The combined use of probiotic E. coli with strain-specific bacteriophage for decolonization of a multi-drug resistant target (E. coli ST131)
Widespread antibiotic resistance in commensal bacteria creates a persistent challenge for human health. Resident drug-resistant microbes can prevent clinical interventions, colonize wounds post surgically, pass resistance traits to pathogens or move to more harmful niches following routine interventions such as catheterization. Accelerating the removal of resistant bacteria or actively decolonizing particular lineages could therefore have a number of long-term benefits.  However, removing resident bacteria via competition with probiotics, for example, poses a number of ecological challenges. Resident microbes are likely to have physiological and numerical advantages and competition based on bacteriocins or other secreted antagonists gives advantages to the dominant partner, via positive frequency dependence. Since a narrow range of E. coli genotypes (primarily those belonging to the clonal group ST131) cause a significant proportion of multi-drug resistant infections, this group presents a promising target for decolonization with bacteriophage, as narrow host range viral predation could lead to selective removal of particular genotypes. In this study we tested how a combination of a ST131-specific phage and competition from the well-known probiotic E. coli Nissle strain could displace E. coli ST131 under aerobic and anaerobic growth conditions in vitro. We showed that the addition of phage could break the frequency dependent advantage of a numerically dominant ST131 isolate.  Moreover, the addition of competing E. coli Nissle could improve the ability of phage to suppress ST131 by two orders of magnitude. Low-cost phage resistance evolved readily in these experiments and was not inhibited by the presence of a probiotic competitor. Nevertheless, combinations of phage and probiotic produced stable long-term suppression of ST131 over multiple transfers and under both aerobic and anaerobic growth conditions. Combinations of phage and probiotic therefore have real potential for accelerating the removal of drug resistant commensal targets.