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Chapter 34 : Bacterial Strategies for Survival in the Host
Category: Immunology; Clinical Microbiology
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This chapter focuses on the strategies that persistent bacterial pathogens use to evade, subvert, and disarm the host immune system. Bacteria can avoid these host defense mechanisms by presenting the immune system with a continuously evolving antigen repertoire. Two related processes, antigenic variation and phase variation, generate molecular variants that escape antibody detection. The widespread human pathogen Mycobacterium tuberculosis has evolved stress resistance mechanisms that allow it to persist within the phagolysosome of activated macrophages. Bacterial biofilms are now recognized as the cause of many persistent infections that are refractory to antibiotic treatment. Some biofilm infections are associated with surfaces of medical devices such as catheters, shunts, prostheses, and mechanical heart valves. IL-10 is a potent anti-inflammatory and immunesuppressive cytokine that affects antigen presenting cells and T cells. For some bacterial pathogens, persistence in the host depends on the production of protein toxins that interfere with cellular physiology. Intoxication of antigen-presenting cells by CyaA may promote the expansion of regulatory T-cell populations and suppress immunity.
Replication dynamics of transient and persistent bacterial pathogens in animal infection models. The transient pathogen Listeria monocytogenes (gray line) rapidly replicates to high bacterial titers in the mouse gastrointestinal tract, but the infection is quickly resolved by the adaptive immune response. The immune response also generates immunologic memory that largely prevents Listeria replication upon secondary reinfection (gray dashed line). The persistent pathogen Mycobacterium tuberculosis (black line) also replicates to high bacterial loads in the lungs of infected mice, but these bacteria persist despite inducing a robust immune response. In addition, this immune response does not generate effective protective memory, such that infected mice can be superinfected upon secondary exposure to M. tuberculosis (black dashed line).
Mechanisms of antigenic and phase variation of Neisseria gonorrhoeae pilus expression. (A) Antigenic variation. Nonreciprocal recombination of variant pilin sequences from silent pilS cassettes to the expressed pilE locus generates variant PilE pilin subunits. pilS sequences are not expressed because they lack the 5′ coding sequence and promoter of pilE, but they can replace, either in whole or in part, the sequence at pilE to yield mosaic PilE variants that escape recognition by specific host antibodies. (B) Phase variation. Slipped-strand mispairing occurs during DNA replication at a poly-guanosine (poly-G) tract near the 5′ end of the pilC gene, which encodes a minor pilin subunit that is required for pilus assembly. This changes the number of G residues and alters the translational reading frame such that downstream sequences are either in or out of frame for translation. Strains of N. gonorrhoeae that have switched PilC to the “off” phase do not express pili on their surface and therefore escape detection by pilin-specific antibodies.
Staphylococcus aureus complement inhibitors. S. aureus expresses several factors that prevent opsonization by C3b. The staphylococcal complement inhibitors (SCINs) bind to and stabilize C3 convertases to block their activity and prevent C3b deposition. Efb and Ehp bind directly to C3 to interfere with C3 cleavage. The secreted protein Sbi indirectly blocks C3b deposition on the S. aureus surface by cleaving C3 in the fluid phase. S. aureus also recruits host proteases that degrade surface associated C3b: clumping factor A (ClfA) recruits host factor I (fI) that cleaves C3b to C3d, and staphylokinase (SK) recruits host plasminogen and activates it to the protease plasmin that degrades C3b. S. aureus interferes with signaling between complement proteins and cells of the immune system. S. aureus prevents neutrophil migration to the site of infection by blocking neutrophil detection of C5a, a chemoattractant released upon complement activation. CHIPS (chemotaxis inhibitory protein of S. aureus) binds and antagonizes the neutrophil C5a receptor. The C3 binding proteins Efb and Ehp also interfere with communication between the complement system and B cells by blocking the interaction between C3d and complement receptor 2 (CR2).
Antimicrobial factors in the macrophage phagolysosome and Mycobacterium tuberculosis resistance mechanisms. In the maturing phagosome, bacteria encounter reactive oxygen species (ROS) synthesized by the NADPH phagocyte oxidase (NOX2); reactive nitrogen species (RNS) synthesized by the inducible nitric oxide synthase (iNOS); acidic pH resulting from the action of vacuolar ATPase (V-ATPase) proton pumps; and cationic antimicrobial peptides (CAMPs). M. tuberculosis resists ROS and RNS using various detoxification enzymes (described in the text) including degradation of hydrogen peroxide (H2O2) by the catalase/peroxidase KatG. M. tuberculosis repairs oxidative and nitrosative damage to DNA by the nucleotide excision repair pathway (UvrB) and uses the proteasome to degrade proteins damaged by oxidation. The complex M. tuberculosis cell wall serves as a permeability barrier to the influx of protons and the membrane-associated protease Rv367 lc maintains cytoplasmic pH homeostasis during growth in the acidified phagolysosome. M. tuberculosis resists the action of CAMPs by lysinylation of the membrane lipid phosphatidyl glycerol (PG).