Ultrastructure of Serratia liquefaciens Grown at 7 mbar Under Simulated Martian Conditions

Cells of Serratia liquefaciens were grown on trypticase soy agar (TSA) for 28 d under Martian conditions of 7 mbar, 0 °C, and CO₂-enriched anoxic atmospheres (called Mars low-PTA conditions). Earth controls were maintained for 24 h at 1013 mbar, 30 °C, and a standard pN₂/pO₂ gas composition. Cells were harvested at either 24 h or 28 d from TSA surfaces and processed for SEM and TEM imaging. Cells of S. liquefaciens grown under Earth conditions were uniform in shape and size, averaging approximately 1.25 µm in length and 0.5 µm in width. Fimbriae were observed on 10-20% of cells grown under Earth conditions. Key features of low-PTA grown cultures were (1) cells exhibited swollen blunt ends at sites of cell division tapering to unusually constricted points on the distal ends of progeny cells, (2) cell division appeared disrupted with division planes occurring at odd angles often forming right-angle oriented daughter cells, (3) some cells failed to form divisional planes resulting in long spiral and oddly shaped cells measuring up to 6-8 µm in length, and (4) fimbriae were lacking. Cell walls were found to be approx. 17% thinner when cells were grown in low-PTA environments compared to lab-standard conditions.