Hello everyone and welcome back! Some time ago I wrote about a system called a Type 6 Secretion System (T6SS), that was also described as a spring-loaded poison dagger. If you recall, the T6SS helped move proteins from inside the bacterial cell into adjacent cells. Well, there is another system called a Type 3 Secretion System (T3SS), that is also quite important for bacterial colonization.
So, I found a video that does a good job at showing how, particularly Enterohemorrhagic E. coli (EHEC for short) colonizes the gut, and how the T3SS helps. Remember, E. coli species that cause disease will enter your body through ingestion (a fecal-oral route)…well, mostly so anyway. EHEC particularly being a foodborne bacterium, can be acquired by eating contaminated foods.
EHEC itself was isolated from hamburger meat (hence the name of the video, ‘getting to the bottom of the burger bug’), and it has been found that contact with contaminated ruminant feces (either directly or indirectly) to be the culprit of human infection! If you are a student, can you quickly research how many kinds/strains of E. coli there are? Hint…the ballpark answer runs the hundreds!
EHEC can move (using rotating appendages called flagella), and this specific property helps the bacteria navigate through mucus of the gastrointestinal tract…a.k.a. the gut. It is aware that it’s in a favorable/favourable location, and gets ready to attach itself there.
There are several genes and pieces of DNA that control the capability of the organism to bring about disease (a.k.a. bacterial pathogenicity). Timing is very important, and bacterial pathogenicity genes need to be turned on at the right place and right time. Some of these genes code for structural proteins of the T3SS itself.
The T3SS starts to assemble at the inner membrane of the bacterial cell, creating the base structure of the secretion system. Once this is done, EHEC can use other external structures to attach to gut epithelial cell microvilli. After contact is made, further gene expression events happen within the bacterium, and export of proteins from the bacterial inner membrane begins.
If you look at the video, you will observe that the microvilli are effaced/erased/disappears from that spot! The bacterial cell becomes partially embodied in the host membrane itself…as though it’s laying down nicely on a foam mattress! This attachment and effacing lesion that is formed in the gut encourages EHEC persistence here. If you have several bacterial cells all doing this same thing, it can interfere with the natural function of the gut cells, thus promoting diarrhea. EHEC releases shiga toxin, which causes bloody diarrhea, and in some cases, kidney damage.
So, in a nutshell, what is the T3SS? It is a protein appendage/attachment, that many gram-negative bacteria have. It essentially is a sensory probe that helps the bacteria detect eukaryotic cells and secrete proteins that help with bacterial infection. There is direct secretion of proteins from the bacterium into the host cell. These proteins help allow the pathogenic organism to survive and hide from the body’s immune response.
Take care until next time! 🙂
Follow the links to read more on EHEC link 1, EHEC link 2 and serotypes. See link to burger bug video here as well. Credit is given for use of image of mattress cartoon and EHEC.
jtbtrinbagomicrobiologist 