The effect of carbohydrate substrates on Bacillus cereus (pacificus) growth and expression of enterotoxin genes

Bacillus cereus is a common contaminant of improperly stored human milk and food capable of causing emetic and diarrheal disease. Here, we aimed to explore the effect of different temperatures and carbohydrate substrates on the growth of B. cereus and to investigate the effect of substrates on expression of its enterotoxin genes (e.g. nhe, hbl, entFM, and cytK). A mesophilic strain B. cereus (pacificus) ATCC 10987 was used for the growth experiment and transcriptomic profiling. Growth curves and carboxylic acids production at five different temperatures (37 °C, 30 °C, 25 °C, 20 °C, and 15 °C) on six different carbohydrate substrates (xylitol, fructose, glucose, galactose, sucrose, and lactose) were recorded. In addition, transcriptomic analysis of bacterial DNA obtained by B. cereus culture with different carbohydrates at 37 °C was performed by RNAseq technique. Besides the expected effect of temperature on B. cereus growth, we found fermentable carbohydrates (glucose, sucrose, and fructose) to be preferred as energy sources to non-fermentable ones (xylitol, lactose, and galactose). Surprisingly, the enterotoxin genes nhe and entFM were less expressed on glucose and lactose than on other investigated carbohydrates. The enterotoxin genes hbl and cytK were not expressed by this strain at all. In conclusion, lactose-rich environment both slows down the growth of B. cereus ATCC 10987 strain and reduces the gene expression of both enterotoxins (nhe and entFM) by this bacterium. Further research could investigate the therapeutic role of lactose in individuals with B. cereus-caused alimentary disease.