Reposted from the Chasing the Red Queen blog with permission. The concept of cross-resistance is well known in the medical world and in research on bacteria. The idea is that when a bacterial strain becomes resistant to one antibiotic, it can become resistant to another similar antibiotic even though the bacteria has never been exposed to the second antibiotic. Bacteria are the best organisms for investigating this sort of resistance and there is tremendous interest in understanding how bacterial evolution is capable of such a feat. A report in the journal Nature Communications in July 2014 (doi:10.1038/ncomms5352) gave details on a study that tested E. coli exposed to 12 different antibiotics and then looked at genetic changes in response. Based on the data, the researchers created a cross-resistance network of interactions among the drugs in relation to the bacterial genetics. The network suggested some interesting conclusions. First, resistance to one antibiotic can lead to multidrug resistance and not necessarily among drugs that interact. Second, the evolution of resistance does not necessarily rely on new mutations, but can arise from a variety of genetic modifications. Taken together, the results suggest that cross-resistance is not a particularly unusual outcome and may even be predictable.