Abundant  in air, soil and water. Some are beneficial (e.g. those living in  intestine and breaking down food) and some are harmless to humans.  Bacteria that are harmful are known as pathogens. Three main categories:  cocci (spherical), bacilli (rod-shaped) and spirochaetes or spirilla  (spiral-shaped). Bacteria can grow in an inert medium. Susceptible to  antibiotics.

Commensal means the bacteria normally lives in or on the body and is considered medically to be harmless

Nosocomial infection is an infection contracted in hospital.