Lecture 12 Bacterial Survival Strategies

Question 1

Evading phagocytosis

Answer 1

Pathogen specific antibodies aid in opsonisation and uptake by macrophages

Virulence factors can stop antibodies binding - the CAPSULE. Also masks ANTIGENS on the surface of the cell

Many different types of capsule. Capsules are predominantly polysaccharide.

B. cereus G9241 causes anthrax-like disease in immune-competent mice (shares exon with anthrax)
Deletion of the operon encoding the capsule leads to lack of capsule and an increased uptake by macrophages

Question 2

Evading the respiratory burst

Answer 2

Macrophages make toxic oxygen radicals

Host enzymes: Uses NADPH oxidase produces reactive oxygen singlet which can be secreted inside vauoles inside the cell or secreted externally

Superoxide dismutase converts it to H2O2

Fenton reaction makes a OH radical from H202

Also reactive nitrogen species:
Nitroxyl (NO) can be converted to nitrite or peroxynitrite

To evade this bacteria produce enzymes, eg KATs (catalase deactivates h202) AHPs alkyl hydro peroxidase (deactivates peroxynitrite), Sods (superoxide dismutase)

Knocking out these enzymes reduces VIRULENCE

Question 3

Evading the respiratory burst

Answer 3

Bacterial response to the respiratory burst transcriptionally regulated by OxyR and SoxR/S

OxyR senses h2o2, bond forms between 2 sulfhydyl groups on the protein, causing a conformational change on the protein, this allows it to begin transcribing KatG and AHP

SoxR/S - two component system, senses the redox active compounds, causes a conformational change in the sox R dimer which will bing and this causes transcriptional activation of sox S, and sox S is another transcription activator which then transcribles superoxide dismutase, acrA and acrB (efflux pump system which can remove the radical species)

Question 4

Escaping the phagosome

Answer 4

Listeria monocytogenes manages to escape from the phagosome/vacuole

Achieved through the action of Listeriolysin (LLO) a pore-forming toxin belonging to the family of cholesterol-dependent cytolysins (CDCs, are targeted SPECIFICALLY to host membranes). Activity is regulated by pH.

Rocketing into neighbouring cells via actin cytoskeleton results in the listeria being encased in a double membrane. It escapes via:
Phagosome destabilisation is also achieved by two phospholipases (PLC):
PC-PLC and PI-PLC
These work in addition to LLO

Actin ‘comet tails’ Listeria utilise host actin to propel itself once out of the phagosome and into a neighbouring cell. This is regulated by PH also.

PrfA (protein release factor) regulates Listeria virulence
It is a TRANSCRIPTIONAL ACTIVATOR

Question 5

New roles for LLO

Answer 5

LLO also has roles in autophagy - can hijack the autophagy pathway.
Autophagy can degrade intracellular bacteria. Several pathogens can subvert this either by exploiting the autophagosome as a replication niche or by actively avoiding recognition by the autophagic machinery

Inhibition of the respiratory burst - stop the production of ROS and acidification

Question 6

If bacteria cannot escape the phagosome it can INHIBIT PHAGOLYSOSOME FUSION

Answer 6

e.g. M tuberculosis prevents phagosome fusion
Even if fusion occurs, TB phagosomes characterised by
1. Paucity of vacuolar H+ ATPase
2. Subsequent inefficient luminal acidification
3. Inadequate levels of mature lysosomal hydrolases

Inhibition of phagosome acidification possibly due to a secreted tyrosine phosphatase PtpA (secreted by TB): dephosphorylates a component of the tethering complexof VPS proteins that recruits the vacuolar atpase

Question 7

Resistance to antimicrobial peptides

Answer 7

Common theme is to apply positive charges to membrane molecules to repel the +vely charged AMPs (anti microbial peptides)

e.g phoP/Q recognises an acidic environment and causes transcription of genes that cause modification of the outer membrane and adding positively charged molecules to the LPS to repel the cationic anti microbial peptides

Very important defence system

In gram positives: GRAS/GRAR two component system (glycopeptide resistance association) glycopeptides are cationic amps in G+ve
Lysosome contains glycopeptides that will damage these. GRAS/R TCS controls the transcription of the DLT operon (LTA of G+ve cell wall is negatively charged) upregulated transcription of DTA causing +ve alanine to be added on to the LTA (lipoteic acid) of the G+ve cell wall

Both result in the repulsion of the +ve AMPs

Question 8

Iron aquisition

Answer 8

Some bacteria (e.g. TB) secrete Exochelins (they chelate iron)
Salmonella acquires ferric iron by secreting the siderophores enterobactin and salmochelin

Enterobactin uses two membrane receptors IroN and FepA to pull the siderophore back but the receptors can also pull in other species exochelins

Required by salmonella to evade nutritionally immunity in macrophages and cause persistent infection in mice

In TB the siderophore is called Mycobactin.
Deletion of the genes encoding the biosynthetic apparatus leads to attenuation.

Question 9

Many bacteria secrete proteins to modulate host interactions but some

LIVE WITHIN THE PHAGOSOME

Answer 9

Several of these pathogens also used secreted effectors but these are highly elaborate and used to support the intracellular life style

T3 and T4SS are activated by acidification of the phagosome

e.g. salmonella have T3SS which are activated by phagosome acidification
Salmonella pathogenicity island II

e.g. Q fever caused by coxiella has a T4SS
dot/lcm system for intracellular organisation and organelle transport