A meeting of the micros

November 1, 2022 10:11 AM

When I finish one container of yogurt and use the same spoon to scoop some out of a new container, I like to think I'm introducing two long-separated strains of lactobacilli or whatever to each other to live in harmony. What's actually happening? Do they go to war? Do they intermingle? Do they eat each other? And could it possibly affect the yogurt?
posted by BlackLeotardFront to Food & Drink (6 answers total)

Fun tradition!

The specific critters used to make yogurt are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. They are both nonmotile (can't move on their own), and both are pretty inactive at fridge or room temperatures ("S. thermophilus has an optimal growth temperature range of 35–42 °C (95–108 °F), while L. d. bulgaricus has an optimal range of 43–46 °C (109–115 °F)").

Others are bacteria are also used, but I don't think any of that matters much because because I think the boring answer to your question is, they aren't really doing much of anything at all, they are just hanging out. They can't eat each other (either within or between species). What little metabolism is going on doesn't even put them into much resource competition, since they will typically not be resource limited.

Probably the most exciting thing they *might* do is transfer some genes via horizontal gene transfer, as has been documented in several probiotic species as described here, but I think that would be more likely to happen in your gut than in your cup or in the fridge, due to the low temperatures.

The only way it could affect the yogurt that I can think of is the same way using any non-clean spoon might: it could introduce mold spores or other contaminants, but you'd notice if your yogurt has spoiled.
posted by SaltySalticid at 10:41 AM on November 1, 2022 [14 favorites]

This question is AWESOME
posted by rrrrrrrrrt at 11:08 AM on November 1, 2022 [10 favorites]

Just to note: the vast, vast majority of bacteria cannot eat (as in, engulf and digest) each other. And the ones that do are weird and strange, but that's another set of stories. Bacteria just do not range in size enough to the point of being larger than others in order to engulf them (note: crazy exceptions on size both being super large and super tiny, but generally...) Instead, bacteria interact via: resource competition (as SaltySalticid mentions -- ex: if I eat all of the food, then there isn't any for anyone else); through communicating with small molecules (a limited type of communication, sharing information on who is here and in what concentration); or through warfare (!!!), just like your question. Warfare might mean one species making an antibiotic to kill off other species, and then hog the resources for themselves... or eat up the stuff that spills out of the dead bodies of other bacteria that they have killed. The bacteria species in yogurt generally try and kill off each other not through small-molecule-antibiotics, but through short proteins that act just like antibiotics. We're trying to figure out how to use these proteins to help human health; so far, just one is used (Nisin) as an additive to food to prevent spoilage. If you ever see the phrase "Fermented milk solids" on food that really shouldn't have milk -- it's nisin, i.e. one of these warfare proteins. As a part of warfare, bacteria will also load up the equivalent of hypodermic needles and shoot each other up with toxins, as a part of competition. They can be wild!

But SaltySalticid is 100% correct -- they are probably inactive in your yogurt.

-Your friendly local microbiology, who job it is to research interactions like microbial warfare and horizontal gene transfer
posted by Peter Petridish at 12:40 PM on November 1, 2022 [36 favorites]

Response by poster: Fantastic answers. I suspected they were largely inactive but felt like at the very least they must register the presence of a new neighbor. Anyhow I'm taking the occasion to simply learn more about bacteria!
posted by BlackLeotardFront at 3:13 PM on November 1, 2022 [2 favorites]

I don't think the strains from two different batches of the same brand commercial yogurt would be able to sense that the 'others' were any more other than their own siblings and offspring, but maybe Peter Petridish has some input?

If you want more cool bacteria stuff I love that they do quorum sensing and can change behavior based on how many buddies they have and that lets them do all sorts of cool stuff :)
posted by SaltySalticid at 4:47 PM on November 1, 2022 [2 favorites]

I did some looking into this -- bacteria communicating with each other, i.e. quorum sensing. Both of these bacteria ( Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus) have the AI-2 quorum sensing system. This is a 'general communication channel' in bacteria -- tons and tons of species produce this exact signal and can detect this signal. This means that these bacteria can estimate how many other bacteria (regardless of species) are around it. So, these two species are communicating a bit, and potentially modifying their behavior based on the overall concentration of bacteria present. Other (all other?) quorum sensing signals are much, much more specific -- most are within species, like SaltySalticid points out. I liken this to being able to tune into different radio channels: one is open to the public, and the other is encrypted and only within the species members can access it.

My study organism has at least 5 quorum systems, including this general AI-2 system, all which control behavior of the microbes. It make studying group behavior, which we look at, a real pain.
posted by Peter Petridish at 6:19 AM on November 2, 2022 [2 favorites]