A quorum of flavour
If you think bacteria live a low life, knowing about their strategic germ warfare will change your mind – and help you understand what happens to certain foods at the microscopic level.
CERTAIN odd things interest me – and often they are things which most people would never think of, and even if they did, they would still find it boring or tedious. But it doesn’t bother me at all, and hopefully you will soon get to see why some things are a whole lot more interesting than you thought possible.
As a lover of cheeses, it is quite relaxing for me to research how cheeses are created – and in particular, the complex mix of bacterial and fungal cultures needed to create the idiosyncratic characteristics and flavours of various cheeses.
One of my favourite cheeses, St Nectaire Fermier, is produced by an astonishingly complex and active bacterial community made up of debaryomyces hansenii, geotrichum candidum, brevibacterium linens, torulopsis sphaerica, kluyveromyces lactis, candida sake, cladosporium, aspergillus, etc. Some of these same bacteria are also active in the production of blue cheeses as well, with the characteristic blue furrows developed by either penicillium roqueforti or penicillium glaucum.
What was immediately curious is how these various microbial cultures manage to co- exist so peacefully in the cheeses. It would be more plausible to assume that one of the more aggressive organisms would attempt to wipe out the other species and dominate the whole curdy landscape – and very likely introducing a single sour taste to all cheeses.
But it seems that these various bacterial and fungal colonies are perfectly happy to co- exist in cheeses – acting rather like a perfect multi- racial human society where each race contributes subtly yet significantly to the cultural flavour and well- being of the country.
Obviously, this utopian situation very rarely happens, even with seemingly intelligent human beings, so how come simple bacteria and fungi are so much better at co- existence?
The answer is rather fascinating and also explains why our bodies tolerate the many billions of bacteria in the microbial flora in our intestines.
Some evidence of bacterial co- operation was already observed in the mid- 1960s, notably by a Hungarian- born scientist called Alexander Tomasz. By 1994, scientists had determined that bacteria can indeed detect variations in their environment caused by other bacteria – what’s more, the bacterium can understand the implications of such changes and apply specific survival strategies based on the environmental alterations.
A key piece of research penned in 1994 by the American scientists, Fuqua, Winans and Greenberg, sketched out the concept of Quorum Sensing ( QS) by bacteria. The original research was based on marine luminescent bacteria but the principles of QS ( or inter- bacterial communication) has since been validated and found to apply to many other mixed colonies of bacterium, including those in cheeses.
At the simplest level, QS is managed by the production and interplay of groups of signalling molecules known as autoinducers – the individual molecules are known as inducers and they are detected by special receptors in the bacterial cells.
A single cell can emit one or more different kinds of inducers. The interaction between inducers and receptors can be quite simple or very complex, depending on various factors, not least being the proximity of other colonies of different bacterium in the same environment.
There are three general types of inducers – the ones produced by Gram- positive bacteria are based on peptides ( or chains of amino acids) while Gram- negative bacteria use derivatives of fatty acids.
If you’re curious, Gram- positive bacteria have thicker cell walls which can retain the purple dye used in the Gram stain test ( hence the stain effect is positive) – so by inference, Gram- negative bacteria are not stained by the Gram stain test and that’s because their thinner cell walls cannot hold the stain.
The final type of inducers is rather rarer and can be produced and utilised by both Gram- positive and Gram- negative bacteria – these inducers are unusual because they are based on boron, an element seldom associated with biomolecules.
Regardless of the types of inducers, the effect at the cell level is like a non- linear equation; that is, the effect can become suddenly chaotic.
However, it is important to note that autoinducers work not only on external bacterium but also within the same colony of bacteria. Within the same colony, the bacteria may choose to wait and expand until the colony reaches a certain size before breaking out of their enclave – this is how many bacterial infections begin.
The bacteria are always aware of its colony size by the amount of autoinducers around it and won’t break out until it is confident of overwhelming the host’s body defences.
The interaction between different species of bacteria goes along somewhat similar lines. Let’s review a situation where the numbers of autoinducers from an external bacterium are slowly growing in an environment colonised by another single host species of bacteria.
While the numbers of external autoinducers are small, nothing much really happens – the inducer molecules are detected, counted by the host receptors and things continue quietly between both colonies of bacteria. And it stays that way until the quantity of inducers detected reaches a threshold ( also known as the quorum).
Once the quorum is reached, the host bacteria react rather more frenetically and in several ways – one way is by activating a gene which produces more receptors.
Another way is to activate a separate gene which promotes the production of its own autoinducers – almost like a signal to alert its presence to other bacterium.
There are then several options available to the host bacteria – they can attempt to destroy the external invading bacteria, they can choose to tolerate each other as peacefully as possible, or they can form a partnership with the external bacterium.
Within our own bodies, there are examples of all three kinds of behaviour. Foreign virulent bacteria ( also known as pathogens), once detected in the blood or organs, are set upon by the white blood cells in a violent attempt to kill the invaders.
However, in the gut, the externally- introduced microflora work together magnificently with the body – in fact, much of the body’s immune system emanates from, or is enhanced significantly, by the bacterial colonies there.
And within the gut bacteria, there are also countless species which don’t do much for the body but are tolerated either because they are food for the good bacteria or they just don’t have much impact on anything and it is too difficult to rid them from amongst the rest of the gut microflora.
So although there are many kinds of autoinducers swirling around in the gut, it does seem that the various species of gut bacteria generally tend to get along fine with each other.
So from the above, you can probably surmise, quite correctly, that the multiple colonies of bacteria and fungi in cheeses also tolerate each other pretty well. The main difference is that they don’t have any noble aims to benefit a higher organism ( as with human gut microflora) – most of the bacteria in cheeses just co- exist together in blocks of curdled milk, producing their individual microbial end products, and the final results are the idiosyncratic flavours and textures of cheeses.
As an aside, if you have been drinking alcohol, the ethanol can significantly impact the microflora in the gut, mainly by killing them. Hence it is always a good idea to ingest some probiotic products the next day to replace the lost bacterium. The alternative, of course, is to avoid drinking alcohol but that would be as ridiculous as the idea of me converting to a religion.
hospital- associated Methicillin resistant Staphylococcus aureus ( MrSA) bacteria. — Photos:
Probiotics, the natural friendly bacteria are an integral part of the digestive system.