Bacteria are a:
- Prokaryotic microorganism
- Single-celled
- Microscopic organisms
- Lack nuclei and other organized cell structure
- Several species are pathogenic (capable of causing disease)
- Most are non-infectious
- Have a role in decay, fermentation, nutrient recycling, and nitrogen fixation.
Bacteria are usually classified as gram-positive or gram-negative based on a basic microbiological staining procedure called the gram strain.
They come in a variety of shapes and sizes.
Shapes and sizes
- Cocci
Small, spherical, or oval (0.75-1.25µm). They are unicellular. Either they may remain as a single cell or may aggregate together for various configurations.
- Monococcus or Micrococcus: Single, discrete round. Ex. Micrococcus flavus.
- Diplococcus: Cell of diplococcus divides ones in a particular plane and after division; cells remain attached to each other. Ex. Diplococcus pneumonia.
- Streptococcus: The cells divide repeatedly in one plane to form a chain of cells. Ex. Streptococcus pyogens.
- Tetracoccus: Consists of 4 round cells, which defied in 2 planes at a right angle to one another. Ex. Goffkya tetragena.
- Staphylococcus: Cells are divided into 3 planes forming a structured likes bunches of grapes giving an irregular configuration. Ex. Staphylococcus aureus.
- Sarcina: Cells divide into 3 planes but form a cube-like configuration consisting of 8 or 17 cells but they have a regular shape.
- Bacilli
Bacillium meaning Stick. Rod-shaped or cylindrical. Either remains singly or in pairs. Its length is 2-10 times its width. For some bacilli, the length of a cell may be equal to the width. Those are called Coccobacilli (short bacilli). Ex. Bacillus cereus.
- Chain of bacilli: ex. Bacillus anthracis
- Flagellate rods: ex. Salmonella typhi
- Spore-former: ex. Clostridium botulinum
- Vibrios
The comma in shape or curved rods. They have a single genus. Ex. Vibrio cholerae, Vibrio comma.
- Spirals/Spirilla
Spiral or spring-like. Have multiple curvature and terminal flagella. These are longer rigid rods with several curves or coils. Helical in shape. Ex. Spirillum ruprem, Spirillum volutans, Helicobacter pyloria.
On mode on nutrition
- Phototrophs:
Those bacteria gain energy from light.
- Photolithotrophs: gain energy from light and uses reduced inorganic compounds such as H2S as an electron source. Ex. Chromatium okenii.
- Photoorganotrophs: gain energy from light and uses organic compounds such as succinate as an electron source.
- Chemotrophs:
Bacteria gain energy from chemical compounds. It cannot carry out photosynthesis.
- Chemolithotrophs: gain energy from the oxidation of chemical compounds and reduces inorganic compounds such as NH3 as an electron source. Ex. Nitrosomonas.
- Chemoorganotrophs: gain energy from the chemical compound and uses organic compounds such as glucose and amino acids as a source of an electron. Ex. Pseudomonas pseudoflava.
- Autotrophs:
Bacteria use CO2 as a sole source of carbon to prepare their food. There are 2 types on basis of energy utilized to assimilate CO2.
- Photoautotrophs: utilize light to assimilate CO2 basis of the electron source.
- Chemoautotroph’s: utilize chemical energy for the assimilation of CO2.
- Heterotrophs:
Bacteria use the organic compound as a carbon source. They cannot fix CO2. Most human pathogenic bacteria are heterotrophic.
- Simple heterotrophs have simple nutritional requirements.
- Fastidious heterotrophs: require special nutrients for their growth.
Basis of temperature requirements.
- Psychrophiles:
- Bacteria that can grow at 00C(min.) – 150C (opt.) – 200C (max.).
- they have polyunsaturated fatty acids in their cell membrane which gives fluid nature to cell membranes at a lower temperature.
- Ex. Virio psychroerythrus, Vibrio Marinus, Polaromonas vascular, Psychroflexus.
- Psychrotrophs / Facultative psychrophiles:
- Bacteria that can grow even at 00C but optimum temperature are 20-30oc.
- Mesophiles:
- Bacteria grow at 250C (min.) – 370C (opt.) – 400C.
- Most human pathogens are mesophilic.
- Ex. E.coli, salmonella, staphylococcus.
- Thermophiles:
- Bacteria best grow above 45oC.
- Contain saturated fatty acids in their cell membrane so their cell membrane does not become too fluid even at a higher temperature.
- Capable of growing in mesophilic range (400C) are Facultative thermophiles.
- True thermophiles (<450C) are Stenothermophiles
- Ex. Streptococcus thermophiles, Thermus aquaticus, Bacillus stenothermophiles.
- Hypethermophiles:
- Best above 800C.
- Mostly Archeobacteria is this type.
- The monolayer cell membrane of archeobacteria is more resistant to heat and they adapt to grow in higher temperatures.
- Ex. Thermodesulfobacterium, aquifex, Pyrolobus fumari, thermotoga.
On basis of oxygen requirement.
- Obligate aerobes:
- Requires O2 to live.
- Ex. Pseudomonas.
- Facultative anaerobes:
- can use O2, but can grow in its absence.
- Have a complex set of enzymes.
- Ex. E.coli, yeasts, and many intestinal bacteria.
- Obligate anaerobes:
- Cannot use O2, but tolerate the presence of O2.
- Can break down toxic forms of O2.
- Ex. Lactobacillus carries out fermentation regardless of O2 presence.
- Microaerophiles:
- Requires O2, but at low concentration.
- Sensitive to toxic forms of O2.
- Ex. Campylobacter.
On basis of pH of growth.
- Acidophiles:
- Grow best at an acidic pH. (below 7)
- The cytoplasm of these bacteria is acidic.
- Some acidophiles are thermophilic, called Thermoacidophiles.
- Ex. Thiobacillus thioxidans, Thiobacillus, ferroxidase.
- Alkaliphiles:
- Grow best at an alkaline pH. (above 7)
- The optimum pH of growth is 8.2
- Ex. Vibrio cholera
- Neutrophiles:
- Grow best at neutral pH. (6.5 – 7.5)
- Most of the bacteria grow at neutral pH.
- Ex. E.coli.