Controlling Microbial Growth

Sterilization and Disinfection

Sterilization describes the statistically complete (99.9999%) destruction of all microbes, their spores, and endospores, while disinfection is a less extensive process that destroys 99.99% to 99.999% of pathogenic microbes and other microorganisms.

It is often useful for scientists in many different disciplines to control microbial growth in order to prevent contamination and provide safe working spaces. Controlling microbial growth is limiting the growth of microbes or eliminating them entirely through the use of both physical and chemical agents. In some disciplines, such as food microbiology, the goal is to reduce microbial growth below specific thresholds, while in others, such as surgery, the goal is to eliminate microbes entirely. Control of microbial growth is important to many industries and to public health. There are two primary levels of microbial growth control: sterilization and disinfection. Of these two levels, sterilization is the more severe and extensive.

Sterilization is the statistically complete (99.9999%) destruction of all microbes, their spores, and endospores from a surface, object or fluid. Because sterilization entails destroying 99.9999% of all microbes, there are no degrees of sterilization. Thus, sterilization is an "all-or-nothing" process. If any viable microbes, their spores, or endospores are detected, the object, surface, or fluid is not considered sterilized.

In comparison, disinfection is the destruction of 99.99% to 99.999% of all pathogenic microbes and other microorganisms, but not necessarily their spores and endospores, from a surface, object, or liquid. Disinfection tries to kill or inactivate microbes in order to decrease the level of microbes to a safe level. Furthermore, because disinfection only reduces the number of microbes rather than eliminating them completely, degrees of disinfection do exist. An appropriate threshold for disinfection depends on the application of the surface or fluid in question. For example, the acceptable level of microbes present in a wound bandage after disinfection may be lower than the acceptable level of microbes present on a drinking straw. Sanitization is a type of disinfection that reduces levels of microbial pathogens, especially in food and water, to amounts deemed safe by public health agencies. When applied to food and beverages, antimicrobials used for sanitization are less severe, extensive, and hazardous than other disinfectants. Antimicrobials used for sterilization are more severe, extensive, and hazardous than are sanitizers or stronger disinfectants.

Sterilization vs. Disinfection

Sterilization requires the death or removal of all organisms and all forms of organisms. Disinfection requires that the number of living organisms has been reduced to safe levels.
Antisepsis is the practice of using substances to control the growth of microbes on living skin. These substances are known as antiseptics. Disinfectants are used to remove contamination from nonliving objects whereas antiseptics are used to destroy infection-causing microbes on living tissues. The type of microbial control of an antiseptic is based on its name. Substances that affect bacteria begin with the prefix bacteri-, while those that affect fungi begin with the prefix fungi-. Substances that kill microbes end with the suffix -cide, while those that inhibit growth of microbes end with the suffix -static. The prefix germi- is used to denote all forms of microbes, rather than specifying bacteria or fungi.

Antiseptics may be germicides, or they may be microbiostatic. A germicide is a substance that kills microbes. A germicide may be a bactericide, a substance that kills bacteria, or a fungicide, a substance that kills fungi. Microbiostatic means that a substance inhibits growth of a microbe but does not kill it. The organism is not removed from the surface of tissue, it is just prevented from growing. As with germicides, microbiostatic substances may be bacteriostatic, causing microbial replication to fail inhibiting the growth of bacteria, or fungistatic, inhibiting the growth of fungi.

Purposes of Antiseptics

Antiseptic Purpose
Germicide Kill microbes
Bactericide Kill bacteria
Fungicide Kill fungi
Microbiostatic Inhibit growth of microbes
Bacteriostatic Inhibit growth of bacteria
Fungistatic Inhibit growth of fungi

Antiseptics come in several different forms and work to control the growth of microbial agents on different surfaces.

Decontamination refers to the removal of harmful physical, chemical, or biological agents from an object, surface or fluid. In microbiology, decontamination is concerned with removing microbes that could negatively affect an organism or biological process. Harmless microbes are not a concern in decontamination. What is considered harmful varies according to application. For example, the bacterium Staphylococcus aureus is considered a harmful contaminant in medical settings because it can cause infections in patients, while the bacterium Lactobacillus brevis is considered a harmful contaminant in brewing beer because it interferes with the brewing process.

Some microbes are more easily controlled than others. Some gram-positive bacteria can form an endospore, which is a toughened, condensed form of cell that can withstand harsh environmental conditions and is formed completely within the bacterial cell and is released when the cell dies. Endospores have thick, calcium-rich spore coats and little enzyme activity within their resting cells. They are much more difficult to kill than vegetative bacterial cells, which are the cells that typically exist in favorable environmental conditions.

Viruses are more difficult to control than living organisms. Since they are nonliving pathogens only active within living cells, viruses cannot be killed. Instead, they must be inactivated. Because viruses contain few, if any, enzymes and do not carry out metabolic processes, agents that interfere with enzyme function or interrupt metabolic processes are ineffective against them. For this reason, antibiotics are ineffective against viruses and should not be prescribed or taken to combat viral infections.

Prions are the most difficult pathogens to control. Prions are not living things; rather, they are misfolded proteins that cause the proteins around them to become misfolded as well. Prions are responsible for Creutzfeldt-Jakob disease and bovine spongiform encephalopathy, also known as mad cow disease. Like viruses, prions cannot be killed because they are not alive. Control of prion contamination requires agents that denature the prions or break them apart.