Hart C. Anthony, Shears Paul. Color Atlas of Medical Microbiology.pdf

Host–Pathogen Interactions 15

pili at a high rate (Fig. 1.2). The borreliae that cause relapsing fevers have the capacity to change the structure of one of the adhesion proteins in their outer 1 membrane (vmp = variable major protein), resulting in the typical “recurrences” of fever. Similarly, meningococci can change the chemistry of their capsule polysaccharides (“capsule switching”).

& IgA proteases. Mucosal secretions contain the secretory antibodies of the sIgA1 class responsible for the specific local immunity of the mucosa. Classic mucosal parasites such as gonococci, meningococci and Haemophilus influenzae produce proteases that destroy this immunoglobulin.

Clinical Disease

The clinical symptoms of a bacterial infection arise from the effects of damaging noxae produced by the bacteria as well as from excessive host immune responses, both nonspecific and specific. Immune reactions can thus potentially damage the host’s health as well as protect it (see Immunology, p.103ff.).

&Cytopathic effect. Obligate intracellular parasites (rickettsiae, chlamydiae) may kill the invaded host cells when they reproduce.

&Exotoxins. Pathogenic bacteria can produce a variety of toxins that are either the only pathogenic factor (e.g., in diphtheria, cholera, and tetanus) or at least a major factor in the unfolding of the disease. One aspect the classification and nomenclature of these toxins must reflect is the type of cell affected: cytotoxins produce toxic effects in many different host cells; neurotoxins affect the neurons; enterotoxins affect enterocytes. The structures and mechanisms of action of the toxins are also considered in their classification (Table 1.5):

— AB toxins. They consist of a binding subunit “B” responsible for binding to specific surface receptors on target host cells, and a catalytic subunit “A” representing the active agent. Only cells presenting the “B” receptors are damaged by these toxins.

— Membrane toxins. These toxins disrupt biological membranes, either by attaching to them and assembling to form pores, or in the form of phospholipases that destroy membrane structure enzymatically.

— Superantigens (see p. 72). These antigens stimulate T lymphocytes and macrophages to produce excessive amounts of harmful cytokines.

&Hydrolytic exoenzymes. Proteases (e.g., collagenase, elastase, nonspecific proteases), hyaluronidase, neuraminidase (synonymous with sialidase), lecithinase and DNases contribute at varying levels to the pathogenesis of an infection.

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&Cell wall. The endotoxin of Gram-negative bacteria (lipopolysaccharide)

1plays an important role in the manifestation of clinical symptoms. On the one hand, it can activate complement by the alternative pathway and, by releasing the chemotactic components C3a and C5a, initiate an inflammatory reaction at the infection site. On the other hand, it also stimulates macrophages to produce endogenous pyrogens (interleukin 1, tumor necrosis factor), thus inducing fever centrally. Production of these and other cytokines is increased, resulting in hypotension, intravasal coagulation, thrombocyte aggregation and stimulation of granulopoiesis. Increased production of cytokines by macrophages is also induced by soluble murein fragments and, in the case of Gram-positive bacteria, by teichoic acids.

&Inflammation. Inflammation results from the combined effects of the nonspecific and specific immune responses of the host organism. Activation of complement by way of both the classic and alternative pathways induces phagocyte migration to the infection site. Purulent tissue necrosis follows. The development of typical granulomas and caseous necrosis in the course of tuberculosis are the results of excessive reaction by the cellular immune system to the immunogens of tuberculosis bacteria. Textbooks of general pathology should be consulted for detailed descriptions of these inflammatory processes.

Regulation of Bacterial Virulence

Many pathogenic bacteria are capable of living either outside or inside a host and of attacking a variety of host species. Proliferation in these differing environments demands an efficient regulation of virulence, the aim being to have virulence factors available as required. Four different regulatory mechanisms have been described:

&DNA changes. The nucleotide sequences of virulence determinants are changed. Examples of this include pilin gene variability involving intracellular recombination as described above in gonococci and inverting a leader sequence to switch genes on and off in the phase variations of H antigens in salmonellae (see p. 284).

&Transcriptional regulation. The principle of transcriptional control of virulence determinants is essentially the same as that applying to the regulation of metabolic genes, namely repression and activation (see p. 169f.):

— Simple regulation. Regulation of the diphtheria toxin gene has been thoroughly researched. A specific concentration of iron in the cytoplasm activates the diphtheria toxin regulator (DtxR). The resulting active repressor prevents transcription of the toxin gene by binding to the promoter

—Complex regulation, virulence regulon. In many cases, several virulence genes are switched on and off by the same regulator protein. The virulence determinants involved are either components of the same operon or are located at different genome sites. Several vir (virulence) genes with promoter regions that respond to the same regulator protein form a socalled vir regulon. Regulation of the virulence regulon of Bordetella pertussis by means of gene activation is a case in point that has been studied in great detail. This particular regulon comprises over 20 virulence determinants, all controlled by the same vir regulator protein (or BvgA coding region) (Fig. 1.4).

&Posttranscriptional regulation. This term refers to regulation by mRNA or a posttranslational protein modification.

&Quorum sensing. This term refers to determination of gene expression by bacterial cell density (Fig. 1.5). Quorum sensing is observed in both

Regulation of Bacterial Virulence: Two-Component Regulator System

Fig.1.4 A sensor protein integrated in the cytoplasmic membrane receives signals from a receiver module extending into the external milieu, activating the transmitter module. These signals from the external milieu can carry a wide variety of information: pH, temperature, osmolarity, Ca2+, CO2, stationary-phase growth, hunger stress, etc. The transmitter module effects a change in the receiver module of the regulator protein, switching the functional module of the regulator to active status, in which it can then repress or activate the various virulence determinants of a virulence regulon by binding to the different promoter regions. Phosphorylation is commonly used to activate the corresponding sensor and regulator modules.

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Quorum-Sensing Communication in Bacteria (Cell-to-Cell Signals)

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Fig.1.5 Cell-to-cell signaling is made possible by activation of two genes. The I gene codes for the synthase responsible for synthesis of the autoinducer. The autoinducer (often an N-acyl homoserine lactone) can diffuse freely through the cell membrane. The R gene codes for a transcriptional regulator protein that combines with the autoinducer to become an activator for transcription of various virulence genes.

Gram-positive and Gram-negative bacteria. It denotes a mode of communication between bacterial cells that enables a bacterial population to react analogously to a multicellular organism.

Accumulation of a given density of a low-molecular-weight pheromone (autoinducer) enables a bacterial population to sense when the critical cell density (quorum) has been reached that will enable it to invade the host successfully, at which point transcription of virulence determinants is initiated.

The Genetics of Bacterial Pathogenicity

The virulence genes of pathogenic bacteria are frequently components of mobile genetic elements such as plasmids, bacteriophage genomes, or conjugative transposons (see p. 170ff.). This makes lateral transfer of these genes between bacterial cells possible. Regions showing a high frequency of virulence genes in a bacterial chromosome are called pathogenicity islands (PI).

Host–Pathogen Interactions 21

PIs are found in both Gram-positive and Gram-negative bacteria. These are

DNA regions up to 200 kb that often bear several different vir genes and have 1 specific sequences located at their ends (e.g., IS elements) that facilitate lat-

eral translocation of the islands between bacterial cells. The role played by lateral transfer of these islands in the evolutionary process is further underlined by the fact that the GC contents in PIs often differ from those in chromosomal DNA.

Defenses against Infection

A macroorganism manifests defensive reactions against invasion by microorganisms in two forms: as specific, acquired immunity and as nonspecific, innate resistance (see also Chapter 2, Basic Principles of Immunology, p. 43).

Nonspecific Defense Mechanisms

Table 1.6 lists the most important mechanisms.

&Primary defenses. The main factors in the first line of defense against infection are mechanical, accompanied by some humoral and cellular factors. These defenses represent an attempt on the part of the host organism to prevent microorganisms from colonizing its skin and mucosa and thus stave off a generalized invasion.

&Secondary defenses. The second line of defense consists of humoral and cellular factors in the blood and tissues, the most important of which are the professional phagocytes.

&Phagocytosis. “Professional” phagocytosis is realized by polymorphonuclear, neutrophilic, eosinophilic granulocytes—also known as microphages— and by mononuclear phagocytes (macrophages). The latter also play an important role in antigen presentation (see p. 62). The total microphage cell count in an adult is approximately 2.5 ! 1012. Only 5% of these cells are located in the blood. They are characterized by a half-life of only a few hours. Microphages contain both primary granules, which are lysosomes containing lysosomal enzymes and cationic peptides, and secondary granules. Both microphages and macrophages are capable of ameboid motility and chemotactic migration, i.e., directed movement along a concentration gradient toward a source of chemotactic substances, in most cases the complement components C3a and C5a. Other potentially chemotactic substances include secretory products of lymphocytes, products of infected and damaged cells or the N-formyl peptides (fMet-Phe and fMet-Leu-Phe).

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Table 1.6 The Most Important Mechanisms in Nonspecific Defenses Against

Phagocytes are capable of ingestion of both particulate matter (phagocytosis) and solute matter (pinocytosis). Receptors on the phagocyte membrane initiate contact (Fig. 1.6). Particles adhering to the membrane are engulfed, ingested and deposited in a membrane-bound vacuole, the so-called phagosome, which then fuses with lysosomes to form the phagolysosome. The bacteria are killed by a combination of lysosomal factors:

—Mechanisms that require no oxygen. Low pH; acid hydrolases, lysozyme; proteases; defensins (small cationic peptides).

Fig.1.6 Encapsulated bacteria can only be effectively phagocytosed if IgG-class antibodies (Fc ligand) or the complement component C3b, or both, are located on their surfaces. The Fc and C3b ligands bind to their specific receptors on the phagocyte surface.

—Mechanisms that require oxygen. Halogenation of essential bacterial com-

ponents by the myeloperoxidase-H2O2–halide system; production of highly reactive O2 radicals (oxidative burst) such as superoxide anion (O2–), hydroxyl radical (!OH), and singlet oxygen (1O2).

Specific Defense Mechanisms

Specific immunity, based on antibodies and specifically reactive T lymphocytes, is acquired in a process of immune system stimulation by the corresponding microbial antigens. Humoral immunity is based on antitoxins, opsonins, microbicidal antibodies, neutralizing antibodies, etc. Cellular immunity is based on cytotoxic T lymphocytes (T killer cells) and T helper cells. See Chapter 2 on the principles of specific immunity.

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Defects in Immune Defenses

1

Hosts with defects in their specific and/or nonspecific immune defenses are prone to infection.

&Primary defects. Congenital defects in the complement-dependent phagocytosis system are rare, as are B and T lymphocyte defects.

&Secondary defects. Such effects are acquired, and they are much more frequent. Examples include malnutrition, very old and very young hosts, metabolic disturbances (diabetes, alcoholism), autoimmune diseases, malignancies (above all lymphomas and leukemias), immune system infections (HIV), severe primary diseases of parenchymatous organs, injury of skin or mucosa, immunosuppressive therapy with corticosteroids, cytostatics and immunosuppressants, and radiotherapy.

One result of progress in modern medicine is that increasing numbers of patients with secondary immune defects are now receiving hospital treatment. Such “problem patients” are frequently infected by opportunistic bacteria that would not present a serious threat to normal immune defenses. Often, the pathogens involved (“problem bacteria”) have developed a resistance to numerous antibiotics, resulting in difficult courses of antibiotic treatment in this patient category.

Normal Flora

Commensals (see Table 1.3, p. 9) are regularly found in certain human microbiotopes. The normal human microflora is thus the totality of these commensals. Table 1.7 lists the most important microorganisms of the normal flora with their localizations.

Bacteria are the predominant component of the normal flora. They proliferate in varied profusion on the mucosa and most particularly in the gastrointestinal tract, where over 400 different species have been counted to date. The count of bacteria per gram of intestinal content is 101–105 in the duodenum, 103–107 in the small intestine, and 1010–1012 in the colon. Over 99% of the normal mucosal flora are obligate anaerobes, dominated by the Gram-neg. anaerobes. Although life is possible without normal flora (e.g., pathogen-free experimental animals), commensals certainly benefit their hosts. One way they do so is when organisms of the normal flora manage to penetrate into the host through microtraumas, resulting in a continuous stimulation of the immune system. Commensals also compete for living space with overtly pathogenic species, a function known as colo-