Classification of Bacteria

Classification is the organization of organisms into different groups on the basis of either phenotypic similarity or evolutionary relationships. In the hierarchical nature of classification, a species is made up of strains, and similar species are grouped into genera (genus-singular). Similar genera are grouped into families, and families into orders, orders into classes, up to the domain (the highest level of biological classification).  The three domains of biological organisms are the Bacteria, the Archaea, and the Eukarya.

Unlike viruses, there is no formal system for the classification of bacteria, and the classification system most widely accepted by microbiologists is that of Bergey’s Manual of Systematic Bacteriology. According to the binomial Linnean scheme, bacteria are classified into genera and species. For example, in the case of Staphylococcus aureus, Staphylococcus is the name of the genus, and aureus is the species designation.

For convenience, bacteria have been classified into various classes on the basis of phylogenetic, morphological, or physiological (biochemical) relatedness.

Classification of bacteria on the basis of the cell wall and staining reaction

The nature of the cell wall (i.e., is it rigid, flexible, or absent?) is the primary criterion used in the classification of bacteria. On the basis of cell wall characteristics, bacteria are of three types, those with rigid cell walls (most of the bacteria), those with flexible, thin walls (the spirochetes), and those without cell walls (the mycoplasma).

1. Wall-less cells: Mycoplasma
2. Flexible, thin-walled cells (spirochetes): Treponema, Borrelia, Leptospira
3. Rigid, thick-walled cells: Most of the free-living bacteria (Gram-positive, Gram-negative, and acid-fast) and obligate intracellular parasites such as Rickettsia

Bacteria with rigid, thick walls can be subdivided into free-living bacteria and non-free-living bacteria (obligate intracellular parasites). Free-living bacteria are further subdivided according to shape and staining reactions into a variety of gram-positive and gram-negative rods with different oxygen requirements and other attributes (spore-forming, capsule forming, flagella, etc.).

Gram-Positive Bacteria

• Gram-positive cocci: Streptococcus, Staphylococcus
• Gram Positive spore-forming rods
  • Aerobic: Bacillus
  • Anaerobic: Clostridium
• Non-spore forming rods
  • Non-filamentous: Corynebacterium, Listeria
  • Filamentous: Actinomyces, Nocardia

Gram-Negative Bacteria

1. Gram-negative cocci: Neisseria
2. Gram negative coccobacilli: Haemophilus, Bordetella, Legionella, Brucella, Francisella, Pasteurella, Yersinia
3. Gram-negative rods (Enterics): Escherichia, Enterobacter, Serratia, Klebsiella, Salmonella, Shigella, Proteus
4. Gram-negative curved rods: Campylobacter, Helicobacter, Vibrio

Classification of bacteria on the basis of oxygen requirements

Based on the oxygen requirement, bacteria are classified into four types: aerobes, facultative anaerobes, anaerobes, and microaerophiles.

1. Aerobic bacteria (aerobes): Aerobic bacteria can grow when incubated in atmospheric air containing 21% oxygen. E.g., Bacillus spp., Pseudomonas, Micrococcus, Staphylococcus spp.
2. Anaerobic bacteria (anaerobes): Anaerobic bacteria cannot grow in atmospheric air. Oxygen is toxic to them.
   • Bacteria that can tolerate low oxygen levels are called non-stringent or aerotolerant anaerobes. E.g., Lactobacillus, Streptococcus pyogenes, etc.
   • Bacteria that cannot tolerate low oxygen levels are called strict or obligate anaerobes. E.g.: Gram-positive (Clostridium, Propionibacterium, Bifidobacterium), Gram-negative: (Bacteroids, Fusobacterium)
3. Facultatively anaerobic bacteria: These bacteria do not require oxygen for growth, nor is oxygen inhibitory. If oxygen is present, they may use it for energy production. E.g., E. coli, Klebsiella, Salmonella, Shigella, Proteus, Vibrio cholerae etc.
4. Microaerophilic bacteria: They cannot tolerate the atmospheric air but require oxygen at a low level of about 2-10% for growth. E.g., Treponema pallidum, Streptococcus pneumoniae, Helicobacter pylori, Campylobacter, Streptococcus pyogenes, and Borrelia burgdorferi. 

Classification of bacteria on the basis of pH Requirements

Based on the pH requirement, bacteria are classified into acidophiles, neutrophiles, and alkalophiles.

1. Acidophiles: These bacteria grow at low pH with the pH optimum of pH 0 to 5.5. E.g., Sulfolobus, Picrophilus, Ferroplasma, Cyanidium caldarium
2. Neutrophiles: These bacteria grow at neutral pH with a pH optimum of 5.5 to 8.0. E.g.,Escherichia coli, Salmonella etc.
3. Alkalophiles: These bacteria grow at high pH with a pH optimum of 8.0 to 11.5. E.g., Bacillus alcalophilus, Natronobacterium

Classification of bacteria on the basis of temperature requirements

Based on the temperature requirement for growth, bacteria are classified into five types: psychrophiles, psychrotrophs, mesophiles, thermophiles, and hyperthermophiles.

1. Psychrophiles: The minimum growth temperature of psychrophiles is 0°C, the optimum is 15°C, and the maximum is 20°C. E.g., Pseudomonas, Vibrio, Alcaligenes, Bacillus, Arthrobacter, Moritella, Photobacterium, and Shewanella.
2. Psychrotrophs: The minimum growth temperature of psychrotrophs is 0°C-7°C, the optimum is 20-30°C, and the maximum is 35°C. They are also called facultative psychrophiles. (Xanax) Psychrotrophic bacteria spoil the foods kept in a refrigerator. E.g., Listeria monocytogenes, Pseudomonas fluorescens.
3. Mesophiles: The minimum growth temperature is 15°C-20, the optimum is 37°C, and the maximum is about 45°C. Most human pathogens are mesophiles. E.g., Escherichia coli, Neisseria gonorrhoeae etc. 
4. Thermophiles: The minimum growth temperature is 45°C, and the optimum is 55- 65°C. Thermophiles are found in composts, self-heating hay stacks, hot water lines, and hot springs. E.g., Bacillus stearothermophilus, Thermus aquaticus, etc.
5. Hyperthermophiles: The minimum growth temperature is 55°C, and the optimum temperature is 80°C-113°C. E.g., Sulfolobus, Pyrococcus, etc. 

Classification of bacteria on the basis of salt and tolerance

Based on the salt requirement, bacteria are classified into halophilic bacteria (halophiles) and non-halophiles.

1. Non-halophiles: Bacteria that cannot survive in this saline condition. It can grow in less than 0.2 M, i.e., less than 1% of NaCl.
2. Halophilic bacteria (halophiles): Bacteria that can grow in the presence of salt (NaCl).

Based on salt tolerance, halophiles are divided into the following types:

• Halotolerant bacteria: Bacteria that can grow at moderate salt concentrations and also grow the best well in the absence of NaCl. E.g., Staphylococcus aureus (can tolerate 5-10% NaCl)
• Slight halophiles: Optimum salt concentration is 0.2-0.85M (1-5%) NaCl. E.g., Halomonas
• Moderate halophiles: Optimum salt concentration is 0.85-3.4M (5-20%) NaCl. E.g., Bacillus, Marinococcus
• Extreme halophiles: Optimum salt concentration is 3.4-5.1M (20-30%) NaCl. E.g., Halobacterium, Haloarcula, Halococcus

Classification of bacteria on the basis of nutritional requirements

Based on the mode of obtaining nutrition, i.e., source of carbon, energy, and electron, bacteria are classified into the following types.

Source of Carbon  

• Autotrophic bacteria (autotrophs): Bacteria can make their organic compounds themselves. Its principal source of carbon utilization is CO_2. E.g., Purple and green sulfur bacteria
• Heterotrophic bacteria (heterotrophs): Bacteria depend on the others performed organic compounds. E.g, Escherichia coli, Salmonella Typhi, Proteus spp., Staphylococcus aureus, Lactobacillus acidophilus, Azospirillum spp. etc.

Source of Energy

• Phototrophs: They can utilize light as their source of energy. E.g, Chromatium okenii, Rhodospirillum rubrum etc.
• Chemotrophs can obtain energy from the oxidation of organic or inorganic compounds. E.g., Nitrosomonas, Pseudomonas pseudoflava

Source of Electrons

• Lithotrophs: They use inorganic compounds as the electron source. E.g., Nitrobacter, Thiobacillus denitrificans, Nitrosomonas etc.
• Organotrophs: They use organic compounds as the electron source. E.g., Pseudomonas pseudoflava

Classification of bacteria on the basis of shape and arrangement

Bacteria are classified into five groups based on the basis of their shapes. They are; cocci (round), bacilli (rod), spiral, vibrios (comma), and spirochetes.

You can find more about the shapes of bacterial cells and their arrangements in this post: Size, Shape, Arrangement of Bacteria

Classification of bacteria on the basis of flagella

Bacteria are classified into two groups based on the presence of flagella: atrichous (non-flagellated) and flagellated bacteria. Flagellated bacteria are further classified into monotrichous, lophotrichous, amphitrichous, and peritrichous based on flagellar arrangements. Flagella (singular: flagellum) is a locomotory organ of bacteria.

1. Flagella absent: Atrichous bacteria. E.g., Staphylococcus aureus 
2. Flagella present: It is further classified into different types based on the number of the flagella and their location in the body:

• Monotrichous: flagellum is present only at one end of bacteria, i.e., polar. E.g., Pseudomonas aeruginosa
• Lophotrichous: a cluster of flagella is present only at one end of bacteria. E.g., Pseudomonas fluorescens
• Amphitrichous: flagella are present in both ends of the bacteria in single or clusters. E.g., Aquaspirillum serpens
• Peritrichous: flagella are present all around the body. E.g., Salmonella Typhi.

Ability to form spores

Based on the presence and absence of spores, bacteria are classified into non-spore formers and spore former. In unfavorable conditions like desiccation, bacteria form the spores which is a dormant form. When the condition gets normal, it germinates and forms the vegetative cell.

1. Non-spore formers:  E. coli, Staphylococcus aureus
2. Spore former: Bacteria produces two types of spores: exospores and endospores
   • Exospores: Spore is produced outside of the cell. E.g., Methylosinus
   • Endospores: Spore is produced inside the cell but at different locations.

Oval and central spore E.g., Bacillus spp.

Oval and sub-terminal spore: E.g., Clostridium spp. except C. tetani (round and terminal) and C. bifermentans (oval and central)

Classification of bacteria on the basis of capsule

Based on the presence of capsules, bacteria are classified into two types: non-capsulated bacteria and capsulated bacteria.

1. Non-capsulated bacteria: absence of capsule. E.g., Mycobacterium tuberculosis, Shigella spp. 
2. Capsulated bacteria: presence of a capsule. Examples, Streptococcus mutans, Klebsiella pneumoniae, and Bacillus anthracis.

References

• Madigan, M. T., Martinko, J. M., Stahl, D. A., & Clark, D. P. (2011). BROCK Biology of Microorganisms (13th edition). Benjamin Cumming.
• Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th edition). Tata McGraw-Hill.