Who’s for port and ecosystem? The secret life of cheese

Having survived the festive season dinner party circuit, here’s my top tip for killing the conversation stone dead: point out to your host/ess that part of your dinner is not only still alive, but is in fact a teeming ecosystem. Yes, sitting on your table is a microcosm whose denizens are busily engaged in the economy of Nature, right under your very nose. Welcome to the world of cheese ecology.

While the idea that cheeses contain bacteria and other microbes is hardly news—after all, they do the lion’s share of the work in cheesemaking—the intricate web of interactions between them is now becoming apparent. Rather than just looking at microbial species individually, researchers are now studying how they all work together as a system and are applying the same principles of ecology that they once trained on rock pools and rainforests to the contents of your cheeseboard.

As I discuss in my New Scientist article, this kind of research promises to help cheesemakers improve the food safety of their products and create new or more refined kinds of cheeses. Unfortunately, I didn’t have space to talk about the world of basic ecology–the kind you may have done on school trips with nets, quadrats and jam jars–that’s quietly happening in a round of cheese that you can hold in your hands.

The other neat thing about this story is that it works both ways. Ecology can tell scientists a lot about cheese, but cheese can also tell scientists a lot about ecology. Relatively simple, discrete and easy to work with, these little islands of life are now being used as models in ecology labs.

While “ecosystem” might sound like a rather grandiose term for a common or garden cheese, it is in fact perfectly apt. The reeking slice of Camembert oozing over your oatcake has more in common with those rock pools and rainforests than you might expect.

Imagine, for example, a bare outcrop of rock. It’s barren at first, but then lichens gradually colonise it, slowly crumbling its surface and attracting wind-blown dust. When they die, their rotting fronds merge with the dust and rock crumbs to make the beginnings of soil, creating a new habitat that other species, such as plants, can move in to. These newcomers eventually oust the pioneers who built the habitat and build a stable community of species, be it a lowly patch of weeds or a great forest.

This is a classic piece of ecology known as succession, and it often takes years. But you can watch it happening over a matter of days on your cheeseboard in the form of a squidgifying triangle of Brie, or indeed any ripening cheese.

A cheese begins life as an unripe block of moulded, salted curds. These curds are very acidic, thanks to the lactic acid produced by the bacteria used to ferment the milk. Most bacteria can’t tolerate this environment—only those with the right adaptations survive.

But in surface-ripened cheeses such as Brie or Camembert, the yeasts and moulds the cheesemakers spray on to the curd blocks break down this lactic acid and release ammonia, making the cheese progressively more neutral. This allows a succession of other microbes to move in and start breaking down the fats, proteins and other molecules in the tart, chalky curd, transforming it into a tasty runny goo.

Ripening also means that different areas of a cheese can harbour a range of habitats that host different microbial communities, an ecological feature known as habitat diversity. As I explain in the article, researchers at the University of Nottingham have looked at this in Stilton cheese and shown how this affects its smell and flavour.

The cornerstone of ecology, of course, is the relationships between the species in a community. Although microbiologists have long studied these interactions, the arrival of new technologies such as next-generation DNA sequencing, genomics and metabolomics has transformed the scope and depth of their data. The wealth of information they are uncovering shows that even the most innocuous-looking cheese is red in tooth and claw.

Some microbial species, for example, supply nutrients to others without benefiting or suffering from it themselves, arrangement known as commensalism. Others visit famine and death on rivals by hogging nutrients and resources, a phenomenon known as competitive exclusion. There is war, waged with chemical weapons called bacteriocins, and, during cheesemaking, there is even pestilence, in the form of viruses called phages that prey on bacteria. The question is: how do these relationships work?

Cheeses typically contain tens of microbial species, making the interactions between them complex and hard to unpick. So biologists have begun to look at this by studying simpler systems containing only two interacting species, such as the two species of bacteria used to make yoghurt, Streptococcus thermophilus and Lactobacilus delbrueckii subsp bulgaricus.

S. thermophilus and L. delbrueckii‘s relationship is a form of mutualism called called protocooperation. Like two lovers feeding each other pudding, each produces molecules that promote their partner’s wellbeing and improve the stability and composition of the final yoghurt. A recent study showed that S. thermophilus altered the behaviour of no less than 77 different genes or proteins involved in its metabolism in the presence of L. delbrueckii, and that the two bacteria might be swapping even more nutrients than previously thought .

Other teams are simplifying matters by stripping down the ecosystem in a cheese (see the NS article) to the minimal set of species needed to create it. Almost all the yeast and bacteria in this minimal system have had their genomes sequenced, which should help answer a number of mysteries about cheese.

One of these mysteries is the strange case of the missing cultures. Commercial cheesemakers usually inoculate milk and curds with cultures–mixtures of microbial species–to trigger fermentation or ripening. Many of these cultures are made from lab-grown strains of bacteria, carefully selected to try to give resulting cheese batches a uniform quality. Trouble is, these species often disappear from the cheese, which ends up being ripened by wild microbes hanging around in the creamery.

One possibility is that these lab-grown bacteria simply don’t have the right adaptations to survive in the acidic, salty, fatty and iron-poor environment of cheese.
Before the invention of lab-produced cultures, this wasn’t a problem. Cheesemakers would smear microbes from old, ripe cheeses on to young, unripened ones–unwittingly encouraging the evolutionary adaptation of those microbes to the cheese environment.

A lovely illustration of this kind of adaptation is the recently published genome sequence of Arthrobacter arilaitensis, one of the major species found on the surface of cheeses. Compared with other Arthrobacter species, A. arilaitensis has lost genes needed for consuming carbohydrates (cheese contains only a limited amount) and gained genes that help it snatch iron (vital for most microbes) from its environment at the expense of its rivals. It also has robust salt-tolerance mechanisms and the ability to break down lactic acid, fats and proteins.

As well as helping to produce more robust starter cultures, understanding microbial adaptations to cheese could help biologists work out why food poisoning bacteria, such as Listeria monocytogenes, manage to colonise some cheeses, but not others.
It’s also where another ecological idea could come in to play: biodiversity.

A team of French researchers, for example, has studied the ability of the microbes isolated from the surface of Saint-Nectaire, a raw milk cheese from Auvergne, to inhibit the growth of Listeria. Intriguingly, complex mixtures containing the greatest range of species were much better at inhibiting Listeria than simpler, less diverse ones, suggesting that complex diverse ecosystems could be key to keeping out the bad guys.

Succession, community structure, mutualism, competition, adaptation and biodiversity; it all adds up a unique ecosystem for each cheese, helping to create its characteristic flavour, smell and texture. So if you’re stuck next to a dinner party bore and who tries to test your knowledge of what makes Gorgonzola different from Roquefort or distinguishes Limburger from Livarot, you know what to say.

It’s the ecology, stupid.

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