Breaking down the microbiology world one bite at a time
Competition is the rule in culturable microbes.
In all environments, individual species of microbes don’t stay together, but rather live in multi-species communities where many cells of different species interact with each other. The study of these interactions in microbial communities has been a growing field for the past decades. They are usually represented by networks with species as nodes (circles) and interactions as edges (lines).
One key question when studying microbial interaction networks is: Do species in the community cooperate, or compete instead?
Indeed, both types of interactions have already been seen by scientists studying microbial communities. Cooperation can take many forms. Examples include cross-feeding, in which a microbe eats a nutrient, produces “waste” and this waste is used by another microbe as a nutrient; or biofilms, in which the microbe community produces mucus to protect the cells from predators and external perturbations. Competition typically involves a “battle” for a nutrient needed by two (or more) different species to survive, or the production of antibiotics to kill other species.
In their article, Foster and Bell tried to give a first insight into the aforementioned question. As only a very small fraction of known microorganisms can be grown in the lab by current techniques (see for example here and here), they had to focus on these culturable bacteria.
They first isolated bacteria from “pools” in beech trees: a place where rainwater accumulates in natural holes, serving as a perfect place for bacteria to develop. They conducted two separate experiments with respectively 72 (Exp1) and 32 species (Exp2) and measured the productivity of the resulting communities. That resulted in 683 mixtures in Exp1 and 480 in Exp2 (including monocultures, i.e. each individual species cultured alone in order to compare with multi-species cultures). They then assessed productivity by measuring the quantity of CO2 released by each community: the more CO2 is produced, the more “productive” the community is.
They found that the vast majority of pairwise interactions (interactions between two species), in both experiments was competitive. Only a small minority was cooperative, and most of them were associated with only a modest gain in productivity. Also, cooperation was most of the time beneficial for one microbe only, and rarely mutually beneficial. The multi-species communities generally performed better than the average of the monocultures, but this only suggests that competition between different species is less strong than competition between different individuals of the same species. In the end, the authors didn’t find any evidence supporting the existence of positive interactions implying more than two species (what we call “higher-order interactions” in microbial communities studies).
Based on data obtained from monocultures, it is predicted that cooperation enhances productivity, which is predicted to increase if more species are added to the mix (represented here by the red line). However, during the experiments, the researchers saw no increase in productivity, leading to the conclusion that there was no cooperation between the different species (black line).
These results are surprising because we might expect that bacteria in nature have more interest in cooperating to survive than to compete. Why didn’t they observe more positive interactions? Foster and Bell stated two arguments. First, there is a potential for competition for resources among microbes: food can be scarce in the environment, and microbes don’t necessarily have a wide range of edible components in their diet. Second, microbial ecosystems in nature are extremely diverse when compared to animal or plant ecosystems in terms of temperature, pressure, light and so on. In such environments, they may not have the possibility to exhibit cooperation at all.
Foster and Bell concluded that if interactions with other species are fleeting and unreliable, there will be little interest (and potential big cost) in investing time and resources for cooperation with these species over short or long timescales. That could be the explanation for the dominance of negative interactions observed here, but microbiologists will need more studies to confirm these results and check if they are also observed in nature, outside the more controlled environment of a laboratory.
Featured image: Alice van Helden (personal work, 19-08-2021), with kind permission from the author.