Temperature can affect the three-dimensional shape of biological molecules such as proteins.  Bacteria can be separated into several groups based on the temperature range best suited to their metabolism.

Psychrophiles grow best at temperatures between -5 and +20 oC.  This group includes those responsible for the spoilage of food in refrigerators.

Mesophiles grow best between 15 and 45 oC.  All major human and animal pathogens are mesophiles.  Some are thermoduric, meaning that they can withstand slightly higher temperature ranges.

Thermophiles have an optimal temperature range between 45 and 80 oC.  Hyperthermophiles such as some of the Archea can live at temperatures between 65 and 105 oC.


Acidophiles are bacteria that grow best at a pH range between 0 and 6.5.

Neutralophiles require a pH between 6.5 and 7.5.  The pathogens of humans and animals belong to this group.  Some neutralophiles are more tolerant of pH changes.  For example, Helicobacter pylori, a pathogen of the gastrointestinal tract, produces bicarbonate and urea waste products that neutralize stomach acid.  This results in a gradual erosion of the mucosa and eventually an ulcer.  Approximately 80% of stomach ulcers in humans are caused by H. pylori infection.

Alkalinophiles require a pH between 7.5 and 14.


Water is essential to all living cells as the primary solvent in which chemical activities take place and as a reactant in metabolic reactions such as hydrolysis.

Some microorganisms can produce resting stages such as endospore and cysts when in the absence of water, but all must have water in order to be metabolically active.

Osmotic pressure is the pressure exterted on a cell by the movement of water toward the highest concentration of dissolved solutes across a semi-permiable membrane.  Cells that can survive in high concentrations of salt are called halophiles.

Obligate halophiles such as Halobacterium, a member of the Archea, require high salinity.  Facultative halophiles such as Staphylococcus aureus can survive in salt concentrations between 7.5% and 20%, allowing them to colonize the skin and nasal cavity of mammals- places too salty for most other microbial flora to survive.

Hydrostatic pressure is the force placed on an object by the column of water above it.  Water pressure increases by a factor of 1 atmosphere for each 10 meters of depth.  Barophiles are microorganisms that have evolved to live in conditions of high hydrostatic pressure such as in deep ocean trenches.