By Brooke Napier
Salmonella enterica serovar Typhi (S. Typhi), made infamous by Typhoid Mary, is a strictly human pathogen and the causative agent of typhoid fever (which still kills around 200,000 people a year).
Interestingly, we do not know what drives the species-specification of S. Typhi – even though there is a very similar mouse pathogen, Salmonella Typhimurium (Typhi-murium, murium = mouse, yah!). The species-restrictions carry all the way down to the cellular level, even though human and mouse macrophages (lean, mean innate immune system machines) are a very similar cell types, S. Typhi cannot infect mouse macrophages.
Both S. Typhi and S. Typhimurium are taken up by host innate immune cells, macrophages, and survive within an intracellular compartment called the Salmonella containing vaculole (SCV). It’s curious that S. Typhi and S. Typhimurium have such similar life cycles, but retain species-specifications.
Perhaps the key to species-restriction can be found in the differences within the SCVs?
While in the SCV, S. Typhi (human) and S. Typhimurium (mouse) communicate with host cells through a variety of proteins, called effector proteins, which are secreted from the bacteria into the host cell.
S. Typhi and S. Typhimurium have subtle differences in what effector proteins are secreted and this results in different SCV environments. Specifically, species-specific S. Typhi recruits host protein Rab29 to the SCV, however S. Typhimurium does not because it secretes an effector protein GtgE that degrades Rab29.
Whoa whoa whoa, what is a Rab protein anyway?
Rab proteins are a family of G-proteins, or guanine nucleotide-binding proteins, a superfamily of proteins involved in transmitting chemical signals throughout host cells. Specifically Rab proteins are involved in regulating steps of vesicle formation and trafficking – vesicles like the Salmonella-containing vesicles (SCVs). REALLY briefly, they are proteins that are anchored to the membranes of these vesicles in our case interact with bacterial effector protein GtgE.
Could the key to the species-restriction of S. Typhi be related to the recruitment of host proteins to the SCV?
Stefania Spanò and Jorge E. Galán found that if they expressed gtgE within the human-specific S. Typhi they could bypass host-restriction and S. Typhi could survive and replicate within macrophages and other tissues of mice.
Breaking it down:
The expression of just one single effector protein belonging to another bacterial species allowed S. Typhi to overcome host-cell restriction and survive in a nonpermissive host cell.
I guess this is particularly interesting because we always hear about flu viruses jumping from one species to the next (ex: Avian flu or Swine flu), but we rarely hear about the evolution of species-specification in bacteria.
Not only that, but papers like this have identified it’s almost as simple as viruses jumping from one species to the next – it’s a matter of one gene. Considering many species of bacteria can readily pick up floating DNA and/or exchange DNA easily with other bacterial strains and that these DNA floaters could very well encode just one gene needed for a pathogen to jump species – that’s SCARY!
Back to the story though, remember how I mentioned that GtgE acts as a protease to degrade host protein Rab29, but this degradation does not happen in S. Typhi? Perhaps…
…is the degradation of Rab29 in the SCV by GtgE responsible for host-restriction?
Oddly they found that Rab29 was not very important in host-specificity, but that another host protein, Rab32, is perhaps this is the key to host-restriction.
They first noticed that like Rab29, Rab32 was being recruited to the membrane of SCVs during infection of human macrophages with S. Typhi; however, Rab32 is not recruited to the SCV in an infection of mouse macrophages with S. Typhimurium (hint: Rab32 is most likely being degraded by GtgE, like Rab29).
So if the presence of Rab32 in the SCV membrane might be restricting S. Typhi from surviving and replicating in mouse macrophages – what if we just delete Rab32 from mouse macrophages? Could S. Typhi then survive and replicate in mouse macrophages? Could we have found the key to host restriction?!
Eureka! They depleted Rab32 (by siRNA) from mouse macrophages and S. Typhi could not only survive but also THRIVE.
So why on Earth would Rab32, a signaling protein, be required for host-restriction?
Their hypothesis is that Rab32 has been implicated in the biogenesis of very specific cellular compartments that harbor antimicrobial peptides; therefore, perhaps Rab32 may restrict S. Typhi growth in mouse macrophages by delivering antimicrobial peptides to the SCV. They found this was actually happening, and S. Typhi was actively dying in the SCVs of mouse macrophages because of the presence of antimicrobial peptides.
Want to know something really cool? Their last few sentences:
“…a recent genome-wide association study has uncovered a genetic polymorphism (or mutation) in (human) Rab32 that is linked to increased susceptibility of Mycobacterium leprae infection. Furthermore, Rab32 has been reported to be present in the Mycobacterium tuberculosis-containing vacuole.”
In short, this could be the reason behind species-restrictions in other multiple intracellular bacterial pathogens.
Spano, S., & Galan, J. (2012). A Rab32-Dependent Pathway Contributes to Salmonella Typhi Host Restriction Science, 338 (6109), 960-963 DOI: 10.1126/science.1229224