Write short notes.
Useful and harmful bacteria.

Hint: Bacteria is a type of prokaryotic organism. The shape and size of the bacterial cell vary from minute spheres, spiral threads, and cylinders, to filamentous chains and flagellated rods. They are microscopic in nature and are found everywhere on earth.

Complete answer:
Bacteria is a prokaryote that is devoid of well-defined nuclei and lacks membrane-bound organelles. The bacterial cell consists of a chromosome having circular double-stranded DNA molecules. Some species of bacteria are useful whereas some species of bacteria are harmful. Bacteria are used as antibiotics to treat bacterial infection and disease. Lactobacillus is a bacterium present in milk and milk products. Vitamins are produced by the bacteria present in the colon of the large intestine.

Bacteria called Rhizobium present in the root nodules of the legumes are useful for the fixation of atmospheric nitrogen which is essential for the growth and development of the plants. Bacteria that are harmful to human beings can cause disease and infections. Such bacteria are called pathogenic bacteria. Streptococcus pneumoniae causes pneumonia disease. Some species of bacteria like Myxococcus xanthus feed on other organisms for their food by killing them. Such species of bacteria are called predatory bacteria, e.g., Streptococcus. Harmful bacteria present in the mouth can cause tooth decay. Bacteria called Salmonella typhi causes a disease called typhoid.

Note: Bacterial cells can be further classified as aerobic bacteria and anaerobic bacteria. Various species of bacteria are symbiotic in nature. Bacterial cells can grow rapidly and increase in number quickly. Some species of bacteria reproduce by budding which involves the formation of protrusion that breaks and falls off to produce a daughter cell.