Bacteria of the genera Bacillus and Staphylococcus are frequent inhabitants of the International Space Station (ISS) and represent possible opportunistic pathogens. The effect of simulated microgravity on growth and the frequency of mutation to antibiotic resistance in the model surrogate organisms Bacillus subtilis (B. subtilis) and Staphylococcus epidermidis (S. epidermidis) were investigated. The test organisms were cultivated for six days in Rotating Wall Vessel (RWV) clinostats either in the vertical (simulated microgravity) or horizontal (1 g control) orientation. Parameters measured were: optical densities (ODs); viable counts; frequencies of resistance to rifampicin (RFM); and frequencies of double resistance to RFM and trimethoprim (TMP). The results indicated that the response to simulated microgravity differed in the two microorganisms. Both B. subtilis and S. epidermidis grew to higher ODs and cell numbers in simulated microgravity. However, the frequencies of mutation, both to RFM resistance and double resistance to RFM and TMP, were observed to increase significantly in simulated microgravity-grown B. subtilis but not in S. epidermidis.
