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Psychrotrophic strains of the foodborne pathogen Bacillus cereus can multiply during the refrigerated storage of food products. The aim of this study was to determine the impact of anaerobiosis on the growth of two psychrotrophic B. cereus strains exposed to acidic pH at a cold temperature in a laboratory medium. At 10 °C, growth occurred at pH values equal to or higher than 5.7 during anaerobiosis, whereas aerobic growth was observed from pH 5.4. Growth rates during aerobiosis were similar at pH 5.4 and pH 7. No growth was observed for the two tested strains at 8 °C without oxygen regardless of the pH; however, both strains grew at this temperature from pH 5.4 in the presence of oxygen. These pH growth limits in aerobiosis are consistent with those reported for different strains and different foods or media, but no other studies have described anaerobic growth at acidic pH values. The maximal B. cereus concentration was approximately 6.0 log 10 CFU/ml for cultures in the absence of oxygen and approximately 8.0 log 10 CFU/ml for cultures in the presence of oxygen. In conclusion, we found that the combination of anaerobiosis, pH < 5.7 at 10 °C, or anaerobiosis and temperatures ≤8 °C prevent psychrotrophic B. cereus growth. [ABSTRACT FROM AUTHOR]

This study was performed to evaluate the effect of anaerobiosis on the formation of Bacillus cereus spores and their resulting properties. For this purpose, an appropriate sporulation medium was developed (MODs). Sporulation of 18 strains from different phylogenetic groups of B. cereus was studied in MODs medium in aerobiosis and anaerobiosis. In anaerobiosis, sporulation ability was weaker and more heterogeneous than in aerobiosis. Among tested strains, B. cereus AH187 produced the highest level of spores in anaerobiosis. This strain was therefore chosen to study spore properties. Spores produced in anaerobiosis were more resistant to wet heat at 90 °C, 92.5 °C, 95 °C, 97.5 °C and 100 °C. For example, D 90 were 21,09 ± 1.70 and 81.87 ± 2.00 for aerobiosis and anaerobiosis conditions, respectively. Spores produced in anaerobiosis have a z-value of 7.70 °C compared with 10.52 °C for spores produced in aerobiosis. Spores produced in anaerobiosis were also more resistant to 1 M NaOH, 1 M nitrous acid and pulsed light at fluences of 0.34 J cm−2 and 0.49 J cm−2. No difference in resistance to UV-C, 5% hydrogen peroxide or 0.25 mM formaldehyde was observed between these two conditions. In the presence of l-alanine, spores produced in anaerobiosis germinated more efficiently than spore produced in aerobiosis. No difference in germination was observed with inosine as inducer. No difference in the size of spores produced in the different conditions was observed by transmission electron microscopy. However, spores obtained under anaerobic conditions had a damaged exosporium, or in some cases a completely detached exosporium, unlike spores produced under aerobic conditions. This study shows that few spores are formed under anaerobic condition; nevertheless, this condition has an impact on the spore properties of B. cereus AH 187 strain. Spores obtained under anaerobic condition were more resistant to heat and to some chemical compounds. This is an important feature, considering the risk associated with the presence of this pathogen in thermally processed and packaged food in absence of oxygen. [ABSTRACT FROM AUTHOR]

Abstract: In a chemically defined medium and in Luria broth, cold strongly reduced maximal population density of Bacillus cereus ATCC 14579 in anaerobiosis and caused formation of filaments. In cooked spinach, maximal population density of B. cereus in anaerobiosis was the same at cold and optimal temperatures, with normal cell divisions. The lipid containing fraction of spinach, but not the hydrophilic fraction, restored growth of B. cereus under cold and anaerobiosis when added to the chemically defined medium. This fraction was rich in unsaturated, low melting point fatty acids. Addition of phosphatidylcholine containing unsaturated, low melting point, fatty acids similarly improved B. cereus anaerobic growth at cold temperature. Addition of hydrogenated phosphatidylcholine containing saturated, high melting point, fatty acids did not modify growth. Fatty acids from phospholipids, from spinach and from hydrogenated phosphatidylcholine, although normally very rare in B. cereus, were inserted in the bacterium membrane. Addition of phospholipids rich in unsaturated fatty acids to cold and anaerobic cultures, increased fluidity of B. cereus membrane lipids, to the same level as those from B. cereus normally cold adapted, i.e. grown aerobically at 15 °C. B. cereus is therefore able to use external fatty acids from foods or from the growth medium to adapt its membrane to cold temperature under anaerobiosis, and to recover the maximal population density achieved at optimal temperature. [Copyright &y& Elsevier]

Abstract: The impact of fermentative metabolism at low temperature on cell division of Bacillus cereus was studied. Fermentation at 37 °C had no influence on the division of bacteria. Aerobic cultures at 15 °C produced larger cells than at 37 °C, but cell division was normal. In fermentative cultures at 15 °C, no increase in CFU ml−1 was observed. However, A 600 increased, due to formation of long filaments. Transmission electronic microscopy and light microscopy with fluorescent staining showed several nucleic acid entities separated by a hydrophobic membrane, indicating that each filament contained several individual cells attached by peptidoglycan. When left in air at room temperature, one filament gave several daughter cells, this means that one CFU formed by one filament may represent a greater contamination potential than one CFU formed by a single cell. Division was observed in cultures at 15 °C with anaerobic respiration in the presence of nitrates. Possible filamentous growth must thus be taken into account to avoid underestimating B. cereus growth in vacuum or modified atmosphere packaged foods stored at low temperature. [Copyright &y& Elsevier]