Insights into the Structure and Protein Composition of Moorella thermoacetica Spores Formed at Different Temperatures.

The bacterium Moorella thermoacetica produces the most heat-resistant spores of any spoilage-causing microorganism known in the food industry. Previous work by our group revealed that the resistance of these spores to wet heat and biocides was lower when spores were produced at a lower temperature than the optimal temperature. Here, we used electron microcopy to characterize the ultrastructure of the coat of the spores formed at different sporulation temperatures; we found that spores produced at 55 °C mainly exhibited a lamellar inner coat tightly associated with a diffuse outer coat, while spores produced at 45 °C showed an inner and an outer coat separated by a less electron-dense zone. Moreover, misarranged coat structures were more frequently observed when spores were produced at the lower temperature. We then analyzed the proteome of the spores obtained at either 45 °C or 55 °C with respect to proteins putatively involved in the spore coat, exosporium, or in spore resistance. Some putative spore coat proteins, such as CotSA, were only identified in spores produced at 55 °C; other putative exosporium and coat proteins were significantly less abundant in spores produced at 45 °C. Altogether, our results suggest that sporulation temperature affects the structure and protein composition of M. thermoacetica spores.

Temperature impacts the sporulation capacities and spore resistance of Moorella thermoacetica.

Temperatures encountered in cannery allow growth of thermophilic spore-forming bacteria, including the strictly anaerobe Moorella thermoacetica, which grows optimally from 55 °C to 65 °C and is the main cause of spoilage of low-acid canned foods (LACFs) at high temperature. Resistance to wet-heat, biocides and UV-C of spores formed at different temperatures was assessed either for a selection of M. thermoacetica strains or for the strain M. thermoacetica ATCC 39073. Spores formed at 45 °C were significantly more sensitive to wet-heat than spores produced at 55 °C, while spores produced at 65 °C were as heat-resistant as spores produced at 55 °C. Spores of M. thermoacetica ATCC 39073 produced at 45 °C were significantly less resistant to peracetic acid than spores formed at 55 °C, while no difference in sensitivity to H2O2 or to UV-C treatment was observed whatever the sporulation temperature. However, both types of treatment enabled at least a 3.3 log CFU/mL reduction of M. thermoacetica ATCC 39073 spores. M. thermoacetica spores thus showed higher resistance properties when sporulation temperature was close to optimal growth temperature. These findings suggest food spoilage due to M. thermoacetica species could be controllable by holding temperatures below optimal growth temperature from the blanching step to the can filling step.