Microbial melodramas make for good soap

There might be no ‘I’ in team, but there is definitely one in ‘bacteria’. While microbes aren’t traditionally recognised for their altruism, James Collins and a team of researchers from the Howard Hughes Medical Institute have found individual bacteria can pay a price that ultimately benefits others in the colony.

The text-book microbe competes in a bug-eats-bug world, where a subtle physiological variation in a select few can make the difference between a population’s survival and its annihilation. As the mutants come to represent the majority, resistance to old threats increases. Whether it is antibiotics or plain old disinfectants, chemical warfare loses its punch when a few bacterial mutants come to represent the species.

However, these new findings (Nature, Vol467, pg82) demonstrate the bacterial struggle for mutant dominance might not be so selfish. In an effort to observe how genetically identical bacteria developed the initial variations, Collins and his team subjected a population of cloned Escherichia coli to a steady gradient of the antibiotic norfloxacin. Routine tests of sample bacteria were taken to record the minimum concentration of antibiotic that would halt their growth.

On comparing the samples with their home population it was revealed that the very act of removing bacteria decreased their resistance to norfloxacin. Stranger still, individuals gifted with the ability to deal with the chemical attack were in the extreme minority, making up less than a mere one percent of the colony. Their talent lay in their ability to produce tryptophanase, an enzyme that breaks the amino acid tryptophan down into the chemical indole. Fortunately for its less capable siblings, an increased concentration of indole in the environment helps switch on useful metabolic pathways that combat the antibiotic’s effect, allowing the rest of the population to benefit.

Of course, given there is no such thing as a free lunch, the production of this enzyme requires the devotion of precious energy. ‘Kin selection’ is one explanation for this behaviour. A process first suggested in the 1960s by the evolutionary biologist William Hamilton, it suggests individuals act altruistically to increase the chance of survival and reproduction for those with a close genetic relationship.

What, then, of their nefarious counterpart, the bacterial bum? Led by Steven Diggle at the University of Nottingham’s Centre for Biomolecular Sciences, microbiologists have found that individuals within a population of Staphylococcus aureus can opt to coast along for the ride when it comes to contributing to the costs of an infection.

After deliberately infecting waxworms with the bacteria, the researchers eavesdropped on the developing colony by observing a chemical coordination process called ‘quorum sensing’. They discovered those which lacked the means to engage in this microbial forum could also refrain from making toxins, saving them energy which could be devoted instead to reproduction. In effect, these bacteria were relying on their siblings to provide them with their nutrients. Seeding an infection with these freeloaders could present physicians with a novel form of treatment.

While this could be great news for the medical world, it does present a rather perplexing contrast to their more charitable cousins. Understanding how microbes interact with their host’s environment is vital if we are to find additional ways of controlling infection. Just last year, the World Health Organisation released a warning concerning the potential threat posed by the NDM-1 strain of E. coli – a potential superbug that is on the rise across the globe. It could very well join the likes of more familiar foes such as the vancomycin-resistant enterococcus and the rather formidable methicillin-resistant staphylococcus aureus.

Paying attention to the melodramas unfolding in these microbial ‘Days of our Lives’ is certainly better than any daytime soap. What’s more, it might be the key to turning the tide on the twilight of the antibiotic.

Published in: on November 2, 2010 at 9:46 am  Leave a Comment  
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