Microscopic organism (free-living single cells ), sometimes parasitic, that take various forms and often cause disease (infection). Contain no nucleus and are therefore classed as prokaryotes. Bacteria come in one of three shapes : coccal (sperical); bacillary (rod-shaped) or spirochetal (spiral/helical). The shape of a bacteria is determined by the shape of the rigid cell wall. Bacteria are also generally classified into either gram-positive (blue) or gram-negative (pink) depending on their colour following a laboratory procedure called a Gram Stain .

Gram-negative bacteria are characterised by having two outer membranes, which makes them more resistant to conventional treatment. This class of bacteria also easily mutate and transfer these genetic changes to other strains, making them more resistant to antibiotic treatment.
Examples of infections caused by this class of bacteria include the plague, rabbit fever, cholera, thyphoid fever and salmonella.

Gram-positive bacteria are rarer and treatable with penicillin . Gram-positive bacteria can cause damage by either releasing toxic chemicals (e.g. clostridium botulinum) or by penetrating deep into tissue (e.g. streptococci). Infections caused by this class of bacteria include anthrax and listeriosis.