Download Criteria for Classification of Bacteria

Bacterial Classification
Classification, nomenclature, and identification are the three separate but interrelated
areas of taxonomy.
Classification can be defined as the arrangement of organisms into taxonomic groups (taxa)
on the basis of similarities or relationships.
Classification of prokaryotic organisms such as bacteria requires a knowledge obtained by
experimental as well as observational techniques, because biochemical, physiologic, genetic,
and morphologic properties are often necessary for an adequate description of a taxon.
Nomenclature is naming an organism by international rules according to its characteristics.
Identification refers to the practical use of a classification scheme: (1) to isolate and
distinguish desirable organisms from undesirable ones; (2) to verify the authenticity or
special properties of a culture; or, in a clinical setting, (3) to isolate and identify the causative
agent of a disease.
Permit the selection of pharmacologic treatment.
Criteria for Classification of Bacteria:
1: Valuable information can be obtained microscopically by observing cell shape
2: Presence or absence of specialized structures such as spores or flagella.
3: Staining procedures such as the Gram stain.
4: Some bacteria produce characteristic pigments,
5: Zones of hemolysis in agar medium containing red blood cells.
6: Specific antibodies.
7: Tests such as the oxidase test, which uses an artificial electron acceptor, can be used to
distinguish organisms on the basis of the presence of a respiratory enzyme, cytochrome c.
8: Simple biochemical tests.
Identification & Classification Systems:
Keys:
Keys organize bacterial traits in a manner that permits efficient identification of
organisms. Groups are split into smaller subgroups on the basis of the presence (+) or
absence (–) of a diagnostic character.
for example, for a key to bacteria to include a group such as "bacteria forming red
pigments" even though this would include such unrelated forms as Serratia
marcescens.
Numerical Taxonomy
Numerical taxonomy (also called computer taxonomy, phenetics, or taxometrics)
became widely used in the 1960s. Numerical classification schemes use a large
number (frequently 100 or more) of unweighted taxonomically useful characteristics.
Such data provide a basis for the construction of a frequency matrix for identification
of unknown strains against the defined taxa.
Phylogenetic Classifications: Understanding of Evolutionary Relationships
among Bacteria
Phylogenetic classifications are measures of the genetic divergence of different phyla
(biologic divisions). Close phylogenetic relatedness of two organisms implies that
they share a recent ancestor.
There is considerable genetic diversity among bacteria. Chemical characterization of
bacterial DNA revealed a wide range of nucleotide base compositions when DNA
from different bacterial sources was compared. The G (guanine) and C (cytosine)
compositions of DNA from a single source were always equal, as were the A
(adenine) and T (thymine) compositions.
DNA sequencing has become a routine laboratory procedure, and comparison of the
DNA sequences of divergent genes can give a measure of their relatedness.
Bergey's Manual of Systematic Bacteriology:
First published in 1923, the Manual is an effort to classify known bacteria and to
make this information accessible in the form of a key.
In 1980, the International Committee on Systematic Bacteriology published an
approved list of bacterial names. This list of about 2500 species replaces a former list
that had grown to over 30,000 names. since January 1, 1980, only the new list of
names has been considered valid.
Description of the Major Categories & Groups of Bacteria:
There are two different groups of prokaryotic organisms: eubacteria and
archaebacteria. Eubacteria contain the more common bacteria, ie, those with which
most people are familiar. Archaebacteria do not produce peptidoglycan, a major
difference between them and typical eubacteria. They also differ from eubacteria in
that they live in extreme environments (eg, high temperature, high salt, or low pH)
and carry out unusual metabolic reactions, such as the formation of methane.
Subtyping & Its Application
Classically, subtyping has been accomplished by biotyping, serotyping,
antimicrobial susceptibility testing, bacteriophage typing, and bacteriocin typing. For
example, more than 130 serogroups of Vibrio cholerae have been identified based on
antigenic differences in the O polysaccharide of the LPS; however, only the O1 and
O139 serogroups are associated with epidemic and pandemic cholera. Within these
serogroups, only strains that produce cholera toxin are virulent and cause the disease
cholera; nontoxigenic V cholerae O1 strains, which are not associated with epidemic
cholera, have been isolated from environmental specimens, from food, and from
patients with sporadic diarrhea.
Clone: they are the progeny of a single cell and thus, for all practical purposes, are
genetically identical.
Subtyping plays an important role in identifying these particular microorganisms.
Multilocus enzyme electrophoresis (MLEE), which has been a standard method for
investigating eukaryotic population genetics, has also been used to study the genetic
diversity and clonal structure of pathogenic microorganisms.
restriction endonucleases to cleave DNA into discrete fragments is one of the most
basic procedures in molecular biology.
Digestion of DNA with these enzymes generally results in 5 to 20 fragments ranging
from approximately 10 kb to 800 kb in length. Separation of these large DNA
fragments is accomplished by a technique called pulsed field gel electrophoresis
(PFGE).
Ribotyping is a method that uses Southern blot analysis to detect polymorphisms of
rRNA genes, which are present in all bacteria.