Effect of simulated microgravity on the virulence properties of the opportunistic bacterial pathogen Staphylococcus aureus

Extended manned space flight will result in a diminution of immune status and cause profound changes in the human bacterial microbiota, leading to increased risk of infection. Experiments conducted during short-term flight suggest that growth in microgravity leads to increases in bacterial antibiotic resistance and to cell wall changes. Growth under low-shear modelled microgravity (LSMMG) indicated that a reduced gravitational field acts as an environmental signal for expression of enhanced bacterial virulence in Gram-negative pathogens. We examined the effect of simulated microgravity on parameters of virulence in clinical isolates of Staphylococcus aureus. Three strains were grown under LSMMG in a High Aspect Ratio Vessel and compared with cells grown under normal gravity (NG) in the same vessel. There were no significant differences in the antibiotic susceptibility, growth rate or morphology of staphylococci grown under LSMMG compared to NG. LSMMG-induced reductions in synthesis of the pigment staphyloxanthin were noted. Strains secreted less protein under LSMMG and reductions in haemolysin secretion were found. Reduced expression of the major virulence determinant "-toxin in the microgravity environment was found by gene amplification. Thus, in contrast to published data on Gram-negative pathogens, simulated microgravity reduces the expression of key virulence determinants of S. aureus