Effect of carbohydrate substrates on growth and enterotoxin gene expression in Bacillus cereus (pacificus)

Bacillus cereus sensu lato is a common contaminant of improperly stored food and human milk, capable of causing severe emetic and diarrheal diseases, particularly in infants and immunocompromised individuals. In this in vitro study, the effects of various temperatures and carbohydrate substrates on the growth of B. cereus (pacificus) strain ATCC 10987 and its expression of enterotoxin genes were explored. Bacterial growth at several temperatures and with six different carbohydrate substrates was evaluated. In cultures grown with various substrates at 37 °C, selected metabolites were analyzed using capillary electrophoresis and RNA-seq transcriptomic profiling was performed. At the same temperature, glucose, fructose, sucrose, and xylitol-enriched environments were preferable for the bacterial growth to lactose and galactose-enriched media. Lactate production markedly increased in bacterial cultures grown with glucose, fructose, or sucrose, accompanied by a drop in pH and increased substrate consumption. The lowest expression of non-hemolytic enterotoxin genes (nhe) was observed in glucose and lactose-enriched media, while downregulation of enterotoxin FM gene (entFM) expression was found in bacteria cultured with lactose. Other known enterotoxin genes, such as hemolysin BL (hbl) and cytotoxin K (cytK), were not expressed by this bacterial strain under any of the tested conditions. In conclusion, a lactose-rich environment slowed the growth of B. cereus (pacificus) ATCC 10987 and was associated with a downward trend in the expression of the enterotoxin genes nhe and entFM. These findings indicate that the presence of lactose, a dominant carbohydrate of human milk, may influence the pathogenic potential of B. cereus s. l.