Survival of Avian Pathogenic Escherichia coli in an In Vitro Contamination Model of Eggs

Laying chickens infected with avian pathogenic Escherichia coli (APEC) may lay eggs contaminated with this organism. However, whether APEC has a superior ability than non-APEC strain to survive and proliferate in egg contents remains unknown. Here, an in vitro egg contamination model was used to assess this matter. A total of 10 APEC and 11 non-APEC E. coli strains, determined according to the results of an embryo lethality assay, were used. A total of 35-190 colony-forming units (CFU) of one of the E. coli strains were experimentally inoculated onto the vitelline membrane of eggs collected from specific-pathogen-free chickens, following which the albumen from a single egg was poured into a plastic beaker to surround the yolk and incubated at 25 °C for three days. Albumen samples from the egg contamination model were enriched using buffered peptone water, and 10 μl of the culture was spread onto a DHL agar plate. Percentage of the albumen samples inoculated with APEC strains that yielded growth was significantly higher (P < 0.05) than that of those inoculated with non-APEC E. coli strains. When E. coli strains were inoculated in separated albumen samples, percentage of the samples inoculated with APEC strains that yielded growth was significantly higher than that of those inoculated with non-APEC E. coli strains. Treatment of strains with suspension containing lysozyme and ovotransferrin, which simulates the properties of albumen postoviposition, resulted in a decrease in viable cell counts in both the non-APEC E. coli and APEC strains. Several adhesin-associated genes were significantly prevalent in APEC strains than in non-APEC E. coli. The results of this study suggest that APEC strains associated with the vitelline membrane are capable of surviving in albumen in the egg contamination model. These findings lay a foundation for studying the mechanisms of survival of APEC strains in the albumen, which may help in controlling colibacillosis in chickens.