Bacteria come in a variety of shapes and arrangements. They are classified based on their morphology, which includes whether they are cocci (spherical), bacilli (rod-shaped), spirals, or other miscellaneous shapes. Bacteria are also classified based on other characteristics like their flagella, motility, nutritional requirements, temperature tolerance, pH tolerance, response to osmotic pressure, and salt concentration requirements. Morphological classification looks at bacterial cell shape, arrangement, cell wall composition, and presence of flagella or motility structures.

1. BACTERIA

2. • Bacteria are microscopic prokaryotic unicellular
living organisms found everywhere and can be
harmful, as in infections or beneficial, as in
fermentation or decomposition
• They occur as a few micrometer in length and have
a number of shapes

3. Classification of Bacteria
Based on….
• Shape
• Arrangement
• Composition of cell wall
• Flagella & Motility
• Nutrition
• Oxygen
• Temperature
• pH
• Osmotic pressure
• Water Activity
• Salt Concentration

4. MORPHOLOGICAL CLASSIFICATION
OF BACTERIA

5. Morphological classification of bacteria
• Bacteria have characteristic shapes & arrangements
• The common microscopic morphologies are
– COCCI (spherical or ovoid cells)
– BACILLI (rod-shaped cells)
– SPIRALS
– Miscellaneous
• RICKETTSIA
• ACTINOMYCETES (ACTINOBACTERIA)
• MYCOPLASMAS

6. Cocci (spherical or ovoid cells)
• “KOKKOS” means “BERRY”
• Single: Monococci Eg: Micrococcus flavus
• Pairs: Diplococci Eg: Streptococcus pneumoniae
• Chains:Streptococci Eg. Streptococcus pyogenes
• Groups: Consists of 4 round cells which defied in 2 planes at a
right angles to one another
– 4: Tetrads (Tetracoccus) Eg. Gaffkya tetragena
– 8: Sarcina Eg. Sarcina ventriculi, Sarcina lutea
– Grape-like clusters: Staphylococci Eg. Staphylococcus
aureus

9. Bacilli (rods)
• “BACULUS” means “ROD”
• Single: Monobacilli
– Bacillus cereus
• Pairs: Diplobacilli
– Klebsiella pneumonia
– K. rhinoscleromatis
– Coxiella burnetii
– Moraxella bovis
• Bacilli in chains: Streptobacilli
– Streptobacillus moniliformis
– Bacillus anthracis
• Bacilli in clusters Eg: Bacillus anthracis

10. • Coccobacilli
– Short and stumpy that they appear ovoid
– Haemophilus influenzae
– Gardnerella vaginalis
– Chlamydia trachomatis
• Palisades
– bend at the points of division following the cell divisions, resulting in a
palisade arrangement resembling a picket fence and angular patterns
that look like Chinese letters
– Corynebacterium diphtheriae
• Flagellated rods
– Salmonella typhi

11. Spirals
• Vibrios
– Comma-shaped or curved rod shaped bacteria with less
than one complete turn or twist in the cell
– Having vibratory mobility
– Example
• Vibrio cholerae

12. Spirals
• Spirilla
– Rigid spiral structure
– Spirillum with many turns can superficially resemble
spirochetes
– They do not have outer sheath and endoflagella, but have
typical bacterial flagella
– Examples
• Campylobacter jejuni
• Helicobacter pylori
• Spirillum winogradskyi

13. Spirals
• Spirochetes
– Spirochetes have a helical shape and flexible bodies
– They move by means of axial filaments, which look like
flagella contained beneath a flexible external sheath but
lack typical bacterial flagella
– Examples
• Leptospira species (Leptospira interrogans)
• Treponema pallidum
• Borrelia recurrentis

15. Based on Flagella
• Flagellated Bacteria & Non- Flagellated Bacteria
• Monotrichous
– One flagellum at one end of the cell
– Eg. Vibrio cholerae
• Lophotrichous
– Bunch of flagella or single flagellum at one end of the cell
– Eg. Spirilla spp.
• Amphitrichous
– Bunch of flagella or single flagellum arising from both ends
– Eg. Rhodospirillum rubrum
• Peritrichous
– Evenly distributed surrounding the entire bacterial cell
– Eg. Salmonella typhi (All members of Enterobacteriaceae family)
Atrichous
– No flagella
– Eg. Cornybacterium diptherae

16. Types of Motility
• Tumbling Motility
• Gliding Motility
• Stately Motility
• Darting Motility
• Swarming Motility
• Cork Screw Motility
Flagella &
Motility

17. Based on Nutritional Requirement
• Autotrophs – can synthesise all their organic compounds by
utilising atmospheric CO2 & N2
• Heterotrophs – unable to synthesise their own metabolites &
depend on preformed organic compounds
• Phototrophs – Bacteria which derive their energy from
sunlight
• Chemotrophs – Bacteria which derive energy from chemical
reactions
• Organotrophs – require organic sources of hydrogen
• Lithotrophs – require inorganic sources of hydrogen like NH3,
H2S

19. Based on Oxygen Requirement
• Obligate aerobes require O2 for growth; use O2 as a final electron acceptor
in aerobic respiration
• Obligate anaerobes (aerophobes) do not need or use O2 as a nutrient;
O2 - either kills or inhibits their growth. Obligate anaerobic procaryotes
may live by fermentation, anaerobic respiration, bacterial photosynthesis,
or the novel process of methanogenesis.
• Facultative anaerobes (or facultative aerobes) are organisms that can
switch between aerobic and anaerobic types of metabolism. Under
anaerobic conditions they grow by fermentation or anaerobic respiration,
but in the presence of O2 they switch to aerobic respiration.

20. Based on Oxygen Requirement
• Aerotolerant anaerobes are bacteria with an exclusively anaerobic
(fermentative) type of metabolism but they are insensitive to the presence
of O2. They live by fermentation alone whether or not O2 is present in
their environment.
• Microaerophiles require O2 but at levels below 0.2 atm

21. Based on Temperature Requirement
• Mesophiles: organisms with an optimum temperature near 37 degrees
(the body temperature of warm-blooded animals)
• Eg. Escherichia coli, Mycobacterium tuberculosis, Pseudomonas
aeruginosa, Streptococcus pyogenes
• Thermophiles: Organisms with an optimum T between about 45 degrees
and 70 degrees
• Eg. Thermoplasma acidophilum, Thermus aquaticus
• Extreme thermophiles or Hyperthermophiles: Some Archaea with an
optimum T of 80 degrees or higher and a maximum T as high as 115
degrees
• Eg. Desulfurococcus (85ᴼC), Methanothermus(83ᴼC)

22. Based on Temperature Requirement
• The cold-loving organisms are psychrophiles defined by their ability to
grow at 0 degrees
• A variant of a psychrophile (which usually has an optimum T of 10-15
degrees) is apsychrotroph, which grows at 0 degrees but displays an
optimum T in the mesophile range, nearer room temperature.

23. Effect of pH
• The pH, or hydrogen ion concentration, [H+], of natural environments
varies from about 0.5 in the most acidic soils to about 10.5 in the most
alkaline lakes
• Most free-living procaryotes can grow over a range of 3 pH units, about a
thousand fold change in [H+]
• The range of pH over which an organism grows is defined by three
cardinal points:
– Minimum pH, below which the organism cannot grow
– Optimum pH, at which the organism grows best
– Maximum pH, above which the organism cannot grow
Organism Minimum pH Optimum pH Maximum pH
Staphylococcus aureus 4.2 7.0-7.5 9.3
Escherichia coli 4.4 6.0-7.0 9.0
Pseudomonas
aeruginosa
5.6 6.6-7.0 8.0

24. • Based on pH, the microorganisms are categorized as follows.
– Acidophiles - grow at an optimum pH well below neutrality (7.0)
• Among eukaryotes, many fungi are acidophiles
• Among Archaea Eg. Sulfolobus and Thermoplasma
– Neutrophiles - grow best at neutral pH
– Alkaliphiles - grow best under alkaline conditions

25. Effect of Osmotic Pressure
• There is a wide range of osmotic tolerance found in bacteria. 0.5% NaCl is
added in culture media to provide suitable osmolarity
• PLASMOLYSIS = Sudden exposure to hypertonic solutions may cause
osmotic withdrawal of water and shrinkage of protoplasm. This occurs
more readily in gram negative than in gram positive bacteria.
• PLASMOPTYSIS = Sudden transfer from a concentrated solution to distilled
water may cause plasmoptysis (excessive osmotic imbibition leading to
swelling and rupture of the cell).

26. Effect of Water Availability
• The availability of water for a cell depends upon its presence in the
atmosphere (relative humidity) or its presence in solution or a substance
(water activity - Aw)
• Aw of pure H2O is 1.0 (100% water)
• Aw is affected by the presence of solutes such as salts or sugars that are
dissolved in the water
• The higher the solute concentration of a substance, the lower is the water
activity and vice-versa
• Microorganisms live over a range of Aw from 1.0 to 0.7

27. Based on their growth response to salt
• Halophiles (osmophiles) require some NaCl for growth
• Mild halophiles require 1-6% salt
• Moderate halophiles require 6-15% salt
• Extreme halophiles require 15-30% NaCl for growth (archaea)
• Halotolerant (osmotolerant) are the bacteria that are able to
grow at moderate salt concentrations, even though they grow
best in the absence of NaCl,
• osmophiles is usually reserved for organisms that are able to
live in environments high in sugar
• Xerophiles are the organisms which live in dry environments
(made dry by lack of water)

28. • The concept of lowering water activity in order to prevent
bacterial growth is the basis for preservation of foods by
drying (in sunlight or by evaporation) or by addition of high
concentrations of salt or sugar
• Minimum Aw for growth
– Pseudomonas, Salmonella, E. coli (0.91)
– Staphylococcus (0.85)

29. • Symbiosis: growth of one organism favours growth of another
organism
– Eg. Staphylococcus aureus streaked across Haemophilus influenza
favours its growth
• Antagonism: growth of one organism is detrimental to the
other
– Eg. Pseudomonas hampers growth of Gonococci