Food borne pathogens and commercial eggs in Australia

Abstract

A low level of bacterial contamination on an eggshell is important from both food safety and storage perspectives. A high number of bacteria present on the eggshell surface increases the chances of eggshell penetration and contamination of internal contents. Overall, food quality and sanitary processing conditions can be judged by Enterobacteriaceae populations. In chapter 2, no significant differences were detected in the Enterobacteriaceae counts of the eggshell surface or in the eggshell pores, of visibly clean eggs collected from Australian layer flocks at various stages of lay. Out of all eggs tested, 4.51% (14/310) eggshell samples were reported Salmonella positive. The rate at which Salmonella infected flocks produce Salmonella contaminated eggs is unclear. In chapter 3, the possible transmission of Salmonella from the environment to the egg was investigated with the help of longitudinal studies on commercial egg farms. Results indicated that the odds of an eggshell being positive for Salmonella were 91.76, 61.5 and 18.2 times higher when faecal, egg belt and dust samples, respectively, tested positive for Salmonella. On other hand, a one log increase in the load of Salmonella detected in faecal, egg belt and in the floor dust samples, respectively, resulted in 35%, 43% and 45% increase (p<0.001) in the odds of an eggshell testing positive for Salmonella. In chapter 4, the shedding of Salmonella in a single age commercial egg layer flock was also investigated at the onset of lay (18 weeks) followed by two longitudinal samplings at 24 and 30 weeks. At the age of 18 weeks, when the first sampling was performed, the prevalence of Salmonella in faeces was 82.14%. However, in later samplings, at the age of 24 and 30 weeks, the prevalence of Salmonella in faeces was significantly reduced (p<0.001) to 38.88% and 12.95% respectively. The prevalence of Salmonella in faeces collected from the low tier cages was significantly higher (p=0.009) as compared samples from the high tier cages. There are various methods to decontaminate the eggshell surface; egg washing is one of them. Egg washing can reduce the level of bacteria on the eggshell surface and horizontal transmission across the eggshell. However, egg washing can damage the cuticle which is the outmost layer on the eggshell surface. The effect of egg washing on Salmonella Typhimurium (S. Typhimurium) penetration was investigated using agar and whole egg penetration techniques. The results in chapter 5 indicated that eggshell penetration was higher in washed eggs as compared to unwashed eggs. Hence, appropriate attention is essential to make sure eggs are kept at appropriate storage and drying conditions after washing. Statistical analysis also indicated that eggshell penetration by S. Typhimurium was related to the incidence of various eggshell ultrastructural features such as cap quality, alignment, erosion, confluence, Type B bodies and cuticle cover. All the S. Typhimurium strains used in this study were able to survive on the eggshell surface and in egg internal contents 21 days after infection. Other egg industry associated Salmonella serovars such as S. Singapore, S. Adelaide, S. Worthington and S. Livingstone had the capacity to penetrate the eggshell. However, these serovars had little or no capacity to survive in the egg internal contents 21 days after inoculation (Chapter 6). Eggshell quality and safety are important for the consumer’s impression of the product. A good quality eggshell protects the egg internal contents from bacterial penetration. A cracked or damaged egg encourages bacteria to move across the eggshell. Mycoplasma synoviae (M. synoviae) have been found to be associated with poor egg shell quality. The association between egg shell quality parameters and the seroprevalence of M. synoviae in eggs collected from Australian commercial layer flocks was investigated in chapter 7. Seroprevalence of M. synoviae was found to be high at 69% (95% confidence interval (CI) = 41.3–89.0). Statistical analysis showed an association between serological status for M. synoviae and the incidence of egg quality parameters such as translucency, shell breaking strength, % shell reflectivity and shell deformation. Thus, M. Synoviae infection could compromise eggshell quality and as a consequence the eggs from M. Synoviae positive flocks may be at risk of becoming contaminated by potentially pathogenic bacteria.