Escherichia coli at the Intestinal Mucosal Surface

Escherichia coli is the predominant facultative anaerobe in the gastrointestinal tracts of mammals. A large and growing body of evidence indicates that commensal E. coli strains grow in the intestine on nutrients acquired from mucus. Fluorescence in situ hybridization analysis of mouse intestinal thin slices showed that E. coli BJ4, a rat commensal isolate, is dispersed in the mucus layer but is not associated with the epithelium. Colonization resistance is the ability of a complete intestinal microflora to resist colonization by an invading bacterium. When E. coli and serovar Typhimurium enter the mouse cecum, they initially encounter cecal contents. Since E. coli and serovar Typhimurium fail to grow in cecal contents but grow rapidly in cecal mucus, they must be able to enter the mucus layer and grow by using available nutrients. Genetic screens based on signature-tagged mutagenesis identify individual mutants that are lost from pools of mutants used to infect suitable animals. It is becoming increasingly clear that once E. coli strains reach the large intestine, in order to colonize, they must enter the mucus layer and utilize nutrients there for growth.