Protective role of spore structural components in determining Bacillus subtilis spore resistance to simulated mars surface conditions

Abstract

Spores of wild-type and mutant Bacillus subtilis strains lacking various structural components were exposed to simulated Martian atmospheric and UV irradiation conditions. Spore survival and mutagenesis were strongly dependent on the functionality of all of the structural components, with small acid-soluble spore proteins, coat layers, and dipicolinic acid as key protectants.

Fig 1

Survival of B. subtilis spores in response to simulated Martian environmental conditions, as determined by the ability to form macroscopic visible colonies. The strains used are indicated below the bars; the wild-type strain is listed to the left of the corresponding mutant strain. Spores were exposed as air-dried spore multilayers for 24 h to ambient laboratory conditions (white bars; panel A), Mars(−)UV conditions (hatched bars; panel B), or Mars(+)UV conditions (shaded bars; panel C). Data are expressed as averages and standard deviations (n = 3). The lowercase letters above the bars denote groups significantly different by ANOVA (P < 0.05).

Fig 2

Impacts of the spore-specific structural attributes (exterior and interior) are displayed as relative sensitivities of spores lacking pigmentation, spore coat assembly, dipicolinic acid (DPA) formation, core dehydration, and α/β-type small, acid-soluble spore protein (SASP) formation to Mars(−)UV conditions (hatched bars; panel A) or Mars(+)UV conditions (shaded bars; panel B). Relative spore sensitivity was expressed as the ratio of the survival of each mutant strain with respect to the survival of the corresponding wild-type strain from the respective exposure to simulated Martian conditions (from Fig. 1B and C) to that of the ambient control (Fig. 1A). Data are averages and standard deviations (n = 3). The actual data values are shown above the corresponding columns (Fig. 1).

Fig 3

Frequencies of mutation to 4-azaleu^r of spores of different strains exposed to the full suite of simulated Martian conditions shielded (white hatched bars) and including UV radiation (gray bars). Data are averages and standard deviations (n = 3). The lowercase letters above the bars denote groups significantly different by ANOVA (P < 0.05). The spontaneous frequency of mutation to 4-azaleu^r of the wild-type strain (i.e., the mean spontaneous 4-azaleu^r mutation frequencies of strains 168, PY79, and PS832 ± the standard deviation) was (9.7 ± 3.6) × 10⁻⁷, in good agreement with previous data (35).