Lesson 2: Public Health Microbiology

Section I: Introduction

2-4

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2-4. CLASSIFICATION OF ORGANISMS

a. General. Classification is an orderly arrangement of organisms with similar physical, biochemical, and genetic characteristics into groups. Taxonomy, the science of classification, is subject to change as new and more precise information becomes available.

b. Five Kingdom System. The five-kingdom system is based on the difference among eucaryotic and prokaryotic forms of life, including their cellular organization (unicellular, unicellular-colonial, or multicellular) and their nutrition (absorptive, ingestive, photosynthetic, or combination of these). The characteristics of the kingdoms are shown in Table 2-1. You will notice that viruses are not included in any kingdom, that is because they are not cells nor are they living organisms.

c. Scientific Names of Organisms. Every living organism is assigned a scientific name (usually in Latin) by which it is identified uniformly by scientists worldwide. The scientific name of an organism, which is always italicized, consists of two identifiers-the genus and the species. The first letter of the genus is capitalized. For example, bos taurus denotes a member of the genus Bas (cattle), and taurus indicates the species taurus (common European cattle). It is customary, when discussing additional species of a genus previously mentioned, to indicate the genus by the first letter of the genus name. For example, B. indicus refers to Indian, or Zebu (Brahman) cattle. When referring collectively to a number of species within the same genus, a writer frequently uses the genus name, followed by the abbreviation "spp" in lower case letters. For example, Salmonella spp refers collectively to species of bacteria within the genus Salmonella.

Table 2-1. Characteristics of the five kingdoms.

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