Bacteria come in two flavors: Gram-positive and Gram-negative. Gram is a violet dye, named for its discoverer, that is more readily taken up by some microbes (the Gram-positive ones) than by others (you guessed it, the Gram-negative ones). Gram-negative bacteria are surrounded by two cell membranes, and the outer one is really tough to get across. In addition to keeping out the Gram dye, this outer membrane also keeps out many commonly used antibiotics.

We are desperately in need not only of new antibiotics, but of new types of antibiotics. The last new class of antibiotics effective against Gram-negative bacteria—including bacteria that cause Whooping cough, Legionnaire’s disease, typhoid fever, and bubonic plague—was last introduced in 1968. And this is not for lack of trying. In 2007, GlaxoSmithKline reported screening 500,000 compounds for activity against E. coli and came up with a grand total of none.

Now, a team of researchers may have figured out a way of getting antibiotics that normally don't work on Gram-negative bacteria inside those cells. Once inside, the antibiotics seem to be as effective as drugs specific to Gram-negative bacteria.

Part of the problem has been that we don’t really know what makes an antibiotic effective against Gram-negative bacteria. Retrospective analysis of antibiotics that do work suggests that the antibiotics should be pretty small and fairly polar, meaning they should have areas on the molecule that carry a slight charge. This makes sense since they must enter the cell through small, polar channels called "porins."

But plenty of small, polar antibiotics don’t get in, so those can’t be the only requirements. In order to generate a set of rules that could be used to make antibiotics that will target Gram-negative bacteria, the team screened 180 different chemicals for their ability to penetrate E. coli.

Charge makes the difference

Researchers on the team found that a positive charge is the primary determinant of whether a molecule gets in to E. coli. Twelve out of the 41 positively charged molecules they tried got in; zero neutral or negatively charged molecules did. All 12 had an amine group (which has a positively charged nitrogen), which is important but insufficient for accumulation. The flexibility and shape of the molecule also mattered; more rigid, flatter (as opposed to rounder) molecules got into E. coli more readily. Despite the importance of charge, some area of charge-free surface seemed necessary as well.

The importance of the amine had been previously noted, and other scientists have tried to turn Gram-positive-active antibiotics into Gram-negative-active antibiotics by adding an amine. It often didn’t work, possibly because the starting molecule did not have the proper flexibility and shape. In this newer work, the researchers targeted a drug that had the right shape and flexibility, and they succeeded in turning a Gram-positive-only agent into a broad-spectrum antibiotic by adding an amine. Their new molecule retained activity against Staphylococcus aureus, a Gram-positive bug, and it gained activity against four out of five Gram-negative pathogens, including a multi-drug resistant strain.

The authors hope that this set of rules—molecules should be positively charged, contain an amine group, and be rigid and non-globular—will spur the conversion of other Gram-positive-only antibacterials into broad-spectrum drugs. They also suggest that knowing these rules might help in the synthesis of new compounds specific for Gram-negative bacteria. This latter route might become necessary, since amines are fairly rare in natural compounds and are “vanishingly rare” in the standard collections of chemicals that are screened for drug activity.

Nature, 2017. DOI: 10.1038/nature22308 (About DOIs).