Researchers at Tufts University have found that those distinctly funky
smells from cheese are one way that fungi communicate with bacteria, and
what they are saying has a lot to do with the delicious variety of flavors
that cheese has to offer. The research team found that 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 flavor and quality
of the cheese, so figuring out how that can be controlled or modified adds
science to the art of cheese making.
The discovery, published in Environmental Microbiology, also 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, or VOCs, as they interact with their environment. A widely
recognized 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 that 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. All of those biological products contribute
to the flavor and aroma of cheese and they are the reason why Camembert,
Blue cheese and Limburger have their signature smells.
The Tufts researchers found that VOCs don't just 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 5 common cheese rind fungi, the researchers found that
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 common cheese bacteria decreased significantly
in number when exposed to VOCs produced by Galactomyces.
The researchers found that the VOCs altered the expression of many genes in
the bacteria, including genes that affect the way they metabolize nutrients.
One metabolic mechanism that was enhanced, called the glyoxylate shunt,
allows the bacteria to utilize more simple compounds as "food" when more
complex sources such as glucose are unavailable. In effect, they enabled the
bacteria to better "eat" some of the VOCs and use them as sources for energy
and growth.
"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."
There are direct implications of this research for cheese producers around
the world. When you walk into a cheese cave there are many VOCs released
into the air as the cheeses age. These VOCs may impact how neighboring
cheeses develop by promoting or inhibiting the growth of specific microbes,
or by changing how the bacteria produce other biological products that add
to the flavor. A better understanding of this process could enable cheese
producers to manipulate the VOC environment to improve the quality and
variety of flavors.
The implications of the research can even extend much further. "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," said Wolfe.
More information:
Casey M. Cosetta et al, Fungal volatiles mediate cheese rind microbiome
assembly, Environmental Microbiology (2020). DOI:
10.1111/1462-2920.15223