Fundamentals of Microbiology

2.4.6 Chemical requirements

Carbon

One of the most important requirements for microbial growth is the source of carbon. According to their source of energy and source of carbon microorganisms can be classified. 

Combining the energy and carbon source the organisms an be classified as follows:

Nutritional type	Energy source	Carbon source	Example
Photoautotrophs	Light	CO2	Photosynthetic bacteria cyanobacteria, algae, plants
Photoheterotrophs	Light	Organic compounds	Green non- sulphur bacteria
Chemoautotrophs	Electrons from inorganic compounds	CO2	Nitrifying bacteria, sulphur bacteria, iron bacteria
Chemoheterotrophs	Electrons from organic compounds	Organic compounds	Most bacteria all fungi, protozoa and animals.

Nitrogen, Sulfur and Phosphorus

Nitrogen, sulfur and phosphorus are needed by microbes for synthesis of cellular materials.

Protein synthesis requires nitrogen and sulfur

DNA or RNA synthesis requires nitrogen and phosphorus

ATP synthesis requires nitrogen and phosphorus

Nitrogen makes up about 12 – 15% of the dry weight of bacterial cells; sulfur and phosphorous together constitute 3%.

Many bacteria derive nitrogen by decomposing protein containing material

Some bacteria derive nitrogen from ammonium ions in organic material

Some bacteria derive nitrogen from nitrates

Some bacteria derive nitrogen directly from the atmosphere (N-fixation)

Important natural sources of sulfur include the sulphate ion, H₂S and sulfur containing amino acids. Important source of phosphorous is the phosphate ion (PO₄^{- -}). Other elements that are required by microorganisms are potassium, magnesium, calcium, etc.

Trace elements

Microorganisms require very small amounts of other minerals such as iron, copper, molybdenum and zinc. These are referred to as trace elements. Most of the trace elements are essential for activity of enzymes.

Oxygen

On the basis of oxygen requirements, organisms are classified into five different categories.

1. Obligate aerobes
2. Facultative anaerobes
3. Obligate anaerobes
4. Aerotolerant anaerobes
5. Microaerophiles

Organisms that require oxygen to live are called obligate aerobes. Facultative anaerobes are those that can grow both in the presence and absence of oxygen. In fact, they do not require oxygen for growth but grow better in its presence. Eg. E.coli. Obligate anaerobes are those that do not tolerate oxygen at all and die in its presence. Only in the absence of oxygen they can grow. Eg. Clostridium spp. Aerotolerant anaerobes do not use oxygen but can tolerate it. They grow equally well whether it is present or not. Microaerophilic organism are basically aerobes but grow only in oxygen concentrations lower that those in air. The normal atmosphere oxygen level is about 20%. These microaerophiles grow at a range of 2 to 10%.

Generally organisms can be harmed by oxygen. Oxygen is toxic because of the production of toxic forms like Hydrogen peroxide (H₂O₂), super oxide radical (O₂-) and hydroxyl radical (OH). Aerobes, facultative anaerobes, and microaerophiles must possess enzymes that destroy the toxic forms of oxygen namely super oxide dismutase and either catalase or per oxidase

2O₂- + 2H — O₂ + H₂O₂

   Super oxide dismustase

2H₂O₂ — 2H₂O + O₂

       Catalase

H₂O₂ + NADH+H^{+ —} 2H₂O + NAD⁺

                     Peroxidase