No man is an island, said John Donne. Each of us is a community of billions, nearly all of them bacteria. The human skin, gut, mouth, vagina, etc., all harbour complex ecologies of many different micro-organisms, all living in seeming harmony. But how is this harmony maintained between selfish species? Moulds such as Penicillium take selfishness to extremes, and attack their bacterial rivals with deadly antibiotics. But more subtle discouragements must be more widespread. Thus, while it is quite easy to contract either syphilis or gonorrhoea, it is much harder to get both. Each disease seems to protect against the other. Similarly, few people carry more than one tapeworm; the first arrival somehow ‘warns off’ later ones. And invading bacteria are also usually warned off by our permanent flora. Clearly, says Daedalus, the chemical signalling behind our bacterial harmony is a set of gentlemen's agreements rather than gang warfare.
So DREADCO bacteriologists are now studying these agreements. They are culturing simple mixtures of gut and skin organisms, observing their equilibria and isolating the metabolites they produce. A signalling metabolite would be expected to shift the equilibrium of a culture to which it was added in excess, or from which it was rigorously excluded. Gradually the project should build up a library of such signalling metabolites, and discover their scope and mode of action.
The final goal is a new and milder form of antibacterial therapy. Many diseases, from vague inflammations and stomach upsets to sinusitis and genital infections, are upsets of bacterial equilibria. The medics move in with antibiotics, kill off goodies and baddies alike, and leave the field open for a new colonization. With luck it re-establishes the original equilibrium. But how much neater to drop in a nicely chosen mixture of chemical signals, warning the rioters to calm down, telling the persecuted to stand up for their rights, intimidating the foreigners, and thus restoring the equilibrium directly!
Yet Daedalus may have been anticipated. He now wonders if our immune system keeps our benevolent microbial ecologies in balance by multiple chemical signals. If so, all multicellular species may use such signals in their negotiations with bacteria. Hence, possibly, the bafflingly complex multi-component nostrums of the old herbalists and animal-product therapists.