Bacteria (the singular is bacterium) are tiny, living, one-celled organisms that can live inside the body, or outside if conditions are right. They can multiply rapidly if conditions are warm (i.e. anything between room temperature and body temperature), damp, dark, and has some type of organic material that can be used as nutrients.
For example, if laundry is left damp and dirty in a pile, the number of bacterium would increase drastically before the wash was done.
The number of organisms can double in as little as 15 minutes.
At low temperatures (e.g. freezing or refrigeration temperature), some bacteria may die, but others increase in numbers. High temperatures and some chemical germicides will kill them.
Many bacteria cause disease in humans. The bacterium that causes TB (Mycobacterium tuberculosis) is one of the germs most resistant to killing by microbicides.
Bacteria that are growing and multiplying are vegetative bacteria. In this stage they are relatively easy to kill, with heat or the appropriate chemicals. However, a few species of bacteria may occasionally form bacterial spores, when conditions are poor. A bacterial spore is a very tough, dry form of the bacterium, which is still alive, but inactive. It does not grow or multiply or cause disease, but it is extremely difficult to kill with heat or chemicals. It waits until conditions are favourable again (e.g., warm, moist and dark), then it will transform into the vegetative form and start to multiply again.
Most bacterial species are classified as either Gram positive or Gram negative. This simply means that when a lab technician applies certain dyes or stains to bacteria on a glass slide, the bacteria will end up coloured violet (Gram positive) or red (Gram negative). The word gram was the name of the inventor of the procedure. While Gram negative organisms are, in general, somewhat harder for disinfectant cleaners to kill, both Gram positive and negative organisms can cause nosocomial infections.
The Centres for Disease Control and Prevention (CDC) defines a nosocomial infection (NI) as an infectious event that is diagnosed >48 hours after admission without evidence that the pathogen was already in the incubation phase.