How Salmonella defend themselves

Well, it’s about time for another microbiology review, and I got a good one for y’all. I finally got around to browsing through the past weeks’ issues of Science during my lunch breaks this past week, and I found this article and review on how Salmonella defend themselves inside host cells. You see, pathogenic bacteria have developed several strategies for overcoming the wonderful defenses of our bodies. One of the most notorious of these is the Type III Secretion System (T3SS) encoded by several Gram-negative pathogens. This includes: Salmonella species, causative agents of food poisoning and Typhoid fever; Shigella species, causative agents of dysentery; Enterhemorrhagic Escherichia coli, or EHEC, causative agent of the dreaded food poisoning outbreaks from spinach and at Jack in the Box, Yersinia pestis, causative agent of plague, Pseudomonas species, causing a slew of infections in humans and plants, and the list goes on and on…

IMHO, the T3SS is the most amazing of engineering feats among bacteria. It’s basically a huge needle complex, the base of which sits in the envelope of the bacterium. After constructing the base, the bacterium secretes the needle components through the base. Once reaching a critical length, components of the “translocon” are secreted to the very tip of the needle. The translocon is capable of forming pores in eukaryotic membranes, ultimately allowing the bacteria to inject its own proteins, termed “effectors”, into target host cells. These secreted effectors vary greatly among T3SS-encoding bacteria, allowing different bugs to perform a diverse range of functions, which include: killing the targeted host cell with secreted toxins so that the bacteria can invade into deeper tissues; inducing sessile cells to engulf the bacteria – host cells make great hiding spots; modifying macrophages, which are meant to kill invading bacteria, to make them more hospitable environments for the bacteria; or inducing macrophage lysis to escape killing.

Figure 1: Diagram and Electron Microscopy Image of T3SS from P. aeruginosa

One of the mysteries of these crazy systems is the temporal separation between construction of the needle and secretion of its effectors. Secretion in many cases is host-cell dependent, while construction of the needles begins as soon as the bacteria are switched to permissive temperature (37°C, or body temperature). It makes sense, since the bacteria wouldn’t want to secrete their protein effectors willy-nilly into the extracellular milieu. But the mechanism by which this occurs has been very difficult to unravel.

Until now…

In the above referenced article, Yu and colleagues show that secretion of effector proteins in Salmonella requires a sequential, two-step process of pH detection. Upon engulfment by a macrophage, the Salmonella are separated from the cytoplasm by a membrane in what is termed a Salmonella-containing vacuole, or SCV. The SCV undergoes a drop in pH, to about 5.0, and is targeted for destruction of its contents by the host cell. To combat this, engulfed Salmonella use their T3SS to secrete effector proteins into the eukaryotic cytoplasm, thereby altering the fate of the SCV.

To mimic the environment of the SCV, the authors grew Salmonella in media at pH 5.0, during which they found three proteins – SsaL, SsaM, and SpiC – that formed a ternary complex at the entryway to the secretion channel. The authors showed that this complex allowed secretion of the translocon components (the tip of the needle which forms a pore in the eukaryotic membrane) while preventing secretion of the effector proteins. While interesting, this didn’t answer how the Salmonella switch from translocon to effector protein secretion, saving their sickening little asses from certain destruction…

The big clue came when the authors shifted Salmonella grown in acidic medium back to neutral pH, at which point the bacteria began secreting effector proteins. This finding was very interesting, since you can imagine that Salmonella engulfed by a host cell might sense a similar shift in pH once the needle pokes a hole through the SCV membrane. The authors then showed that switching acidic-grown Salmonella to neutral pH led to dissociation of the SsaL-SsaM-SpiC complex, and that acidification of Salmonella-infected macrophages blocked secretion of effectors. Combined, these results indicate that engulfed Salmonella first detect the lower pH of the SCV, which allows the T3SS needle translocon to poke a hole in the SCV membrane, then sense the increased pH of the eukaryotic cytoplasm, which causes dissociation of the SsaL-SsaM-SpiC complex and effector protein secretion:

Figure 2. Regulation of Salmonella effector protein secretion. From Collier, Science 2010.

So now a complex piece of machinery has a complex set of operating instructions to go with it. Not all that suprising if you ask me…but very cool.

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