Generally, food spoilage microorganisms are classified as yeast and molds or bacteria.
Yeasts and molds predominate in foods with a low water activity or pH. Many yeast species are psychrotrophic, which means they can grow at low temperatures. Others are extremely resistant to weak acid preservatives, such as sorbate or benzoate.
While many spoilage yeasts are able to grow under anaerobic conditions, molds are generally inhibited in oxygen-free atmospheres.
Bacteria are responsible for the rapid spoilage of high-protein foods such as meat, poultry, fish, shellfish, milk and some dairy products. The range of food-spoiling bacteria is huge, including lactic acid bacteria, Gram-negative rods (Pseudomonas, Shewanella), Gram-positive spore-formers (Bacillus, Clostridium) and other Gram-positive bacteria (Brochothrix, Micrococcus).
The spoilage floras typical of diverse food commodities and products are described in detail in recent publications by the International Commission on Microbiological Specifications of Foods (ICMSF, 1998, Microorganisms in Foods, Blackie Academic and Professional, London)