There are several terms used for microorganisms that are adapted to various environments. For example, a word ending, '-phile', is used for organisms which are adapted to grow well in an 'extraordinary' conditions. These organisms grow poorly or cannot grow in moderate conditions that are familiar to human beings. These examples include thermophiles, barophiles, halophiles and psychrophiles. On the other hand, microorganisms that are able to grow in extreme conditions as well as in ‘normal’ conditions are called, '-resistant ' bacteria or '-tolerant ' bacteria. These include barotolerant bacteria, halotolerant bacteria, and radioresistant bacteria. These bacteria can grow at 'normal' conditions. However, most microorganisms are resistant to lower temperature. In addition, there is no known bacterium that is resistant to higher temperature. Therefore, the terms “cold tolerant bacterium” and “thermotolerant bacterium” are not used.
Thermophiles, psychrophiles, and halophiles are subcategorized. Many bacteria grow fastest from 30 to 40ºC. Although many bacteria can grow below 30ºC, their growth rates become slower. Bacteria that grow fastest below 30ºC are called cold-adapted bacteria. A bacteria that grow only below 20ºC are called psychrophiles. On the other hand, many mesophiles cannot grow over 50ºC. Bacteria that can grow over 50ºC are called thermophiles. Thermophiles that can grow below 50oC is called a moderate thermophile (or facultative thermophile). Thermophiles that can grow only over 50ºC are called extreme thermophiles (or obligate thermophiles). In particular, extreme thermophiles whose optimum growth temperatures are over 80ºC are called hyperthermophiles. In a similar way, halophiles that requires more than 1.5 M NaCl for the growth are called an extreme halophiles.