classification of Bacteria

1. CLASSIFICATION OF
BACTERIA
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2. BACTERIAL TAXONOMY
 All organisms have a name consisting of two parts:
the
genus followed by the species (i.e., Homo sapiens).
 This is no different from bacteria
Genus species
Bacillus subtilis
Clostridium tetani
Staphylococcus aureus
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3. Basis of classification
• Phenotypic classification
❖ Staining
❖ Morphological
❖ Anatomical
❖ Cultural characteristics
❖ Nutrition
❖ Environmental factors
❖ Biochemical reactions
❖ Antigenic structure
• Genotypic classification
❖ DNA-DNA hybridization
❖ G+C content 2
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4. Based on Staining reaction
 Because bacteria are colourless and usually invisible to
light microscopy, colourful stains have been developed
to
visualize them.
 The most useful is the Gram stain, which separates
organisms into 2 groups:
 gram-positive bugs
 gram-negative bugs
Note: The different stains are the result of differences in
the
cell walls of gram-positive and gram-negative bacteria.
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5. Based on Staining reaction
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• GRAM’S STAIN
– Gram-positive cocci – Staphylococcus aureus
– Gram-negative cocci – Neisseria gonorrhoeae
– Gram-positive rods – Clostridium spp.
– Gram-negative rods – E. coli
• ACID FAST STAIN
– Acid-fast bacilli –Mycobacterium tuberculosis
– Non-acid-fast bacilli – Staphylococcus aureus

6. Morphological classification
• Bacteria can be classified into FOUR major groups on
morphological basis.
 Cocci: These are spherical or oval cells.
 Bacilli: rods. Short bacilli are called coccobacilli.
 Spiral forms: comma-shaped, S-shaped, or spiral-
shaped.
 Pleomorphic: lacking a distinct shape (like jello)
The different shaped creatures organize together into
more complex patterns, such as pairs (diplococci),
clusters, strips, and single bacteria with flagella.
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9. Gram-Positive Bacteria
 There are 6 classic gram positive bugs that cause disease in
humans, and basically every other organism is gram-negative.
 Of the gram-positives, 2 are cocci, and the other 4 are rod-
shaped (bacilli).
 The 2 gram-positive cocci both have the word coccus
in their names:
1) Streptococcus forms strips of cocci.
2) Staphylococcus forms clusters of cocci.
 Two of the 4 gram-positive rods produce spores. These are:
3) Bacillus
4) Clostridium
 The last 2 gram-positive rods do not form spores:
5) Corynebacterium
6) Listeria 9

10. Gram-Negative Bacteria
 Of the gram-negative organisms, there is only one group of gram-
negative cocci. It is actually a diplococcus. Example: Neisseria spp.
 There is also just 1 group of spiral-shaped organisms: the
Spirochetes. This group includes the bacterium Treponema spp.,
Borrelia spp. and Leptospira spp.
 The rest are gram-negative rods or pleomorphic.
EXCEPTIONS
1) Mycobacteria are weakly gram-positive but stain better with a
special stain called the acid-fast stain
2) Spirochetes have a gram-negative cell wall but are too small to be
seen with the light microscope and so must be visualized with a
special darkfield microscope.
3) Mycoplasma do not have a cell wall. They only have a simple cell
membrane, so they are neither gram-positive nor gram-negative
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11. Based on Anatomical features
• Capsule
– Capsulate– Streptococcus pneumoniae
– Non-capsulate – Viridans streptococci
Flagella
– Flagellate – vibro cholerae
– Aflagellate – Shigella spp.
Spore
– Spore-forming – Bacillus spp.
– Non-sporing – Escherichia coli
•
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12. Based on Cultural characteristics
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• Extra growth factors requirements
– Fastidious – Hemophilus influenzae
– Non-fastidious – Escherichia coli
• Hemolysis on Sheep Blood Agar
– Alpha-hemolysis – Streptococcus pneumoniae
– Beta-hemolysis – Streptococcus pyogenes
• Utilization of carbohydrates
– Oxidative - Micrococcus
– Fermentative – Escherichia coli

13. Based on Cultural characteristics
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• Growth rate
– Rapid growers– Vibrio cholerae
– Slow growers – Mycobacterium tuberculosis
• Pigment production
– Pigment producer – Staphylococcus aureus
– Pigment non-producer – Escherichia coli

14. Based on Nutrition
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• Autotrophs: use inorganic sources, such
as ammonium and sulphide as energy
sources.
Heterotrophs: use organic carbon
sources as energy sources

15. Based on environmental factors
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• Temperature
• Oxygen dependence
• pH
• Salt concentration
• Atmospheric pressure

16. Temperature
• Psychrophiles (15-200
C) – Pseudomonas
fluorescens
• Mesophiles (20-400
C) – Escherichia coli,
Salmonella enterica, Staphylococcus aureus
• Thermophiles (50-600
C)- Bacillus
stearothermophilus
• Extremely thermophiles (as high as 2500
C)
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17. Oxygen dependence
• Aerobe (grow in ambient temperature, which
contains 21% O2 and a small amount of CO2
,
0.03%)
• Obligate aerobes – Strictly require O2for their
growth (Pseudomonas aeruginosa)
• Microaerophilic (grow under reduced O2
, 5-
10% and increased CO2
, 8-10%)- Campylobacter
jejuni, Helicobacter pylori
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18. Oxygen dependence
• Facultative anaerobe (capable of growing
either in presence or absence of O2
)- E. coli
• Obligate anaerobe – Clostridium spp.
• Capnophilic (require increased concentration
of CO2
, i.e., 5-10%) –
H. influenzae,
N. gonorrhoeae
• Aerotolerant 18

19. pH
• Acidophiles (Lactobacillus acidophilus)
• Alkaliphiles (Vibrio)
• Neutralophiles (pH 6-8)
Majority of the medically important bacteria grow
best at neutral or slightly alkaline reaction (pH
7.2-7.6)
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20. Salt concentration
• Halophiles
• Non-halophiles
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21. Other ways of classification
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•
•
•
•
Motile/Non-motile
Pathogenic/Non-pathogenic
Sensitive/Resistant (to particular antibiotic/ chemicals)
Lactose fermenter/Lactose non-fermenter

22. 5
Time (Minutes)
Bacterial Growth Curve
1 10
Lag phase
Stationary phase
Death
phase
Log phase
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