Indeed, the most common bacterial contaminant of food and water supplies belong to these natural contaminants, Pseudomonas and Klebsiella . Any moist surface can support growth of these organisms. They are both Gram negative short rods. But they can contribute to food spoilage. Overgrowth, even during extended refrigeration, can produce a slimy surface, rendering the food unappetizing.
Molds are also natural contaminants.
When considering animals as sources of food, the natural contaminants include organisms associated with the animals, such as Lactobacillus sp. Limiting the growth of these natural contaminants is the goal of food preservation techniques. Food storage procedures must take into account food preservation techniques. Many microbes prefer a neutral pH for growth. Procedures such as pickling rely on growth of natural contaminants which produce acid as a by-product of their metabolism, thus lowering the pH below the organism s optimum range, killing the natural contaminant, and prohibiting growth of other organisms.