In this article we will discuss about the harmful and useful activities of bacteria.
Some bacteria are harmful to human affairs in different ways:
a. Pathogenic Bacteria:
These bacteria cause great losses to plant and animal population by causing several diseases. Some important bacterial diseases of plants and animals, including human beings is given below (Table 2.6-2.8 and 2.11).
b. Reduction of Soil Fertility:
Some facultative anaerobic bacteria are available mostly in the oxygen deficient soil which reverse the nitrifying process, thereby causing the loss of a part of its combined nitrogen. They break down the nitrates in a stepwise manner either to ammonia through assimilatory nitrate reduction or to N2 through dissimilatory nitrate reduction.
The two pathways have a common intermediate, the nitric oxide:
The free nitrogen goes to the atmosphere, resulting in the reduction of soil fertility by loosing valuable N2 source, the nitrate. The bacteria those cause denitrification are called denitrifying bacteria, e.g., Bacillus cereus, Paracoccus denitrificans, and Pseudomonas aeruginosa.
c. Spoilage of Food:
In favourable temperature and humidity, bacteria can grow luxuriantly in many food materials. They change the flavour, appearance and smell of food.
Different species of Streptococcus, Micrococcus and Lactobacillus cause spoilage of milk and different milk products. The exotoxin produced by Clostridium botulinum causes botulism disease showing the symptoms like swelling of tongue, double vision and respiratory disturbances.
d. Pollution of Water:
Some bacteria pollute water and make it unsuitable for drinking. Diseases like cholera (Vibrio cholerae), typhoid fever (Salmonella typhi) and bacillus dysentery (Shigella dysenteriae) are commonly transmitted by drinking water.
Some species of Salmonella cause abortion of sheep, horse, goat and other animals.
f. Biological Warfare:
Bacteria causing diseases like anthrax, black-leg, tuberculosis, etc., are used as secret war-agents.
a. Increase Soil Fertility:
Plants do not have the efficiency of direct utilisation of huge amount (78%) of atmospheric nitrogen. However, some free-living and symbiotic bacteria are able to fix free nitrogen into nitrogenous compounds.
These bacteria are of the following three categories:
(i) Nitrifying bacteria:
This group of bacteria oxidises ammonia to nitrate in two steps. The first step involves the oxidation of ammonia to nitrous acid by bacteria like Nitrosomonas, etc. and the second step involves oxidation of nitrous acid to nitric acid by Nitrobacter.
Step I : NH3 + 1½O2———> HNO2 + H2O + Energy
Step II : 2HNO2 + O2———— > 2HNO3 + Energy
(ii) Ammonifying bacteria:
Some saprophytic bacteria hydrolyse the proteins of dead animals and plants into amino acids and other organic nitrogenous substances. Some ammonifying bacteria like Bacillus ramosus, Proteus vulgaris, Clostridium sp. etc., convert amino acids into ammonia, which combines with water and carbon dioxide to form ammonium carbonate, used by many crops as nitrogen source.
(iii) Nitrogen fixing bacteria:
These bacteria can fix atmospheric nitrogen.
They are of two types:
(a) Symbiotic nitrogen fixers like Rhizobium, found in the root nodules of leguminous plants which fix N2 about 100-400 kg/hectare/year. The filamentous Actinomycete, Frankia found in root nodules of Alnus rugosa can fix N2 about 150 kg/ hectare/year,
(b) Free-living fixers like Clostridium and Azotobacter can fix N2 25-50 kg/hectare/year.
b. Industrial Importance:
From ancient time men have used microorganisms in the preparation of their food, drinks etc.
Some of the uses are:
(i) Dairy industry:
The lactic acid bacteria are used in the preparation of butter, cheese, curd etc. The curd is prepared from milk by bacteria in two steps. They convert lactose sugar to glucose and then glucose to lactic acid, which sours the milk and coagulates the milk protein (casein) forming curd. Some bacteria used in dairy are Lactobacillus plantarum, L. acidophilus, L. bulgaricus, Streptococcus lactis, S. thermophilic etc.
(ii) Vinegar industry:
Clostridium aceto- butylicum and Acetobacter aceti, the acetic acid bacteria are used in the conversion of alcohol to acetic acid, the vinegar. This was done in wooden vats and takes several weeks to complete the process.
(iii) Butanol and acetone production:
Butanol and acetone are produced by the action of species of Clostridium on molasses. They are used as solvent in different industry.
(iv) Fibre retting:
Butyric acid bacteria such as Clostridium tetani, C. botulinum etc., are used in the retting of jute, hemp and flax fibres, thereby the bast-fibres become loosen and can be extracted easily.
(v) Curing of tea and tobacco:
The curing of tea and tobacco is done by some bacteria which give particular taste, flavour or smell.
c. Biological Control of Insect:
Many bacteria like Bacillus thuringiensis etc., secrete proteins which are highly toxic to caterpillars and insects belonging to Lepidoptera. They are, however, nontoxic to other animals and also plants, thereby the bacteria are used as ideal agent for biocontrol of insect.
d. Degradation of Petroleum:
Many bacteria like Pseudomonas, Micrococcus, Candida and Achromobacter are able to degrade petroleum hydrocarbon in water from different vehicles, thereby they check pollution.
e. Decomposition of Dead Animals:
Some bacteria are, able to decompose the dead organic matters into inorganic form which mix with soil and increase the soil fertility. These inorganic substances are then absorbed by the plants as nutrients.
f. Bacteria in Medicine:
Bacteria are the sources of different antibiotics and they are also used as serum and vaccine.
(i) Sources of antibiotic:
Antibiotics are the chemical substances secreted by microorganisms, which inhibit the growth and development of other microorganisms. Antibiotics like bacitracin, polymyxin B, terramycin, streptomycin etc., are of bacterial origin (Table 2.10).
(ii) Serum and vaccine:
Sera (sing, serum) are used by men as therapeutic measure before infection or after development of diseases like pneumonia, diphtheria etc. Vaccines are commonly used for immunisation against diseases like cholera, typhoid etc.
(iii) Production of vitamins:
Some bacteria produce vitamins of commercial importance. Pseudomonas denitrificans is used to produce Cobalamin (Vitamin B12) and Clostridium butylicum is used in the production of Riboflavin (Vitamin B2).
g. In Sewage Work:
Putrifying bacteria are used to remove the solid and semi-solid constituent of sewage under anaerobic conditions. After treatment the constituents get decayed and liquefied. Those are filtered and the liquid is drained out in the river.
h. Preservation of Green Fodder:
Some bacteria are used in the preservation of green fodder in pits by the ensilage process.