Download Dr.A.K.AL-Yassari lect.2 2016-2017 Microbiology Year:third

Dr.A.K.AL-Yassari lect.2
2016-2017
Microbiology
Year:third
Bacterial classification
 Introduction
 Classification is the categorization of organisms into taxonomic groups.
Classification of bacteria requires experimental and observational techniques;
this is because biochemical, physiologic, genetic, and morphologic properties
are often necessary for an adequate description of a taxon.
 Nomenclature refers to the naming of an organism by international rules
(established by a recognized group of medical professionals) according to its
characteristics.
 Identification refers to the practical use of a classification scheme to:
a) Isolate and distinguish desirable organisms from undesirable ones,
b) Verify the authenticity or special properties of a culture in a clinical setting,
c) Isolate and identify the causative agent of a disease.
 Why we need to classify bacteria?
 Facilitate proper laboratory identification of clinical isolates.
 Necessary for determining etiology of infectious diseases during epidemiological
investigation.
 Essential to bacterial nomenclature.
 Criteria for Classification of Bacteria
 Growth on Media
a) complex media: undefined compositions media that contain a carbon source such
as glucose for bacterial growth, water, and various salts needed for bacterial
growth.
b) Nonselective Media: a define media can stimulate the growth of more than one
type of bacteria like blood agar and chocolate agar media.
c) Selective Media: a define media are used to eliminate (or reduce) the large
numbers of irrelevant bacteria in the same samples. The basis for selective media is
the incorporation of an inhibitory agent that specifically selects against the growth
of irrelevant bacteria. Examples of such agents are:
 a-Sodium azide: selects for gram-positive bacteria over gram-negative bacteria.
 b-Bile salts: (e.g., sodium deoxycholate) select for gram-negative enteric bacteria,
and inhibit gram-negative mucosal and most gram-positive bacteria.
 c-Colistin and nalidixic acid: inhibit the growth of many gram-negative bacteria.
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Dr.A.K.AL-Yassari lect.2
2016-2017
Microbiology
Year:third
d) -Differential Media: media that include some sort of added indicator that allows for
the differentiation of particular chemical reactions occurring during growth such as
the production of some characteristic pigments.
 Bacterial Microscopy: consist of
a) Cell and colony morphology: rod, cocci, bacilli, …etc
b) Cell wall structure: differences in cell wall compositions
 Biochemical Tests: there are many examples of biochemical tests that can
ascertain the presence of characteristic metabolic functions and be used to group
bacteria into a specific taxon such as oxidase test, catalase test, ….etc.
 Immunologic Tests: Serotypes, Serogroups, and Serovars
a) The designation "sero" simply indicates the use of antibodies that react with specific
bacterial cell surface structures such as lipopolysaccharide (LPS), flagella, or capsular
antigens.
b) The terms "serotype," "serogroups," and "serovars" are, for all practical purposes,
identical, they all utilize the specificity of these antibodies to subdivide strains of a
particular bacterial species.
 Genetic Instability:
a) Many organisms are difficult to cultivate, and in these instances techniques that
reveal relatedness by measurement of nucleic acid hybridization or by DNA
sequence analysis may be of particular value.
b) DNA-DNA hybridization generally refers to a molecular biology technique that
measures the degree of genetic similarity between pools of DNA sequences. It is
usually used to determine the genetic distance between two species.
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Dr.A.K.AL-Yassari lect.2
2016-2017
 Classification Systems:
There are two classification systems of bacteria:

a)
b)
c)
d)
e)
Phenotypic classification systems: include
Gram stain and bacterial morphology:
Growth Requirements
Biochemical reactions:
Serologic reactions:
Environmental Reservoirs
 Genotypic systems:
a) Universal Phylogenetic Tree:
b) Ribosomal RNA (rRNA) sequence analysis
c) Molecular subtyping
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Microbiology
Year:third