and fatty acids are called heterotrophs. Heterotrophic organisms acquire or feed
on organic compounds from other organisms.
Organisms can also be categorized according to whether they use chemicals
or light as a source of energy. Organisms that acquire energy from redox re-
actions involving inorganic and organic chemicals are called chemotrophs.
Organisms that use light as their energy source are called phototrophs.Chemical Requirements for Microbial Growth
A microorganism requires two chemical elements in order to grow. These ele-
ments are carbon and oxygen.CARBON
Carbonis one of the most important requirements for microbial growth. Carbon
is the backbone of living matter. Some organisms, such as photoautotrophs, get
carbon from carbon dioxide (CO 2 ).OXYGEN
Microorganisms that use oxygen produce more energy from nutrients than micro-
organisms that do not use oxygen. These organisms that require oxygen are called
obligate aerobes. Oxygen is essential for obligate aerobes because it serves as
a final electron acceptor in the electron transport chain, which produces most
of the ATP in these organisms. An example of an obligate aerobe is micrococ-
cus. Some organisms can use oxygen when it is present, but can continue to
grow by using fermentation or anaerobic respiration when oxygen is not avail-
able. These organisms are called facultative anaerobes. An example of a fac-
ultative anaerobe is E. colibacteria, which is found in the large intestine of
vertebrates, such as humans.
Some bacteria cannot use molecular oxygen and can even be harmed by it.
Examples includeClostridium botulinum, the bacterium that causes botulism,
and Clostridium tetani, the bacterium that causes tetanus. These organisms are
called obligate anaerobes. Molecular oxygen (O 2 ) is a poisonous gas to obligate
anaerobes. Toxic forms of oxygen are:(^104) CHAPTER 6 Microbial Growth and Controlling It