Control of Microbial Growth (page 2)
(This chapter has 6 pages)
© Kenneth Todar, PhD
usually destroys or distorts nucleic acids. Ultraviolet light is
commonly used to sterilize the surfaces of objects, although
x-rays, gamma radiation and electron beam radiation are also used.
lamps are used to sterilize
workspaces and tools used in microbiology laboratories and health care
facilities. UV light at germicidal wavelengths (two peaks, 185 nm and
causes adjacent thymine molecules on DNA to dimerize, thereby
inhibiting DNA replication (even though the organism may not be killed
outright, it will not be able to reproduce). However, since
microorganisms can be shielded from
ultraviolet light in fissures, cracks and shaded areas, UV lamps should
only be used as a supplement to other sterilization techniques.
An ultraviolet sterilization cabinet.
Gamma radiation and electron
beam radiation are
forms of ionizing radiation used primarily in the health care
Gamma rays, emitted from cobalt-60, are similar in many ways to
microwaves and x-rays. Gamma rays delivered during sterilization break
chemical bonds by interacting with the electrons of atomic
constituents. Gamma rays are highly effective in killing
microorganisms and do not leave residues or have sufficient energy to
Electron beam (e-beam) radiation,
a form of ionizing energy, is
generally characterized by low penetration and high-dose rates. E-beam
irradiation is similar to gamma radiation in that it alters various
chemical and molecular bonds on contact. Beams produced for e-beam
concentrated, highly-charged streams of electrons generated by the
acceleration and conversion of electricity.
e-beam and gamma radiation
are for sterilization of items ranging from syringes to
involves the physical removal (exclusion) of all cells in a liquid or
gas. It is especially important for sterilization of solutions which
by heat (e.g. antibiotics, injectable drugs, amino acids, vitamins,
etc.). Portable units can be used in the field for water purification
and industrial units can be used to "pasteurize" beverages.
Essentially, solutions or gases are passed through a filter of
sufficient pore diameter (generally 0.22 micron) to remove the smallest
known bacterial cells.
This water filter for hikers and
backpackers is advertised to "eliminate Giardia, Cryptosporidium and
most bacteria." The filter is made from 0.3 micron pleated glass fiber
with a carbon core.
A typical set-up in a
microbiology laboratory for filtration sterilization of medium
components that would be denatured or changed by heat sterilization.
filter is placed
(aseptically) on the glass platform, then the funnel is clamped and the
fluid is drawn by vacuum into a previously sterilized flask. The
recommended size filter that will exclude the smallest bacterial
cells is 0.22 micron.
Chemical and gas
Chemicals used for sterilization include the
gases ethylene oxide and formaldehyde, and liquids such as
glutaraldehyde. Ozone, hydrogen peroxide
and peracetic acid are also examples of
chemical sterilization techniques are based on oxidative capabilities
of the chemical.
(ETO) is the most commonly used form of chemical sterilization. Due to
its low boiling point of 10.4ºC at
atmospheric pressure, EtO) behaves as a gas at room
chemically reacts with amino acids, proteins, and DNA to prevent
microbial reproduction. The sterilization
process is carried out in a specialized
gas chamber. After sterilization, products are transferred to an
aeration cell, where they remain until the gas disperses and the
product is safe to handle.
ETO is used for cellulose and plastics irradiation,
usually in hermetically sealed packages. Ethylene
oxide can be used with a wide range of plastics (e.g. petri dishes,
pipettes, syringes, medical devices, etc.) and other materials
without affecting their integrity.
An ethylene oxide sterilization gas chamber.
has been recently approved for use in the U.S. It uses oxygen that is
subjected to an intense electrical
field that separates oxygen molecules into atomic oxygen, which then
combines with other oxygen molecules to form ozone.
Ozone is used as a disinfectant for water and
food. It is used in both
gas and liquid forms as an antimicrobial agent in the treatment,
storage and processing of foods, including meat, poultry and eggs. Many
municipalities use ozone technology to purify their water and sewage.
Los Angeles has one of the largest municipal ozone water treatment
plants in the world. Ozone is used to
disinfect swimming pools, and some companies
bottled water use ozonated water to sterilize containers.
An ozone fogger for sterilization of egg surfaces.
The system reacts ozone with water vapors to create powerful oxidizing radicals. This
system is totally chemical free and is effective against
bacteria, viruses and hazardous microorganisms which are deposited on
An ozone sterilizer
for use in the hospital
or other medical environment.
Gas Plasma (LTGP) is used as an alternative to ethylene oxide.
It uses a small amount of liquid hydrogen peroxide (H2O2),
which is energized
with radio frequency waves into gas plasma. This leads to the
of free radicals and other chemical species, which destroy organisms.
An LTGP sterilizer
that pumps vaporized H2O2
into the chamber.