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Antibiotic resistance typically confers a biological fitness cost on bacteria that can be manifested as a decreased growth rate in culture media and experimental animals. However, there are limited experimental data on the relative fitness of resistant and susceptible bacteria during growth in their natural environment.We have developed a human competition model to investigate the relative fitness of antibiotic-resistant and -susceptible bacteria.A non-epidemic Staphylococcus epidermidis strain was isolated from skin, and a rifampicin-resistant (RifR) clone was selected. The RifR marker was used to distinguish the inoculated strains from the resident population of coagulase-negative staphylococci. The RifR strains were further selected for resistance to ciprofloxacin (CipR) and fusidic acid (FusR). A 1:1 mix of susceptible and resistant bacteria was applied on the forearms of 12 volunteers. Competition was monitored by sampling bacteria from skin and determining their relative numbers.Resistance to ciprofloxacin due to parC mutations did not decrease the growth rate in vitro, and the CipR/CipS ratio was close to 1 during day 1 and 3 in the in vivo competition experiments. In contrast, fusidic acid resistance due to fusA mutations resulted in a decrease in the growth rate in vitro and a considerable loss of fitness in the competition. The FusR/FusS ratio diminished from 1.3 to 0.023 in 3 days.These data show that human volunteers can be used as a simple and relevant model to study the biological cost of resistance.