Bacteria are classified based on their phenotypic and genotypic characteristics. Phenotypically, bacteria are classified based on staining reactions like Gram staining which separates bacteria into gram-positive and gram-negative groups. Morphological characteristics like shape (cocci, bacilli, spiral) are also used. Genotypically, DNA-DNA hybridization and G+C content are used. Gram-positive bacteria are further classified based on whether they form spores, like Bacillus and Clostridium. Environmental factors like temperature, oxygen requirements, pH and salt tolerance also determine bacterial classification.

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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