Download Bacterial Classification (The second lecture)

Bacterial Classification
(The second lecture)
Dr.Sahar Mahdi
Classification Systems
The classification of bacteria serves a variety of different functions.
Because of this variety, bacteria may be grouped using many
different typing schemes. The critical feature for all these
classification systems is an organism identified by one individual
is recognized as the same organism by another individual. At
present the typing schemes used by clinicians and clinical
microbiologists rely on phenotypic typing schemes. These
schemes utilize the bacterial morphology and staining
properties of the organism, as well as O2 growth requirements
of the species combined with a variety of biochemical tests.
Scientists interested in the evolution of microorganisms are
more interested in taxonomic techniques that allow for the
comparison of highly conserved genes among different species.
As a result of these comparisons a phylogenetic tree can be
developed that displays the degree of relatedness of different
organisms.
Phenotypic classification systems
Gram stain and bacterial morphology:
This method was Discovered by. Gram, H.C in 1884 it
remains an important and useful technique to this day.
It allows a large proportion of clinically important
bacteria to be classified as either Gram positive or
negative based on their morphology and differential
staining properties. Slides are sequentially stained with
crystal violet, iodine, then destained with alcohol and
counter-stained with safranin. Gram positive bacteria
stain blue-purple and Gram negative bacteria stain red.
The difference between the two groups is believed to
be due to a much larger peptidoglycan (cell wall) in
Gram positives.
As a result the iodine and crystal violet
precipitate in the cell wall and are not eluted
by alcohol in contrast with the Gram negatives
where the crystal violet is readily eluted from
the bacteria. As a result bacteria can be
distinguished based on their morphology and
staining properties. Some bacteria such as
mycobacteria (the cause of tuberculosis) are
not stained due to the large lipid content of
the peptidoglycan. Alternative staining
techniques (acid fast stain) are therefore used
for stained bacteria
Growth Requirements:
Microorganisms can be grouped on the basis of their
need for oxygen to grow. Facultative anaerobic bacteria
can grow in high oxygen or low oxygen content. In
contrast, strictly anaerobic bacteria grow only in
conditions where there is minimal or no oxygen present
in the environment. Bacteria such as bacteroides found
in the large bowel are examples of anaerobes. Strict
aerobes only grow in the presence of significant
quantities of oxygen. Pseudomonas aeruginosa, an
opportunistic pathogen, is an example of a strict
aerobe. Microaerophilic bacteria grow under conditions
of reduced oxygen and sometimes also require
increased levels of carbon dioxide. Neisseria species
(e.g., the cause of gonorrhea) are examples of
microaerophilic bacteria.
Biochemical reactions:
Clinical microbiology laboratories typically will identify a
pathogen in a clinical sample, purify the microorganism
by plating a single colony of the microorganism on a
separate plate, and then perform a series of
biochemical studies that will identify the bacterial
species.
Serologic systems:
Selected antisera can be used to classify different
bacterial species. This may be based on either
carbohydrate or protein antigens from the bacterial cell
wall or the capsular polysaccharide. (Group A
streptococcal M proteins or O and H polysaccharide
antigens of Salmonella)
Environmental Reservoirs:
When considering likely pathogens it is also important
to know which of the different species are found in
different locations. Environmental reservoirs are
generally divided into those that are endogenous
(i.e., on or within the human or animal body) and
exogenous (somewhere in the environment).
Requirement for oxygen like aerobic bacteria as
Bacilli and Anaerobic bacteria, like Clostridium, also
Facultative anaerobe like Staphylococci ,In addition
sometime bacteria were Lactose fermenters as E.
coli, Klebsiella and Non lactose fermenters as
Salmonella, Shigella
Genotypic systems:
Universal Phylogenetic Tree(‫(النشوء والتطور‬: depend of
ribosomal RNA sequences. These sequences are highly
conserved and undergo change at a slow, gradual and
consistent rate. They are therefore useful for making
comparisons among the different living organisms.
Molecular subtyping: Sometimes it is necessary to
determine whether strains from the same species are
the same or different. For example, if there is an
outbreak of infections that appear due to the same
bacterial species, the hospital epidemiologist will want
to know if all of the infections are due to the same
strain. Clues can be obtained by examining the
biochemical studies or the antibiotic susceptibility
profile, but a more reliable method is by molecular
analysis.
A microaerophile
is a microorganism that requires oxygen to
survive, but requires environments
containing lower levels of oxygen than
are present in the atmosphere (i.e. <21%
O2; typically 2-10% O2).Many
microaerophiles are also capnophiles,
( requiring high concentration of carbon
dioxide )e.g. 10% CO2 in the case
of Campylobacter spp.)
Linnaeus hierarchical sequences(‫ (تسلسل هرمي‬proceed
this way
Old classification
Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species
New classification
ex:
Kingdom
prokaryota
division
bacteria
family
Streptococcaceae
genus
Streptococcus
species
equi
Scientific nomenclature
Binomial nomenclature
A binomen or a scientific name; more informally it is also called
a Latin name. The first part of the name identifies
the genus to which the species belongs; the second part
identifies the species within the genus. For example, humans
belong to the genus Homo and within this genus to the
species Homo sapiens.
The binomial names of species are usually typeset in italics;
for example, Staphylococcus aureus . Generally, the binomial
should be printed in a font style different from that used in
the normal text; for example; Streptococcus . When
handwritten, each part of a binomial name should be
underlined; for example, Staphylococcus aureus . The first
part of the binomial, the genus name, is always written with
capital letter, the second name (species ) is written with small
letter.
ex: Pasteulla maltucida
.
The binomial name should generally be written in full. The
exception to this is when several species from the same
genus are being listed or discussed in the same paper or
report, or the same species is mentioned repeatedly; in
which case the genus is written in full when it is first used,
but may then be abbreviated to an initial (and a period/full
stop). For example, a list of members of the
genus Canis might be written as "Canis lupus, C. aureus, C.
simensis". In rare cases, this abbreviated form has spread
to more general use; for example, the
bacterium Escherichia coli is often referred to as just E. coli,
The abbreviation "sp." is used when the actual specific name
cannot or need not be specified. The abbreviation "spp."
(plural) indicates "several species". These abbreviations are
not italicized (or underlined).
Most genus and species name are chosen to tell us
something about the organism its:
▀
Appearance ex: Streptococcus spp.
▀ Source ex : Canis Spp.
▀ Characteristic property ex: Staphylococcus aureus
▀ The scientist associated with it ex : Shigella
For example : Escherishia coli is named for microbiologist Escherich and
its habitat is the colon
Shigella is named for Japanes microbiologist Shiga
Staphylococcus aureus is named for the appearance and characteristics
feature (staphyl means cluster) and (aureus means golden).
Also the species is used for :
▀ The infection like Bacillus anthracis & Mycobacterium tuberculosis
▀ The host ex: Streptococcus eque & Brucella ovis
▀ the locality like the name of town or country ex. Salmonella Dublin &
S. London.
The characters used to classify bacteria:
Bergeys manual of determinative Bacteriology
► The characters used to classify bacteria fall into two
groups
A- Traditional
1-morphology
2-cultural characteristics
3- gram staining reaction
4- spore forming
5- flagellation
6- biochemical tests
7- antigenic properties
8- pathogenisity & toxin production
B- Genomic
1- present G+C (Guanine- Cytosine).
2- Sequences bases in DNA.
3- DNA Hybridization.