Fundamentals of Microbiology

Lesson 20. CONTROL OF MICROBIAL GROWTH BY CHEMICAL METHODS

Module 5. Microbial growth and nutrition

Lesson 20

CONTROL OF MICROBIAL GROWTH BY CHEMICAL METHODS

20.1 Introduction

Chemical agents are the disinfectants that kill microorganisms, but not necessarily their spores, but are not safe for application to living tissues; they are used on inanimate objects such as tables, floors, utensils, etc. e.g. hypochlorites, chlorine compounds, lye, copper sulfate, quaternary ammonium compounds, formaldehyde and phenolic compounds (Table 20.1).

Table 20.1 Chemical methods of control of microbial growth

20.1.1 Phenol and phenolics

Phenolics exert their action by injuring the lipid-containing plasma membrane which results in leakage of cellular contents. Mycobacteria are susceptible to phenolics due to their rich lipid content e.g. Cresols (O-phenylphenol, main ingredient in Lysol), bisphenols (Hexachlorophene, used in pHisoHex, effective against Gram positive cocci), triclosan (soap, toothpaste, plastics kitchenware; Gram positive and fungi).

20.1.2 Biguanides

Chlorohexedine damages plasma membranes of vegetative cells and is broad spectrum. These are commonly used for surgical hand scrubs. These are effective against most vegetative bacteria and fungi. Mycobateria, endospores, and protozoan cysts are not affected.

20.1.3 Halogens

Some halogens (iodine and chlorine) are used alone or as components of inorganic or organic solutions. Iodine may combine with certain amino acids to inactivate enzymes and other cellular proteins. Iodine is available as a tincture (in solution with alcohol) or as an iodophor (combined with an organic molecule) like in Betadine. The germicidal action of chlorine is based on the formation of hypochlorous acid when chlorine is added to water. It is an excellent oxidizing agent. Chlorine is used as a disinfectant in gaseous form (Cl₂) or in the form of a compound, such as calcium hypochlorite, sodium hypochlorite, sodium dichloroisocyanurate, and chloramines.

20.1.4 Alcohol

Alcohols exert their action by denaturing proteins and dissolving lipids. In tinctures, they enhance the effectiveness of other antimicrobial agents. Aqueous ethanol (60-90%) and isopropanol are used as disinfectants. Not effective against spores or non-enveloped viruses.

20.1.5 Heavy metals and their compounds

Silver, mercury, copper, and zinc are used as germicidals. They exert their antimicrobial action through oligodynamic action. When heavy metal ions combine with sulfhydryl (-SH) groups, proteins are denatured. Examples are 1% Silver nitrate solution, mercuric chloride, copper sulfate (algicide).

20.1.6 Surface-active agents-soaps and acid anionic detergents

The agents decrease the surface tension among molecules of a liquid; soaps and detergents are examples. Soaps have limited germicidal action but assist in the removal of microorganisms through scrubbing. Acid-anionic detergents are used to clean dairy equipment.

Quaternary Ammonium Compounds: Quats are cationic detergents attached to NH4⁺. By disrupting the plasma membranes, they allow cytoplasmic constituents to leak out of the cell (Fig. 20.1). Quats are most effective against Gram-positive bacteria. They do not kill endospores or mycobacteria. Examples include Zephiran (benzalkonium chloride) and Cepacol (cetylpyridinium chloride). Pseudomonads are highly resistant, can even live in quats.

Fig. 20.1 Mode of action of surfactant

20.1.7 Organic acids and derivatives

This class of organic compounds is commonly used as food preservatives. These are effective mostly against mold as they interfere with mold metabolism or the integrity of the plasma membrane.

20.1.7.1 Nitrates

It can be found in some cheeses, adds flavor, maintains pink color in cured meats and prevents botulism in canned foods. Can cause adverse reactions in children, and potentially carcinogenic.

20.1.7.2 Sulfur dioxide and sulfites

These are used as preservatives and to prevent browning in alcoholic beverages, fruit juices, soft drinks, dried fruits and vegetables. Sulfites prevent yeast growth and also retard bacterial growth in wine. Sulfites may cause asthma and hyperactivity. They also destroy vitamins.

20.1.7.3 Benzoic acid and sodium benzoate

These are used to preserve oyster sauce, fish sauce, ketchup, non-alcoholic beverages, fruit juices, margarine, salads, confections, baked goods, cheeses, jams and pickled products. They have also been found to cause hyperactivity.

20.1.7.4 Propionic acid and propionates

These are used in bread, chocolate products, and cheese for lasting freshness.

20.1.7.5 Sorbic acid and sorbates

It prevents mold formation in cheese and flour confectioneries

Some common food preservatives along with their applications are listed in Table 20.2.

Table 20.2 Common food preservatives and their application

20.1.8 Aldehydes

Aldehydes such as formaldehyde and glutaraldehyde (Gidex) exert their antimicrobial effect by inactivating proteins. They are among the most effective chemical disinfectants.

20.1.9 Gaseous chemosterilants

This class of chemosterilants includes chemicals that sterilize in a closed chamber. Chemicals used for sterilization include the gases ethylene oxide and formaldehyde, and liquids such as glutaraldehyde. Ozone, hydrogen peroxide and peracetic acid are also examples of chemical sterilization techniques are based on oxidative capabilities of the chemical.

20.1.9.1 Ethylene oxide (ETO)

It is the most commonly used form of chemical sterilization. Due to its low boiling point of 10.4°C at atmospheric pressure, ETO behaves as a gas at room temperature. ETO chemically reacts with amino acids, proteins, and DNA to prevent microbial reproduction. The sterilization process is carried out in a specialized gas chamber. After sterilization, products are transferred to an aeration cell, where they remain until the gas disperses and the product is safe to handle. ETO is used for cellulose and plastics irradiation, usually in hermetically sealed packages. Ethylene oxide can be used with a wide range of plastics (e.g. petri dishes, pipettes, syringes, medical devices, etc.) and other materials without affecting their integrity (Fig.20.2).

Fig. 20.2 An ethylene oxide sterilization gas chamber

Ozone sterilization has been recently approved for use in the U.S. It uses oxygen that is subjected to an intense electrical field that separates oxygen molecules into atomic oxygen, which then combines with other oxygen molecules to form ozone. Ozone is used as a disinfectant for water and food. It is used in both gas and liquid forms as an antimicrobial agent in the treatment, storage and processing of foods, including meat, poultry and eggs. Many municipalities use ozone technology to purify their water and sewage. Los Angeles has one of the largest municipal ozone water treatment plants in the world. Ozone is used to disinfect swimming pools, and some companies selling bottled water use ozonated water to sterilize containers. An ozone fogger for sterilization of egg surfaces is depicted in (Fig.20.2). The system facilitates reaction of ozone with water vapors to create powerful oxidizing radicals. This system is totally chemical free and is effective against bacteria, viruses and hazardous microorganisms which are deposited on egg shells (Fig. 20.3).

Fig. 20.3 Ozone fogger

20.1.9.2 Low temperature gas plasma (LTGP)

It is used as an alternative to ethylene oxide. It uses a small amount of liquid hydrogen peroxide (H₂O₂), which is energized with radio frequency waves into gas plasma. This leads to the generation of free radicals and other chemical species, which destroy organisms. (Fig. 20.4)

Common antiseptics and disinfectants and their uses are summarized in (Table 20.3)

Table 20.3 Common antiseptics and disinfectants and their uses

Fig. 20.4 Relative effectiveness of chemical biocides