PHARMACOLOGY:

SUMMARY: Drugs that selectively target resistant bacteria may help to counter the antibiotic resistance phenomenon.

Antibiotics generally select for resistant bacteria by killing off only those which are susceptible to the drug, thereby ensuring a continual need for new drugs. This is good news only for the job prospects of medicinal chemists.

Recent research has approached the antibiotic resistance challenge from a fundamentally new direction. A small number of bacteria can impart resistance to a large population; perhaps targeting these resistant bacteria may help to kill the entire population.

Roy Kishony (Harvard University, United States) and coworkers recently reported an approach for selectively targeting drug resistant bacteria. They have now developed a general screening protocol, based upon a standard assay, for drugs that specifically act against resistant bacteria.

The concept.

The basis of the scientists' protocol is to attack bacteria with a mixture of two compounds. One of them is an antibiotic, and the other is a toxin that is suppressed by the antibiotic.

When a drug-resistant bacterium neutralizes the antibiotic, the deleterious properties of the toxin are unleashed. When the toxin is more harmful than the antibiotic, drug-resistant bacteria are killed to a greater extent than the remaining bacteria.

Can this trick be pulled off with a wide range of antibody/toxin combinations, and if so, can a method be devised to identify them? These questions were investigated by Kishony and coworkers.

Assay development and validation.

The scientists' protocol is based on the agar diffusion assay of antibiotic resistance. Here, bacteria are evenly spread across a sugar (agar) growth medium, an antibiotic is spotted onto the agar, and the antibiotic spreads a bit through the agar.

Over time, bacteria generally die off near the zone spotted by the antibiotic. The distance from the nearest living bacteria to the center of the antibiotic zone is a measure of antibiotic resistance of the bacteria under study; a larger dead zone implies a greater sensitivity.

Kishony and coworkers built upon this common assay. They first mix drug-resistant (red fluorescence) and drug-sensitive (green fluorescence) bacterial strains on an agar plate. They spot the antibiotic at one location, and an antibiotic/toxin mixture at another location.

Everywhere else on the plate, neither bacterial strain is favored. Consequently, the plate appears yellow, a mixture of red and green.

The location spotted with the antibiotic is red, since this location favors drug-resistant bacteria. The location spotted with the antibiotic/toxin combination is green, since this location favors drug-sensitive bacteria.

It's important to note again that not every antibiotic/toxin combination works in this manner. It only works for toxins that are inhibited by the antibiotic, e.g. tetracycline antibiotic and fusaric acid toxin.

Tetracycline used to be a very useful antibiotic, but many bacterial strains are now resistant to it. Identifying tetracycline/toxin combinations that target drug-resistant bacteria will breathe new life into tetracycline as a clinically-useful antibiotic.

The scientists screened over 1000 soil isolates (each one likely possessing many different microbial species), searching for toxins that are inhibited by tetracycline. They found at least six isolates that select against tetracycline resistance, and at least 14 isolates that select for tetracycline resistance.

Future directions.

Immediate future research should be the isolation and molecular identification of the toxins that select against resistance to the antibiotic tetracycline. This may be easier said than done, especially considering that the toxins' chemical composition is entirely unknown, but mass spectrometry may be the way to go if the soil microbes secrete them in a sufficiently high concentration.

NOTE: The scientists' research was funded by the Bill and Melinda Gates Foundation, the National Institutes of Health, and Harvard University.

Chait, R., Shrestha, S., Shah, A. K., Michel, J.-B., & Kishony, R. (2010). A Differential Drug Screen for Compounds That Select Against Antibiotic Resistance PLoS ONE, 5 (12) : 10.1371/journal.pone.0015179