What’s that cheese smell? Just bacteria and fungi communicating
The scientists found common bacteria essential to ripening cheese can sense and respond to compounds produced by fungi in the rind and released into the air, enhancing the growth of some species of bacteria over others.
The composition of bacteria, yeast and fungi that make up the cheese microbiome is critical to both the flavor and quality of the cheese, so understanding how that can be controlled or modified is important in cheesemaking.
The discovery, published in Environmental Microbiology, provides a model for the understanding and modification of other economically and clinically important microbiomes, such as in soil or the gastrointestinal tract.
“Humans have appreciated the diverse aromas of cheeses for hundreds of years, but how these aromas impact the biology of the cheese microbiome had not been studied,” said Benjamin Wolfe, professor of biology in the School of Arts and Science at Tufts University and corresponding author of the study.
“Our latest findings show that cheese microbes can use these aromas to dramatically change their biology, and the findings’ importance extends beyond cheese making to other fields as well.”
Many microbes produce airborne chemical compounds called volatile organic compounds (VOCs), as they interact with their environment. An example of a microbial VOC is geosmin, which is emitted by soil microbes and can often be smelled after a heavy rain in forests.
As bacteria and fungi grow on ripening cheeses, they secrete enzymes, which break down amino acids to produce acids, alcohols, aldehydes, amines, and various sulfur compounds, while other enzymes break down fatty acids to produce esters, methyl ketones, and secondary alcohols.
The resulting biological products contribute to the flavor and aroma of cheese and they are the reason why cheeses have their signature smells.
The Tufts researchers found VOCs not only contribute to the sensory experience of cheese but also provide a way for fungi to communicate with and “feed” bacteria in the cheese microbiome.
By pairing 16 different common cheese bacteria with five common cheese rind fungi, the researchers found the fungi caused responses in the bacteria ranging from strong stimulation to strong inhibition. One bacteria species, Vibrio casei, responded by growing rapidly in the presence of VOCs emitted by all five of the fungi.
Other bacteria, such as Psychrobacter, only grew in response to one of the fungi (Galactomyces), and two cheese bacteria decreased significantly in number when exposed to VOCs produced by Galactomyces.
The researchers found the VOCs altered the expression of genes in the bacteria, including those that affect the way they metabolize nutrients. One metabolic mechanism that was enhanced, the glyoxylate shunt, allows the bacteria to utilize more simple compounds as “food” when more complex sources such as glucose are unavailable.
“The bacteria are able to actually eat what we perceive as smells,” said Casey Cosetta, post-doctoral scholar in the department of biology at Tufts University and first author of the study.
“That’s important because the cheese itself provides little in the way of easily metabolized sugars such as glucose. With VOCs, the fungi are really providing a useful assist to the bacteria to help them thrive.”
The researchers said a better understanding of the process could enable cheese producers to manipulate the VOC environment to improve the quality and variety of flavors.
“Now that we know that airborne chemicals can control the composition of microbiomes, we can start to think about how to control the composition of other microbiomes, for example in agriculture to improve soil quality and crop production and in medicine to help manage diseases affected by the hundreds of species of bacteria in the body,” Wolfe said.
Fungal volatiles mediate cheese rind microbiome assembly
Cosetta, C.M., Kfoury, N., Robbat, A., and Wolfe, B.E.