Bacteria are prokaryotic organisms that do not contain chlorophyll. They are unicellular and do not show true branching. They differ from eukaryotes in not having a nuclear membrane, a nucleolus, and cell organelles like mitochondria, golgi apparatus and endoplasmic reticulum. They have a single circular chromosome.
For growth and multiplication of bacteria, they require nutrition include a source of carbon, nitrogen, hydrogen, Oxygen and some inorganic salt. Water is the vehicle of entry of all nutrients into the cell and for the elimination of waste products. The bacterial cell contains water (80% of total weight), proteins, polysaccharides, lipids, nucleic acids, mucopeptides and low molecular weight compounds.
Bacteria can be classified nutritionally based on their energy requirements and on their ability to synthesize essential metabolites.
On the basis of requirements for bacterial growth, these are divided into two categories on the basis of
A. Physical (Temperature & osmotic pressure)
B. Chemical (source of C, N, S, P, O, trace elements & organic growth factors)
Classification of bacteria on the basis of their preferred range of temperature
Bacteria can be classified into the following major types on the basis of their temperatures response as indicated below:
1.Psychrophiles:
Bacteria that can grow at 0°C or below but the optimum temperature of growth is 15 °C or below and maximum temperature is 20°C are called psychrophiles. Psychrophiles have polyunsaturated fatty acids in their cell membrane which gives fluid nature to the cell membrane even at lower temperature.
Examples: Vibrio psychroerythrus, vibrio marinus, Polaromonas vaculata, Psychroflexus.
2.Psychrotrops (facultative psychrophiles):
Those bacteria that can grow even at 0°C but optimum temperature for growth is (20-30)°C
3. Mesophiles:
Those bacteria that can grow best between (25-40)oC but optimum temperature for growth is 37oC Most of the human pathogens are mesophilic in nature.
Examples: E. coli, Salmonella, Klebsiella, Staphylococci.
4. Thermophiles:
Those bacteria that can best grow above 45C. Thermophiles capable of growing in mesophilic range are called facultative thermophiles.True thermophiles are called as Stenothermophiles, they are obligate thermophiles. Thermophils contains saturated fattyacids in their cell membrane so their cell membrane does not become too fluid even at higher temperature.
Examples: Streptococcus thermophiles, Bacillus stearothermophilus, Thermus aquaticus.
5. Hypethermophiles:
Those bacteria that have optimum temperature of growth above 80C. Mostly Archeobacteria are hyperthermophiles.
Monolayer cell membrane of Archeobacteria is more resistant to heat and they adopt to grow in higher tremperature.
Examples: Thermodesulfobacterium, Aquifex, Pyrolobus fumari, Thermotoga
- pH: suitable for most bacteria- 6.5-7.5
- Acidophiles these bacteria tolerate the acidity so can group can grow in the acidic medium.
- Optimum pH for growth of yeast and mould:-5-6
Classification of bacteria on the basis of Nutrition
A. Autotrophs / lithotroph
B. Heterotrophs
Autotrophs
Bacteria that can synthesize all their organic compounds are called autotrophs. They are able to use atmospheric carbon dioxide and nitrogen. They are capable of independent existence in water and soil.
Phototrophs
Bacteria that derive energy from sunlight are called phototrophs.
Chemotrophs
Those that obtain energy from chemical reactions are called chemotrophs.
Heterotrophs
Some bacteria are unable to synthesize their own metabolites. They depend on preformed organic compounds. They are
called heterotrophs.
Classification of bacteria on the basis of oxygen requirement
1. Obligate aerobes
Bacteria/organism that requires oxygen to live are called obligate aerobes oxygen must
2. Facultative anaerobes
The such-type organism can grow in the presence or absence of oxygen but greater growth in the presence of oxygen. E.coil
3. Obligate anaerobes
Such Microorganisms are unable to use oxygen for growth. Growth is restricted in the presence of oxygen (cannot tolerate oxygen), only grow under anaerobic. These bacteria obtain oxygen atoms from water.
Example: Clostridium bacteria that cause tetanus and botulism.
4. Aerotolerant anaerobes
Aerotolerant anaerobes do not require oxygen for grow but tolerate oxygen fairly well
Example lactic acid-producing lactobacillus
5. Microaerophiles
These are aerobic required (required oxygen but in lower concentration) but grow only in oxygen concentration lower than those in air.
Capnophilic:-
Some bacteria like Brucella abortus require much higher levels of carbon dioxide (5-10%) for growth. They are called capnophilic.
In case of aerobes, atmospheric oxygen is the final electron acceptor in the process of respiration (aerobic respiration). Anaerobic bacteria use compounds like nitrates or sulphates instead of oxygen as final electron acceptors in the process of respiration (anaerobic respiration).
oxidative phosphorylation & anaerobic respiration
In this case, the carbon and energy source completely oxidised to carbon dioxide and water. Energy is provided by the production of energy-rich phosphate bonds and the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). This process is called oxidative phosphorylation.
Anaerobic bacteria use compounds like nitrates or sulphates instead of oxygen as final electron acceptors in the process of respiration (anaerobic respiration). A more common process used by these bacteria in anaerobic metabolism is fermentation. It is defined as the process by which complex organic compounds, such as glucose, are broken down by the action of Enzymes into simpler compounds without the use of oxygen. During the process of fermentation, energy-rich phosphate bonds are produced by the introduction of organic phosphate into intermediate metabolites. This process is known as substrate-level phosphorylation.
Classification of bacteria on the basis of pH of Growth
Acidophiles:
- These bacteria grow best at an acidic pH.
- The cytoplasm of these bacteria are acidic in nature.
- Some acidopiles are thermophilic in nature, such bacteria are called Thermoacidophiles.
- Examples: Thiobacillus thioxidans, Thiobacillus, ferroxidans, Thermoplasma, Sulfolobus
Alkaliphiles:
- These bacteria grow best at an alkaline pH.
- Example: Vibrio cholerae optimum ph of growth is 8.2.
Neutrophiles:
- These bacteria grow best at neutral pH (6.5-7.5).
- Most of the bacteria grow at neutral pH.
- Example: E. coli
Classification of bacteria on the basis of Osmotic Pressure Requirement
Halophiles
- Require moderate to large salt concentrations.
- Cell membrane of halophilic bacteria is made up of glycoprotein with high content of negatively charged glutamic acid and aspartic acids. So high concentration of Na+ ion concentration is required to shield the -ve charge.
- Ocean water contains 3.5% salt. Most such bacteria are present in the oceans.
- Archeobacteria, Halobacterium, Halococcus.
Extreme or Obligate Halophiles:
- Require a very high salt concentrations (20 to 30%).
- Bacteria in Dead Sea, brine vats.
Facultative Halophiles:
- Do not require high salt concentrations for growth, but tolerate upto 2% salt or more.
Classification of bacteria on the basis of Number of Flagella
- Atrichos: – These bacteria has no flagella. Example: Corynebacterium diptherae.
- Monotrichous: – One flagellum is attached to one end of the bacteria cell. Example: – Vibro cholerae.
- Lophotrichous: – Bunch of flagella is attached to one end of the bacteria cell.
Example: Pseudomonas. - Amphitrichous: – Bunch of flagella arising from both end of the bacteria cell.
Example: Rhodospirillum rubrum. - Peritrichous : – The flagella are evenly distributed surrounding the entire bacterial cell.
Example: Bacill
Important Key Point
- Depending on requirements of temperature for growth, bacteria can be classified as mesophilic (25-40ºC), psychrophilic (below 20ºC) and thermophilic (55-80ºC).
- Moisture and drying, hydrogen ion concentration, light, osmotic effect and mechanical and sonic stress may also influence the growth and multiplication of bacteria.
- Bacteria divide by binary fission.
- When the bacterial cell reaches a certain size, it divides to form two daughter cells.
- Nuclear division is followed by cell division.
- The time interval between two cell divisions is the generation time or the population doubling time. It may vary from 20 minutes (coliform bacilli) to 20 hours (tubercle bacilli) to 20 days (lepra bacilli).
- The bacterial growth curve consists of a lag phase, a log phase, a stationary phase and a decline phase. This is seen in a liquid medium.
- In the lag phase, the bacteria adapt to the environment. There is no appreciable increase in cell number.
- In the log phase, there is an exponential increase in the number of bacterial cells.
- In the stationary phase, there is no increase or decrease in the number of bacterial cells.
- In the decline phase, there is a decrease in the bacterial population due to cell death.
