Types of Bacteria

SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 1
TYPES OF BACTERIA (BASED ON VARIOUS PARAMETERS)
Bacteria and Archae can be categorized based on the following parameters:
 Domain
 Optimum temperature
 Oxygen requirement
 Flagellar arrangement
 Mode of nutrition
 Optimal pH for growth
 Shape
 Spore formation
 Extreme nutrient conc.
 Salt requirement
 Gram staining
 Capsule
TYPES OF PROKARYOTES BASED ON DOMAIN
1. ARCHAEA
 Archaea / Arachaeobacteria are primitive unicellular prokaryotes that are found in
extreme environments (they are extremophiles) such as in deep-sea, hot springs,
alkaline or acid water.
 They have membrane lipids that are branched hydrocarbon chains attached to glycerol
by ether linkages.
 The presence of these ether linkages in Archaea adds to their ability to withstand
extreme temperatures and highly acidic conditions, although many archeae live in mild
environments.
 Archaea evolved many cell sizes, but all are relatively small. Their size ranges from 0.1
μm to 15 μm diameter and up to 200 μm long.
 They are about the size of bacteria, or similar in size to the mitochondria found in
eukaryotic cells.
 Members of the genus Thermoplasma are the smallest of the Archaea.
 Archae also differ from bacteria in their cell wall composition, i.e. their cell walls are
made up of pseudomurein, proteins, heteropolysaccharides, etc.
Figure: Cell walls of various archaea.
 Archaea are of three types:
o Methanogens: These archaea can produce methane (by reducing carbon dioxide)
and are obligate anaerobes. They are found in the intestinal tracts of animals and

pg. 2
humans, and sewage water. Examples include Methanococcus spp. and
Methanobacterium spp.
o Halophiles: These archaea are found in salt environments such as the Dead Sea and
the Great Salt Lake. Examples include Halococcus spp. and Halobacterium spp.
o Thermoacidophiles: These archaea thrive in extremely hot and acidic environments
such as volcanoes and hydrothermal vents. Examples include Sulfolobus spp. and
Thermoplasma spp.
2. EUBACTERIA:
 Eubacteria include all prokaryotes except Archaea.
 They are more complex and comparatively more modern than Archaea.
 They can live in both harsh conditions and normal conditions.
 Eubacteria membrane lipids are ester-linked.
 Eubacteria have photosynthetic members, as opposed to Archaea which don’t.
 Transcription factors are needed for protein synthesis in eubacteria, as opposed to
Archaea.
 Members of the genus Mycoplasma are the smallest eubacteria and the smallest living
cells (they are also the only bacteria that lack a cell wall).
 The largest bacterium ever discovered is Thiomargarita namibiensis is a Gram-negative
coccoid which is found in the ocean sediments of the continental shelf of Namibia. It is
0.1 to 0.3 mm (100 to 300 μm) in diameter, but can sometimes attain a size of 0.75 mm
(750 μm). The cells are large enough to be visible to the naked eye.
 The second-largest bacterium is Epulopiscium fishelsoni, or "epulo" for short, a Gram-
positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are
best known for their large size, ranging from 0.2 to 0.7 mm (200–700 μm) in length. It
grows slightly longer, but narrower than the biggest bacterium Thiomargarita
namibiensis.
Figures: (Left to Right) Colony of Mycoplasma spp. – the smallest bacterium, Epulopiscium
fishelsoni – the second-largest bacterium in comparison with Paramecium spp. (C), and
Thiomargarita namibiensis – the largest bacterium.

pg. 3
TYPES OF BACTERIA BASED ON MODE OF NUTRITION
NOTE: STUDY THESE POINTS BEFORE-HAND
Types of organisms based on energy source
 Phototrophs: Organisms that gain energy from light.
 Chemotrophs: Organisms that gain energy from chemical compounds.
Types of organisms based on electron source
 Lithotrophs: Organisms that use reduced inorganic substances (hydrogen-containing)
as their electron source.
 Organotrophs: Organisms that extract electrons from organic compounds (hydrogen-
containing).
Types of organisms based on carbon source
 Autotrophs: Organisms that make their food, by fixing CO2.
 Heterotrophs: Organisms that consume others for food, i.e use carbon-containing
molecules.
1. AUTOTROPHS:
 These bacteria use CO2 as the sole source of carbon to prepare their food.
 Autotrophs are of two types:
o Photo-lithotrophic Autotrophs: They are also called Photo-litho-autotrophs and
Photo-autotrophs. They perform photosynthesis. They get their energy from light,
electrons from reduced inorganic hydrogen-containing molecules, and carbon from
CO2, i.e. Cyanobacteria.
o Chemo-lithotrophic Autotrophs: They are also called Chemo-litho-autotrophs and
Chemo-autotrophs. They use inorganic molecules as their energy source, reduced
inorganic hydrogen-containing molecules as electron source, and CO2 as a carbon
source, i.e. Thiobacillus spp.
2. HETEROTROPHS:
 These bacteria use organic compounds as carbon source.
 They cannot fix CO2.
 Most of the human pathogenic bacteria are heterotrophic.
 Based on the source of energy, they are subdivided into two types:
o Photo-organotrophic Heterotrophs: They are also called Photo-organo-heterotrophs
and Photo-heterotrophs. They get their energy from light, electrons from reduced
organic hydrogen-containing molecules, and carbon from organic carbon sources,
i.e. Purple and green non-sulfur bacteria.
o Chemo-organotrophic Heterotrophs: They are also called Chemo-organo-
heterotrophs and Chemo-heterotrophs. They get their energy from organic
molecules, electrons from reduced organic hydrogen-containing molecules, and

pg. 4
carbon from organic carbon sources, i.e. most non-photosynthetic bacteria
(including most pathogens).
 Based on nutritional requirements, heterotrophs are subdivided into two types:
o Simple Heterotrophs: They have simple nutritional requirements.
o Fastidious Heterotrophs: They require special nutrients for their growth, i.e.
Neisseria spp. needs hemolyzed blood to grow.
TYPES OF BACTERIA BASED ON EXTREME NUTRIENT
CONCENTRATION (TOLERANCE)
1. OLIGOTROPHIC BACTERIA:
 These bacteria live in an environment that offers very low levels of nutrients.
 They are characterized by slow growth, low rates of metabolism, and generally low
population density.
 Their environments include deep oceanic sediments, caves, glacial and polar ice, deep
subsurface soil, aquifers, ocean waters, and leached soils.
 Examples: Pelagibacter ubique (the most abundant organism in the oceans); Collimonas
spp. (bacteria) is capable of living in the oligotrophic soil; the most abundant species in
the frozen soil (of Arctic and Antarctic) are Actinobacteria, Proteobacteria,
Acidobacteria, and Cyanobacteria.
2. COPIOTROPHIC BACTERIA:
 These bacteria are found in environments rich in nutrients, particularly carbon.
 They are the opposite of oligotrophs, which survive in much lower carbon
concentrations.
 They tend to grow in high organic substrate conditions, i.e. sewage lagoons.
 They grow in organic substrate conditions up to 100X higher than oligotrophs.
 Examples: Acidobacteria, β-Proteobacteria, Bacteroides spp., etc.
TYPES OF BACTERIA BASED ON OPTIMUM TEMPERATURE
1. PSYCHROPHILES:
 These Bacteria can grow at 0°C or below.
 The optimum temperature of growth is 15°C or below.
 The maximum tolerable temperature is 20°C.
 They have polyunsaturated fatty acids in their cell membrane which gives fluid nature to
the cell membrane even at low temperatures.
 Examples: Vibrio psychroerythrus, Vibrio marinus, Polaromonas vaculata, etc.
 Psychrotrophs: They are facultative psychrophiles, they can grow in cold temperatures
(0°C), but the optimum temperature for their growth is 20-30°C, they spoil refrigerated
goods, i.e. Pseudomonas spp., Bacillus spp., Clostridium spp., etc.

pg. 5
2. MESOPHILES:
 These bacteria can grow best between 25-40 °C.
 The optimum temperature for growth is 37°C.
 Most of the human pathogens are mesophilic.
 Examples: Escherichia coli, Salmonella spp., Klebsiella spp., Staphylococci spp., etc.
3. THERMOPHILES:
 These bacteria can best grow above 45°C.
 They contain saturated fatty acids in their cell membrane so their cell membrane does
not become too fluid even at higher temperature.
 There are three types of thermophiles:
o Facultative Thermophiles: These thermophiles are capable of growing in the
mesophilic range as well, i.e. Streptococcus thermophilus.
o Steno-thermophiles: These are obligate/true thermophiles and can't grow in the
mesophilic range, i.e. Thermus aquaticus.
o Hyper-thermophiles: These bacteria have optimum temperature of growth above
80°C, mostly archeobacteria are hyperthermophiles (monolayer cell membrane of
archeobacteria is more resistant to heat), i.e. Thermodesulfobacterium spp., Aquifex
spp., Pyrolobus fumari, Thermotoga spp., etc.
TYPES OF BACTERIA BASED ON OPTIMUM PH
1. ACIDOPHILES:
 These bacteria grow best in acidic pH (below 5.0).
 The cytoplasm of these bacteria is acidic.
 Examples: Thiobacillus thioxidans, Thiobacillus ferroxidans, etc.
 Thermoacidophiles: These acidophiles are also thermophilic, i.e. Thermoplasma spp.,
Sulfolobus spp., etc.
2. ALKALIPHILES:
 These bacteria grow best at alkaline pH (above 8.0).
 Example: Vibrio cholera (optimum pH = 8.2)
3. NEUTROPHILES:
 These bacteria grow best at neutral pH (6.5-7.5).
 These are the most common of the three types of bacteria (based on pH).
 Example: Escherichia coli
PAGE TURN OVER

pg. 6
TYPES OF BACTERIA BASED ON SALT REQUIREMENT
1. HALOPHILES:
 These bacteria require a high concentration of NaCl for growth.
 The cell membrane of halophilic bacteria is made up of glycoproteins with high content
of negatively charged glutamic acid and aspartic acids, hence a high concentration of Na+
ion concentration is required to shield the negative charge.
 Red Extreme Halophiles: These are archaeobacteria that can't grow with less than 12-
15% NaCl in their environment (they need it for the integrity of their cell walls and the
stability and activity of some of their enzymes). Their colonies have a red-orange color
due to the carotenoids present in their cells.
 Examples: Halobacterium spp., Halococcus spp., etc.
2. HALOTOLERANT:
 These bacteria do not require NaCl but can tolerate a low concentration of it, i.e. in the
environment or growth media.
 Example: Many cyanobacteria that live in hyper-saline waters.
TYPES OF BACTERIA BASED ON OXYGEN REQUIREMENT
1. OBLIGATE AEROBES:
 These bacteria require oxygen and cannot grow in its absence.
 They can only carry out oxidative type of metabolism.
 Examples: Mycobacterium spp., Bacillus spp., etc.
2. FACULTATIVE ANAEROBES:
 These bacteria do not require O2 but can use it if available.
 The growth of these bacteria becomes better in presence of O2.
 They can carry out both oxidative and fermentative type of metabolism.
 Examples: Escherichia coli, Klebsiella spp., Salmonella spp., etc.
3. AEROTOLERANT ANAEROBES:
 These bacteria do not require O2 for growth but can tolerate its presence.
 The growth of these bacteria is not affected by the presence of O2.
 They can only carry out a fermentative type of metabolism.
 Example: Lactobacillus spp.
4. MICROAEROPHILES:
 These bacteria require O2 for growth but only in lower concentrations (less than that of
the atmosphere).

pg. 7
 At the atmospheric level of oxygen, their growth is inhibited.
 They can only carry out oxidative type of metabolism.
 Capnophiles: These bacteria require carbon dioxide for growth (they thrive in high CO2
concentrations). Most of the micro-aerophiles are capnophilic in nature, i.e.
Campylobacter spp.
 Examples: Campylobacter spp., Helicobacter pylori, Brucella abortus, etc.
5. OBLIGATE ANAEROBES:
 These bacteria can only grow in the absence of oxygen.
 Oxygen is harmful to them.
 They can only carry out a fermentative type of metabolism.
 Examples: Clostridium spp., Methanococcus spp., etc.
Figure: Different types of bacteria, based on oxygen requirement.
TYPES OF BACTERIA BASED ON SHAPE
1. COCCI:
 These bacteria are spherical or oval.
 Based on arrangement, cocci are further classified as:
o Coccus: A single coccus.
o Diplococcus: Cocci in pair, i.e. Streptococcus pneumonia.
o Streptococcus: Cocci in chain, i.e. Streptococcus salivarius.
o Tetrad: Cocci in a group of four, i.e. Tetragenococcus spp.
o Sarcina: Cocci in cubical arrangement of cell, i.e. Sarcina ureae.
o Staphylococcus: Cocci in bunch (irregular), i.e. Staphylococcus aureus.

pg. 8
Figure: Arrangements of cocci.
2. BACILLI:
 These are rod-shaped bacteria.
 Based on arrangement, bacilli are further classified as:
o Bacillus: A single rod-shaped bacillus, i.e. Bacillus cereus.
o Diplobacilli: A pair of bacilli joined end to end, i.e. Klebsiella rhinoscleromatis.
o Streptobacilli: It is a chain of rod shape bacteria, i.e. Bacillus subtilis.
o Palisades: The bacilli 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, i.e. Corynebacterium diphtheriae.
o Cocco-bacilli: Their shape is intermediate to cocci and bacilli, i.e. Haemophilus
influenzae.
Figure: Arrangements of bacilli.

pg. 9
3. SPIRAL SHAPED BACTERIA:
 These are curved and spiral-shaped bacteria.
 They are of the following types:
o Vibrio: These are comma-shaped bacteria
with less than one complete turn or twist
in the cell, i.e. Vibrio cholarae.
o Spirilla: They have a rigid spiral structure,
they lack an outer sheath and endoflagella,
but have typical bacterial flagella, i.e.
Campylobacter jejuni.
o Spirochetes: They have a helical shape and
flexible body, they move using axial
filaments, which look like flagella, i.e.
Treponema pallidum.
Figure: Types of spiral-shaped
bacteria.
4. FILAMENTOUS BACTERIA:
 They have filamentous or branching structure
(mycelium like structure).
 They resemble more closely to fungi than
bacteria.
 Examples: Streptomyces spp., Candidatus
spp., etc. Figure: Filamentous structure of
bacteria.
5. STAR SHAPED BACTERIA:
 These bacteria have a star-like shape.
 Example: Stella spp.
Figure: Star-shaped bacteria.
6. RECTANGULAR SHAPED BACTERIA:
 These bacteria have a rectangular shape.
 Example: Haloarcula spp.
Figure: Rectangular shaped bacteria.

pg. 10
7. PLEOMORPHIC BACTERIA:
 These bacteria do not have any characteristic
shape.
 They can change their shape.
 They lack a cell wall.
 Example: Mycoplasma pneumonia
Figure: Pleomorphic bacteria.
TYPES OF BACTERIA BASED ON GRAM STAINING
1. GRAM-POSITIVE BACTERIA:
 The cell wall of these bacteria is mainly composed of peptidoglycan.
 In gram staining, they retain crystal violet dye (primary dye) and stain dark violet or
purple, they remain colored with gram stain when washed with alcohol solution.
 They have virtually no lipid or lipoprotein content in their cell walls.
 Teichoic acids may be present.
 Periplasmic space is absent.
 They mainly produce exotoxins.
 They are much more susceptible to antibiotics.
 Examples: Staphylococcus spp., Streptococcus spp., Micrococcus spp., etc.
2. GRAM-NEGATIVE BACTERIA:
 The cell wall of these bacteria is composed mainly of lipopolysaccharides.
 In gram staining, they can be decolorized to accept counterstain (Safranin or Fuchsine)
and stain red or pink, they don't retain the gram stain when washed with alcohol
solution.
 They have a high lipid and lipoprotein content.
 Teichoic acids are absent.
 Periplasmic space is present.
 They mainly produce endotoxins.
 They are comparatively more resistant to antibiotics.
 Examples: Escherichia coli, Salmonella spp., etc.
3. GRAM VARIABLE BACTERIA:
 These bacteria, after staining with the Gram stain, yield a Gram-variable pattern: a mix
of pink and purple cells are seen.

pg. 11
 It is caused by a decrease in peptidoglycan thickness during growth, which coincides
with an increase in the number of cells that stain Gram-negative.
 Examples: Bacillus spp., Butyrivibrio spp., and Clostridium spp.
4. GRAM INDETERMINATE BACTERIA:
 These bacteria do not respond predictably to Gram staining.
 They cannot be determined as either Gram-positive or Gram-negative, but can be
detected via acid-fast staining, hence they are referred to as "acid-fast bacteria".
 Examples: Many species of the genus Mycobacterium, including M. tuberculosis and M.
leprae.
Figure: Comparison between Gram-positive and negative bacteria.

pg. 12
TYPES OF BACTERIA BASED ON ARRANGEMENT OF FLAGELLA
1. MONOTRICHOUS BACTERIA:
 These bacteria have a single flagellum at one end of
the cell.
 Examples: Vibrio cholera, Pseudomonas
aerogenosa, etc.
2. LOPHOTRICHOUS BACTERIA:
 These bacteria have a bundle of flagella at one end
of the cell.
 Example: Pseudomanas fluroscence
3. AMPHITRICHOUS BACTERIA:
 These bacteria either have a single flagellum or a
cluster of flagella at both ends of the cell.
 Example: Aquaspirillium spp.
4. PERITRICHOUS BACTERIA:
 These bacteria have flagella all over the cell surface.
 Examples: Escherichia coli, Salmonella spp.,
Klebsiella spp., etc.
5. ATRICHOUS BACTERIA:
 These bacteria don’t possess any flagella.
 Example: Shigella spp.
PAGE TURN OVER

pg. 13
TYPES OF BACTERIA BASED ON SPORE FORMATION
1. SPORE FORMING BACTERIA:
 These bacteria produce spores during unfavorable conditions.
 These are comparatively tougher.
 They are mostly gram-positive.
 They are generally pathogenic.
 They are further subdivided into two groups:
o Endospore Forming Bacteria: They produce spores within the bacterial cell, i.e.
Bacillus spp., Clostridium spp., Sporosarcina spp., etc.
o Exospore Forming Bacteria: They produce spores outside the cell body, i.e.
Methylosinus spp.
2. NON-SPORE FORMING BACTERIA:
 These bacteria do not produce spores.
 They are not as tough as spore-forming bacteria.
 They are mostly gram-negative.
 They are not generally pathogenic.
 Examples: Escherichia coli, Salmonella spp., etc.
TYPES OF BACTERIA BASED ON CAPSULE
1. CAPSULAR BACTERIA:
 These bacteria have a polysaccharide
layer that lies outside the cell envelope.
 Sometimes, the secretion that makes
up the capsule, diffuses into the
surrounding medium and forms a loose
matrix known as a slime layer.
 Capsule and slime layers are sometimes
summarized under the term glycocalyx.
 Capsule is a virulence factor, i.e. it
enhances the ability of bacteria to
cause disease (e.g. prevents
phagocytosis).
Figure: A typical capsular bacterium.
 Capsules also contain water which protects the bacteria against desiccation.
 They also exclude bacterial viruses and most hydrophobic toxic materials such as
detergents.
 Capsules also help cells adhere to surfaces.

pg. 14
 Examples: Gram-negative capsular bacteria include: Escherichia coli (in some strains),
Neisseria meningitides, Klebsiella pneumonia, and Haemophilus influenzae. Gram-
positive capsular bacteria include Bacillus megaterium, Streptococcus pyogenes,
Streptococcus pneumonia, and Staphylococcus aureus.
2. NON-CAPSULAR BACTERIA:
 These bacteria lack a capsule.
 They can hide in complex biofilms with other organisms that produce a capsule or
polysaccharide matrix.
 Alternatively, they may produce surface proteins that are antiphagocytic for protecting
themselves.
 Examples: Most Gram-negative Haemophilus influenzae strains are unencapsulated.

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