Low gravity and inertial effects on the growth of E. coli and B. subtilis in semi-solid media.
BACKGROUND: Several published experimental results have shown that cultures of suspended bacteria exhibit increased growth in the spaceflight environment. HYPOTHESIS AND METHODS: To test whether these differences were due to fluid mechanics and not cellular effects, E. coli and B. subtilis were grown on agar cultures under static, agitated, and rotated conditions in the laboratory, and under low-gravity conditions on four Space Shuttle flights. Growth experiments were terminated with glutaraldehyde, and individual cells were counted after quantitative elution from the agar. RESULTS: The spaceflight results, in conjunction with static, rotation, and agitation experiments indicate that E. coli and B. subtilis cultures on agar, unlike their suspension grown counterparts, do not experience heightened final cell concentration when the inertial environment is changed. CONCLUSIONS: This finding points to fluid dynamics and extracellular transport phenomena and not cellular dynamics as the most likely cause of previously reported increases in bacterial growth in microgravity.