Chemical Control of Microorganisms

1. Introduction to Chemical Methods of Controlling Microbial Growth

You have learned about the physical methods to control microbes, but other approaches are used as well. Chemical methods are commonly used to control microbes and vary considerably in their mechanisms of action and preferred applications.

When choosing a chemical method of control, it is important to consider the effect that the chemical will have on the microbe. However, it is also important to consider how the chemical will work in the particular environment needed and how it will affect surfaces and equipment. Additionally, the safety of chemical agents varies considerably.

2. Phenolics

As you learned in other lessons, phenolics were the first surgical disinfectants used through the pioneering work of British surgeon Joseph Lister (1827–1912). Based on Lister’s work, American chemist Joseph Lawrence (1836–1909) developed an alcohol-based mixture called Listerine that is still used as an oral antiseptic.

Phenol (carbolic acid) is no longer used as a surgical disinfectant because it causes skin irritation, but related chemicals called phenolics are found in mouthwashes and throat lozenges.

In general, phenolics are stable, persistent on surfaces, and less toxic than phenol. They inhibit microbial growth by denaturing proteins and disrupting membranes. Phenol itself consists of a benzene ring with an alcohol (OH) group attached, and phenolics all have this as part of their structure.

EXAMPLE

At present, the most commonly used examples of phenolic compounds are hexachlorophene and triclosan. Hexachlorophene is a bisphenol commonly used as a disinfectant and as a handwashing agent in laboratory and clinical settings. Triclosan is also a bisphenol and has been used in a wide range of products. It was used in many soaps, but many of these uses were banned by the FDA because of a lack of evidence of efficacy or safety and concerns about possible health risks. However, triclosan is still used in some products such as toothpaste.

3. Heavy Metals

Heavy metals were some of the first chemical disinfectants and antiseptics but are rarely used now because of their toxicity and ability to build up in the body. Heavy metals act by binding to proteins and inhibiting enzymes.

EXAMPLE

Heavy metals that have been used for the chemical control of microbes include mercury, silver, copper, nickel, and zinc. Silver is still sometimes used in creams (Silvadene) and as an antimicrobial component in catheters and bandages.

4. Halogens

Halogens are elements that are all found in the same column of the periodic table (the 17th column from the left, also called group 17).

EXAMPLE

The halogens commonly used for disinfection are iodine, chlorine, and fluorine.

Chlorine is added to water to help make it safe to drink. Chlorine gas added to water produces the strong oxidant hypochlorous acid that can enter cells. Sodium hypochlorite is the active ingredient in household bleach, which is used for a wide range of disinfectant purposes but should not be mixed with other chemicals because of the risk of releasing dangerous chlorine gas.

Chloramines are relatively stable compounds that resemble ammonia (nitrogen bound to three hydrogen atoms) except that one to three of the hydrogen atoms are replaced by chlorine. These molecules are very stable and slowly release chlorine over time. Chloramines are also added to drinking water.

Other chlorine compounds used to disinfect water include sodium dichloroisocyanurate (NaDCC) and chlorine dioxide gas. NaDCC appears to be helpful for water disinfection, and tablets are available for general use and in the military.

Despite their value as disinfectants, chlorinated compounds can cause irritation for some individuals and may not eliminate certain pathogens (e.g., the protozoan parasite Cryptosporidium).

Fluorine also has antimicrobial properties and is most commonly used to maintain oral health. Fluoride is often added to water supplies and is found in most toothpaste. When fluoride becomes incorporated into the hydroxyapatite of tooth enamel, it helps the tooth resist erosion by acidic fermentation products of oral microbes. It appears to have bacteriostatic qualities when present on teeth.

5. Alcohols

Alcohols are very commonly used as both disinfectants and antibiotics.

EXAMPLE

The alcohols most frequently used are ethyl alcohol (ethanol) and isopropyl alcohol (isopropanol or rubbing alcohol). They are a common sight in medical offices and first aid kits. They are ingredients in hand sanitizers and are used to clean injection sites.

These chemicals work by denaturing proteins and disrupting cell membranes. They are most effective at concentrations of about 70%, not 100%, because they more effectively penetrate cells at these concentrations.

Alcohols are generally bactericidal and fungicidal but may be virucidal for enveloped viruses because they disrupt the envelope. They can inhibit sporulation and germination but are not sporicidal.

In addition to their use independently, alcohols are often used to make tinctures with other antiseptics such as iodine.

6. Surfactants

Surfactants (surface-active agents) are chemical compounds that lower the surface tension of water.

EXAMPLE

They are major ingredients in soaps and detergents.

Soaps are salts of long-chain fatty acids and have both polar and nonpolar regions, as shown in the image below. In the image, there is a polar carboxylic acid group on the left consisting of carbon double-bonded to one oxygen atom and single-bonded to an oxygen atom carrying a negative charge. A sodium ion () is attracted by the negative charge. The rest of the molecule is a long, nonpolar hydrocarbon tail.

Because they have polar and nonpolar regions, these molecules can surround nonpolar molecules such as oils to help them travel in water (which would otherwise repel them). Soaps can help to carry away microbes even though they do not kill microbes or inhibit growth. However, chemicals can be added to soaps to give them disinfectant or antiseptic properties.

Soaps tend to form films that can be difficult to wash away, but detergents containing synthetic surfactant molecules do not leave these deposits. Anionic detergents have a negatively charged anion at one end, whereas cationic detergents have a positively-charged cation at one end.

Cationic detergents include quaternary ammonium salts (quats), which are antiseptics and detergents that disrupt cell membranes. The cationic charge appears to contribute to their antimicrobial properties. These molecules are stable, nontoxic, inexpensive, colorless, odorless, and tasteless. They are bactericidal because of their ability to disrupt membranes. Although they can also be helpful against fungi, protozoans, and enveloped viruses, they do not harm endospores. Benzalkonium chlorides are examples that are found in a variety of products.

terms to know

Surfactants (Surface-Active Agents)

Chemical compounds that lower the surface tension of water.

Quaternary Ammonium Salts (Quats)

Antiseptics and detergents that disrupt cell membranes.

7. Bisbiguanides

Bisbiguanides are cationic molecules with antiseptic products.

EXAMPLE

Chlorhexidine is an important example that is effective against a variety of bacteria and yeasts. Chlorhexidine disrupts cell membranes and can be bacteriostatic or bactericidal depending on the concentration. It causes the cytoplasmic contents of cells to congeal and also has some effectiveness against enveloped viruses.

Chlorhexidine is most commonly used as a surgical scrub, for handwashing in medical settings, and for topical antisepsis before surgery. It is also used in some oral rinses. It provides long-lasting antimicrobial activity when used on the hands.

Although less commonly used than chlorhexidine, alexidine is a faster-acting bisbiguanide that is sometimes used as a surgical scrub and oral rinse.

8. Alkylating Agents

Alkylating agents are strong disinfecting agents that act by replacing a hydrogen atom within a molecule with an alkyl group (a combination of carbon and hydrogen atoms with n carbon atoms for every 2n + 1 hydrogen atoms). These agents inactivate nucleic acids.

A formaldehyde solution (sometimes called formalin), sometimes used in a gaseous form, can kill bacteria, viruses, fungi, and endospores. Because it can sterilize at a low temperature, it can be valuable. It also cross-links proteins and is useful as a preservative. However, it is very irritating to living tissues and is carcinogenic (cancer-causing), which limits its usefulness.

Glutaraldehyde is also a carcinogen and is similar to formaldehyde, but has two reactive aldehyde groups instead of one and acts more quickly. It is commonly used for disinfection but, like formaldehyde, is irritating and not used on the skin.

Another alkylating agent is o-phthalaldehyde (OPA), which has two reactive aldehyde groups linked by an aromatic bridge. It is less irritating than glutaraldehyde, has a minimal odor, does not require processing before use, and is more effective against mycobacteria.

Ethylene oxide is an alkylating agent that is highly useful for sterilizing materials that cannot be subjected to high temperatures. It can penetrate plastic bags to sterilize medical and laboratory supplies. However, it must be used with care as it is carcinogenic and highly explosive.

β-Propionolactone is an alkylating agent with a different chemical structure from formaldehyde, glutaraldehyde, and OPA. It binds to DNA, and this inactivates the DNA. It can be used as a clear liquid or vapor and can kill endospores. It can be used to sterilize medical instruments, tissue grafts, nutrient broths, blood plasma, milk, and water. It is sometimes used in small quantities in vaccines to prevent microbial growth. However, it can also be an irritant and is known to be carcinogenic in animals. It can cause permanent damage to the eyes, kidneys, and liver. Therefore, it must also be used with caution.

9. Peroxygens

Peroxygens are strong oxidizing agents that can be used as disinfectants or antiseptics.

EXAMPLE

The most commonly used example is hydrogen peroxide, which can be used as a solution or gas. It produces highly reactive oxygen species that damage cellular macromolecules. It can be very effective against a range of bacteria, fungi, viruses, and endospores, but microbes that produce catalase have a tolerance to low concentrations because they can convert it to water and oxygen. High concentrations are needed to kill endospores.

10. Supercritical Fluids

There has been increasing interest in recent years in using supercritical fluids, especially supercritical carbon dioxide, as a sterilant. Materials are placed in a chamber in which carbon dioxide has been brought to about 10 times atmospheric pressure, at which point it reaches a supercritical state and is highly capable of penetrating surfaces. Once it enters cells, supercritical carbon dioxide forms carbonic acid and lowers the pH.

EXAMPLE

Supercritical carbon dioxide kills a range of vegetative cells and can kill endospores when combined with peracetic acid. Increasing the temperature or using rapid cycles of pressurization and depressurization can increase its effectiveness.

Benefits include its ability to be used at a low temperature, its low reactivity, and its trait of not being toxic or flammable. It is often used to treat foods and medical devices. Additionally, it is being used more often to disinfect tissues prior to transplantation and can be used to kill eggs and larvae in agricultural products.

11. Food Preservatives

In another lesson, you learned how desiccation and increasing solute concentration (e.g., by candying or salting) can help to preserve food. However, chemicals can also be used as food preservatives. These chemicals must be able to inhibit microbial growth but be safe to consume.

EXAMPLE

These preservatives include sorbic acid, benzoic acid, and propionic acid. Sorbic acid appears to inhibit cellular enzymes. Benzoic acid appears to lower intracellular pH, interfering with metabolic processes and the uptake of molecules. Propionic acid is thought to inhibit enzymes and decrease intracellular pH. It is more effective at a higher pH than benzoic acid.

Nitrates are added to processed food and prevent Clostridium botulinum endospores from germinating, reducing the risk of food poisoning. Nitrates are reduced to nitric oxide, and this appears to interfere with ATP synthesis by bacteria. During cooking, nitrosamines can be formed. Because these are carcinogenic, this is a limitation of the use of nitrates.

Some natural antimicrobial products can be used as food preservatives. Nisin is produced by Lactococcus lactis and disrupts cell wall production. Natamycin is an antifungal antibiotic produced by Streptomyces natalensis. Natamycin is used to preserve dairy products and meats.

The table below provides an overview of all of the chemical disinfectants mentioned, including their mode of action and examples of their uses.