A recent paper by Benjamin Wolfe et al. at Tufts University showed that a slow-growing bacterium can outcompete its relatives with the help of fungi in the genus Scopulariopsis.
In the study, they evaluated the relative abundance of three species Staphylococcus bacteria (see image). Staphylococcus equorum was the slowest grower in lab tests where the bacteria were grown in isolation, without the addition of fungi. With the addition of the fungus Scopulariopsis, S. equorum lowered its expression of genes involved in iron uptake and metabolism. The fungus may provide the bacterium with freely available iron needed for growth, allowing the bacterium to stop expending energy to acquire and process iron. This would then allow the bacterium to outcompete other related bacteria that do not receive iron from the fungus. The other bacteria in the experiment did not receive the same benefit of iron acquisition from the fungus, because the relationship between S. equorum and Scopulariopsis is species-specific, meaning that this interaction is unique between them.
This study is significant because it shows that there is a decoupling of the interactions between bacteria and fungi in situ (the original habitat) and in vitro (a controlled test container). While the studies comparing bacterial growth and competition in vitro showed S. equorum to be the worst of the three bacteria, it was actually the best when growing with its in situ fungal partner. The ecological processes and molecular mechanisms that shape the distributions of bacteria in agriculture, medicine, industry, etc. are not well understood; we need more studies that can show realistic in situ interactions in order to have the best comprehensive understanding of these microorganisms.
Specifically, one of the bacteria in this study, S. saprophyticus, is a common causal agent of urinary tract infections and is often found in cheese. By working toward a more comprehensive understanding of the interactions between the bacteria in these cheeses and other organisms in situ, cheese producers can better control for safety and quality of their products. Staphylococcus bacteria occur frequently in mammals, including humans, so understanding these bacterium-fungus interactions could have implications beyond cheese production.
Original paper by Kastman et al., published in Nature 2016:
Article at nature.com: