Bacteria (singular: bacterium) are single-celled prokaryotic organisms. They can be further subdivided according to various differences that occur between them. Two common systems are as follows: gram-staining based (gram-positive vs. gram-negative) and shape-based (which gives bacilli, cocci, coccobacillus and spiral bacteria). They can also be classified by their dependence on the presence of oxygen for growth and survival. Aerobic organisms require oxygen for growth and survival; whereas anaerobic organisms do not require oxygen for their growth or survival, some are even obligated to survive in the absence of oxygen (with the presence of oxygen they die or otherwise injured).
Humans and other animals interact with bacteria in a complex way: sometimes the relationship can be symbiotic (e.g., bacterial organisms living in a cow's stomach can allow them to breakdown cellulose in their food) whilst other times it can be harmful and parasitic.
Interaction with humansEdit
Both symbiotic and parasitic relationships exist between humans and bacterial organisms; parasitic relationships include those they have with bacterial pathogens like staphylococci whereas symbiotic relationships include those with microbial flora residing in the human digestive tract.