The microbial groups important in foods consist of several species and types of bacteria, yeasts, molds, and viruses. Although some algae and protozoa as well as some worms (such as nematodes) are important in foods, they are not included among the microbial groups in this chapter. Some of the protozoa and worms associated with health hazards, and several algae (fish and shellfish poisoning) associated with health hazards and bioprocessing (sources of vitamins, single-cell proteins), are discussed in Chapters 17 and 28. Bacteria, yeasts, molds, and viruses are important in food for their ability to cause foodborne diseases and food spoilage and to produce food and food ingredients. Many bacterial species and some molds and viruses, but not yeasts, are able to cause foodborne diseases.
Most bacteria, molds, and yeasts, because of their ability to grow in foods (viruses cannot grow in foods), can potentially cause food spoilage. Several species of bacteria, molds, and yeasts are considered safe or food grade, or both, and are used to produce fermented foods and food ingredients. Among the four major groups, bacteria constitute the largest group. Because of their ubiquitous presence and rapid growth rate, even under conditions in which yeasts and molds cannot grow, they are considered the most important in food spoilage and foodborne diseases.
Prion or proteinaceous infectious particles have been identified as causing transmissible spongiform encephalopathies (TSEs) in humans and animals (see Chapter 28). However, their ability to cause foodborne diseases is not clearly understood.
In this chapter, a brief discussion is included initially on the methods currently used in the classification and nomenclature of microorganisms and later on the important characteristics of microorganisms predominant in food. Living cellular organisms, on the basis of phylogenetic and evolutionary relationships, were grouped originally in five kingdoms, in which bacteria belonged to prokaryotes (before nucleus), and the eukaryotic (with nucleus) molds and yeasts were grouped under fungi.
1 In the 1970s, the prokaryotic domain was changed to Eubacteria (with murein or peptidoglycan on cell wall) and Archaebacteria (without murein on cell wall). In the 1990s, this was changed to Bacteria and Archaea, respectively.2,3 Figure 2.1 depicts a phylogenetic tree representing three major divisions: Bacteria, Archaea, and Eukarya.
The Archaea include most extremophiles and are not important to food microbiology. Fungi and parasites are included under Eukarya and are responsible for foodborne disease and/or spoilage. Viruses are not included in this classification system.