Chapter 22: Pathogenesis of Bacterial Infections

• Pathogenicity—The ability of any bacterial species to cause disease in a susceptible human host.

• Pathogen—A bacterial species able to cause such disease when presented with favorable circumstances (for the organism).

• Virulence—A term which presumes pathogenicity, but allows expression of degrees from low to extremely high, for example:

  • Low virulence—Streptococcus salivarius is universally present in the oropharyngeal flora of humans. On its own, it seems incapable of disease production, but if during a transient bacteremia it lands on a damaged heart valve, it can stick and cause slow but steady destruction.

  • Moderate virulence—Escherichia coli is universally found in the colon, but if displacement to other sites such as adjacent tissues or the urinary bladder regularly causes acute infection.

  • High virulence—Bordetella pertussis, the cause of whooping cough, is not found in the resident flora, but if encountered it is highly infectious and causes disease in almost every nonimmune person it contacts.

  • Extremely high virulence—Yersinia pestis, the cause of plague, is also highly infectious, but in addition leads to death in a few days in over 70% of cases.

As discussed in Chapter 1, humans have a rich microbiota, and the composition of that flora is mostly bacterial. Of these bacteria, most in humans are commensal; that is, they eat from the same table that we do. These microbes are constant companions and often depend on humans for their existence. We also encounter transient species, which are just passing through, but some of these may be opportunistic pathogens. That is, they can cause disease only when one or more of the defense mechanisms designed to restrict them from the usually sterile internal tissues are breached by accident, by intent (eg, surgery), or by an underlying metabolic or an infectious disorder (eg, AIDS). Nevertheless, a small group of bacteria regularly cause infection and overt disease in seemingly healthy persons. These are the primary pathogens such as the typhoid bacillus, gonococcus, and the tubercle bacillus, which are never considered members of the normal microbiota.

Long-term survival in a primary pathogen is absolutely dependent on its ability to replicate, survive, and be transmitted to another host. To accomplish this, the primary pathogens have evolved the ability to breach human cellular and anatomic barriers that ordinarily restrict or destroy commensal and transient microorganisms. Thus, pathogens can inherently cause damage to cells to gain access by force to a new unique niche that provides them with less competition from other microorganisms, as well as a ready new source of nutrients. Thus, pathogens have not only acquired the capacity to breach cellular barriers, they also have, by necessity, learned to circumvent, exploit, subvert, and even manipulate our normal cellular mechanisms to their own selfish need to multiply at our expense.

For pathogens not adapted to humans, other animals, or insects, survival in the environment is a requirement for continued disease production. As the most adaptable living forms on the planet it is not surprising that pathogens are part of the free-living forms common among bacteria. Extended survival is enhanced by the formation of biofilms. These extracellular polysaccharide slimes act to bind an entire community of bacteria to an environmental site, for example, water pipes. Endospores provide the most extended survival form for gram-positive bacteria.

The emergence of many seemingly new bacterial diseases has as much to do with human behavior as bacterial adaptability. The Legionnaires disease outbreak of 1976 was eventually traced to Legionella pneumophila, which is widely found in aquatic environments as an infectious agent of amoebae. However, without the aerosolization created by modern systems (cooling towers) designed to humidify large buildings, transmission to humans would not have occurred. The development of super absorbent tampons had the unintended consequence of providing conditions favorable for the production of a toxin by some strains of S aureus. The result was a national outbreak of toxic shock syndrome. Food poisoning by E coli O157:H7, Campylobacter, and Salmonella arise as much from food technology and modern food distribution networks as from any fundamental change in the virulence properties of the bacteria in question. No part of our planet is more than 3 days away by air travel, a fact known and feared by all public health officials.

Whether a microbe is a primary or opportunistic pathogen, it must be able to enter a host; find a unique niche; avoid, circumvent, or subvert normal host defenses; multiply; and injure the host. For long-term success as a pathogen, it must also establish itself in the host or somewhere else long enough to eventually be transmitted to a new susceptible host. This competition between the pathogen and the host can be viewed as similar to the more familiar military or athletic struggles—that is, the offense against the defense. The more we learn about bacterial pathogens, the more it seems that the most successful ones not only have an excellent offense; they are also particularly able to confound the host defense.

ENTRY: BEATING INNATE HOST DEFENSES

Each of the portals in the body that communicates with the outside world becomes a potential site of microbial entry. Human and other animal hosts have various protective mechanisms to prevent microbial entry (Table 22–1). A simple, though relatively efficient, mechanical barrier to microbial invasion is provided by the epithelial borders of the internal and external body surfaces. Of these, the skin is the most formidable with its tough keratinized superficial layer. Organisms can gain access to the underlying tissues only by breaks or by way of hair follicles, sebaceous glands, and sweat glands that traverse the stratified layers. The surface of the skin continuously desquamates and thus tends to shed contaminating organisms. The skin also inhibits the growth of most extraneous microorganisms because of low moisture, low pH, and the presence of substances with antibacterial activity. Bacteria have no known mechanism for passing the unbroken skin.

For the internal surfaces a viscous mucin secreted by goblet cells protects the epithelium lining of the respiratory tract, the gastrointestinal tract, and the urogenital system. Microorganisms become trapped in this thick network of protein and polysaccharide and may be swept away before they reach the epithelial cell surface. Secretory IgA (sIgA) secreted into the mucus and other secreted antimicrobials such as lysozyme and lactoferrin aid this cleansing process. Some bacteria excrete an enzyme sIgA protease, which cleaves human sIgA1 in the hinge region to release the Fc portion from the Fab fragment. This enzyme may play an important role in establishing microbial species at the mucosal surface. Ciliated epithelial cells constantly move the mucin away from the lower respiratory tract. In the respiratory tract, particles larger than 5 μm are trapped in this fashion. The epithelium of the intestinal tract below the esophagus is a less efficient mechanical barrier than the skin, but there are other effective defense mechanisms. The high level of hydrochloric acid and gastric enzymes in the normal stomach kill many ingested bacteria. Other bacteria are susceptible to pancreatic digestive enzymes or to the detergent effect of bile salts.

How efficiently bacterial pathogens navigate all these barriers before their initial encounter with their target cell type is in some ways measured by their infecting dose. How many organisms must be given to a host to ensure infection in some proportion of the individuals? Estimates of the infectious doses for several pathogens are shown in Table 22–2. In general, pathogens that have environmental or animal reservoirs can overwhelm innate defenses with large numbers. Those that are amplified by growth in food may also deliver high numbers with or without a reservoir. Pathogens with no reservoir or amplification mechanism must be transmitted human-to-human and thus require the lowest infecting doses. Without this advantage, these pathogens would eventually die out in the population.

ADHERENCE: THE SEARCH FOR A UNIQUE NICHE

The first major interaction between a pathogenic microorganism and its host entails attachment to a eukaryotic cell surface. In its simplest form, adherence requires the participation of two factors: an adhesin on the invading microbe and a receptor on the host cell (Figure 22–1). The adhesin must be exposed on the bacterial surface either alone or in association with appendages like pili. Pili seem to be “sticky” by themselves which may be enhanced by specific adhesin/receptor molecular relationships mediated by molecules at their tips. In gram-negative bacteria, the outer membrane is a major site for adhesins. Most adhesins are proteins, but carbohydrates and teichoic acids may also be involved. The chemical nature of the host receptors is less well known because of the greater difficulty in their isolation (bacteria can be grown by the gallon), but they may be thought of as general or specific. For example, two of the most common receptors, mannose and fibronectin, are widely present on human epithelial cell surfaces. Pili that bind to them can mediate attachment at many sites. Specific receptors are those unique to a particular cell type such as human enterocytes or uroepithelial cells. Where known, these receptors are usually sugar residues that are part of glycolipids or glycoproteins on the host cell surface.

Many bacteria have more than one mechanism of host cell attachment. In some instances, pili mediate initial attachment, which is followed by a stronger, more specific binding mediated by another protein. This may allow implementation of a second function such as cytoskeleton rearrangement or invasion. Multiple adhesins may also allow bacteria to use one set at the epithelial surface, but a different set when encountering other cell types or the immune system. The role of pili may be more than a simple adhesive one. The pili of Neisseria gonorrhoeae, the cause of gonorrhea, mediate an active twitching motility on the cell surface with the formation of mobile microcolonies (Figure 22–2). Biofilms may also act as an adherence mechanism by binding to catheters, prosthetic devices, or mucosal surfaces.

Strategies for Survival

Once the bacterial pathogen attaches, it must persist if it is to produce disease. Survival is less complicated if the organism can produce injury without moving from its initial niche. This is the case with some exotoxin-mediated bacterial diseases (diphtheria, whooping cough), but most pathogens must move either into the cell or beyond it. To do so requires a new set of survival strategies which include either multiplying in the intracellular milieu or avoiding the attack of complement and phagocytes in the submucosa.

INVASION: GETTING INTO CELLS

A few bacteria, like viruses, are obligate intracellular pathogens. Other bacteria are facultative intracellular pathogens and can grow as free-living cells in the environment as well as within host cells. Generally, invasive organisms adhere to host cells by one or more adhesins but use a class of molecules, called invasins, which interact with integrins or other families of cell adhesion molecules. The integrins in turn interact with elements of the cell cytoskeleton stimulating modifications which end in uptake of the bacterial cell. Invasive bacteria seem to be exploiting cell uptake mechanisms that are there for other purposes such as nutrition.

Bacteria enter cells initially within a membrane-bound, host-vesicular structure but then follow one of two pathways (Figure 22–3). Some bacteria (Listeria, Shigella) enzymatically lyse the phagosome membrane and escape to the nutrient-rich safe haven of the host cell cytoplasm. These bacteria may continue to multiply there, infect adjacent cells, or move through the cell to the submucosa. Other invasive pathogenic species (Salmonella serotype Typhi, Mycobacterium tuberculosis) remain in the phagosome and replicate even in professional phagocytes. Their survival in this usually perilous location is due to thwarting normal host cell trafficking patterns and avoiding the killing action of the phagolysosome. There are multiple known mechanisms for this including preventing phagosome–lysosome fusion or, if fused, blocking acidification to the optimum pH for digestive enzyme activity. Some bacteria are able to neutralize the phagocytes’ oxidative burst by the production of neutralizing enzymes (catalase, superoxide dismutase).

In gram-negative bacteria with injection secretion systems (types III, IV, VI), a variation on the above scenarios is possible. The secretion systems inject many proteins, some of which disrupt cellular signaling and the cell’s cytoskeleton. The cytoskeleton rearrangements may leave the bacteria tightly bound to an altered surface or trigger invasion. One pathogen even injects its own receptor, which is processed to the outer membrane where it mediates tight binding of its parent bacterial strain.

PERSISTING IN A NEW ENVIRONMENT

Bacteria that reach the subepithelial tissues are immediately exposed to the extracellular tissue fluids, which have defined properties that inhibit multiplication of many bacteria. For example, most tissues contain lysozyme in sufficient concentrations to disrupt the cell wall of gram-positive bacteria. Tissue fluid itself is a suboptimal growth medium for most bacteria and is deficient in free iron. In humans the iron not found in hemoglobin is chelated to a series of iron-binding proteins (lactoferrin, transferrin). Because virtually all pathogenic bacteria require iron they have evolved their own set of iron-binding proteins called siderophores which effectively compete with the human proteins for available iron.

Confounding the Immune System

The host immune system evolved in large part because of the selective pressure of microbial attack. To be successful, microbial pathogens must escape this system at least long enough to be transmitted to a new susceptible host or to take up residence within the host in a way that is compatible with mutual coexistence.

Innate Immunity

Manipulating PAMPs and AMPs

The early warning and response system in which pathogen-associated molecular patterns (PAMPs) are recognized by Toll-like receptors (TLRs) (see Chapter 2) is subject to evasion by successful pathogens. This has been studied in regard to gram-negative bacterial LPS whose pattern is typically detected by TLR-4. In some pathogens (Helicobacter, Legionella, Yersinia) the lipid A (toxic) component of LPS is simply a variant poorly recognized by TLR-4; other pathogens (Salmonella, Pseudomonas) are able to modulate their lipid A pattern. The result of both is a head start by evading a major innate immune mechanism. Modification of lipid A in a way that modifies their surface charge is also a way that gram-negative bacteria escape the action of antimicrobial peptides (AMPs) which attack bacterial membranes by electrostatic force. Gram-positive bacteria may accomplish a similar benefit by altering their cell wall teichoic acids.

Disrupting Complement

A fundamental requirement for many pathogenic bacteria is escape from phagocytosis by macrophages and polymorphonuclear leukocytes. The most common bacterial means of avoiding phagocytosis is an antiphagocytic capsule, which is possessed by almost all principal pathogens that cause pneumonia and meningitis. These polysaccharide capsules of pathogens interfere with effective complement deposition on the bacterial cell surface by binding regulators of C3b that are present in serum. When one of these, serum factor H, is concentrated on the capsular surface, it accelerates the degradation of C3b deposited from the host’s serum. This negates both direct complement injury and makes the receptors recognized by phagocytes unavailable (Figure 22–4). This mechanism is not restricted to polysaccharide capsules. Surface proteins able to bind factor H have the same biologic effect. Antibody directed against the capsular antigen reverses this effect because C3b can then bind in association with IgG. Another mechanism for complement disruption is through surface acquisition of sialic acid, a common component of capsular polysaccharides. Some bacteria are able to incorporate sialic acid from the host on their surfaces with an effect similar to capsules.

Induction of Apoptosis

One of the most common tactics of pathogens is to produce proteins which induce programmed cell death (apoptosis). This microbial tactic not only inactivates the killing potential of the phagocyte, but also reduces the number of defenders available to inhibit other bacterial invaders. The invading bacteria that induce apoptosis obtain the added benefit that death by apoptosis nullifies the normal cellular signaling processes of cytokine and chemokine signaling of necrotic death.

Adaptive Immunity

Antigenic Variation

Another method by which microorganisms avoid host immune responses is by varying surface antigens. Gonorrhea is a disease in which there appears to be no natural immunity and reinfections are common. In fact, an immune response can be mounted to the pathogenically significant surface pili and outer membrane proteins (OMPs) of N gonorrhoeae, but the organism is continuously varying them. This can happen even in the course of a single acute infection. The genetic mechanism for antigenic variation of pili involves recombination between multiple silent and expressing genes in the gonococcal chromosome. For OMPs multiple genes are turned on and off by the status of a frame-shift mutation. These mechanisms are illustrated in Figure 22–5 and discussed further in Chapter 30. The effect is that when the immune system delivers specific IgG to the site of infection, it will bind its homologous antigen, but a subpopulation with an antigenically different surface can multiply and continue the infection. Therefore, the pathogen escapes immune surveillance. A number of other bacteria and parasites also undergo antigenic variation.

INJURY

The successful pathogen must survive and multiply in the face of multiple host defenses. Although this is a formidable achievement, by itself it is not enough to cause disease. Disease requires some disruption of host function by the bacteria. Bacterial toxins are the most obvious mechanism of injury and are exported by the secretion systems described in Chapter 21 often along with multiple other virulence factors. In some diseases the only injury appears to be due to the inflammatory response to the invader.

Exotoxins

The longest known and best studied virulence factors are bacterial exotoxins. They are proteins toxic to the human host which are secreted by the bacteria into the surrounding body fluids. Their action may be local or systemic if absorbed into the bloodstream. These exotoxins usually possess some degree of host cell specificity, which is dictated by the nature of the binding of one or more toxin components to a specific host cell receptor. The distribution of host cell receptors often dictates the degree and nature of the toxicity.

A–B Exotoxins

The best-known pathogenic exotoxin theme is represented by the A–B exotoxins. These toxins are divided into two general domains. The B subunit(s) contains the binding specificity of the holotoxin to the host cell. Generally speaking, the B region binds to a specific host cell surface glycoprotein or glycolipid. The specificity of this binding determines the host cell specificity of the toxin. The A (active) subunit, catalyzes an enzymatic reaction characteristic for the toxin. After attachment of the B domain to the host cell surface, the A domain is transported by direct fusion or by endocytosis into the host cell. In the cell, the A unit carries out the enzymatic modification of a protein called its target protein. The most common enzymatic reaction is ADP-ribosylation, which attaches the ADP-ribose moiety from NAD to the target protein. This ADP-ribosylated protein is then unable to carry out its function or behaves abnormally. There are multiple other enzymatic reactions carried out by A–B exotoxins.

The net effect of the toxin depends on the intracellular function of the target protein and the biologic function of the cell in humans. If it is crucial for the protein-synthesizing apparatus of the cell (diphtheria toxin), protein synthesis ceases and the cell dies (see Figure 1–7). However, cell death is not the inevitable outcome of toxin action. One of the major targets of the ADP-ribosylating A–B toxins are guanine nucleotide-binding proteins (G proteins), which are involved in signal transduction in eukaryotic cells. In this case, the inactivation of the regulatory G protein can inhibit or stimulate some activity of the cell. Cholera toxin inactivates a G protein that downregulates a secretory pathway. If the cell is an intestinal enterocyte, the end result is hypersecretion of electrolytes and diarrhea. Cholera toxin applied to cells from the adrenal gland stimulates steroid production.

Membrane-Active Exotoxins

Some exotoxins act directly on the surface of host cells to lyse or to kill them. Many were first observed in the laboratory by their ability to cause hemolysis of erythrocytes. The most common action is to create pores by direct insertion into eukaryotic membranes of a wide range of cells including phagocytes (Figure 22–6). These pore-forming toxins are produced by some of the most aggressive pathogens (S aureus, group A streptococcus, E coli) and cause cellular death by loss of cellular integrity and leakage through the pore. Some are called the RTX (repeats in toxin) group because of a recurrent amino acid sequence in their structure. Another type of membrane-active toxin acts through direct enzymatic activity destroying the integrity of plasma membrane lipids. The α-toxin of Clostridium perfringens is a lecithinase causing hemolysis of RBCs.

Hydrolytic Enzymes

Many bacteria produce one or more enzymes that are nontoxic per se, but facilitate tissue invasion or help to protect the organism against the body’s defense mechanisms. For example, various bacteria produce collagenase or hyaluronidase or convert serum plasminogen to plasmin, which has fibrinolytic activity. Although the evidence is not conclusive, it is reasonable to assume that these substances may facilitate the spread of infection. Some bacteria also produce deoxyribonuclease, elastase, and many other biologically active enzymes, but their function in the disease process or in providing nutrients for the invaders is uncertain.

Superantigen Exotoxins

Some microbial exotoxins have a direct effect on cells of the immune system, and this interaction leads to disease. The most dramatic of these are the toxins causing the toxic shock syndromes of S aureus and group A streptococci. These syndromes are evoked when toxin is produced at an infected site and absorbed into the circulation. These toxins are able to bind directly to class II major histocompatibility complex (MHC) molecules on antigen-presenting cells (without processing) and directly stimulate production of cytokines such as interleukin 1 (IL-1) and tumor necrosis factor (TNF) (Figure 22–7). These molecules are called superantigens because they act as polyclonal stimulators of T cells. This means a significant proportion of all T cells respond by dividing and releasing cytokines, which makes the cytokine release massive enough to cause systemic effects such as shock. When ingested preformed in food, some of these toxins cause diarrhea and vomiting.

Endotoxin

In many infections caused by gram-negative bacteria, the LPS endotoxin of the outer membrane is a significant component of the disease process. LPS can cause local injury, but the major effects come when gram-negative bacteria enter the bloodstream and circulate. The lipid A portion causes fever through the release of IL-1 and TNF from macrophages and dramatic physiologic effects associated with inflammation. These include hypotension, lowered polymorphonuclear leukocyte and platelet counts from increased margination of these cells to the walls of the small vessels, hemorrhage, and sometimes disseminated intravascular coagulation (DIC) from the activation of clotting factors. Rapid and irreversible shock may follow passage of endotoxin into the bloodstream.

The term endotoxin comes from the fact that LPS is an inherent structural component of the gram-negative cell wall, not a secreted product of the bacteria. A comparable event with gram-positive and gram-negative bacteria can occur with the release and circulation of peptidoglycan cell wall fragments. This also leads to cytokine release and systemic manifestations. Although the biology is similar, the terms endotoxin or endotoxemia are not used because they have long been reserved for the LPS endotoxin of gram-negative bacteria.

Damage Caused by Inflammation and Immune Responses

Many successful pathogens produce disease without using any of the known virulence factors just described. In these instances, injury can still be produced by acute or chronic inflammation or a misdirected immune response triggered by antigenic components of the pathogen.

Persistent Inflammation

The normal inflammatory response is a two-edged sword in both acute and chronic infections. Although the enzymes of PMNs are killing the invader, they still cause some damage to host tissues or compromise organ function. Pulmonary alveoli filled with PMNs and macrophages are not effective in the absorption of oxygen. In the closed space of the central nervous system, the swelling caused by inflammation may directly lead to brain injury. In some chronic infections, the pathologic and clinical features are due largely to delayed-type hypersensitivity (DTH) reactions to the organism or its products. In tuberculosis if the host is unable to halt the growth of M tuberculosis by activation of cell-mediated immunity, persistent growth of the pathogen will continue to stimulate DTH-mediated injury.

Misdirected Immune Responses

Reactions between high concentrations of antibody, soluble microbial antigens, and complement can deposit immune complexes in tissues and cause acute inflammatory reactions and immune complex disease. In poststreptococcal acute glomerulonephritis, for example, the complexes are sequestered in the glomeruli of the kidney, with serious interference in renal function from the resulting complement deposition and tissue reaction. Antibody produced against bacterial antigens can cross-react with certain host tissues and initiates an autoimmune process. This molecular mimicry is felt to be the explanation for poststreptococcal rheumatic fever.

KEY CONCLUSIONS

All of the genetic tools described in the previous chapter are put to use in service of the complex business of being a pathogen. The multiple and sequential deployment of adherence, evasion, and injury-related virulence factors have evolved in ways that make them efficient and persistent. Part of this is the use of the plasmid and regulatory systems already described in unique ways such as pathogenicity islands. Our understanding of others remains at a more descriptive level, such as the emergence and spread of clones with enhanced virulence by unknown mechanisms.

PLASMIDS

Many of the essential determinants of pathogenicity are actually replicated as part of the bacterial chromosome, but a surprising number are carried in plasmids. This often includes multiple virulence factors in the same plasmid. For example, one type of diarrhea-causing E coli carries the genes for pili mediating adherence to enterocytes and for the enterotoxin it delivers to those enterocytes on the same plasmid. The term virulence plasmid has been used for plasmids whose loss or modification causes loss of pathogenicity for the host strain. Since plasmids are inherently a less secure home for genes than the chromosome, this location must provide some efficiency for the pathogen. Perhaps the excess baggage of the plasmid is a trade-off for avoiding disruption of the organization of the bacterial chromosome.

REGULATION OF VIRULENCE GENES

In addition to the multiple steps of pathogenesis, some pathogens lurk in locations like seawater (cholera) or fleas (plague) until their opportunity to cause human disease presents itself. As it is not economical to produce virulence factors when they are not needed it is not surprising that bacteria have evolved mechanisms for their more timely deployment. The control involves regulatory genes and their products activating genes, operons, regulons (see Chapter 21), and more complex systems. One of the longest known of these are the “on” and “off” states of flagellar genes explaining their phase variation. It turns out that the motility mediated by these flagella is a virulence factor in E coli urinary tract infections. Many pathogens have evolved regulatory systems, which link sensing of environmental cues (temperature, osmolarity, iron concentration) to activation of their virulence apparatus. These signals can “tell” the pathogen whether it is in a benign environment, inside an insect vector, in body fluids, or even inside a phagocyte. The virulence factor deployment then proceeds often in a multistep manner, synthesizing the adhesin or toxin just at the time it is needed. An example of this is shown in Figure 22–8, which illustrates the two-component regulatory system used by B pertussis in whooping cough. At the resting state B pertussis produces no virulence factors. Sensing a physiologic temperature it starts to produce its multiple virulence factors in two stages. The first is the factors needed in the early stages of infection such as the adherence protein Fha. After a delay the toxins which mediate the disease itself (pertussis toxin, adenylate cyclase) are produced. This just-in-time production is energy efficient and effective in producing disease.

QUORUM SENSING

The quantitative aspects of pathogenicity suggest there could be value in timing the deployment of virulence factors in relation to the size of the population ready to attack. Success may depend on a cell population large enough to produce disease before the host mounts an effective defense. For bacteria this would require the cell to be able to sense the local presence of other members of the same species and respond accordingly. Such cell-to-cell communication systems have been described. This communication is called quorum sensing. It has been shown to regulate the expression of adherence factors, toxin production, secretion systems, and biofilm formation. In the species studied the communication is by secretion of small autoinducer molecules which can readily diffuse and cross cell membranes much like hormones in higher organisms. In gram-negative bacteria acylated homoserine lactones and ketones (α-hydroxyketone) have been shown to carry out these functions. In gram-positive bacteria small peptides are more common. The sending and receiving cells have transcription regulators which modulate the product of the target gene. Each cell in the population has a synthesis/receptor pair that generates and responds to the autoinducer molecule. The end result is transcription of the relevant virulence factor proteins by the entire bacterial population in unison.

PATHOGENICITY ISLANDS

In recent years, large blocks of genes found on the bacterial chromosome have been given the name pathogenicity island (PAI) to describe unique regions exclusively associated with virulence (Figure 22–9). The “island” component of the name comes from the fact that the PAI regions themselves usually have fundamental characteristics such as guanine + cytosine content, codon usage, and tRNA genes that are different from the rest of the genome of the current host organism. This suggests that gene transfer from a foreign species sometime in the distant past is the likely origin. Many PAIs have strikingly similar homologs in bacteria that are pathogenic for plants and animals. The PAIs typically contain the complete package required for delivery of the pathogenic trait, even those that are the most complex involving 20 to 30 genes. In organisms that deploy injection secretion systems, the genes for the injection apparatus, the secreted proteins, and regulatory elements are all included in the PAI.

CLONALITY

Bacteria cannot be a helter-skelter amalgam of genes brought about by promiscuous genetic exchange. If this were so, there would be no bacterial specialization, and all would possess a consensus chromosomal sequence. Thus, most bacteria have some degree of built-in reproductive isolation, except for members of their own or very closely related species. In this way, diversity within the species through mutation can be maximized (usually by transformation or transduction) while conserving useful gene sequences. The end result of husbanding of important genes during evolution is that at any given time in the world, many bacteria are representatives of a single or, more often, a relatively few clonal types that have become widespread for the (evolutionary) moment. The study and definition of clonality require more than the presence or absence of virulence factors. They require examining the specific alleles of various genes and the subtle differences in amino acid sequence in batteries of multiple housekeeping enzymes.

One of the discoveries to come from the application of molecular diagnostic tools to infectious diseases is the clonal nature of many infectious diseases. That is, over long periods and large geographic distances, the organisms of a given species isolated from clinical samples tend to be so similar in genetic makeup that one is forced to envision that a clone of bacteria descended from a relatively recent common ancestor is responsible for all or most of the disease incidence. The results have been striking. For example, isolates of B pertussis from the United States represent a single clone, whereas in Japan there is a slightly different clone. Another study has determined that only 11 multilocus genotypes (clones) of Neisseria meningitidis have been responsible for the major epidemics of serogroup A organisms worldwide over 60 years. When microbes establish a unique niche, they protect their selective advantage.

Although the human bacteria pathogens represent only a tiny percentage of the microbial world, they are among the most ingenious in the ways they produce disease. The independence and power of the bacterial cell are translated into some of the most feared of all diseases. The bacteriology, disease mechanisms, and clinical aspects of these diseases are explored in the following chapters.