Microscopic organism (free-living single ), 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 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 .
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.