Salmonella. Ask anyone about salmonella and you will get an immediate response. “I’ve had salmonella before. I was keeled over in a fetal position for days”, they say. Although there are many other forms of bacteria that can cause foodborne illnesses, it seems salmonella has the most notoriety.
Salmonella is one of the most devastating bacterial infections worldwide. Mortality rates hover around 2 million people per year. If we look into statistics from the USA, we find approximately 1.2 million illnesses and about 450 deaths annually from this pathogen. Think of how many illnesses go unreported. When was the last time you called your local health department after catching the “24” hour flu/bug? Probably never, so that number is actually quite higher.
Salmonella has over 2500 different serotypes (think strains, but with antigenic properties on the cell wall), but 32 of these are more well-known and much more studied. The most recognized species are Salmonella enteritidis, S. enterica, S. typhi, and S. typhimurium. Salmonella is more common in summer months and the core groups most susceptible for infection include: Under 5 or over 65, the immunocompromised, and those on a steady regimen of medication that reduce stomach acid.
For such a devastating pathogen, the cell-hijacking machinery behind its malicious behavior was only recently understood. One single protein allows the bacterium to both evade cells lining the intestine and hijack cellular functions to avoid destruction. Salmonella causes disease when it takes control of cells lining the intestine by using its own specialized “nano-syringe”. This syringe injects proteins that mimic the proteins of the host cell.
Let’s take a closer look into the deviousness of this process. This protein, SopB, works with the plasma membrane to coax the cell into taking in the pathogen.
“Knock, knock. Who’s there? Salmonella protein. Salmonella protein, who? I’m sorry. I meant, pizza guy. Sure. Come on in!”
Once inside, the host cell wraps it in a vesicle. That’s like inviting in your killer and offering them a bullet proof vest.
Secondly, once inside the vesicle the pathogenic protein evades the lysosome (an organelle) that degrades proteins that are no longer needed. It does this by moving from the plasma membrane to the membrane of the vesicle that contains the bacterium. Oh yeah… You didn’t think the bacterium didn’t get welcomed in with open arms, did you?
Once the bacterium and the protein are wrapped up all snug like a bug in a rug, Salmonella coaxes the cell to mark, SopB, with a tag that will identify it as a host cell.
This “nano-syringe” deserves a more rounded explanation of how it works. Yale university and the University of Texas Medical School-Houston created a cryo-electron tomography to reveal the molecular structure of how this device works.
This syringe, or the more aptly named Type III secretion machine, features an injection point on one end and a rotating staging area (think bullets in a revolver) on the bottom where proteins are rotated and selected for the delivery into target cells. “The device is like a stinger and injects ready-made bacterial proteins into mammalian cells to commandeer them for the benefit of the pathogen” (Jorge Galan).
(image credit: Yale university)
It’s fascinating to think all this goes on in a microscopic world that we rarely give a second thought about. Bacteriology is an interesting field within Microbiology that merits further reading. It highlights the importance of scientific grants and the pursuit of knowledge. By further understanding this “nano-syringe”, researchers could devise new anti-infective strategies against a variety of bacterial pathogens.
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