For my microbiology class final paper, I am planning to write about the microbiology of cheese rinds. Until recently, I didn't appreciate that nearly all cheese rinds (with the except of wax-coated cheeses), are thick layers of microbial communities, known as biofilms. For the past year, I was studying biofilms in a rather different context. I was examining a single strain of bacteria, known as Bacillus subtilis, to try to understand the mechanics of its growth. The cheese rinds are far more complicated, with successive waves of colonization by various microbes over the course of several months:
- Lactic acid bacteria are first to the scene and convert lactose to lactic acid.
- Yeast cells eat the lactic acid, which de-acidifies the cheese.
- New waves of bacteria can now colonize the curd. If the cheese is brined, then these are predominantly salt-tolerant bacteria.
- Fungi can also colonize the cheese later in the aging process.
Each of these microbes has its own mix of peptases and lipases, which can break down the proteins and fats, respectively, in the curd to produce flavorful compounds and change the texture of the cheese. One of the first people to thoroughly study cheese from a microbiological perspective was Sister Noella Marcellino, also known as the Cheese Nun, who got a Fulbright grant to travel around France and study the micro-ecology of all types of artisan cheeses. Below is a figure from one of her papers (N Marcellino and D R Benson, Applied and Environmental Microbiology, Nov. 1992, p. 3448-3454), which shows some of the microbial diversity living on the surface of a piece of St. Nectaire cheese:
At the top (a) are fungal spores and collapsed hyphae (chains of cells). Below that (b) is a layer of yeast and bacterial colonies. More fungal hyphae can be seen growing further inwards (c). The boundary with the curd (d) can be seen near the bottom of the figure. The total thickness of the rind is about 1.5 mm.
Categorizing this microbial diversity is just the first step. In my paper for the class, I am proposing a study to figure out how these microbes are interacting. One well-known method is through quorum-sensing, in which bacteria send out small signal molecules into their environment and listen for the concentration of these same molecules. If there are a lot of the same species around, the concentration of these molecules is high, so the bacteria know they are not alone and start behaving in new ways, such as forming a biofilm.
Cheese is a great system for studying microbial ecology, since it's more interesting a single species on an agar plate, but still far simpler than the overwhelming diversity found in nature. It's another potential topic for my future gastro-science PhD.