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A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, typically between 20 and 45 °C (68 and 113 °F). The optimal temperature is 37 °C.The term is mainly applied to microorganisms. Organisms that prefer extreme environments are known as extremophiles. Mesophiles have diverse classifications, belonging to three kingdoms: Bacteria, Archaea, and Fungi. Mesophiles belonging to the domain Bacteria can either be gram-positive or gram-negative. Gram-positive bacteria have a cell layer made of peptidoglycan and stains purple. Gram-negative bacteria does not contain peptidoglycan and stains red or pink. Oxygen requirements for mesophiles are not just confined to aerobic or anaerobic. There are three basic shapes of mesophiles: coccus, bacillus, and spiral.

Oxygen Requirements
Due to the diversity of mesophiles, oxygen requirements greatly vary. Aerobic respiration requires the use of oxygen and anaerobic does not. There are three types of anaerobes. Facultative anaerobes grow in the absence of oxygen, using fermentation instead. During fermentation, sugars are converted to acids, alcohol, or gasses. If there is oxygen present, it will use aerobic respiration instead. Obligate anaerobes cannot grow in the presence of oxygen. Aerotolerant aerobes can withstand oxygen.

Habitat
The habitats of mesophiles can include cheese and yogurt. They are often included during fermentation of beer and wine making. Since normal human body temperature is 37 °C, the majority of human pathogens are mesophiles, as are most of the organisms comprising the human microbiome.

Extremophiles
Organisms that prefer cold environments are termed psychrophilic, those preferring warmer temperatures are termed thermophilic or thermotropic and those thriving in extremely hot environments are hyperthermophilic. Hyperthermophiles are a type of extremophile. All bacteria have their own optimum environmental surroundings and temperatures in which they thrive the most. A genome-wide computational approach has been designed by Zheng, et al. to classify bacteria into mesophilic and thermophilic.

Adaptations
Many factors are responsible for a given organism's optimal temperature range, but evidence suggests that the expression of particular genetic elements (alleles) can alter the temperature-sensitive phenotype of the organism. A recently published study demonstrated that mesophilic bacteria could be genetically engineered to express certain alleles from psychrophilic bacteria, consequently shifting the restrictive temperature range of the mesophilic bacteria to closely match that of the psychrophilic bacteria.

Due to the less stable structure of mesophiles, it has reduced flexibility for protein synthesis. Mesophiles are not able to synthesize proteins in low temperatures. It is more sensitive to temperature changes, and the fatty acid composition of the membrane does not allow for much fluidity. Decreasing the optimal temperature of 37 °C to 0 °C to 8 °C leads to a gradual decrease in protein synthesis. Cold-induced proteins (CIPs) are induced during low temperatures, which then allows cold-shock proteins (CSPs) to synthesize. The shift back to the optimal temperature sees an increase, indicating that mesophiles are highly dependent on temperature. Oxygen availability also has an impact on microorganism growth.

Roles of Mesophiles
Microorganisms play an important role in decomposition of organic matter and mineralization of nutrients. In aquatic environments, the diversity of the ecosystem allows for the diversity of mesophiles. The functions of each mesophile rely on the surroundings, most importantly temperature range.

Examples of Mesophiles
Some notable mesophiles include Listeria monocytogenes, Staphylococcus aureus, and Escherichia coli. Other examples of species of mesophiles are Clostridium kluyveri, Pesudomonas maltophilia, Thiobacillus novellus, Streptococcus pyrogenes, and Streptococcus pneumoniae. Different types of diseases and infections typically have pathogens from mesophilic bacteria such as the ones listed above.

Listeria monocytogenes
Listeria monocytogenes is a gram-positive bacteria. It is closely related to Bacillus and Staphylococcus. It is rod-shaped, facultative anaerobe that is motile by peritrichous flagella. L. monocytogenes motility is limited from 20 °C to 25 °C. At the optimal temperature, it loses its motility. This bacteria is responsible for listeriosis which derives from contaminated food.

Staphylococcus aureus
Staphylococcus aureus was first identified in 1880. They are responsible for different infections stemming from an injury. The bacteria overcomes the body's natural mechanisms. Long lasting infections of S. aureus includes pneumonia, meningitis, and osteomyelitis. S. aureus is commonly contracted in hospital settings.

Escherichia coli
Escherichia coli is a gram-negative, rod-shaped facultative anaerobic bacteria that does not produce spores. The bacteria is a member of Enterobacteriaceae. It is capable of producing enterotoxins which are thermolabile or thermostable. Other characteristics of E. coli are that it is oxidase-negative, citrate-negative, methyl-red positive, and Voges-Proskauer-negative. To sum up E. coli; it is a coliform. It is able to use glucose and acetate as a carbon source for fermentation. E. coli is commonly found in the gut of living organisms. E. coli has many capabilities such as being a host for recombinant DNA and being a pathogen.