Egg penetration by 'Salmonella' Typhimurium in washed and unwashed eggs

Egg or egg product related 'Salmonella' poisoning is a major concern for the Australian egg industry. Salmonellosis can be acquired by the ingestion of raw or undercooked eggs. 'Salmonella' Typhimurium (ST) is the most common serovar notified in Salmonella food poisoning cases in Australia. The objectives of the current study were to examine the effect of egg washing on the survival of 'Salmonella' on the eggshell surface, to investigate the penetration ability of four different 'Salmonella' Typhimurium phage types (ST PT), previously isolated from Australian layer farms, and to study the effect of egg washing on bacterial eggshell penetration. Our results indicated that there was no significant difference in survival of ST PTs on the egg shell surface of washed and unwashed eggs. Survival rate on inoculated eggshell surface was highest for ST PT 9 (83.33%) followed by ST PT 44 (53.33%), ST PT 193 (43.33%) and ST PT 170 (43.33%). All these phage types are able to penetrate the eggshell and they can survive in the egg internal contents at 20°C and 37°C in both washed and unwashed eggs. ST PT 44 penetration was significantly higher in washed eggs as compared to unwashed eggs. However, for other ST PT (PT 9, 170 and 193), we did not find any significant difference in the penetration of washed and unwashed eggs. The internal contents of whole eggs were most frequently contaminated by ST PT 44 (23.33%) followed by PT 170 (20%), PT 9 (10%) and PT 193 (10%). It was also found that there was no significant effect of incubation temperature (20°C and 37°C) on 'Salmonella' penetration.