DRUG DISCOVERY:

Bacteria not killed by a drug are more likely to survive and propagate their genetic resistance to subsequent generations. In a nutshell, this is how bacteria acquire antibiotic resistance, and is why new drugs are always needed.

Consequently, much research has focused on developing new drugs, e.g. through correlating small similarities in protein structure with biochemical pathway similarities. Less research has focused on how bacteria may cooperate to effect antibiotic resistance.

James Collins (Boston University, United States) and coworkers have shown that small numbers of Escherichia coli bacteria, in a large population, can benefit the collective (at cost) when faced with antibiotic stress. Given that these "charitable" bacteria are small in number, the bacterial population can readily return to its pre-antibiotic state when resistance is no longer needed.

Helping the collective.

The scientists stressed a population of genetically-identical bacteria with the antibiotic norfloxacin. This is an uncommonly used drug, as it is generally reserved for when other antibiotics have failed.

To stress the bacteria without killing most of them off, they used no more antibiotic than necessary to keep at least 40% of the bacteria alive. Every day, the scientists collected twelve bacterial samples and checked their antibiotic resistance via standard assays.

Interestingly, their first discovery was that individual bacterial collections were often less resistant than average, and that a small number were far more resistant than the rest. Note that less-observant scientists may have attributed this result to experimental error; instead, they saw its possible implications, and ran with it.

The scientists then subjected their most resistant bacterial population (colony 12, isolated on day 10) to further testing. They found that these bacteria produce large quantities of tryptophanase, an enzyme that converts the amino acid tryptophan into indole, an essential molecule that is well-known to contribute to antibiotic resistance.

This bacterial strain is the only one that, under antibiotic stress, was able to maintain indole production up to a concentration of 300 micromolar. When the cell medium was supplemented with 300 micromolar indole, even the least-resistant day 10 strain increased its resistance to norfloxacin by 75%, within 5% of the drug resistance of the most resistant bacterial strain.

It's possible that other proteins in the cell medium, besides tryptophanase, contribute to bacterial survival. However, tryptophanase is clearly one of them (and likely the primary contributor).

However, in the absence of antibiotic stress, formerly resistant bacterial populations genetically modified to no longer produce indole outgrew the original resistant strain by a factor of 2.6. Drug resistance comes with a cost (reduced growth), but facilitates survival when under pressure.

Molecular mechanism of antibacterial resistance.

What is the role of indole in antibacterial resistance? The scientists' findings were that indole induced the bacteria to export the antibiotic norfloxacin out of the cell and to activate intracellular repair mechanisms.

Thy further found several DNA mutations in the most resistant bacteria. The two most common mutations were in a region of the DNA known to be affected by norfloxacin, and another was in a region of the DNA which codes for an electron transfer (detoxification) enzyme.

These DNA mutations are not directly related to the production of indole. It is likely that they instead enable the bacteria to keep on producing the essential molecule indole even in the presence of the antibiotic norfloxacin.

Overall evaluation.

Escherichia coli bacteria are typically killed by the antibiotic norfloxacin, because they are no longer able to produce the essential molecule indole. However, small numbers of the bacteria are resistant to the antibiotic, and are still able to produce indole, keeping the rest of the bacteria alive.

The scientists observed broadly similar results for the antibiotic gentamicin. It is therefore likely that their bacterial "charity" results are general (i.e. not limited to the antibiotic norfloxacin).

Unlike in Star Trek ("resistance is futile" against the Borg collective), there is hope. Next-generation antibiotics should perhaps focus on drug cocktails which selectively target the most resistant bacteria in a population. By killing the bacteria which help the remainder to survive, it may be easier to wipe out the entire infection.

NOTE: The scientists' research was funded by the National Institutes of Health, the National Science Foundation, and the Howard Hughes Medical Institute.

for more information:
Lee H. H., Molla M. N., Cantor C. R., & Collins J. J. (2010). Bacterial charity work leads to population-wide resistance. Nature, 467 (7311), 82-5 PMID: 20811456