Growth of Salmonella in egg white.
Six non-S. Enteritidis strains and eleven S. Enteritidis strains of which 8 were isolated from eggs, were tested on their growth behaviour in fresh eggs and in eggs of 2 and 3 weeks old hold at 20 degrees C. The growth of Salmonella at 20 degrees C was measured at days 6, 13 and 23 after inoculation. Taking the average growth of the different S. Enteritidis and of the non-S. Enteritidis strains, we found that both groups grew very well in fresh eggs. S. Enteritidis strains grew as good in eggs of 2 weeks as in fresh eggs, non-S. Enteritidis strains grew less. When keeping the eggs 3 weeks before inoculation, both groups showed still a relatively high growth. Additional experiments were done on the growth of Salmonella in a minimal medium with an iron source together with the iron chelator conalbumin, and in a medium without iron and conalbumin, to overcome variations in viscosity and in egg composition. After 12 hours incubation at 37 degrees C, both S. Enteritidis and non-S. Enteritidis strains grew better in the presence of iron than in the absence. S. Enteritidis egg isolates behaved more variable than S. Enteritidis non-egg isolates in the presence of iron. To check the influence of pH changes during ageing of eggs, we measured the growth of Salmonella in minimal media (with iron) of pH 8, 8.5, 9 and 9.5. All salmonellae seemed to grow well independently of the pH. Thus the fact that Salmonella grew best in fresh egg white and showed a decreasing growth in stored egg white, is probably not due to a pH effect. We also investigated if the presence of the yolk had an influence on the growth of Salmonella. We saw no general correlation so it seems that no components that could promote the growth of Salmonella diffuse from the yolk. We studied the influence of the egg sources on the variability in growth by comparing experiments done in eggs of 3 different sources. For the different strains we found a variable growth. But in general, the most important conclusion of our experiments is that all Salmonella strains do grow better in fresh egg white than in egg white of 2 or 3 weeks old. These results are contradictory with findings in literature (Humphrey et al., 1993) and must be checked with the European guidelines concerning the storage of eggs. Moreover, we found that even non-S. Enteritidis strains can grow well in egg white. These are not found in naturally contaminated eggs. It doesn't seem probable that S. Enteritidis could penetrate the egg shell easier than other serotypes so we suppose that horizontal transmission of Salmonella in eggs is of less importance than the vertical transmission.