The hen's egg is susceptible to microbial attack in a number of ways. The yolk or the albumen may be contaminated before the egg is laid. After the egg has been laid the possibility exists of microbial penetration from the outside. In this review, both these possibilities are discussed together with the defences, both physical and chemical, that the egg has against microbial contamination. Most eggs contain no bacteria when they are laid and only become contaminated subsequently. The shell membrane offers the best protection against bacterial penetration, but once inside the egg their growth and multiplication is slowed due to the viscous nature of the egg white proteins, their pH, and the bactericidal properties of lysozyme and conalbumen.

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