Today we will discuss control of bacterial or microbial growth.
Let's start with antimicrobial agents. A multi-million dollar industry has been created
around the idea of keeping unwanted microorganisms from invading our homes and our bodies.
Anytime you walk down the cleaning aisle of your local grocery market,
you are bombarded with products having words like, 'sanitizer', 'disinfectant', and 'antibacterial'.
Let's look at what these words really mean. Sanitation is a process that reduces the
microbial population to what is considered to be a "safe level", according to public health
agencies. The FDA sets regulations for what is considered acceptable amounts of feces, mold,
bacteria, rodent hair, bug parts, and larvae! The Food and Drug Administration, also known as
the FDA, establishes maximum levels of natural or unavoidable defects in foods for human consumption
that, according to the FDA, present no health hazard. Let's look at some of these allowable
contaminants in our food. White fish is allowed to have 50 parasitic cysts per 1000 pounds. Fig
paste is allowed to contain no more than 13 insect heads per 100 grams of fig paste. Peanut butter is
allowed to have no more than 30 insect fragments per 100 grams. Raisins are allowed to have
no more than 10 whole or equivalent insects and 35 drosophila eggs per 8 ounces. STERILIZATION.
Sterilization is the complete destruction of any and all types of microorganisms. Therefore,
you can sterilize a countertop or a medical instrument, but you cannot sterilize living
tissue. For example, it would be incorrect to say that the nurse had "sterilized" a wound.
DISINFECTION. Disinfection refers to the destruction of pathogens or
microorganisms that cause disease. You can disinfect living or non-living surfaces.
Disinfectants are used to disinfect non-living surfaces. A disinfectant is an antimicrobial agent
that is used on non-living surfaces or an object. Antiseptics are used to disinfect living surfaces.
An antiseptic is an antimicrobial agent that is used on living tissue, such as the skin's surface.
The control of microbial growth. Why do we need or want to control microbial growth? In short,
we must control the growth of microorganisms to prevent pathogens from reaching the body.
Methods of controlling bacterial growth. There are many different methods by which microorganism
growth can be controlled. The methods of microorganism growth control are grouped into two
broad categories: physical methods and chemical methods. The first method that we will explore,
is extreme temperatures. Control of microbial growth, as used in this tutorial, means to
inhibit or prevent growth of microorganisms. This control is affected in two basic ways:
first, by killing microorganisms or second, by inhibiting the growth of microorganisms.
Agents which kill cells are called -cidal agents. -Cidal agents are agents which inhibit the growth
of cells killing them. Agents that inhibit the growth of cells without killing them
are referred to as static agents. For example, the term bactericidal refers to killing bacteria
and bacteriostatic refers to inhibiting the growth of bacteria. A bactericide kills
bacteria. A fungicide kills fungi and so on. In microbiology, sterilization refers to the
complete destruction or elimination of all living organisms on a surface or a medical instrument.
For a substance being sterilized, there are no degrees of sterilization; an object or a
substance is either sterile or not. Sterilization procedures involve the use of heat, radiation,
chemicals, or physical removal of cells. One of the most popular methods of sterilization
is the use of heat. For sterilization, one must consider the type of heat, and most importantly,
the time of application, and temperature, to ensure destruction of all microorganisms.
Endospores of bacteria are considered the most resilient to high temperatures of all cells,
so their destruction guarantees sterility. One method of applying heat, is incineration.
Incineration burns organisms and physically destroys them. This is used for needles,
inoculating wires, glassware, and objects not destroyed in the incineration process.
Another method of applying heat, is boiling. For example, when we boil a substance at 100
degrees Celsius for 30 minutes, this kills everything except some endospores. In order
to kill endospores, and therefore sterilize the solution, you need to have a very long time of
boiling of more than six hours, or intermittent boiling is required. Another useful method of
heat application to control microbial growth, is autoclaving. Autoclaving is steam under pressure
using an autoclave or a pressure cooker. Autoclaving is the most effective and most
efficient means of sterilization. All autoclaves operate on a time-temperature relationship.
These two variables are extremely important. Higher temperatures ensure more rapid killing of
microorganisms. The usual standard temperature - pressure that is employed, is 121 degrees Celsius
and 15 psi for 15 minutes. Longer times are needed for larger loads, large volumes of liquid,
and more dense materials. Autoclaving is ideal for sterilizing biohazardous waste,
surgical dressings, glassware, microbiologic media, liquids, and medical instruments, as well.
However, certain items, such as plastics, and certain medical instruments, such as fiber
optic endoscopes, cannot withstand the autoclave process. These items should be sterilized with a
chemical or gas sterilant. When proper conditions and time are employed, no living organisms will
survive the trip through the autoclave. Ovens can be used that produce dry heat. The spores of
a bacterium are eliminated with dry heat after exposure to 160 degrees Celsius for two hours,
versus those same spores of a bacterium can be eliminated using an autoclave in only 15 minutes.
We can also use refrigeration and freezing to control microbial growth. Refrigerator incubation
slows down microbial growth, but does not kill microorganisms. This is why things in
your refrigerator might go bad after some time. Freezing temperatures effectively halt microbial
growth, but it does not kill microorganisms. This is why we can store things in the freezer for some
time, but after thawing the decaying process will renew. In laboratories, liquid nitrogen
is often used to preserve cells and tissues. Liquid nitrogen is usually between negative 210
degrees Celsius and negative 196 degrees Celsius. Another method of controlling microbial growth
is dehydration. Dehydration involves taking water out of the environment, which will halt microbial
growth. This method is widely used in the preservation of foodstuffs. Methods of dehydration
include sun drying, such as we see in dried fruits, and beef jerky, and the addition of salt,
or copious amounts of sugar, which draws out the water using osmosis. These foods are not sterile,
but they are considered preserved. Let's look at some of the Chemical methods for controlling
bacterial growth. We'll first look at ionizing radiation or UV light. UV light or ultraviolet
light forms free radicals which damages microbial DNA and proteins. However, if the microbes are
in the form of a spore, it may experience some protection from the effects of ionizing radiation.
Ethyl alcohol or isopropyl alcohol at 70% or higher concentrations, are great anti-microbial
agents that are routinely used in hospitals and laboratories. Ethyl alcohol kills bacteria
through two mechanisms: protein degeneration and dissolving the lipid membrane. Detergents are also
often used to control microbial growth. Detergents act by inserting themselves into the plasma
membrane of microorganisms. When this occurs, the cytoplasm of the microorganism leaks out and
death follows. FORMALIN. Formalin, formaldehyde, and glutaraldehyde are toxic and carcinogenic,
but they have good antimicrobial qualities. This is commonly used for tissue preservations in
hospitals and laboratories. Halogens are also used. Halogens include bleach, chlorine,
and iodine. Peroxidases are another commonly used chemical to fight microbial growth. Peroxides
release free oxygen, which leads to many chemical reactions in the microorganism that ultimately
leads to death. Organic matter can interfere with antimicrobial agents. The presence of organic
matter, such as the feces, urine, or blood, reduces the effectiveness of antimicrobial agents.
This is because, they react with the antimicrobial agent so that none of the agent is available to
kill the unwanted microorganism. Another reason for this, is that the physical coding on the
microorganism may provide a protective barrier between the microbe and the antimicrobial agent.