Controlling Microbial Growth

Types of Chemical Agents

Phenols, halogens, alcohols, heavy metals, and detergents are the main types of chemical agents used in controlling microbial growth.

Chemical agents include phenols, halogens, alcohols, heavy metals, and detergents. Each has one or more specific mechanisms of action, and each has benefits and drawbacks.

When selecting the chemical agent to use for disinfection or sterilization, it is important to consider several factors. These include the agent's solubility, penetration, resistance, corrosiveness, environmental impact, and affordability.

  • Solubility refers to the agent's ability to dissolve in water or mix with it. Depending on the use, either water-soluble or water-insoluble agents might be preferred.
  • Penetration refers to how far the agent will reach, especially with regard to living tissue. It is important that the agent be able to act in all areas where contamination can occur.
  • Resistance refers to how likely it is that the target microbe(s) will not be affected by the agent. The short generation times of bacteria create significant risk that incorrect use of agents such as antibiotics and chemical cleansers will select for bacteria that are resistant to those antibiotics and cleansers.
  • Corrosiveness refers to the extent to which the agent will damage metal. In surgical settings, where metal tools are often used on living tissue, it is vital that the tools remain undamaged by the application of disinfectants.
  • Environmental impact refers in this instance to how easy it is to get rid of the disinfectant following use. Some disinfectants pose a public health hazard if dumped into the environment and must be sequestered or neutralized before being disposed.
  • Affordability refers to how much the agent costs. The user must balance the need for quality against the need for low cost, depending on application.

Scientists use many methods to evaluate the effectiveness of various agents. One popular method is the use-dilution test, which tests how a disinfectant performs against a high concentration of a known microorganism in a nonporous object. A nonporous material, such as glass beads, is dipped in a colony of the microorganism and incubated. After the object is populated by the microorganism, it is submerged in the disinfecting agent. It is then incubated again so any remaining microbes can be seen. Another method is the disk diffusion test. A disk diffusion test is used to test the sensitivity of bacteria. Bacteria are grown on an agar plate, and wafers containing the antimicrobial being tested are also placed on the plate, which is left to incubate. The area around the wafer where the bacteria have not grown is called the zone of inhibition, and its size can point to whether the bacteria are susceptible or resistant to the antimicrobial.

Phenols

Phenols are aromatic organic compounds that provide long-lasting disinfection by denaturing microbial proteins.

Phenols are aromatic (containing a ring-shaped molecular structure) organic compounds that denature proteins. They are generally insoluble in water and are corrosive. Phenols are good disinfectants to use because their action lasts for a long time, they are able to disinfect surfaces for several days at a time, and they are not inhibited by organic materials. Phenol is commonly used in throat lozenges, surface cleaning sprays, and antibacterial soaps and gels. Some phenols, such as amylphenol, are particularly effective against vegetative bacteria, fungi, and some viruses.

Phenol's remarkable ability to kill microbes makes it the agent against which other chemical agents are measured. The phenol coefficient of any chemical agent is the agent's ability to kill microbes as compared to phenol. To calculate this number, the degree of dilution of the agent in question for which it kills the microbe within a given time is divided by the degree of the dilution of phenol that kills the same microbe in the same time under the same conditions. If the phenol coefficient is greater than 1, the agent is more effective than phenol. If the phenol coefficient is less than 1, the agent is less effective than phenol.

Phenols

Phenols are aromatic organic compounds that denature proteins. They make good disinfectants because their action lasts for a long time and they are not inhibited by other organic materials.

Halogens

Halogens (group 17 elements) are highly electronegative and disrupt the structure of microbes by reacting with essential biomolecules.

Halogens are the elements in group 17 of the periodic table—fluorine, chlorine, bromine, iodine, and astatine. They have antimicrobial properties because they are highly electronegative, meaning they have a strong tendency to attract electrons towards themselves. This tendency causes halogens to react readily with other elements. These reactions alter the chemical structure of the microbe, disrupting key structures and functions such as enzymes, proteins, or lipid membranes.

Fluorine has limited antimicrobial properties. It is most effective at controlling the growth of bacteria in the mouth. For this reason, fluorine is added to drinking water in many places.

Chlorine is commonly added to water to form hypochlorite, the active ingredient in bleach. It both kills bacteria and inactivates viruses and is often added to swimming pools and drinking water. However, chlorine is inactivated by the presence of organic materials, carbon-based compounds.

Bromine is an effective antimicrobial agent in gaseous form and so is often used to fumigate soil. Bromine is also used as an alternative to chlorine in pools and hot tubs because it has a less offensive smell.

Iodine tinctures were among the first antiseptics ever used. Today, iodine preparations are often used as medical antiseptics. They take several minutes to activate and are active against bacteria, viruses, fungi, and amoebas. However, many people are allergic to iodine, so care must be taken when using this agent.

Astatine is radioactive and is therefore not commonly used as an antiseptic.

Halogens

Halogens (the elements in group 17 of the periodic table) have antimicrobial properties because they are highly electronegative, reacting quickly with other elements to produce toxic effects in microbes. Fluorine, chlorine, bromine, and iodine are the most commonly used halogens.

Alcohols

Alcohols are hydroxyl-containing organic compounds that are accessible and inexpensive disinfectants.

Alcohols are organic compounds containing a hydroxyl group (OH\mathrm{-OH}) bound to a carbon. The most common alcohols are ethanol (found in drinking alcohol), isopropanol (rubbing alcohol), and methanol (used in windshield wash). Alcohols are useful disinfectants that can evaporate quickly without leaving a residue. However, this quick evaporation produces flammable fumes and often means they have limited use because they evaporate before having enough time to kill the microbes. Alcohols are accessible and inexpensive, which makes them useful for everyday use and in a hospital setting.

Alcohols work both as surfactants and also as protein denaturation agents. Alcohols work best when diluted with water. At 100% concentration, alcohol absorbs water from cells, dehydrating them and causing the cell walls and membranes to form protective layers around the cell. This can preserve bacterial cells, reducing the antiseptic properties of the alcohol. However, when alcohols are diluted with water, this problem is overcome, and the alcohol can penetrate the cell and denature the proteins within it.

Due to the use of ethanol in alcoholic beverages, it is more regulated by governments than isopropanol. Thus, isopropanol, not ethanol, is more commonly used as a disinfectant. Solutions of 70% isopropanol are often used to disinfect wounds and surfaces.

Alcohols

Alcohols are organic compounds that contain a hydroxyl group (OH\mathrm{-OH}). The most common are ethanol, isopropanol, and methanol. Alcohols work both as surfactants and as protein denaturation agents.

Heavy Metals

Heavy metals (metallic elements with high atomic weights) covalently bond with microbial proteins and denature them.

Heavy metals are metallic elements with high atomic weights. The heavy metals often used as antimicrobials include selenium, mercury, copper, and silver. Heavy metals bond covalently with the proteins and enzymes of microbes, thus denaturing and inactivating them.

Selenium is an effective antifungal agent. It is often used in ointments, soaps, and shampoos to treat fungal infections of the skin.

Mercury kills vegetative bacteria on the skin but is not effective against endospores. Thimerosal, an organic mercury compound, has often been used as a preservative in vaccines, though controversy has reduced its use in modern vaccines. Organic mercury compounds are still used to treat wounds on the skin.

Copper is often used to control algal growth, especially in swimming pools. Because organic materials can inhibit the antimicrobial properties of chlorine, the use of copper sulfate to control algal growth in combination with hypochlorite to control bacterial growth is very common. Copper sulfate is also used to control algal growth in air conditioning ducts.

Silver has excellent antibacterial properties and has been used in many hospitals to prevent gonococcal infections. Silver can also be used to coat surfaces, such as countertops, dishwashers, medical instruments, and bandages, where it provides significant antimicrobial protection. New uses for silver as an antiseptic agent are currently under investigation.

Detergents

Detergents are common surfactants such as soaps that remove microbes, dirt, and oils.

A detergent is a surfactant or mixture of surfactants that can remove microbes, dirt, and oils. Detergents are usually the first option in antisepsis because of their affordability and high effectiveness. The effectiveness of a detergent is greatly enhanced by scrubbing. Simply washing hands with a detergent such as soap and water removes over 90% of bacteria on the hands. Many soaps contain additional antibacterial agents, but these have not been shown to increase their effectiveness in eliminating bacteria as compared to soaps without these agents.

The efficacy of detergents comes from multiple modes of action. Primarily, when diluted in small amounts in water, detergents allow water to penetrate small crevices in the skin, physically washing microbes away. Furthermore, detergents are surfactants and thus can disrupt the cell membranes of bacteria. They can also alter the structure of the protein coats of viruses, inactivating them.

Detergents are often used in combination with other chemical agents, such as alcohols (using hand-sanitizing solution after hand washing) and halogens (adding bleach to a load of laundry). This can significantly enhance the antimicrobial effectiveness of both chemical agents.

Results of Detergents and Other Chemicals for Good Hand Hygeine

Detergents are surfactants (or a mixture of surfactants) that can remove microbes, dirt, and oils. They can be effective antiseptic and antibacterial agents. Combining a detergent, such as soap, with other chemical agents, such as alcohols or halogens, increases the detergent's effectiveness. Washing hands with soap removes 92% of germs, and following up with hand sanitizer can remove most of the remaining germs.