Disinfection: Methods and Uses

Disinfection is a process whereby pathogenic organisms, but not necessarily all microorganisms or spores, are destroyed. Disinfection may be accomplished by physical or chemical means.

A number of factors influence the activity of disinfectants such as the following:

1. Types of organisms present: Bacterial spores such as Bacillus spp., are the most resistant, followed by mycobacteria (acid-fast bacilli). Younger cells are usually more readily destroyed than mature organisms.
2. Number of organisms present (microbial load): Time necessary for killing microorganisms increases in direct proportion with the number of organisms (microbial load). This is particularly true of instruments contaminated with organic material such as blood, pus, or mucus.
3. Concentration of disinfectant: Generally, higher concentration of chemical disinfectants has higher killing power. Over a short-range, a small increase in concentration leads to an exponential rise in effectiveness but beyond a certain point, an increase in concentration may not raise the killing rate. Sometimes, an agent is more effective at lower concentrations. For example, 70% ethyl alcohol is more effective as a disinfectant than 95% ethyl alcohol.
4. Amount of organics (blood, mucus, pus) present: The organic material should be mechanically removed before chemical sterilization to decrease the microbial load. This is analogous to removing dried food from utensils before placing them in a dishwasher and is important for cold sterilization of instruments such as bronchoscopes.
5. Length of contact time: More is the exposure time to sterilant/disinfectant, the better is the efficacy.
6. Type of water available (hard vs. soft): Hard water may reduce the rate of killing of microorganisms.
7. Temperature and pH of the process: An increase in the temperature at which chemical acts often enhances its activity. A lower concentration of disinfectants can be used at a higher temperature. Heat kills more readily in acidic pH.
8. Nature of surface to be disinfected (e.g., potential for corrosion; porous vs. nonporous surface)

Physical Methods of Disinfection

The three physical methods of disinfection are:

1. Boiling at 100°C for 15 minutes kills vegetative bacteria.
2. Pasteurizing at 63°C for 30 minutes or 72°C for 15 seconds kills food pathogens.
3. Using nonionizing radiation such as ultraviolet (UV) light. UV rays are long wavelength and low energy. They do not penetrate well and organisms must have direct surface exposure, such as working surfaces of a biological safety cabinet (BSC), for this form of disinfection to work.

Chemical Methods of Disinfection

When chemicals are used to destroy all life forms they are called chemical sterilants or biocides; however, these same chemicals used for shorter periods are disinfectants. Disinfectants are chemicals that kill microorganisms and are used on inanimate objects. Chemicals used on living tissue (skin) are called antiseptics.

Chemical disinfectants can be classified into four groups based on their microbicidal activity;

• Low-level disinfectants
• Intermediate-level disinfectants
• High-level disinfectants
• Chemical sterilant

Activity Level of Selected Chemical Disinfectants

Chemical disinfectants comprise many classes, including the following:

1. Alcohols: They are among the most widely used disinfectants and antiseptics. Ethyl or isopropyl alcohol is non-sporicidal (does not kill spores) and evaporates quickly. Alcohols are best used on the skin as an antiseptic (surgical spirit). Clinical thermometers and small instruments can be disinfected by soaking in isopropyl alcohol for 10-15 minutes.
2. Aldehydes: Formaldehyde, glutaraldehyde, and ortho-phthalaldehyde are commonly used aldehydes. They are sporicidal and can be used as chemical sterilants. Formaldehyde (formalin; i.e. formaldehyde in water) is used to preserve anatomical specimens and for fumigation of closed areas, such as operation theaters. They are generally not used as surface disinfectants because of their irritating fumes.
3. Halogens: Halogens, especially chlorine and iodine, are frequently used as disinfectants as they possess antimicrobial activity. They exist in a free state and form a salt with sodium and most other metals.
   1. Chlorine: It is the most commonly available disinfectant. Chlorine is most often used in the form of sodium hypochlorite (NaOCl), the compound known as household bleach. CDC recommends that tabletops be cleaned following blood spills with a 1:10 dilution of bleach. It is also used for municipal water supplies, swimming pools, dairy and food industries, etc.
   2. Iodine: Iodine compounds are widely employed antiseptics. Iodine is prepared either as tincture with alcohol or as an iodophor coupled with a neutral polymer, for example, povidone-iodine.
      
      “70% ethyl alcohol, followed by an iodophor, is the most common compound used for skin disinfection before drawing blood cultures or surgery”.
4. Heavy metals: Salts of heavy metals, such as mercury, silver, zinc, and copper were widely used in the past as germicides but their use has been replaced by less toxic chemicals over the period. Eyedrop solution containing 1% silver nitrate is still instilled in the eyes of newborns to prevent ophthalmia neonatorum (an infection with Neisseria gonorrhoeae).
   “Heavy metals containing mercury are no longer recommended because mercury is toxic to the environment”.
5. Quaternary ammonium compounds (Quats): Quaternary ammonium compounds such as benzalkonium chloride, are used to disinfect bench-tops or other surfaces in the laboratory. However, organic materials, such as blood, may inactivate heavy metals or quaternary ammonium compounds, thus limiting their utility.
6. Phenolics: Phenolics, such as the common laboratory disinfectant amphyl, are derivatives of carbolic acid (phenol). The addition of detergent results in a product that cleans and disinfects at the same time, and at a concentration between 2%-5% these products are widely used for cleaning benchtops.

References and further readings

1. Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
2. Procop, G. W., & Koneman, E. W. (2016). Koneman’s Color Atlas and Textbook of Diagnostic Microbiology (Seventh, International edition). Lippincott Williams and Wilkins.
3. Tille, P. (2017). Bailey & Scott’s Diagnostic Microbiology (14 edition). Mosby.
4. Willey, Joanne M, Sherwood, Linda M, & Woolverton, Christopher J. (2016). Prescott’s Microbiology (10 edition). McGraw-Hill Education.