50% in some series; among live-born neonates treated with antibiotics, mortality rates are much lower (~20%). Granulomatosis infantiseptica is an overwhelming listerial fetal infection with miliary microabscesses and granulomas, most often in the skin, liver, and spleen. Less severe neonatal infection acquired in utero presents at birth. “Late-onset” neonatal illness typically develops ~10–30 days post-partum. Mothers of infants with late-onset disease are not ill.

Many individuals who are promptly diagnosed and treated recover fully, but permanent neurologic sequelae are common in patients with brain abscess or rhombencephalitis. Focal infections of visceral organs; the eye; the pleural, peritoneal, and pericardial spaces; the bones; and both native and prosthetic joints have all been reported. Of 100 live-born, treated neonates in one series, 60% recovered fully, 24% died, and 13% had long-term neurologic or other complications.

Healthy persons should take standard precautions to prevent food-borne illness: fully cooking meats, washing fresh vegetables, carefully cleaning utensils, and avoiding unpasteurized dairy products. In addition, persons at risk for listeriosis, including pregnant women, should avoid soft cheeses (hard cheeses and yogurt are not problematic) and should avoid or thoroughly reheat ready-to-eat and delicatessen foods.