Post - survival of the fittest?
Bacteria have always been very adaptable when it comes to surviving evolutionary stressful situations, such as exposure to antibiotics. Usually some form of mutation will arise leading to the creation of resistant strains of bacteria. These will be selected for via ‘natural selection’ processes and go on to replicate to produce a whole population of resistant bacteria that are able to survive.
However new research looked at a colony of wild type E.coli bacteria in a bioreactor under exposure to increasing levels of the antibiotic norfloxacin and found that no more than 60% of growth was inhibited to maintain a sizable population. The resistance levels of the population as a whole, and of 12 random individuals, was checked every day and it was found that they did not correspond to one another.
The majority of the individual isolates were less resistant than the population as a whole but there was one mutant found that was highly resistant. By isolating the supernatant from the high resistance individual, and conducting gel electrophoresis to separate out the intracellular components, a protein was found that was produced in very high numbers.
This was the enzyme tryptophanase which has the main job of breaking down tryptophan to ammonia, pyruvate and indole. Experiments were done to show that the third molecule, Indole, provided an obvious survival benefit under antibiotic conditions. It upregulated multi-drug efflux pumps which helped in the physical export of the drug and it also had a role in activating various oxidative stress protective mechanisms. The mass production of Indole by the highly resistant mutant allows more vunerable cells in the surrounding area to survive.
a) no antibiotic stress, bacteria naturally produce indole.
b) dead and dying bacteria fail to produce indole
c) mutant appears and supplies indole at a fitness cost to itself.
The resistant bacteria is therefore not selfishly replicating to outgrow the rest of the population but, in helping others to survive, is enduring a fitness cost of its own by mass producing Indole.
This experiment was also carried out using various different antibiotics and the same bacterial altruism was found to exist. The survival of the weaker bacteria does have some advantages as it allows further exploration of mutations that could be even more beneficial to the population. Also, it keeps the opportunity for the bacteria to return to their original state if the stress is temporary, rather than keeping up the energetically wasteful production of antibiotic resistance genes.
So, bacteria working as a team to ensure not just temporary survival but long term advantages for the whole population. Not just survival of the fittest.
Lee HH, Molla MN, Cantor CR, & Collins JJ (2010). Bacterial charity work leads to population-wide resistance. Nature, 467 (7311), 82-5 PMID: 20811456