Intracellular Bacterial Infections

Question 1

Advantages of intracellular lifestyle

Answer 1

• Nutritional:
  
  • Pathogens avoid competition for space and nutrients with other microbes
• Downregulate genes important for citric acid cycle - rely on hosts for those
• Protection from immune system:
  
  • Avoid a number of innate and humoral defenses
  • Lack of sterilization allows some bacteria to lay dormant for decades, only to emerge under immunosuppressive conditions
• Such protection allows easy dissemination around the body

Question 2

Disadvantages of intracellular lifestyle within phagocytic cells

Answer 2

• Can be eaten up and digested by cell
• Phagolysosome: membrane-enclosed organelle that forms when phagosome fuses with lysosome
• After fusion, food particles/pathogens contained within phagosome usually digested by enzymes contained within lysosome
  
  • Phagolysosome formation follows phagocytosis
• Harsh environment encountered inside phagocytes + slow growth characteristic of some intracellular pathogens –> chronicity of many of these infections

Question 3

Advantages of residing within non-phagocytic host cell

Answer 3

• Limited phagocytic and bactericidal activity by host cell
• Free ATP, free food, some good enzymes to play with and nothing is trying to actively kill it = good life.

Question 4

Strategies used by different intracellular pathogens to avoid antimicrobial defenses

Answer 4

• NADPH oxidase complex inhibition: prevents formation of complex
  
  • e.g. Legionella pneumophila
• Detoxify reactive O2 species
  
  • e.g. Salmonella
• Escape into cytoplasm from phagosome
  
  • e.g. Listeria
• Increase natural resistance associated macrophage protein (NRAMP) homologues to help maintain nutrients within cell
• Limit expulsion of Fe2+ by phagosome
  
  • Activated phagocytes usually reduce concentrations of iron available to intracellular pathogens to levels non-compatible with life

Question 5

Antibiotic considerations of intracellular bacteria

Answer 5

• Intracellular location of some microbes: critical aspect for failure of some abx for treatment of pathogens from infected hosts
• Intracellular activity of abx depends on:
  
  • Penetration of eukaryotic membrane
  • Subcellular localization
  • Deleterious interactions with intracellular milieu, including pH
  • Abx susceptibility of intracellular pathogen
• Weak base abx (aminoglycosides, macrolides) concentrated within lysosomes by pH-dependent mechanism
  
  • Preferential localization within lysosomes and partial inactivation by acidic pH: major disadvantages
• Lack of peptidoglycan in cell wall of chlamydiae + intracellular lifestyle = poor activity of b-lactams

Question 6

Intracellular pathogens that escape into cytosol

Answer 6

• Listeria lyses phagocytic vacuole, escaping into cytosol
• Advantages:
  
  • Access to large amounts of nutrients
  • Minimize exposure to antimicrobial defenses associated within vesicles/phagolysosomes
• Disadvantages:
  
  • Cytosolic antimicrobial peptides + cytosolic inducible NO synthase can target them
  • Cytotoxic CD8+ T cells can lyse infected cells expressing microbial peptides on MHC I
• Intracellular movement: once in cytosol, Listeria hijacks actin (polymerizes) and uses it to motor around cytosol as well as invade neighboring cells without being exposed to extracellular compartment

Question 7

Intracellular pathogens that remain in vacuoles: fusogenic

Answer 7

• Fusogenic = fusion of phagosome with lysosome
  
  • Bug is taken up and goes through entire maturation ultimately ending up in phagolysosome
• Coxiella burnetii, obligate intracellular Gram (-) bacterium
  
  • Only intracellular pathogen to reside within phagolysosome
• Nascent phagosome matures through endocytic pathway, eventually acquiring properties of lysosomes
• Pathogen-containing vacuole acidifies
• Coxiella not cytopathic –> reaches very high numbers within single vacuole

Question 8

Intracellular pathogens that remain in vacuoles: non-fusogenic

Answer 8

• Several intracellular pathogens contained in membrane-bound compartments that avoid fusion with lysosome
• Mycobacterium, Legionella, Chlamydia
• No processing of bacteria to put on MHC
• Causes granuloma formation

Question 9

Genetic basis for classification of obligate vs. facultative intracellular pathogens

Answer 9

• Facultative microorganisms:
  
  • Capable of both intra- and extracellular growth
• Obligate intracellular pathogens
  
  • Have established such an intimate relation with their host that they cannot reproduce outside intracellular environment
  • Loss of essential biosynthetic metabolic pathways = obligate dependency of host cells

Question 10

Obligate intracellular pathogens

Answer 10

• Chlamydia spp.
• Coxiella burnetii
• Ehrlichia spp.
• Mycobacterium leprae
• Rickettsia spp.

Question 11

Facultative intracellular pathogens

Answer 11

• Bartonella spp.
• Brucella spp.
• Francisella tularensis
• Legionella pneumophilia
• Listeria monocytogenes
• Mycobacterium spp.
• Nocardia spp.
• Salmonella enterica
• Shigella spp.
• Yersinia

Mnemonic: Listen Sally Yer Friend Bruce/Bart Must Leave Now

Listeria, Salmonella/Shigella, Yersinia, Francisella, Brucella/Bartonella, Mycobacterium, Legionella, Nocardia

Question 12

Zipper vs. trigger mechanisms contributing to invasion and dissemination of intracellular pathogens

Answer 12

• Can be taken by professional and non-professional phagocytes by zipper or trigger mechanism
• In both cases, reorganization of actin cytoskeleton results in changes at membrane surface, allowing uptake of bacteria
• Zipper:
  
  • Tight interaction between bacterial cell surface ligands and host cell receptors –> closure of host cell surface around bacterium
• Trigger:
  
  • Bacterial products induce cell surface to ruffle, projecting membrane extensions that surround bacteria
  • Products secreted by type III secretory systems of Salmonella and Shigella induce this kind of uptake