Enterococcus cecorum lesion strains are less sensitive to the hostile environment of albumen and more resistant to lysozyme compared to cloaca strains

Enterococcus cecorum lesion strains cause more embryonic mortality after inoculation into the albumen of embryonated eggs compared to cloaca strains. We hypothesized that these strain differences are a result of differences in sensitivity to the antimicrobial effects of the albumen. In this study, the sensitivity of 14 lesion strains and 14 cloaca strains to albumen from 12-day incubated and non-incubated eggs was assessed. A major antimicrobial protein of the albumen is lysozyme and, therefore, the lysozyme resistance of all strains was determined. Albumen from 12-day incubated and non-incubated eggs was inoculated with four cloaca strains and four lesion strains (10(4) CFU/tube). Based on the results, in a subsequent experiment, all 28 strains were inoculated only into albumen from non-incubated eggs. For all strains, the minimal inhibitory concentrations (MIC) of lysozyme were determined using an agar dilution method and the growth rates in broth with (500 and 2000 mu g/ml) or without lysozyme were assessed. Compared to cloaca strains, lesion strains had 320 times higher odds of being reisolated from albumen (95% confidence interval: 55-3962) and had 8.5 times higher reisolation ratios (reisolation dose/inoculation dose) (95% confidence interval: 4.8-15.3). Thirteen cloaca strains had a MIC ranging from 1000 to 8000 mu g/ml, while one cloaca strain and all lesion strains were resistant to the lysozyme concentrations tested. Growth rates of cloaca strains were decreased more by lysozyme compared to lesion strains. In conclusion, lesion strains had higher survival rates in egg albumen and were more resistant to lysozyme compared to cloaca strains.