Alternating Antibiotics Render Resistant Bacteria Beatable

By now we have all heard of the dangerous rise of antibiotic-resistant bacteria. While many search to identify new antibiotics and define their mechanisms, others wonder if there are better ways to use the antibiotics already available and approved for use in humans. This is doubly important given the rate at which bacteria adapt, as they will likely develop resistance to new antibiotics. Techniques to better utilize current antibiotics include combination therapies and changes to dosing regimens. One example is sequential treatment, whereby bacterial infections are treated by alternating the use of different antibiotics. Sequential therapies have been used to treat cancer and have been successful in some instances in treating Helicobacter pylori infections. Using sequential therapies increases the optimization space of the treatment, meaning that there are exponentially more ways to design an effective treatment, and that increases the possibility of drugs cooperating in unanticipated ways. In the study presented here, published in PLOS Biology, Ayari Fuentes-Hernandez, Jessica Plucain, Fabio Gori, Robert Beardmore, and colleagues demonstrate that two antibiotics known to act synergistically can be used in a specially designed sequential treatment to kill bacteria at dosages that, when the drugs are administered alone or in combination, cause rapid development of drug resistance and sustained bacterial growth. The two drugs used were erythromycin (ERY) and doxycycline (DOX), both of which target the bacterial ribosome and act as inhibitors of translation. To help mimic more challenging clinical scenarios, the bacteria used in the in vitro model of infection contained a gene that encodes a multidrug efflux pump, the genomic amplification of which results in increased drug resistance to both drugs. Despite the presence and amplification of the efflux pump, the authors identified sequential treatments that killed the bacteria when monotherapies and combination therapies failed to do so. The bacteria were cultured with medium and the antibiotic(s) for 12 h (one “season”), after which 1% of the culture was transferred to new medium with either fresh or different antibiotic (s). This process was continued for as many seasons as the experiment required; thus, for example, over eight seasons there would be 2 (256) possible treatments, two of which are the monotherapies. First, the authors tested whether a sequential therapy could outperform monotherapy or combination therapy (Fig. 1). The combination therapy resulted in the greatest single-season inhibition (a nearly 95% reduction in cell density), but, by the end of the experiment, the cell densities rebounded, consistent with drug resistance. However, they did identify 16 sequential treatment patterns that cleared the bacteria. A follow-up experiment revealed that five of these treatments truly eliminated the entire bacterial population, whereas the other 11 had varying results. By contrast, neither the combination therapy nor either of the monotherapies achieved bacterial clearance.