Growth of psychrophilic bacteria.

On the basis of the temperature range in which growth can occur, bacteria are usually divided into 3 groups, namely: thermophiles, mesophiles, and psychrophiles (sometimes called cryophiles or rhigophiles). Thermophiles have temperature optima (generally quite close to temperature maxima) 10 to 25 degrees above the temperature optima of mesophiles. Thermophiles and mesophiles are easily distinguished on this basis. Possibly by analogy to the distinction between thermophiles and mesophiles, most textbook definitions of psychrophiles are also based on temperature optima or maxima. Psychrophiles are usually defined as organisms that "grow best" below a certain temperature, usually 20 C (Lamanna and Mallette, 1953) or 12 to 18 C (Oginsky and Umbreit, 1954), and sometimes as low as 5 to 10 C (Kelly and Hite, 1949). Actually, however, there are very few reports of organisms with temperature optima below 20 C. Only three such organisms are listed in Bergey's Manual (Breed et al., 1948) and these are questionable, since the temperature maxima were 30 to 40 C. One would not expect optimum and maximum temperature to differ so greatly. ZoBell and Conn (1940) state that they have never encountered "true psychrophiles" or organisms which grow best at relatively low temperatures. Hess (1934) reports three organisms with optima below 20 C, but his criterion of optimum growth is maximum total cell yield rather than maximum growth rate. From a study of the literature, one must conclude that there are very few or possibly no bacteria that fit the textbook definitions of psychrophiles. Irrespective of temperature optima, there is an important and large group of bacteria which grows relatively rapidly at low temperatures, i. e., produces visible colonies in approximately one week at 0 C. Some are capable of growth at temperatures as low as -7.5 C (Bedford, 1933). The ability of bacteria to grow at 0 C has been