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Some pathogenic bacteria are inherently able to resist the bactericidal components of host tissues, usually as a function of some structural property. For example, the poly-D-glutamate capsule of Bacillus anthracis protects the organisms against action of cationic proteins (defensins) in sera or in phagocytes. The outer membrane of Gram-negative bacteria is a permeability barrier to lysozyme and is not easily penetrated by hydrophobic compounds such as bile salts in the GI tract that are harmful to the bacteria. Pathogenic mycobacteria have a waxy cell wall that resists attack or digestion by most tissue bactericides. And intact lipopolysaccharides (LPS) of Gram-negative pathogens may protect the cells from complement-mediated lysis or the action of lysozyme.

Most successful pathogens, however, possess additional structural or biochemical features that allow them to resist the host cellular defense against them, i.e., the phagocytic and immune responses. If a pathogen breaches the host's surface defenses, it must then overcome the host's phagocytic response to succeed in an infection.

Bacterial pathogens have devised numerous and diverse strategies to avoid phagocytic engulfment and killing. Most are aimed at blocking one or more of the steps in phagocytosis, thereby halting the process. The process of phagocytosis is discussed in the chapter on Innate Immunity against bacterial pathogens.

Avoiding Contact with Phagocytes

Bacteria can avoid the attention of phagocytes in a number of ways.

1. Pathogens may invade or remain confined in regions inaccessible to phagocytes. Certain internal tissues (e.g. the lumens of glands, the urinary bladder) and surface tissues (e.g. unbroken skin) are not patrolled by phagocytes.

2. Some pathogens are able to avoid provoking an overwhelming inflammatory response. Without inflammation the host is unable to focus the phagocytic defenses.

3. Some bacteria or their products inhibit phagocyte chemotaxis. For example, Streptococcal streptolysin (which also kills phagocytes) suppresses neutrophil chemotaxis, even in very low concentrations. Fractions of Mycobacterium tuberculosis are known to inhibit leukocyte migration. The Clostridium ø toxin also inhibits neutrophil chemotaxis.

4. Some pathogens can cover the surface of the bacterial cell with a component which is seen as "self" by the host phagocytes and immune system. Such a strategy hides the antigenic surface of the bacterial cell. Phagocytes cannot recognize bacteria upon contact and the possibility of opsonization by antibodies to enhance phagocytosis is minimized. For example, pathogenic Staphylococcus aureus produces cell-bound coagulase and clumping factor which clots fibrin on the bacterial surface. Treponema pallidum, the agent of syphilis, binds fibronectin to its surface. Group A streptococci are able to synthesize a capsule composed of hyaluronic acid. Hyaluronic acid is the ground substance (tissue cement) in connective tissue. Some pathogens have or can deposit sialic acid residues on their surfaces which prevents opsonization by complement components and impedes recognition by phagocytes.

Inhibition of Phagocytic Engulfment

Some bacteria employ strategies to avoid engulfment (ingestion) if phagocytes do make contact with them. Many important pathogenic bacteria bear on their surfaces substances that inhibit phagocytic adsorption or engulfment. Clearly it is the bacterial surface that matters. Resistance to phagocytic ingestion is usually due to a component of the bacterial cell surface (cell wall, or fimbriae, or a capsule). Classical examples of antiphagocytic substances on bacterial surfaces include:

1. Polysaccharide capsules of S. pneumoniae, Haemophilus influenzae, Treponema pallidum and Klebsiella pneumoniae

2. M protein and fimbriae of Group A streptococci

3. Surface slime (polysaccharide) produced as a biofilm by Pseudomonas aeruginosa

4. O polysaccharide associated with LPS of E. coli

5. K antigen (acidic polysaccharides) of E. coli or the analogous Vi antigen of Salmonella typhi

6. Cell-bound or soluble Protein A produced by Staphylococcus aureus. Protein A attaches to the Fc region of IgG and blocks the cytophilic (cell-binding) domain of the Ab. Thus, the ability of IgG to act as an opsonic factor is inhibited, and opsonin-mediated ingestion of the bacteria is blocked.

 Table 1. BACTERIAL INTRACELLULAR PATHOGENS

Organism	Disease
Mycobacterium tuberculosis	Tuberculosis
Mycobacterium leprae	Leprosy
Listeria monocytogenes	Listeriosis
Salmonella typhi	Typhoid Fever
Shigella dysenteriae	Bacillary dysentery
Yersinia pestis	Plague
Brucella species	Brucellosis
Legionella pneumophila	Pneumonia
Rickettsiae	Typhus; Rocky Mountain Spotted Fever
Chlamydia	Chlamydia; Trachoma

Some intracellular parasites have special genetically-encoded mechanisms to get themselves into host cells that are nonphagocytic. Pathogens such as Yersinia, Listeria, E. coli, Salmonella, Shigella and Legionella possess complex machinery for cellular invasion and intracellular survival. These systems involve various types of non-toxin virulence factors. Sometimes these factors are referred to as bacterial invasins. Still other bacteria such as Bordetella pertussis and Streptococcus pyogenes, have recently been discovered in the intracellular habitat of epithelial cells.

Legionella pneumophila enters mononuclear phagocytes by depositing complement C3b on its surfaces and using that host protein to serve as a ligand for binding to macrophage cell surfaces. After ingestion, the bacteria remain in vacuoles that do not fuse with lysosomes, apparently due to the influence of soluble substances produced by the bacteria.

Salmonella bacteria possesses an invasin operon (inv A - H) that encodes for factors that regulate their entry into host cells. Mutations in the operon yield organisms that can adhere to target cells without being internalized. This suggests that one or more of the inv proteins stimulates signal transduction in the host cell that results engulfment of the salmonellae. A similar invasin gene in Yersinia is known to encode a protein that both promotes adherence and activates the cytochalasin-dependent engulfment process. This invasin can confer invasive capacity on noninvasive E. coli, and even latex particles.

Intracellular parasites survive inside of phagocytes by virtue of mechanisms which interfere with the bactericidal activities of the host cell. Some of these bacterial mechanisms include:

1. Inhibition of fusion of the phagocytic lysosomes (granules) with the phagosome. The bacteria survive inside of phagosomes because they prevent the discharge of lysosomal contents into the phagosome environment. Specifically, phagolysosome formation is inhibited in the phagocyte. This is the strategy employed by Salmonella, M. tuberculosis, Legionella and the chlamydiae.

-With M. tuberculosis, bacterial cell wall components (sulfatides) are thought to be released from the phagosome that modify lysosomal membranes to inhibit fusion.

-In Chlamydia, some element of the bacterial (elementary body) wall appears to modify the membrane of the phagosome in which it is contained.

-In L. pneumophila, as with the chlamydia, some structural feature of the bacterial cell surface, already present at the time of entry (ingestion), appears to modify the membranes of the phagosomes, thus preventing their merger with lysosomal granules. In Legionella, it is known that a single gene is responsible for the inhibition of phagosome lysosome fusion.

-In Salmonella typhimurium, the pH that develops in the phagosome after engulfment actually induces bacterial gene products that are essential for their survival in macrophages.

2. Survival inside the phagolysosome. With some intracellular parasites, phagosome-lysosome fusion occurs, but the bacteria are resistant to inhibition and killing by the lysosomal constituents. Also, some extracellular pathogens can resist killing in phagocytes utilizing similar resistance mechanisms. Little is known of how bacteria can resist phagocytic killing within the phagocytic vacuole, but it may be due to the surface components of the bacteria or due to extracellular substances that they produce which interfere with the mechanisms of phagocytic killing. Some examples of how certain bacteria (both intracellular and extracellular pathogens) resist phagocytic killing are given below.

-Mycobacteria (including M. tuberculosis and Mycobacterium leprae) grow inside phagocytic vacuoles even after extensive fusion with lysosomes. Mycobacteria have a waxy, hydrophobic cell wall containing mycolic acids and other lipids, and are not easily attacked by lysosomal enzymes.

-Cell wall components (LPS?) of Brucella abortus apparently interfere with the intracellular bactericidal mechanisms of phagocytes.

-B. abortus and Staphylococcus aureus are vigorous catalase and superoxide dismutase producers, which might neutralize the toxic oxygen radicals that are generated by the NADPH oxidase and MPO systems in phagocytes. S. aureus also produces cell-bound pigments (carotenoids) that "quench" singlet oxygen produced in the phagocytic vacuole.

-The outer membrane and capsular components of Gram-negative bacteria (e.g. Salmonella, Yersinia, Brucella, E. coli) can protect the peptidoglycan layer from the lytic activity of lysozyme.

-Some pathogens (e.g. Salmonella, E. coli) are known to produce extracellular iron-binding compounds (siderophores) which can extract Fe⁺⁺⁺ from lactoferrin (or transferrin) and supply iron to cells for growth.

-Bacillus anthracis resists killing and digestion by means of its capsule which is made up of poly-D-glutamate. The "unnatural" configuration of this polypeptide affords resistance to attack by cationic proteins or conventional proteases and prevents the deposition of complement on the bacterial surface.

Escape from the phagosome. Early escape from the phagosome vacuole is essential for growth and virulence of some intracellular pathogens.

-This is a clever strategy employed by the Rickettsiae. Rickettsia enter host cells in membrane-bound vacuoles (phagosomes) but are free in the cytoplasm a short time later, perhaps in as little as 30 seconds. A bacterial enzyme, phospholipase A, may be responsible for dissolution of the phagosome membrane.

-Listeria monocytogenes relies on several molecules for early lysis of the phagosome to ensure their release into the cytoplasm. These include a pore-forming hemolysin (listeriolysin O) and two forms of phospholipase C. Once in the cytoplasm, Listeria induces its own movement through a remarkable process of host cell actin polymerization and formation of microfilaments within a comet-like tail.

-Shigella also lyses the phagosomal vacuole and induces cytoskeletal actin polymerization for the purpose of intracellular movement and cell to cell spread.

Products of Bacteria that Kill or Damage Phagocytes

One obvious strategy in defense against phagocytosis is direct attack by the bacteria upon the professional phagocytes. Any of the substances that pathogens produce that cause damage to phagocytes have been referred to as aggressins. Most of these are actually extracellular enzymes or toxins that kill phagocytes. Phagocytes may be killed by a pathogen before or after ingestion.

Killing Phagocytes Before Ingestion

Many Gram-positive pathogens, particularly the pyogenic cocci, secrete extracellular substances that kill phagocytes, acting either as enzymes or "pore-formers" that lyse phagocyte membrane. Some of these substances are described as hemolysins or leukocidins because of their lethal action against red blood cells or leukocytes.

-Pathogenic streptococci produce streptolysin. Streptolysin O binds to cholesterol in membranes. The effect on neutrophils is to cause lysosomal granules to explode, releasing their lethal contents into the cell cytoplasm.

-Pathogenic staphylococci produce leukocidin, which also acts on the neutrophil membrane and causes discharge of lysosomal granules.

-Extracellular proteins that inhibit phagocytosis include the Exotoxin A of Pseudomonas aeruginosa which kills macrophages, and the bacterial exotoxins that are adenylate cyclases (e.g. anthrax toxin EF and pertussis toxin AC) which decrease phagocytic activity through disruption of cell equilibrium and consumption of ATP reserves needed for engulfment.

Killing Phagocytes After Ingestion. Some bacteria exert their toxic action on the phagocyte after ingestion has taken place. They may grow in the phagosome and release substances which can pass through the phagosome membrane and cause discharge of lysosomal granules, or they may grow in the phagolysosome and release toxic substances which pass through the phagolysosome membrane to other target sites in the cell. Many bacteria that are the intracellular parasites of macrophages (e.g. Mycobacterium, Brucella, Listeria) usually destroy macrophages in the end, but the mechanisms are not completely understood.

A summary of bacterial mechanisms for interference with phagocytes is given in the table below.
 

Table 2. BACTERIAL INTERFERENCE WITH PHAGOCYTES

BACTERIUM	TYPE OF INTERFERENCE	MECHANISM
Streptococcus pyogenes	Kill phagocyte	Streptolysin induces lysosomal discharge into cell cytoplasm
	Inhibit neutrophil chemotaxis	Streptolysin is chemotactic repellent
	Resist engulfment (unless Ab is present)	M Protein on fimbriae
	Avoid detection by phagocytes	Hyaluronic acid capsule
Staphylococcus aureus	Kill phagocyte	Leukocidin lyses phagocytes and induces lysosomal discharge into cytoplasm
	Inhibit opsonized phagocytosis	Protein A blocks Fc portion of Ab; polysaccharide capsule in some strains
	Resist killing	Carotenoids, catalase, superoxide dismutase detoxify toxic oxygen radicals produced in phagocytes
	Inhibit engulfment	Cell-bound coagulase hides ligands for phagocytic contact
Bacillus anthracis	Kill phagocytes or undermine phagocytic activity	Anthrax toxin EF
	Resist engulfment and killing	Capsular poly-D-glutamate
Streptococcus pneumoniae	Resist engulfment (unless Ab is present)	Capsular polysaccharide
Klebsiella pneumoniae	Resist engulfment	Polysaccharide capsule
Haemophilus influenzae	Resist engulfment	Polysaccharide capsule
Pseudomonas aeruginosa	Kill phagocyte	Exotoxin A kills macrophages; Cell-bound leukocidin
	Resist engulfment	Alginate slime and biofilm polymers
Salmonella typhi	Resist engulfment and killing	Vi (K) antigen (microcapsule)
Salmonella enterica (typhimurium)	Survival inside phagocytes	Bacteria develop resistance to low pH, reactive forms of oxygen, and host "defensins" (cationic proteins)
Listeria monocytogenes	Escape from phagosome	Listeriolysin, phospholipase C lyse phagosome membrane
Clostridium perfringens	Inhibit phagocyte chemotaxis	ø toxin
	Inhibit engulfment	Capsule
Yersinia pestis	Resist engulfment and/or killing	Protein capsule on cell surface
Yersinia enterocolitica	Kill phagocytes	Yop proteins injected directly into neutrophils
Mycobacteria	Resist killing and digestion	Cell wall components prevent permeation of cells; soluble substances detoxify of toxic oxygen radicals and prevent acidification of phagolysosome
Mycobacterium tuberculosis	Inhibit lysosomal fusion	Mycobacterial sulfatides modify lysosomes
Legionella pneumophila	Inhibit phagosome-lysosomal fusion	Unknown
Neisseria gonorrhoeae	Inhibit phagolysosome formation; possibly reduce respiratory burst	Involves outer membrane protein (porin) P.I
Rickettsia	Escape from phagosome	Phospholipase A
Chlamydia	Inhibit lysosomal fusion	Bacterial substance modifies phagosome
Brucella abortus	Resist killing	Cell wall substance (LPS?)
Treponema pallidum	Resist engulfment	Polysaccharide capsule material
Escherichia coli	Resist engulfment	O antigen (smooth strains); K antigen (acid polysaccharide)
	Resist engulfment and possibly killing	K antigen