Fundamentals of Microbiology

Lesson 8. MAJOR CHARACTERISTICS USED IN MICROBIAL TAXONOMY

Module 3. Microbial taxonomy

Lesson 8

MAJOR CHARACTERISTICS USED IN MICROBIAL TAXONOMY

8.1 Introduction

Major characteristics used in microbial taxonomy are morphology, Gram reaction, nutritional classification, cell wall, lipid, cell inclusions and storage products, pigments, carbon source utilization, nitrogen source utilization, sulfur source utilization, fermentation products, gaseous needs, temperature range, pH range, pathogenicity, symbiotic relationships, habitat, etc.

Often Guanine to Cytosine (GC) ratio of deoxyribonucleic acid (DNA) is also used. If two organisms that are thought to be closely related, based on phenotypic criteria, do not have similar GC values, then they are not in fact closely related.

8.2 Dichotomous Key

It is a means of assigning an organism to a specific taxonomic category. It typically involves the use of specific criteria that may be posed as questions (e.g. what does the organism look like?). Relevant criteria may be arranged as a dichotomous key. In a dichotomous key, questions are arranged hierarchically (just as taxonomic categories are) with more general questions (i.e. those arranging organisms into large categories) being asked first, and increasingly specific questions (better suited to arranging organisms into more specific taxa) subsequently. In addition, questions are dichotomous, meaning that each of them have two possible answers, with each answer distinguishing the organisms as well as the path to the next question.

8.3 Strain Differentiation Methods

Very closely related organisms, i.e. members of the same species, are typically sufficiently similar that there exist additional methods that are able to distinguish the small differences seen between them. These methods include protein profiling, immunological reactions, phage typing etc. These methods compare phenotypes and that, though useful, are not as precise as genetic homologies in determining evolutionary relationships.

8.3.1 Protein profile

Various techniques exist for isolating (separating) and then visualizing the proteins from cells. By distinguishing proteins in terms of their size and/or charges one can construct reproducible patterns that are typical of a given organism. Similar organisms display similar protein patterns. The size and other differences between proteins among different organisms may be determined very easily employing methods of protein separation based on the technique known as gel electrophoresis.

8.3.2 Immunological reactions

The ability of antibodies to bind to and/or inactivate microorganisms can be employed to determine evolutionary relationships. The basic premise is that antibodies are highly selective in terms of the proteins to which they bind. Thus, antibodies are able to distinguish the proteins coming from different bacterial species. Two organisms that a single antibody (or antibody preparation) binds to are considered to be more closely related than a third organism to which the antibody preparation does not bind. Various methods based on this principle are agglutination tests, enzyme-linked immunosorbent assays (ELISA) and Western blots.

8.3.3 Flow cytometry

Flow cytometry is a technique employing serological methods that analyze cells suspended in a liquid medium by light, electrical conductivity, or fluorescence as the cells individually pass through a small orifice.

8.3.4 Phage typing

Bacteriophages (or phages) are viruses that infect bacteria. Phage can be very specific in what bacteria they infect and the pattern of infection by many phages may be employed in phage typing to distinguish bacterial species and strains.

8.4 Types of Strains

8.4.1 Type strain

It is the first strain isolated or best characterized. In classifying an organism, it is helpful to have some standard to compare it to. Such standards for a given strain are termed type strains. Often the type strain is the first example of a species or strain. Type strains are kept preserved by the culture collection centres such as American Type Culture Collection (ATCC).

8.4.2 Serovar

A serovar is a strain differentiated by serological means. Individual strains of Salmonella spp. are often distinguished and distinguishable by serological means.

8.4.3 Biovar (biotype)

Biovars are strains that are differentiated by biochemical or other non-serological means.

8.4.4 Morphovar (morphotype)

A morphovar is a strain which is differentiated on the basis of morphological distinctions.

8.4.5 Isolate

An isolate is a pure culture derived from a heterogeneous, wild population of microorganisms. The term isolate is also applicable to eucaryotic microorganisms as well as to viruses.