Pathogens in a parking lot & gratuitous fecal references

Teaching 4/5 classes a semester can leave little time for thinking about your teaching (especially if you have young children). Even with a class release this semester, I find myself with very little time to think ahead more than 1 week. Friday I met with a colleague from one of our other campuses to informally “talk teaching”… it felt like a combination of being super early to a garage sale packed with things you want and a meeting between two spies sharing secrets.

At some point we were trading analogies that can be used in teaching biology and I told him about one that I stumbled into a couple of semesters ago. This post is me thinking about it. Saying this all out loud would work too, but would then I would have to answer a question every 30 seconds from the Cute Little Brain Surgeons.

Competitive exclusion (aka microbial antagonism) is the concept that your body is like the African Savannah (or any ecosystem really). Why don’t gazelles reproduce and reproduce to enormous numbers? Lions. Why don’t hyenas do the same? Well, partly their biology I am sure, but also because they are in competition with lions for food. Any ecosystem is a complex system of interactions that self regulates itself. Your body is no different and microbial diseases are generally (always?) what results from a breakdown in this regulation.  Its true on the skin, inside the vagina and maybe even the surfaces of your teeth.

A different view of your gut.

A different view of your gut.

Here comes the parking lot analogy. Sometimes what prevents you from getting sick, might just be that there is “no room at the inn”, so to speak. Imagine you eat some romaine lettuce contaminated with E coli O157:H7.  In order for those E coli to infect you, they are going to have to stick to the cells lining your gut.  This could be an oversimplification, and I need to chase this down, but I imagine that the flow through the lumen of the gut is a barrier to E coli getting a foot hold.  Flagella are one option that just popped into my head (chase that down Jeff: flagella, chemotaxis, intestine).  Another option is to stick to the cells that line your gut.  This sticking is known to be important for infections with E coli.

bacteria beach 23

An imperfect analogy to be sure, but useful nonetheless.

Just like a real parking lot, your intestines are essentially an enormous flat space to microbes, and there are lots and lots of friendly bacteria already there (chase that down Jeff: commensal, adherence, intestine).  Unlike a parking lot, the pathogen can’t just idle in one place while waiting for a space to open up.  This parking lot is under water and the tide moves regularly (chase that down Jeff: intestine, lumen, flow rate).  Without some way of dealing with those two facts (very few spaces (is that accurate?) and a moving tide), you are going to end up in the dark end of a porta potty.  (Update on the figure after a 4 hour round trip car ride with the fam –> by definition, the cars in the parking lot can NOT represent commensal bacteria in this situation… there is a good essay question buried in there somewhere.)

I imagine probiotics partly working by filling some of those parking spots, at least temporarily (chase that down Jeff: lactobacillus or bfidobacterium, adherence, intestine). Honesty in front of students is good though, so I tell them that I pulling most of this out of my booty (pun intended) but that I would bet hundreds of dollars* that I am not too far off here.

* – the amount of money that I would bet on the truth of any given statement is a way I communicate my confidence in that statement to students.  I calibrate by reminding them that we drive two ten year old cars and can’t afford babysitting.  There are only a few statements (maybe 4 or 5) that I tell them “I would literally put all of my family’s money on this wager, even the kids accumulated allowance money that we keep in cute little wallets in our sock drawer”


About SubOptimist

I am an Associate Professor in the Science Department at Georgia Perimeter College, Clarkston. I teach introductory biology courses at both the majors and non-majors level in addition to microbiology. Previous to that I spent 7 years as a postdoctoral researcher on different viruses. While I don't miss being on the "grant treadmill", I think better when I write and miss writing up data for papers and grants; this blog helps me with that a little. And sometimes my kids' insanely funny and cute antics need to be shared with the world. Any view expressed in this blog is that of me personally and not Georgia Perimeter College or the GPC Clarkston Science Department.
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