Congo red medium to distinguish between invasive and non-invasive Escherichia coli pathogenic for poultry.

In the course of our molecular studies of virulence factors associated with invasive avian Escherichia coli infections, it was first necessary to distinguish between common E. coli and those that cause septicemia in poultry. We found a direct correlation between the ability of clinical isolates of E. coli to bind Congo red dye (CR) and their ability to cause septicemic infection in chickens. This finding was supported by bacteriological studies of 30 broiler flocks (26 sick and 4 healthy) and by virulence studies in chickens and mice. All 144 isolates of E. coli from internal tissues of diseased birds were determined to be CR-positive (red colonies). Congo-red-positive E. coli colonies were isolated from air sacs, pericardium, liver, lung, joint fluid, and heart blood of chickens with lesions of colisepticemia. In contrast, of 170 E. coli isolates from the poultry house environment and from the trachea and cloaca of healthy birds, more than half were CR-negative (white colonies). No CR-negative (white) E. coli colonies were found in internal organs from birds with typical lesions of colisepticemia. We feel that these preliminary findings suggest that the CR dye binding could be used as a phenotypic marker to distinguish between invasive and noninvasive isolates.