or lower (maximal temperature for growth at 20 °C, minimal temperature for growth at 0 °C or lower). Psychrophiles are typically found in Earth's extremely cold ecosystems, such as polar ice-cap regions, permafrost, polar surface, and deep oceans. Mesophiles Mesophiles are bacteria that thrive at moderate temperatures, growing best between 20° and 45 °C. These temperatures align with the natural body temperatures of humans, which is why many human pathogens are mesophiles. Thermophiles Survive under temperatures of 45–80 °C. Acidity Optimal acidity for bacteria tends to be around pH 6.5 to 7.0 with the exception of acidophiles. Some bacteria can change the pH such as by excreting acid resulting in sub-optimal conditions. Water activity Oxygen Bacteria can be aerobes or anaerobes. Depending on the degree of oxygen required bacteria can fall into the following classes: facultative-anaerobes-ie aerotolerant absence or minimal oxygen required for their growth obligate-anaerobes grow only in complete absence of oxygen facultative aerobes-can grow either in presence or minimal oxygen obligate aerobes-grow only in the presence of oxygen Micronutrients Ample nutrients Toxic compounds Toxic compounds such as ethanol can hinder growth or kill bacteria. This is used beneficially for disinfection and in food preservation. See also Monod equation Michaelis–Menten kinetics Cell proliferation References External links An examination of the exponential growth of bacterial populations Science aid: Bacterial Growth High school (GCSE, Alevel) resource. Microbial Growth, BioMineWiki From the Wolfram Demonstrations Project — requires CDF player (free): The Final Number of Bacterial Cells Simulating Microbial Count Records with an Expanded Fermi Solution Model Incipient Growth Processes with Competing Mechanisms Modified Logistic Isothermal Microbial Growth Ratio Generalized Logistic (Verhulst) Isothermal Microbial Growth Microbial Population Growth, Mortality, and Transitions between Them Diauxic Growth of Bacteria on Two Substrates This article includes material from an article posted on 26 April 2003 on Nupedia; written by Nagina Parmar; reviewed and approved by the Biology group; editor, Gaytha Langlois; lead reviewer, Gaytha Langlois ; lead copyeditors, Ruth Ifcher. and Jan Hogle. Bacteriology Population Mathematics in medicine