Recovery of 'Salmonella' and 'Escherichia coli' from commercial egg shells and effect of translucency on bacterial penetration in eggs

Abstract

This experiment was conducted to study the prevalence of 'Salmonella' and 'Escherichia coli' ('E. coli'). from the surface of egg shells, egg shell membranes or pores, and internal contents from unwashed eggs collected from commercial caged layer farms in Australia. Egg shell swabs, shell crush and egg internal contents (yolk and albumen) of an individual egg were processed for bacteriological examination. 'Salmonella' spp. were not detected from any of the egg shell surfaces, egg shell crush or egg internal contents. Thirty five 'E. coli' isolates were isolated from the egg shell surface. Ten 'E. coli' strains were also isolated from shell crush. However, the internal contents of eggs appeared to be sterile. Polymerase chain reaction was performed on forty-five 'E. coli' isolates using primers for heat stable enterotoxin genes A and B (STa and STb) and also for colicin V gene (cvaC). STa gene was detected in four 'E. coli' isolates isolated from egg shell surfaces. All the 'E. coli' isolates were negative for STb and cvaC genes. These data provide useful information regarding the prevalence of virulent 'E. coli' and 'Salmonella' spp. on and in unwashed eggs collected from layer farms. These data also suggest that unwashed eggs collected from caged layer farms are unlikely to be sources of 'Salmonella' outbreaks. Egg shell translucency could be due to changes in the mammillary layer and mamillary cores during the early phases of egg shell formation and has the potential to increase the incidence of microcracks in egg shells, and hence, may be responsible for bacterial penetration. There was a significant correlation between egg shell translucency and egg shell penetration by 'Salmonella' Infantis and 'E coli'. Both strains of bacteria were able to penetrate the translucent egg shells even at very low doses. The penetration, however, was hindered in both translucent and non translucent eggs at 4°C, as compared with room temperature which highlights the importance of storage of eggs at refrigerated temperatures.