- Singlet oxygen, which is in an extremely active, high-energy state. This is
present in cells that use phagocytosis to ingest foreign bacteria cells. - Superoxide free radicals,which is formed during normal respiration of
organisms that use oxygen as a final electron acceptor. In obligate anaer-
obes, they form some superoxide free radicals in the presence of oxygen.
These superoxide free radicals are incredibly toxic to cellular components.
In order for organisms to grow in atmospheric oxygen they must produce
the enzyme superoxide dimutaseor (SOD). Superoxide dimutase neutral-
izes superoxide free radicals. Aerobic bacteria, facultative anaerobes grow-
ing aerobically, and aerotolerant anaerobes produce SOD, which converts
the superoxide free radical into molecular oxygen (O 2 ) and hydrogen per-
oxide (H 2 O 2 ). This can be seen when you place hydrogen peroxide on a
wound infected with bacteria. When hydrogen peroxide is placed on the
colony of bacterial cells that are producing catalase, oxygen bubbles are
released. This is the “foaming” you see when you place hydrogen perox-
ide on a cut. Human cells also produce catalase, which converts hydrogen
peroxide to water and oxygen. - Hydroxyl radical(OH
−); this is a hydroxide ion. Most aerobic respiration
produces some hydroxyl radicals. Aerotolerant anaerobes can tolerate
oxygen, but cannot grow in an oxygen-rich environment. Aerotolerant bac-
teria ferment carbohydrates to lactic acid. Lactic acid inhibits the growth
of aerobic competitors, establishing a good opportunity for the growth of
these organisms. An example of a lactic acid–producing aerotolerant bac-
terium is lactobacillus, used in fermented food such as pickles and yogurt.Culture Media
Aculture mediumis nutrient material prepared in the laboratory for the growth
of microorganisms. Microorganisms that grow in size and number on a culture
medium are referred to as a culture.
In order to use a culture medium must be sterile, meaning that it contains no
living organisms. This is important because we only want microorganisms that
we add to grow and reproduce, not others. We must have the proper nutrients, pH,
moisture, and oxygen levels (or no oxygen) for a specific microorganism to grow.
Many culture media are available for microbial growth. Media are constantly
being developed for the use of identification and isolation of bacteria in the
research of food, water, and microbiology studies.
CHAPTER 6 Microbial Growth and Controlling It^105