Bacterial taxonomy comprises of three separate but interrelated important areas.
1. Classification: It refers to
hierarchy based arrangement of bacteria into taxonomic groups or taxa (singular,
taxon) on the basis of similarities or
differences in their biochemical, physiological, genetic, and morphological properties.
2. Nomenclature: It
refers to the naming of taxa according to their characteristics, by following the
international rules.
3. Identification: It refers to the practical use of a classification scheme such as:
(1) Identification of an unknown taxon by comparing with a defined and named taxon,
(2) To isolate and identify the causative agent of a disease.
 |
BACTERIAL CLASSIFICATION
The most recent taxonomic classification of bacteria is based on Cavalier and Smith's six kingdom classification(1998).
It is the most accepted classification at present, surpassed the previous five
kingdom classification (Whittaker, 1969) and three domain classification
(Woese, 1990)(Table 1.4).
Cavalier and Smith's Classification
It is a molecular classification
which divides all living structures of the earth into six kingdoms-Bacteria, Protozoa,
Chromista, Plantae, Fungi and Animalia.
Kingdom Bacteria is divided successively in decreasing order of hierarchy into phylum/division, class, order, suborder, family, tribe, genus and species. For example, the full taxonomical position of Escherichia coli is given in Table 1.5.
Principle Used to Classify
Bacteria
There is no universally
accepted principle to classify bacteria. There are mainly three approaches,
phylogenetic, Adansonian and molecular.
Phylogenetic
Classification of Bacteria
This is a hierarchical classification representing a branching
tree-like arrangement; one characteristic (or trait) is being employed for division
at each node of the tree (Fig. 1.2).
• This system is called phylogenetic
because it implies an evolutionary arrangement of species.
• Here, the characteristics
are arbitrarily given special weightage. Depending on the characteristic so
chosen, the classificalion would give different patterns.
• The characteristics which
are given importance depend upon various properties of the organisms such as:
•
Morphology of bacteria- cocci or bacilli.
•
Staining property such as gram-positive and
gram negative.
•
Culturalcharacteristics such as lactose fermenting and non-lactose fermenting colonies.
• Biochemical reactions, e.g. coagulase
positive Staphylococci and coagulase negative Staphylococci.
• Antigenic structure, e.g. antigenic structure of somatic antigen present in bacterial cell wall.
Though, this
classification is a convenient and user friendly method, but it is not a perfect
method. Because the weighted characters used may not be
valid all the time
for a given bacterium.
For example,
fermentation of lactose though is an important property to classify family Enterobacteriaceae;
but is not a permanent trait. In due course of the time, bacteria may lose or gain the property to
ferment lactose.
Adansonian Classification of Bacteria
- To avoid the use of weighted characteristics, Michael Adanson proposed another method (1774) that classifies organisms based on giving equal weightage every character of the organism.
- This is also called phenetic classification. It has its greatest application in numeric bacterial taxonomy.
- Numerical bacterial taxonomy: The concept was first developed by Robert R Sokal and Peter HA Sneath in 1963.
- With the advent of computer facilities, the principle of phenetic classification has been extended further, so the very large numbers of characters of several organisms can be compared at the same time.
- They have created a taxonomic system by using numeric algorithms like cluster analysis rather than using subjective evaluation of their properties which are arbitrarily given special weightage.
Molecular Classification of Bacteria
- It is based on the degree of genetic relatedness of different organisms. Guanine+ Cytosine (G + C) content of bacieria is estimated after extracting DNA from pure bacterial culture.
- The nucleotide base composition and the base ratio vary widely among different groups of microorganisms, but for any one particular species, it is constant.
NOMENCLATURE OF BACTERIA
Nomenclature is the branch of bacterial taxonomy, that is
concerned with designating scientific names to taxa, based on a particular
classification scheme and in accordance with agreed international rules and conventions.
Bacterial nomenclature also follows die same rules as proposed
by Swedish botanist Carolus Linnaeus who invented the modern system of binomial
nomenclature.
• Scientific names for
taxonomic levels above genus are always capitalized but not italicized; for
example, Phylum Proteobacteria.
• In binomial nomenclature system,
the scientific name of bacteria comprises of a genus name (starts with a capital
letter) and species name. Both genus and species should be written in italic
or are underlined; e.g. Staphylococcus aureus or Staphylococcus aureus.
• The genus (plural: genera)
is usually a Latin noun whereas the species refers to a defined taxon of organisms
within a particular genus.
• The genus and species are
coined based on some property of the bacteria; for example,
§ Staphylococcus
aureus is named after their arrangement in cluster (Staphyle means
as bunch of grapes) and type of pigmentation they produce (aureus meaning
golden yellow)
§ Neisseria
meningitidis is named after-the discoverer (U. Neisser)
and the disease it causes (meningitis).
§ Brucella
suis and Brucella melitensis (named after the
discoverer (Brucella from David Bruce) and the animal host (suis meaning
pig) and the place of discovery (melitensis from Malta, Europe).
• Typing: The species can
also be classified further by various typing methods.
Type Cultures
There are many international reference laboratories which are designated as type culture reference centres.
• They maintain the representative cultures of die established species, which show all the standard characteristics of the original strain.
• The strains isolated in the laboratories are compared using the standard strains supplied by these type culture centers.
• The original cultures of any new species described are deposited in type collection centers.
• The two most important type collection centers of the world are:
§ ATCC (American Type Culture Collection), USA
§ NCTC (National Collection of Type Cultures), UK.
No comments:
Post a Comment