The first objective of this study was to determine the optimum conditions of temperature, pH values, oxygen availability or absence and time for the best multiplication and penetration of Salmonella typhimurium (ST) (ATCC 14028) into bovan brown (BB) domestic chicken egg components (eggshell, shell membrane, albumen and yolk) at 5, 15, 30 and 45 minutes intervals of egg exposure. To determine the optimum temperature for ST penetration, the eggs were dipped within bacterial broth culture of 1 x 10 cells/ml at 25, 37 and 45°C at neutral pH under aerobic and anaerobic conditions for 5, 15, 30 and 45 minutes, followed by dipping the eggs into S. typhimurium culture at pH 3, 7 and 9 at 37°C under aerobic conditions for 5, 15, 30 and 45 minutes to determine the optimum pH for maximum S. typhimurium penetration. Our results revealed that the average number of ST cells and hence the penetration significantly increased (P<0.01) within eggshell, shell membrane, albumen and yolk at 37°C, pH 7, under aerobic conditions at the end of 45 minutes of egg exposure. However, the optimum penetration was significant within eggshell and shell membrane compared to albumen and yolk, which expressed relatively moderate or slow elevation. The second aim was to study the effect of S. typhimurium penetration on shell membrane structure and on the pH values of albumen and yolk as the time prolonged from 5 to 45 minutes. Our results demonstrated that S. typhimurium penetration through the shell membrane caused the formation of small transparent pores distributed within whole area of shell membrane due to S. typhimurium digestive enzymatic activities to facilitate the penetration towards albumen. The pores increased in number and size as the time extended from 5 to 45 minutes. The pH values of both albumen and yolk increased kalinity at the end of 45 minutes of inoculation, where the average pH values of albumen rose from 7 to 7.90 whereas the average pH values of yolk increased from 5 to 5.7 at 45 minutes.
Moreover, the morphology and intensity of S. typhimurium cells number were visualized and confirmed via scanning electron microscopy (SEM) study. In general, the intensity of S. typhimurium cells rose significantly within each egg components as the time prolonged to 45 minutes. Moreover, S. typhimurium showed two main cellular morphologies, cocci and bacilli within all egg zones. However,
S. typhimurium expressed many aggregates within albumen and yolk at 30 and 45 minutes of infection, which were absent in the controls.