All healthy animals carry a complex microbial flora, part of which may be very specialized and adapted to growth and survival on its host, and part of which may be transient, reflecting the immediate interactions of the animal with its environment. From a topological point of view, the gut is also part of the external surface of an animal but it offers a very specialized environment and the importance of the human gut flora will be dealt with in Chapter 6
The surfaces of humans and other animals are exposed to air, soil and water and there will always be the possibility of contamination of foods and food handling equipment and surfaces with these environmental microbes by direct contact with the animal surface.
However, the surface of the skin is not a favorable place for most micro-organisms since it is usually dry and has a low pH due to the presence of organic acids secreted from some of the pores of the skin. This unfavorable environment ensures that most micro-organisms reaching the skin do not multiply and frequently die quite quickly. Such organisms are only ‘transients’ and would not be regularly isolated from the cleaned skin surface.
Nevertheless, the micro-environments of the hair follicles, sebaceous glands and the skin surface have selected a specialized flora exquisitely adapted to each environment. The bacteria and yeasts making up this ‘normal’ flora are rarely found in other habitats and are acquired by the host when very young, usually from the mother.
The micro-organisms are characteristic for each species of animal and, in humans, the normal skin flora is dominated by Gram-positive bacteria from the genera Staphylococcus, Corynebacterium and Propionibacterium.
For animals which are killed for meat, the hide may be one of the most important sources of spoilage organisms while, in poultry, the micro-organisms associated with feathers and the exposed follicles, once feathers are removed, may affect the microbial quality and potential shelf-life of the carcass.