Tag: bacteria - friend and foe

Bacteria are the component of biological nitrogen fixation which included free living or non-symbiotic soil bacteria. It may be aerobic bacteria (Azotobacter spp), anaerobic bacteria (Clostridium spp), photosynthetic bacteria (Rhodospirillum spp), chemosynthetic bacteria (Thiobacillus spp) and blue-green algae. The finger-like projections on roots of legumes, termed as root nodules, carries Rhizobium bacteria which in turn are responsible for nitrogen fixation; legumes depend on of bacteria for nitrogen fixation. None of the parasitic algae or fungal species can fix nitrogen. Thus, the correct answer is option A.

Azotobacter is a free-living aerobic rod-shaped soil bacteria; it is a member of Gammaproteobacteria and fixes the atmospheric nitrogen into the useable form which makes option A correct. Rhizobium is a Gram-negative, motile, rod-shaped bacteria that forms special nitrogen-fixing organs, called as root nodules, in the roots of legumes and fix the gaseous nitrogen into ammonia. Option B is incorrect. Clostridium is an anaerobic free-living nitrogen fixer which makes option C incorrect. Streptomyces is filamentous free-living aerobic bacterium; S. thremoautotropicus have oxygen insensitive nitrogenase enzyme and ability to fix nitrogen. Since not all the Streptomyces spp can fix nitrogen, options D is incorrect. 

Antibiotics serve as metabolic analogs and control the microbial growth by competing with the necessary enzyme to halt the growth of bacterial cells. Antibiotics occupy the enzyme’s active site thereby blocking the binding of its natural substrate to the enzyme. This puts the whole pathway to halt. Inhibition of enzyme activity by a metabolic analog of the natural substrate is termed as competitive inhibition.

Nitrogen gas is converted to nitrate compounds by nitrogen fixing bacteria in soil or root nodules. The Haber process converts nitrogen gas into ammonia used in fertilizers. Ammonia is converted to nitrates by nitrifying bacteria in the soil. Bacteria of the genus Nitrosomonas oxidize NH$ _3$ to nitrites. Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates. Urea and egested material is broken down by decomposers. This results in nitrogen being returned to the soil as ammonia. Decomposers also break down the bodies of dead organisms resulting in nitrogen being returned to the soil as ammonia. In some conditions denitrifying bacteria in the soil break down nitrates and return nitrogen to the air. Thus, it is summarized that the bacteria play a significant role in nitrogen fixation, nitrification and several other operations in all biogeochemical cycles.

Agrobacterium tumefaciens is a soil plant pathogenic bacterium that carries Ti plasmid. It can transfer a particular segment of the tumor-inducing (Ti) plasmid into the nucleus of infected cells. The transferred T-DNA is then integrated into the host genome and transcribed with it. This ability of Agrobacterium tumefaciens to transfer the T-DNA in the host genome is explored in genetic engineering to transfer the desired DNA segment of up 25kb, carrying the gene of the interest, into the genome of selected organisms. Thus, the correct answer is option A.

Lactobacilli are anaerobic, rod shaped bacteria that produce lactic acid by anaerobic fermentation. They are responsible for digestion of milk sugar lactose into lactic acids at suitable temperatures which in turn add sour taste to milk. 

Free-living and symbiotic nitrogen-fixing bacteria fix atmospheric inert nitrogen into biologically useful ammonia thereby adding nitrogen to the soil and increasing its fertility. Algae increase the organic carbon content of the soil upon their death. Mycorrhizae are the symbiotic fungi that reside in roots of higher plants and increase soil fertility by nitrogen fixation. Phages are the bacterial viruses that replicate within the bacterial cell with a polyhedral head, a helical tail and fibers for attachment to the host cell. Thus, the correct answer is option C.

The antibiotics are substances produced by the natural metabolic processes of spore forming aerobic bacteria and fungi as their natural defence against other microorganisms. Immune host is the one which has acquired immunity against certain antigen by administration of antibiotics. It produces antibodies, not the antibiotics which makes option A incorrect. Commercial production of antibiotics by parasitic fungi needs their culture in live host cells which is a tedious and expensive process; Option B is incorrect. Saprophytic bacteria feed on dead and decaying organic matter and therefore, can easily be cultured on synthetic medium; option C is correct. Since, viruses are metabolically inactive outside the host cell and depend on host for their own nutritional requirements, they do not produce antibodies. Option D is incorrect. Thus, the correct answer is option C. 

Biofertilizer is any living organism that imparts fertility to soil. Azotobacter is a free living aerobic rod shaped soil bacteria; it is a member of Gamma-proteobacteria and fix the atmospheric inter nitrogen into biologically usable ammonia and hence, serves as biofertilizer.