Disappearence of rifampin resistance in a rat model of methicillin-resistant Staphylococcus

16 Orthopedic foreign body-associated infection is often treated with rifampin-based combination 17 antimicrobial therapy. We previously observed that rifampin-resistant methicillin-resistant 18 Staphylococcus aureus (MRSA) isolates were present 2d after cessation of rifampin therapy in 19 experimental foreign body osteomyelitis. Unexpectedly, only rifampin-susceptible isolates were 20 detected 14d after treatment completion. We studied two rifampin-resistant isolates recovered 2d 21 after treatment and one rifampin-susceptible isolate recovered 14d after treatment. Growing these 22 isolates alone in vitro or in vivo demonstrated no fitness defects, however in mixed culture; 23 rifampin-susceptible bacteria out-competed rifampin-resistant. In vivo, two courses of rifampin 24 treatment (25mg/kg every 12h for 21d) yielded a greater decrease in bacterial quantities in bones 25 of treated animals 14d following treatment, compared with animals receiving a single course of 26 treatment (p=0.0398). In infections established with equal numbers of rifampin-resistant and 27 rifampin-susceptible bacteria, one course of rifampin treatment did not affect bacterial quantities. 28 Rifampin-resistant and rifampin-susceptible isolates were recovered both 2d and 14d following 29 treatment completion, however the proportion of animals with rifampin-resistant isolates was 30 decreased at 14d compared to 2d following treatment completion (p=0.024). In untreated animals 31 infected with equal numbers of rifampin-resistant and rifampin-susceptible bacteria for four 32 weeks, rifampin-susceptible isolates were exclusively recovered, indicating out-competition of 33 rifampin-resistant by rifampin-susceptible isolates. The data presented imply that although there 34 is no apparent fitness defect in rifampin-resistant bacteria when grown alone, they are out35 competed by rifampin-susceptible bacteria when the two are present together. The findings also 36 suggest that selected rifampin resistance may not persist in initially rifampin-susceptible 37 infections following the discontinuation of rifampin. 38