Volatile fatty acids and aerobic flora in the gastrointestinal tract of mice under various conditions.

Volatile fatty acids are reported to exert a repressive effect upon Enterobacteriaceae and Pseudomonas species in vitro and in vivo in young mice. The mean total volatile fatty acid concentration in the cecal samples of conventional mice fed ad libitum was 81.7 mumol/g (wet weight), which is antibacterial in vitro, and in the rectal samples it was 41.1 mumol/g (wet weight). The mean count of Enterobacteriaceae in the cecum was only 10(2)/g, whereas in the rectum it was 10(5)/g. Volatile fatty acid levels were influenced by food intake and increased to peak levels approximately 6 to 10 h after eating and then declined. In mice fasted for 17 h, the butyric acid concentration was considerably lower and the number of cecal samples positive for Enterobacteriaceae increased. When fasted for 4 days, mice had extremely low cecal and rectal volatile fatty acid concentrations and the Enterobacteriaceae and enterococci counts increased to mean of 2 x 10(6)/g and 3 x 10(6)/g, respectively, in the cecum and to means of 10(7) and 5 x 10(6)/g in the rectum. We conclude that volatile fatty acids are probably one of the many interference mechanisms which are involved with control of the levels of Enterobacteriaceae (and enterococci) in the large intestine of mice.