Microbes cause infections resulting in diseases among human and animals. On the other side, they help in creating a "disease free world". Where people are saved from the pain of being born with physical or mental deformities. The control of our infectious disease has been the greatest achievement of medical science. Vaccination (the use of attenuated or killed microbes in the preparation of vaccines) reduced the incidence of several epidimic diseases (smallpox, diphtheria, whooping cough, poliomyelitis) but for many organisms vaccination is not effective or feasible. It has been especially important for diseases transmitted by respiratory droplets.

Living cells infected with viruses produce viral proteins (glycoproteins) having a broad spectrum antiviral action called interferon. In addition to the antiviral action, interferon has a number of biological effects, and causes inhibition of parasitic infections due to chlamydia, rickettsiae, protozoa and bacteria.

The powerful methodology was developed and adopted by Robert Koch during " Golden Era" of medical bacteriology (1879-1889) which helped in isolation of Tubercle bacillus, Cholera vibrio, Typhoid bacillus, Diptheriae bacillus, Pneumococcus, Staphylococcus, Streptococcus, Meningococcus, Gonococcus and Tetanus bacillus. Mechanism of pathogenecity of these organisms the host responses, and the methods of prevention and treatment have already been evolved.

Organic composting, increasing soil fertility, reclamation of alkaline user land and use of biofertilizer and microbial pesticides in agriculture are some of the important areas in which different groups of microorganisms participate.

Incomplete and artificially composting in its broadest sense means the organic matter by mixed microbial population in moist, warm and aerobic conditions and the preparation and application of organic anaerobic environment. Compost or organic manure in the agricultural fields is a traditional system. But in the system of composting generally the manure crop residues (wheat straw, rice straw, sugarcane bagasse, etc.) are being highly resistant lignified tissues that are difficult to degrade. Thus, breakdown or microbial conversion of these complex organic materials flavor nutrient rich compost within a short spam. Compost is the store house of major plant nutrients (NPK) as well as micronutrients. Different processes for the recovery of synthetic fuel from various sources of biomass i.e. animal production manures and organic wastes have been developed.

The productivity of leguminous crops largely depends on an efficient and sustainable management of the ecosystems, involving specific rhizobial associations. The symbiotic association of rhizobia (Rhizobium, Bradyrhizobium, Azorhizobium, sinorhizobium and Mesorhizobium) with legumes and free living nitrogen fixing microorganisms (Azotobacter, Azospirillum) offer an advantage in the sense that molecular nitrogen is converted into assimilable form of ammonia thus enabling these crops to grow purely on biological sources of nitrogen. Thus, legume and other cereals are benefitted and such associations mimicks the expensive, energy intensive chemical fertilizer factories.

Bacteria is often associated with human disease and ill health. However, there are several bacteria that are benevolent as well as harmless to human beings. In addition there are certain bacteria which can be of immense benefit to mankind. Bacillus thuringiensis (Bt), B. papillae, B.sphaericus, Xenohrabdus nematophilus, X. fluorescence, etc. are such which kill a wide range of insects like moths, beetles, mosquitoes flies, aphids, ants, termites, midges and butterflies depending on the host strain of the bacterium. Several Peudomonas such as Pseudomonas cepacia, Beauveria brassiana, Paecilomyces fumosporoseus, Verticilluim lecanii are also being used as microbial insecticides. Hirsutella thompsonii and Nomuraea rileyi control lepidopteran insects while most viruses such as occluded viruses namely NPV, cytoplasmic polyhedrosis (CPV), granulosis (GV) and entomopox viruses (EPV) considered for control of insects (usually sauflies and Lepidoptera). Certain plant pathogenic fungi causing diseases (charcoal rot, collar rot, damping off, wilt) can be checked by using various bacteria (Pseudomonas cepacia and P. fluorescence) and fungi (Gliocladium virens and Trichoderma harzianum).

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