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.