Microbiology and Immunology

Strep throat WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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extended time or intermittent boiling can be done, wherein at

least three—and up to 30—periods of boiling are interspersed

with time to allow the fluid to cool.

Steam heat (moist heat) sterilization is performed on a

daily basis in the microbiology laboratory. The pressure

cooker called an autoclave is the typical means of steam heat

sterilization. Autoclaving for 15 minutes at 15 pounds of pres-

sure produces a temperature of 250°F (121°C), sufficient to

kill bacterial spores. Indeed, part of a quality control regiment

for a laboratory should include a regular inclusion of com-

mercially available bacterial spores with the load being steril-

ized. The spores can then be added to a liquid growth medium

and growth should not occur.

Pasteurizationis employed to sterilize fluids such as

milk without compromising the nutritional or flavor qualities

of the fluid.

The final form of heat sterilization is known as dry heat

sterilization. Essentially this involves the use of an oven to

heat dry objects and materials to a temperature of 320–338°F

(160–170°C) for two hours. Glassware is often sterilized in

this way.

Some samples cannot be sterilized by the use of heat.

Devices that contain rubber gaskets and plastic surfaces are

often troublesome. Heat sterilization can deform these materi-

als or make them brittle. Fortunately, other means of steriliza-

tion exist.

Chemicals or gas can sterilize objects. Ethylene oxide

gas is toxic to many microorganisms. Its use requires a special

gas chamber, because the vapors are also noxious to humans.

Chemicals that can be used to kill microorganisms include

formaldehyde and glutaraldehyde. Ethanol is an effective ster-

ilant of laboratory work surfaces. However, the exposure of

the surface to ethanol must be long enough to kill the adherent

microorganisms, otherwise survivors may develop resistance

to the sterilant.

Another means of sterilization utilizes radiation.

Irradiation of foods is becoming a more acceptable means of

sterilizing the surface of foods (e.g., poultry). Ultraviolet radi-

ation acts by breaking up the genetic material of microorgan-

isms. The damage is usually too severe to be repaired. The

sole known exception is the radiation-resistant bacteriaof the

genus Deinococcus.

The final method of sterilization involves the physical

removal of microorganisms from a fluid. This is done by the

use of filters that have extremely small holes in them. Fluid is

pumped through the filter, and all but water molecules are

excluded from passage. Filters—now in routine use in the

treatment of drinking water—can be designed to filter out very

small microorganisms, including many viruses.

See alsoBacterial growth and division; Bacteriocidal, bacte-

riostatic; Laboratory techniques in microbiology

SStrep throatTREP THROAT

Streptococcal sore throat, or strep throat as it is more com-

monly called, is an infection caused by group AStreptococcus

bacteria. The main target of the infection is the mucous mem-

branes lining the pharynx. Sometimes the tonsils are also

infected (tonsillitis). If left untreated, the infection can

develop into rheumatic fever or other serious conditions.

Strep throat is a common malady, accounting for 5–10%

of all sore throats. Strep throat is most common in school age

children. Children under age two are less likely to get the dis-

ease. Adults who smoke, are fatigued, or who live in damp,

crowded conditions also develop the disease at higher rates

than the general population.

The malady is seasonal. Strep throat occurs most fre-

quently from November to April. In these winter months, the

disease passes directly from person to person by coughing,

sneezing, and close contact. Very occasionally the disease is

passed through food, most often when a food handler infected

with strep throat accidentally contaminates food by coughing

or sneezing.

Once infected with the Streptococcus,a painful sore

throat develops from one to five days later. The sore throat can

be accompanied by fatigue, a fever, chills, headache, muscle

aches, swollen lymph glands, and nausea. Young children may

complain of abdominal pain. The tonsils look swollen and are

bright red with white or yellow patches of pus on them.

Sometimes the roof of the mouth is red or has small red spots.

Often a person with strep throat has a characteristic odor to

their breath.

Others who are infected may display few symptoms.

Still others may develop a fine, rough, sunburn-like rash over

the face and upper body, and have a fever of 101–104ºF

(38–40ºC). The tongue becomes bright red with a flecked,

strawberry-like appearance. When a rash develops, this form

of strep throat is called scarlet fever. The rash is a reaction to

toxins released by the streptococcus bacteria. Scarlet fever is

essentially treated the same way. The rash disappears in about

five days. One to three weeks later, patches of skin may peel

off, as might occur with a sunburn.

Strep throat can be self-limiting. Symptoms often sub-

side in four or five days. However, in some cases untreated

strep throat can cause rheumatic fever. This is a serious illness,

although it occurs rarely. The most recent outbreak appeared

in the United States in the mid-1980s. Rheumatic fever occurs

most often in children between the ages of five and 15, and

may have a genetic component, because susceptibility seems

to run in families. Although the strep throat that causes rheu-

matic fever is contagious, rheumatic fever itself is not.

Rheumatic fever begins one to six weeks after an

untreated streptococcal infection. The joints, especially the

wrists, elbows, knees, and ankles become red, sore, and

swollen. The infected person develops a high fever, and possi-

bly a rapid heartbeat when lying down, paleness, shortness of

breath, and fluid retention. A red rash over the trunk may come

and go for weeks or months. An acute attack of rheumatic

fever lasts about three months. Rheumatic fever can cause per-

manent damage to the heart and heart valves. It can be pre-

vented by promptly treating streptococcal infections with

antibiotics. It does not occur if all the Streptococcusbacteria

are killed within the first 10–12 days after infection.

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