Role of competition for substrate in bacterial antagonism in the gut

Small slices of agar, each containing about 10(5.5) Escherichia coli organisms, were preincubated either in the contents of rat cecum or in brain heart infusion broth for 2 h and then were transferred to a small sample of saline. The purpose of these experiments was to examine the ability of E. coli to grow on the substrate that penetrated from the contents of rat cecum into the agar slices. It appeared that preincubation in brain heart infusion broth gave rise to abundant growth, whereas only poor growth occurred after preincubation in the contents of rat cecum. This poor growth was completely reversed by adding brain heart infusion. The same experiments were repeated under anaerobic conditions, in which growth on substrate that penetrated from the contents of rat cecum into the agar slices was extremely poor. This extremely poor growth under anaerobic conditions was reversed to abundant growth by adding brain heart infusion broth. The addition of nitrate as an electron acceptor also stimulated growth of E. coli. Similar results were obtained with other bacteria and with human feces. The results can be interpreted as a demonstration that under anaerobic conditions such as occur in large intestines, bacterial antagonism is caused in a high degree by competition for substrate.