a. Growth Range. Bacteria as a group are able to grow over a range of about
175-Fahrenheit degrees (F). This is a relatively narrow range when one considers the
coldest arctic to the hottest volcano. However, it is a wide range when compared to the
changes acceptable in man's body temperature. No one bacterial species can grow
over the entire 175-degree range but some are capable of growing over about one-half
of it; that is, from near 32F (0C) to 104 to 113F (40 to 45C). Each organism can
grow only within a growth temperature range characteristic of the species.
(1) Minimum growth temperature. The lowest temperature at which growth
occurs is the minimum growth temperature. This is difficult to determine because
physiological activities gradually decrease with the temperature until they can no longer
be detected by ordinary means. The generation time may be increased from minutes to
days or weeks.
(2) Maximum growth temperature. The highest temperature at which
growth can take place is the maximum growth temperature. This is more readily
defined and can occasionally be determined within a Fahrenheit degree.
(3) Optimum growth temperature. The temperature at which most rapid
multiplication occurs is the optimum growth temperature. It is the point at which the
generation time is the shortest. This may also be difficult to define because it may be
altered by both chemical and physical factors. There is no one temperature that is
optimal for all activities of the cell.
b. Three General Groups. Bacteria have been divided into three large groups
on the basis of their growth temperatures. These groups are not sharply defined and
the distinctions are arbitrary, but the classification serves some practical purposes. The
groups are called thermophiles, mesophiles, and psychrophiles.
(1) Thermophiles. Thermophilic bacteria are interesting because they prefer
temperatures intolerable to most forms of animal life. Water at 113F (45C) is hot to
the touch--hotter than the ordinary bath. Thermophilic bacteria grow best at
temperatures above 113F (45C). We arbitrarily say that the temperature range for this
bacterial group is from 113 to 140F (45 to 60C), although they may grow anywhere
within the range of 104 to 176F (40 to 80C). Thermophiles are particularly
troublesome in the dairy industry since they may grow most rapidly at pasteurization
temperatures. They may also cause spoilage of canned foods, which are stored at
elevated temperatures because the spores of some thermophilic bacteria are extremely
resistant to heat and survive the ordinary canning processes.
(2) Mesophiles. Mesophilic bacteria are the ones that are most common,
and they grow best at the moderate temperatures ranging from 65to 105F (18 to
41C). The minimum and maximum growth temperatures vary, for the most part, within
the range 50 to 125F (10 to 52C).