Sterilization, Disinfectants, and Antiseptics

1. Introduction to Controlling Microbial Growth

There are many reasons why it can be useful to control microbial growth. For example, someone may clean a dinner table before eating or clean a surface before preparing food on it. A doctor needs to reduce the risk of one patient spreading a pathogen to the next patient. Surgical instruments need to be especially clean, or infection after surgery would be common.

Control of microbial growth differs depending on the context. In this lesson, the focus is on microbes on surfaces. For example, one way to reduce the spread of disease is by cleaning the inanimate objects (fomites) on which microbes may be left by one person and picked up by another. In other lessons, you will learn about the ways in which medications are used to control pathogens inside the human body.

term to know

Fomite

An inanimate object that may harbor microbes, potentially allowing them to spread to someone else to cause disease.

2. Definitions and Comparisons of Methods of Control

Two major considerations are especially important in choosing a method of microbial control on an object or surface. First, it is important to consider the way in which an object will be used. For example, a surgical instrument requires a much higher level of cleanliness than a kitchen table. For this purpose, items can be classified as critical (meaning that they must be sterile), semicritical (meaning that they require a high level of disinfection), or noncritical (meaning that they need to be clean but not highly disinfected).

EXAMPLE

Surgical instruments are critical items and must be sterile. A medical instrument that contacts intact skin but does not penetrate it (such as an endoscope used to view the esophagus) is semicritical. Bed linens and similar items are noncritical.

Second, it is important to consider what potential pathogens are being targeted and how resistant they are to antimicrobial treatments. Some microbes, especially those that form endospores, require harsher treatments than more sensitive species.

In many clinical and laboratory settings, aseptic technique is used to prevent the contamination of sterile surfaces. Aseptic technique involves a combination of protocols to prevent microbial contamination. This approach is often used in the microbiology laboratory both to prevent contamination of the work area and to prevent the spreading of microbes from the work area.

In medical work, a sterile field is often designated and is kept free of all vegetative (actively growing) microbes, endospores, and viruses. For example, a sterile field is required around a surgical incision. Strict protocols are required to protect the sterility of the area, including the sterilization of equipment and materials. Introducing microbes into a patient’s body produces the risk of sepsis, a systemic inflammatory response to infection that is serious and can be lethal.

There are a variety of terms used to carefully describe the different types of microbial control. These terms are often used much more loosely in everyday conversation but have very specific meanings in microbiology. These terms are summarized below. In other lessons, you will learn more about the specific agents used for each.

The following terms refer to treatments used on inanimate surfaces or items.

1. Sterilization means completely removing or killing all vegetative cells, endospores, and viruses. An agent capable of sterilization is called a sterilant. Sterilization is important for many medical and laboratory procedures where any possible contamination could pose a problem.
2. Commercial sterilization is a food sterilization protocol that removes potential pathogens but uses a lower heat than needed for complete sterilization to preserve food quality. This approach targets microbes that are especially problematic as causative agents of food spoilage and foodborne disease. For especially dangerous pathogens such as Clostridium botulinum, a wide margin of error is used to increase the probability that the pathogens are effectively eliminated.
3. Disinfection inactivates most microbes on an inanimate surface such as a table or laboratory bench. Because some microbes remain, the disinfected item is not considered sterile. The chemicals used for this purpose are called disinfectants. Good disinfectants are fast-acting, stable, easy to prepare, inexpensive, and easy to use. Disinfection is faster and easier than sterilization, so it is useful when the complete elimination of microbes is unnecessary.
4. Sanitization refers to the removal of microbes to a level considered safe by public health standards. For example, commercial dishwashers used for food service must meet this standard.

1. Antiseptics also inactivate microbes without sterilizing a surface, but are safe to use on living tissues. The process of applying an antiseptic is called antisepsis.
2. Degerming means reducing microbial numbers by gently scrubbing living tissue with a mild chemical such as soap (during handwashing) or alcohol. This process removes some microbes, but not all.

try it

You skin your knee after falling off of a bicycle.

What type of microbial control would most likely be appropriate to clean the skin of your knee?

Degerming and, if the skin is sufficiently damaged, antisepsis.

try it

You are preparing surgical instruments for major surgery.

What type of microbial control would most likely be appropriate to prepare the instruments?

Sterilization would be required.

terms to know

Critical

An item that must be sterile, such as an instrument that will be used inside a human body.

Semicritical

An item that requires a high level of disinfection, such as an endoscope to be inserted into the esophagus to view its surface.

Noncritical

An item that must be clean but not highly disinfected, such as bed linens.

Aseptic Technique

A combination of protocols used to prevent the contamination of sterile surfaces.

Sterile Field

A designated area kept free of all vegetative microbes, endospores, and viruses.

Sepsis

A systemic inflammatory response to infection that is serious and can be lethal.

Sterilization

Completely removing or killing all vegetative (actively growing) cells, endospores, and viruses.

Sterilant

An agent capable of sterilization.

Commercial Sterilization

A food sterilization protocol that removes potential pathogens but uses a lower heat than needed for complete sterilization to preserve food quality.

Disinfection

A process that inactivates most microbes on an inanimate surface.

Disinfectant

A chemical used for disinfection.

Antiseptic

Something that inactivates microbes without sterilizing a surface but that is safe to use on living tissues.

Antisepsis

The process of applying an antiseptic.

Sanitization

The removal of microbes to a level considered safe by public health standards.

Degerming

Reducing microbial numbers by gently scrubbing living tissue with a mild chemical.

3. Measuring Microbial Control

There is terminology to describe the extent to which different physical and chemical methods affect microbes. The suffix -cide (or -cidal) indicates that something kills, whereas the suffix -stat (or -static) indicates that something inhibits growth. For example, a virucide kills viruses and a fungicide kills fungi. In contrast, a bacteriostatic chemical inhibits growth without killing the bacteria already present. Whether a particular treatment is -cidal or -static is determined by the types of microorganisms targeted, the concentration of the chemical used, and the type of treatment applied (e.g., the type of chemical or physical method used).

did you know

You may wonder why someone would choose an agent that inhibits growth rather than one that simply kills microbes. Methods that inhibit growth are often less toxic and may be less damaging to objects. For a healthy person, inhibiting the growth of a microbe may be sufficient for their immune system to eliminate it. All of these factors must be considered in choosing the best approach to use in a particular situation.

The image below shows a microbial death curve, which is used to describe the progress and effectiveness of a particular protocol. Over time, exposure to a protocol will lead to a fixed percentage of microbes in a population dying per unit of time. The rate of death is constant regardless of the initial population size, so it is often valuable to know the percentage of the population that has been killed. As shown in the image below, this can be useful to plot death curves as semilog plots because the reduction in microorganisms is typically logarithmic. The D-value labeled on the image is the decimal reduction time (DRT), which is the amount of time it takes for a specific protocol to kill 90% of the population.

terms to know

Microbial Death Curve

A graph used to describe the progress and effectiveness of a particular protocol.

Decimal Reduction Time (DRT)

The amount of time it takes for a specific protocol to kill 90% of the population.

make the connection

If you're taking the Microbiology Lab course simultaneously with this lecture, it's a good time to try the “Bacterial Growth Curves: Experiment with bacterial growth” Activity in Unit 5 of the Lab course. Good luck!