Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Sterilization

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used to describe the process of sterilization by steam. The

function of the sterilizer is to kill unwanted microorganisms

on instruments, in cultures, and even in liquids, because the

presence of foreign microbes might negatively affect the out-

come of a test, or the purity of a sample. A sterilizer also acts

as a test vehicle for industrial products such as plastics that

must withstand certain pressures and temperatures.

Larger chambers are typically lined with a metal jacket,

creating a pocket to trap pressurized steam. This method pre-

heats the chamber to reduce condensation and cycle time.

Surrounding the unit with steam-heated tubes produces the

same effect. Steam is then introduced by external piping or, in

smaller units, by internal means, and begins to circulate within

the chamber. Because steam is lighter than air, it quickly

builds enough mass to displace it, forcing interior air and any

air-steam mixtures out of a trap or drain.

Most sterilization processes require temperatures higher

than that of boiling water (212°F, 100°C), which is not suffi-

cient to kill microorganisms, so pressure is increased within

the chamber to increase temperature. For example, at 15 psi

the temperature rises to 250°F (121°C). Many clinical appli-

cations require a cycle of 20 minutes at this temperature for

effective sterilization. Cycle variables can be adjusted to meet

the requirements of a given application. The introduction of a

vacuum can further increase temperature and reduce cycle

time by quickly removing air from the chamber. The process

of steam sterilization is kept in check by pressure and temper-

ature gauges, as well as a safety valve that automatically vents

the chamber should the internal pressure build beyond the

unit’s capacity.

See alsoInfection control; Laboratory techniques in micro-

biology

SStentorTENTOR

Stentoris a genus of protozoan that is found in slow moving

or stagnant fresh water. The microorganism is named for a

Greek hero in the Trojan War, who was renowned for his loud

voice, in an analogous way to the sound of a trumpet rising up

over the sound of other instruments. The description is fitting

the microorganism because the organism is shaped somewhat

like a trumpet, with small end flaring out to form a much

larger opening at the other end. The narrow end can elaborate

a sticky substance that aids the protozoan in adhering to

plants. At the other end, fine hair-like extensions called cilia

beat rhythmically to drive food into the gullet of the organism.

The various species of stentor tend to be brightly colored. For

example, Stentor coeruleusis blue in color. Other species are

yellow, red, and brown.

Stentorare one of the largest protozoafound in water.

As a protozoan, Stentoris a single cell. Nonetheless, a typical

organism can be 2 mm in length, making them visible to the

unaided eye, and even larger than some multi-celled organ-

isms such as rotifers. This large size and ubiquity in pond

water has made the organism a favorite tool for school science

classes, particularly as a learning tool for the use of the light

microscope. In particular, the various external and internal

features are very apparent under the special type of micro-

scopic illumination called phase contrast. Use of other forms

of microscopic illumination, such as bright field, dark field,

oblique, and Rheinberg illumination, can each reveal features

that together comprise a detailed informational picture of the

protozoan. Thus, examination of stentor allows a student to

experiment with different forms of light microscopic illumi-

nation and to directly compare the effects of each type of illu-

mination of the same sample.

Another feature evident in Stentoris known as a con-

tractile vacuole. The vacuole functions to collect and cycle

back to the outside of Stentorthe water that flows in to balance

the higher salt concentration inside the protozoan. Careful

observation of the individual protozoa usually allows detec-

tion of full and collapsed vacuoles.

For the student, fall is a good time to observe Stentor.

Leaves that have fallen into the water decay and support the

growth of large numbers of bacteria. These, in turn, support

the growth of large numbers of stentor.

See alsoMicroscope and microscopy; Water pollution and

purification

SSterilizationTERILIZATION

Sterilization is a term that refers to the complete killing or elim-

ination of living organisms in the sample being treated.

Sterilization is absolute. After the treatment the sample is either

devoid of life, or the possibility of life (as from the subsequent

germination and growth of bacterial spores), or it is not.

There are four widely used means of sterilization.

Standard sterilization processes utilize heat, radiation, chemi-

cals, or the direct removal of the microorganisms.

The most widely practiced method of sterilization is the

use of heat. There are a number of different means by which

heat can be applied to a sample. The choice of which method

of delivery depends on a number of factors including the type

of sample. As an example, when bacterial spores are present

the heating conditions must be sufficient to kill even these dor-

mant forms of the bacteria.

A common type of heat sterilization that is used many

types each day in a microbiology laboratory is known as incin-

eration. Microorganisms are burned by exposing them to an

open flame of propane. “Flaming” of inoculating needles and

the tops of laboratory glassware before and after sampling are

examples of incineration.

Another form of heat sterilization is boiling. Drinking

water can be sterilized with respect to potentially harmful

microorganisms such as Escherichia coliby heating the water

to a temperature of 212°F (100°C) for five minutes. However,

the dormant cyst form of the protozoan Giardia lambliathat

can be present in drinking water, can survive this period of

boiling. To ensure complete sterility, the 212°F (100°C) tem-

perature must be maintained for 30 minutes. Even then, some

bacterial spores, such as those of Bacillusor Clostridiumcan

survive. To guarantee sterilization, fluids must be boiled for an

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