9.1.1 Immunity to Bacteria
There are four primary steps in bacterial infection.Host-defense mechanisms act at each of these steps, and many bacteria have evolved ways to circumvent some of these host defenses
  • Attachment to host cells
  • Proliferation
  • Invasion of host tissue
  • Toxin-induced damage to host cells
graphic General Aspects of Bacterial
Bacterial adherence and Colonization
Potential Colonization Sites
Ways of adherence
  • fimbriae: rod-shaped, protein
  • pili: adherence to host cell molecule, often CHO (glycoproteins, glycolipids)
  • filamentious hemaglutanin
  • proteins, glycoproteins
  • non-specific interactions
Bacterial virulence factors
  • Adhesins - Adherence
  • Invasins – Invasion of cell
  • Impedins – Avoidance of host defenses (leukocidins)
  • Agressins – Causes direct damage to the host cells (toxins)
  • Modulins – Causes host damage indirectly by perturbing cytokine networks (LPS)
Bacterial infection
Rout of infection
  • fecal-oral (Vibrio cholerae, Escherichia coli)
  • human-human (Mycobacterium tuberculosis)
  • animal-human (Salmonella, Bacillus anthrasis)
  • vector-borne (Yersinia pestis, Borrelia burgdorferi)
  • environmental contact (Clostridium spp.)
  • normal flora (Streptococcus, Staphylococcus)
Spread of Infection
  • Anatomical features inhibit spread
    • “filtration” in lymphatics
    • blood-brain barrier
  • Oxygen: used by facultatives, creates anaerobiosis
  • Temperature: obligate pathogens often have narrow optimum temperatures
  • Nutritional factors: iron limitation
Extracellular vs intracellular bacteria
Extracellular bacteria:
  • Phagocyte avoidance!!
  • Extracellular bacteria are capable of replicating outside of the host cells.
  • They cause disease by two principle mechanisms.
    • induce inflammation.
    • produce toxins: Endotoxins which are commponents of bacterial cell walls like LPS and exotoxins which are actively secreted
Intracellular bacteria:
  • Intercellular bacteria survive/replicate within phagocytes and are inaccessible by antibodies.Cell-mediated is the major immune response against intracellular bacteria.
Host primary physical and chemical barrier defense
  • Keratinized epithelium
    • low pH, drying
    • keratinocytes (phagocytic, low pH)
    • adapted flora, mainly G+
    • cell sloughing
    • lysozyme protects pores, follicles
  • Mucosal epithelium
    • tight junctions connect cells
    • turnover of mucosal cells
    • mucus lubrication, entrapment
    • lung architecture: speed of particle
    • mucociliary ladder in lungs
    • cough, sneeze stimulated by histamine
    • microflora - upper respiratory, lower genitourinary
    • low O2
    • detergent effect of bile salts
    • macrophages Extracellular bacteria
Immunity against extracellular bacteria
  • Antibody response
    • opsonization of bacteria
    • neutralizing the effects of toxins
    • activate compliment system
  • Phagocytosis
    • neutrophils
    • monocytes
    • tissue macrophages
  • Activation of the compliment system
    • in the absence of antibody
innate immune response
  • Phagocytosis by macrophages and neutrophils
  • Complement activation – phagocytosis and lysis
    • gram positive bact. contain peptidoglycan that activate alternative complement cascade
    • mannose binding lectina - homologous to C1q
    • complement components induce inflammation
  • Endotoxins also stimulate the release of cytokines by macrophages and other cells (endothelium)
  • Phagocytes are also activated by binding of microbes to other receptors - TLR, mannose or scavenger
adaptive response
  • Humoral immune response is the principal protective response to extracellular bacteria.
  • Inflammation and septic shock are the principal injurious consequences of host response.
    • mainly caused by cytokine production by activated marophage
    • Some bacterial toxins called superantigens activate many T cells expressing a particular family of Vb T cell receptor.
Invade mechanisms by extracellular bacteria
Avoiding phagocytosis
  • Innate PMNs, monocytes and macrophages engulf (phagocytose) and kill microorganisms with degradative enzymes
  • Bacteria block signaling molecule production or degrade them after production
    • C5a cleaved by C5a peptidaseof S. pyogenes (strep throat)
    • Capsule productionon surface of bacteria: capsule leads to C3b inactivation- ”serum resistance”
    • M proteinof Streptococcus: also inactivates C3b
    • IgA protease: inactivates IgA in secretions
    • Fc receptor agonists: bind antibodies and orient dangerous end away from bacteria
      • Found in Streptococcus (Protein G) and Staphylococcus (Protein A)
  • Extracellular toxins acting on cell surface
    • By binding to certain receptors
    • Superantigens
    • By forming pores in the cytoplasmic membrane
    • Hemolysins
    • Leukotoxins
  • Extracellullar toxins that need to enter cell : A/B toxins
    • Diphtheria toxin
    • Anthrax toxin
    • Shiga toxin
  • Extracellular enzymes: degradative enzymes
    • Hyaluronidase
    • Streptokinases and staphylokinases
    • Collagenases
    • Neuraminidases
    • sIgA proteases
  • Toxins directly delivered into host cell – Type III and IV secretion systems
    • Apoptotic toxins
    • Protein kinases and phosphorylases
Delivery of toxins
Effector Protein “Toxin” Delivery
  • Affects target cell structure and host response
  • Type III/IV Secretion Systems are multi-protein complexes connecting bacteria to host cells
  • Mediate protein secretion and translocation from bacterial cytoplasm to host cell interior
  • Effector proteins subvert cellular functions
  • Important to bacterial pathogenesis and field of eucaryotic cell biology
Effect of toxin
Toxic shock syndrome
  • Staphylococcus aureus
  • Streptococcus pyogenes
  • Cross-linking of MHC II and T-cell receptor by superantigen results in MASSIVE activation of T cells in absence of specific peptide.
  • Massive release of cytokines, IL-1 and TNF-a resulting in systemic reaction of fever, blood clots,  diarrhea, decreased blood pressure and shock.
Toxicity of LPS
  • LPS, in the outer leaflet of Gram negative bacteria
  • Lipid A is toxic if organisms enter bloodstream
    • Massive immune cell infiltration
    • Activation of coagulation
Example of extracellular bacteria:Helicobacter pylori
  • Gram negative bacterium
  • Spiral shaped
  • Colonizes human stomach
  • High prevalence
  • Associated with gastritis, peptic ulcer and gastric cancer
H. pylori Virulence Factors
  • VacA, vacuolating cytotoxin
  • Cag pathogenicity island, cag PaI – T4SS
  • LPS, lipopolysaccharide
  • PGN, peptidoglycan
  • Urease
  • Flagella
  • Adhesins
  • Vaculating cytotoxin.
  • VacA occurs in several forms and induces large acidified vacuoles
  • Secreted form needs to be activated by pH2.
  • Form on bacterial surface is active.
  • At a neutral pH, the cytotoxin self-associates into predominantly dodecameric complexes.
  • T cell suppressive
Model of Hp Type IV Secretion System
Hp LPS – Immune Mimicry
  • Hp LPS has low toxicity.
  • LPS O antigen has carbohydrates identical to hu Lex and Ley.
  • Immune mimicry could allow for immune evasion or elicit cross-reacting abs.
  • Lewis ag plays a role in adhesion.
Peptidoglycan (PGN)
Molecular Signalling Pathways Activated by Intracellular Delivery of H. pylori Peptidoglycan (PGN)
immune response to H.pylori
graphic Intracellular bacteria
Immunity against intracellular bacteria
humoral response
  • Bacteria degraded in macrophage, peptide fragments transferred to surface
  • Display as complex with MHC class II proteins, stimulate helper T cells
  • Stimulated T cells find B cells producing Ab recognizing specific epitope, induce proliferation and antibody production
  • Memory B cells survive, give rapid secondary response
cell mediated response
innate cell-mediated response
The innate immune response to intracellular bacteria consists of phagocytes and NK cells. Innate immunity may limit their growth for some time, but usually fails to eradicate.
  • initally neutrophiles and later macrophaes attempt to destroy,but the intracellular bacteria are resistant to degradation within phagocytes
  • NK cells provides an early defense agianst these microbes.Intracellular bacteria activate NK cells, either directly or by stimulating macrophages production of IL-12
  • PMNs, monocytes, dendritic cells, B-cells and non-activated macrophages ingest and attempt to destroy. Non- activated phagocytes are generally ineffective at degrading these bacteria
    • PMNs
      • immediate response, short-lived
      • PMNs with engulfed bacteria move into lymphatic circulation, carried to lymph node
      • dead PMNs phagocytosed by other PMNs or macrophages
    • Monocytes in blood
      • move into tissue, become macrophages
      • longer lived, less vigorous in phagocytosis than PMNs
      • engulf and carry bacteria to lymph nodes
    • Macrophages are primed to recognize specific bacterium.Macrophages containing intracellular pathogens display complex of MHC I protein and bacterial peptides
adaptive cell-mediated response
  • The major protective immune response against intracellular bacteria is cell- mediated immunity.
  • Cooperation of CD4+ and CD8+ T cells in response to intracellular microbes.
  • CD4 T cell -derived signals CD40 ligand and cytokine IFN-g can lead the activation of macrophage, which can kill the phagocytosed microbe in the phagosomes
  • lysis of infeced cells by CTL, which can kill the microbe escaped from the phagosomes.
  • The macrophage activation that occurs in response to intracellular microbes is also capable of causing tissue injury such as DTH.
  • Th1 and Th2 cells determine outcome of infection.
Invasion mechanism of intracellular bacteria
“Zippering” method of internalization
Tight ligand-receptor interactions direct uptake.“one at a time” uptake.
e.g. Yesinia, Legionella
Ruffling method of internalization
  • General induced cellular response
    • Salmonella
    • Shigella
    • E. coli
  • Can lead to co-invasion of other bacteria in close proximity
M cell Internalization
  • M cells are a portal to the immune system
  • Important site of “antigen sampling
  • Some pathogens use phagocytic nature of M cells to access deeper tissues by transcytosis
Survival Strategies within Phagocytes
  • Blocking lysosomal fusion: prevent delivery of degradative enzymes to bacterial compartment
    • Mycobacterium tuberculosis
    • Salmonella
  • Surviving lysosomal fusion: toxic oxidation (resp. burst), acid pH, degradative enzymes, defensins, NO, iron withholding
    • Coxiella
    • Legionella
Phagosomal escape:lyse out of vacuole and grow in cytoplasm
  • Listeria
  • Shigella
Example of intracellular bacteria: salmonella
Model of intracellular s.Typhimurium population during in vivo infection
graphic Diseases and pathogenesis by bacteria
Damage from immune response
In some cases, disease is caused not by the bacterial pathogen itself but by the immune response to the pathogen. As described in Chapter 12, pathogen-stimulated overproduction of cytokines leads to the symptoms of bacterial septic shock, food poisoning, and toxic-shock syndrome.
The ability of some bacteria to survive intracellularly within infected cells can result in chronic antigenic activation of CD4+ T cells, leading to tissue destruction by a cell- mediated response with the characteristics of a delayed-type hypersensitivity reaction.
  • Inflammation: bacterial meningitis (Neisseria meningitis)
  • Antigen-Ab complexes:
  • compliment activation(Settle in kidney or joints)
    • Glomerulonephritis from S. pyogenes
  • Cross-reactive Ab’s: Ab’s against pathogen may cross-react with host tissues
    • Accute rheumatic fever, complication of Strep throat
    • Arthritis, M. tuberculosis, Y. pestis
  • Septic Shock: lipid A
  • Hyperimmunity: superantigens
  • Granuloma formation: “undigestable”? (Mycobacterium, Streptococcus, Staphylococcus)
Diphtheria (Corynebacterium diphtheriae)
Diphtheria is the classic example of a bacterial disease caused by a secreted exotoxin to which immunity can be induced by immunization with an inactivated toxoid. The causative agent, a gram- positive, rodlike organism called Corynebacterium diphtheriae.
Today, diphtheria toxoid is prepared by treating diphtheria toxin with formaldehyde. The reaction with formaldehyde cross-links the toxin, resulting in an irreversible loss in its toxicity while enhancing its antigenicity. The toxoid is administered together with tetanus toxoid and inactivated Bordetella pertussis in a combined vaccine that is given to children beginning at 6–8 weeks of age.
Tuberculosis (Mycobacterium tuberculosis)
Though several Mycobacterium species can cause tuberculosis, M. tuberculosis is the principal causative agent. The inhaled bacilli are ingested by alveolar macrophages and are able to survive and multiply intracellularly by inhibiting formation of phagolysosomes.The CD4+ T-cell–mediated immune response mounted by the majority of people exposed to M. tuberculosis thus ontrols the infection and later protects against reinfection. However, about 10% of individuals infected with M. tuberculosis follow a different clinical pattern: the disease progresses to chronic pulmonary tuberculosis or extrapulmonary tuberculosis.CD4+ T cells are activated within 2–6 weeks after infection, inducing the infiltration of large numbers of activated macrophages. These cells wall off the organism inside a granulomatous lesion called a tubercle.