UNIT 4 CHEMICAL METHODS OF CONTROLLING
3.0 MAIN CONTENT
Definition of Disinfection Disinfection is the killing, inhibition or removal of organisms that may be capable of causing diseases.
It is the process of destroying infectious agents.
Disinfectants are antimicrobial agents, usually chemicals, used to carry out disinfection; they are normally used on an inanimate object, e.g. disinfection may not lead to the total removal of microorganisms because viable spores and a few microorganisms may remain.
Antimicrobial bleach (sodium hypochlorite) solution, for example, is a disinfectant used to clean and disinfects food preparation areas.
3.1 Antimicrobial Agents
An antimicrobial agent is a natural or synthetic chemical that kills or inhibits the growth of microorganisms. Agents that kill organisms are called -cidal agents, with a prefix indicating the type of microorganisms killed. Thus, they are called bactericidal, fungicidal and viricidal agents because they kill bacteria, fungi and viruses, respectively. Agents that do not kill but only inhibit growth are called - static agents. These include bacteriostatic, fungistatic and viristatic compounds.
3.2 Characteristics of an Ideal Antimicrobial Agent or Disinfectant
i. It should have a broad spectrum of antimicrobial activity i.e. it must be effective against a wide range of infectious agents such as gram positive and Gram negative bacteria, acid fast bacteria, bacterial endospores, fungi and viruses.
ii. It must be active even at low concentration.
iii. It must be active in the presence of organic matter.
iv. Non-toxicity to human and other animals. It should be toxic to the infectious agent.
v. It must be non-corroding and non-staining.
vi. It must be stable upon storage.
vii. Odourless or with pleasant smell.
viii. It must be soluble in water and lipids for proper penetration of microorganisms.
ix. It must be uniform in composition so that active ingredients are present in each application.
x. It must have a low surface tension so as to penetrate cracks in surfaces.
xi. It must be readily available.
xii. It must be relatively inexpensive.
3.3 Factors for the Selection of a Chemical Agent
The major factors that need to be considered in the process of selecting the most appropriate chemical agent for a specific practical application are:
1. The nature of the material to be treated: e.g. a chemical agent used to disinfect contaminated utensils might be quite unsatisfactory for application to the skin.
2. Types of microorganisms: Chemical agents are not all equally effective against bacteria, fungi, viruses, and other microorganisms. Spores are more resistant than vegetative cells.
Differences exist between Gram-positive and Gram-negative bacteria.
3. Environmental condition: Such as temperature, pH, time, concentration, and presence of extraneous organic materials, may all have a bearing on the rate and efficiency of antimicrobial action.
SELF-ASSESSMENT EXERCISE
What is a disinfectant?
3.4 Major Groups of Chemical Antimicrobial Agents
Phenol and phenolic compounds
Alcohols
Halogens
Heavy metals and their compounds
Dyes
Detergents
Quaternary ammonium compounds
Aldehydes
Gaseous agents
3.4.1 Phenolics
Phenol is also known as carbolic acid and is the oldest recognised disinfectant.
Phenol (Carbolic acid) was the first widely used antiseptic and
The mode of the Action of phenol is by disrupting plasma membranes; inactivate enzymes and denaturing proteins of microorganism.
They are stable when heated or dried and retain their activity in the presence of organic material. It is used for disinfection of hospital floors and walls.
Examples of phenol and phenolics (phenol derivatives) are cresols, xylenols and orthophel/phenols which are used as disinfectants in laboratories and hospitals.
Advantage
They are effective against microbial agents of tuberculosis, effective in the presence of organic material and remain on surface long after application.
Disadvantage
They have a disagreeable odour and can cause skin irritation.
3.4.2 Alcohols
Alcohols are widely used as disinfectants and antiseptic. They are bactericidal and fungicidal but not sporicidal. Some viruses are also destroyed by alcohols.
Two most popular alcohols germicides are ethanol and isopropanol usually used at 70 to 80% concentration. Isopropanol has the highest bactericidal activity and is the most widely used. They act by denaturing proteins and by dissolving membrane lipids and acting as a dehydrating agent. 10-15 minutes soaking a thermometer in alcohol is sufficient to disinfect the thermometer
3.4.3 Halogens
A halogen is any of the five elements in group VIIA of the periodic table. They are fluorine, chlorine, bromine, iodine and astatine and are effective microbial elements widely used as disinfectants. The halogens iodine and chlorine are important antimicrobial agents.
Chlorine
Chlorine is the usual disinfectant for municipal water supplies and swimming pools. It is also used in diary and food industries.
Various forms of chlorine are used for disinfection. It may be applied as chlorine gas, sodium hypochlorite (bleach) or calcium hypochlorite. All yield hypochlorous acid (HCLO) followed by
atomic oxygen. It causes oxidation of cellular materials, destruction of vegetative bacteria and fungi by disrupting membranes and inactivating enzymes but does not destroy spores.
The germicidal action of chlorine is based on the formation of hypochlorites when it is added to water. HA releases an active form of death of almost all microorganisms occur within 30 minutes of use.
Chlorine is also an excellent disinfectant for individual use. Small quantities of drinking water can be disinfected with halozone tablets. This tablet (parasulfoen dichloramido benzoic acid) slowly releases chlorite when added to water and disinfects it in about 30 minutes. It is frequently used by campers lacking access to uncontaminated drinking water.
Chlorine is an effective disinfectant because it is inexpensive, effective and easy to employ.
It is commonly used in disinfecting and deodorizing most houses.
Food processing plants and restaurants also use calcium and sodium hypochlorite solution to disinfect utensils.
Hypochlorite is used in hospital to disinfect rooms, surfaces and non-surgical instruments. It used on a consumable product such as drinking water, their concentration must be reduced before product is consumed. The germicidal action of chlorine is based on the formation of hypochlorous acid when it is added to water.
Hypochlorous acid releases an active form of oxygen that reacts with cellular biochemical.
Chlorine forms condensed into liquids is widely used for disinfection and is the standard treatment for disinfecting drinking water in many communities.
It is also used to disinfect effluents from sewage treatment plants to minimise the spread of pathogenic microorganisms. It is also used to disinfect swimming pools. A residual chlorine level of 0.5mg/l will achieve control of microbial population and prevent the multiplication of pathogens in swimming pools. Such levels are harmless to human tissues. Usually, commercial forms of sodium or calcium hypochlorite contain 5.25% chlorine. These need to be diluted ten times to achieve the forms used for disinfection in homes and swimming pools. The commercial types are Chlorox, JIK, Parazone, etc. The powdered form used in swimming pools and water treatment plants is known as HTH (commercially).
3.4.4 Heavy Metals and their Compounds
Heavy metals such mercury, silver, zinc, copper and arsenic are
They act by combining with proteins and inactivating them. They may also precipitate cell proteins.
1% of Silver nitrate is a solution added to the eyes of infants to prevent ophthalmic gonorrhea.
3.4.5 Quaternary Ammonium Compounds (Detergents)
These are detergents that have antimicrobial activity.
Detergents are organic cleaning agents that are amphipathic, having both polar hydrophilic and non-polar hydrophobic components.
They act by disrupting microbial membrane and by denaturing proteins.
If the detergents are electrically charged, they are termed ionic.
Anionic (negatively charged) detergents are only mildly bactericidal and are used as laundry detergents to remove soil and debris. They also reduce number of microorganisms associated with the item being washed.
Cationic (positively charged) detergents are highly bactericidal, i.e. they kill bacteria. They are effective against Staphylococcus and various viruses.
Advantages
They are stable, non-toxic not inactivated by hard water and soap.
Cationic detergents are used as disinfectant for food utensils and small instruments.
3.4.6 Aldehydes
Formaldehyde and glutaraldehyde are useful for disinfection.
They are highly reactive molecules that combine with nucleic acid and proteins and inactive them. They are sporicidal and can be used as chemical sterilants.
Formaldehyde is usually dissolved in water or alcohol before use.
A 2% buffered solution of glutaraldehyde is an effective disinfectant. It is less irritating than formaldehyde and is used to disinfect hospitals and laboratory equipment. Glutaraldehyde usually disinfects objects within minutes but may require as long as 12 hours to destroy all spores.
SELF-ASSESSMENT EXERCISE
List five groups of chemical antimicrobial agents and explain their uses.
3.5 Sterilising Gases
Gases such as ethylene oxide gas are used to sterilise heat sensitive items such as disposable plastic Petri dishes, syringes, heart lung machines components, sutures and catheters.
3.5.1 Ethylene Oxide (EtO)
This is both microbicidal and sporicidal and kills by combining with cell proteins. Sterilisation is carried out in a special ethylene oxide steriliser, very much resembling an autoclave in appearance, that control the EtO concentration, temperature, and humidity. Because pure EtO is explosive, it is usually mixed with either CO2 or dichlorodifluoromethane. The ethylene oxide concentration, humidity and temperature influence the rate of sterilisation. A clean object can be sterilised if treated for 5 to 8 hours at 40 to 50% and the EtO concentration at 700mg/litre. Extensive aeration of the sterilized material is necessary to remove residual EtO because it is very toxic.
3.5.2 Betapropiolactone (BPL)
This is occasionally employed as a sterilising gas. In the liquid form it has been used to sterilise vaccines and sera. BPL decomposes to an inactive form after several hours and is therefore not as difficult to eliminate as EtO. It also destroys microorganisms more readily than ethylene oxides but does not penetrate materials well. It may be carcinogenic. For these reasons, BPL has not been used as extensively as EtO.
3.5.3 Vaporised Hydrogen Peroxide
This can be used to decontaminate biological safety cabinets, operating rooms and other large facilities. These systems introduce vapourised hydrogen peroxide (H2O2) into the enclosure for some time, depending on the size of the enclosure and material within. Hydrogen peroxide is toxic and kills a wide variety of microorganisms. However, during the course of the decontamination process, it breaks down to H2O and oxygen, both of which are harmless. Other advantages of these systems are that they can be used at a wide range of temperatures (4 to 80oC) and they do not damage most materials.
4.0 CONCLUSION
Many different chemicals are available for use as disinfectants and each has its own advantages and disadvantages. In selecting an agent, it is important to keep in mind the characteristics of a desirable disinfectant.
9.0 SUMMARY
Chemical agents are usually used as disinfectants.
Characteristics of an ideal disinfectant include broad spectrum antimicrobial activity, stability, toxicity to humans, non-conoding and non-staining, easy penetration no odour or pleasant odour among others.
Phenolics are used as disinfectants in hospitals and laboratories.
They act by denaturing proteins, disrupting cell membranes and inactivating enzymes of microorganisms.
Halogens (Chlorine and iodine) kill microorganising by oxidizing cellular constituents.
Chlorine is used to disinfect municipal water supply and swimming pools.
The germicidal action of chlorine is based on the formation of hypochlorous acid when it is added to water.
Alcohols are the most effective and most used agents for sterilisation and disinfectant.
Aldehydes such as formaldehyde and glutaraldehyde can sterilise as well as disinfect because they kill spores.
Cationic detergents are used as disinfectants antiseptic; they disrupt the membrane and denature proteins of microorganisms.
Ethylene oxide gas is use to sterilise heat sensitive materials like disposable plastic Petri dishes.
10.0 TUTOR-MARKED ASSIGNMENT
1. List five characteristics of ideal antimicrobial agents.
2. Explain the use of chlorine as a disinfectant.
7.0 REFERENCES/FURTHER READING
Atlas, R.M. (1995). Microorganisms in Our World. Mosby Year Book.
Inc.
Medigan, M.T. et al. (2009). Brock Biology of Microorganisms.
(12th ed.). Pearson Education Inc.
Pelczar, M.J., Chan, E.C.S. & Krieg, R.N. (2001). (5th ed.).
Microbiology. McGraw-Hill.
Willey, J.M., Sherwood, L.M & Woolverton, C.J. (2008). Microbiology.
(7th ed.). Boston Bur Bridge, IL: McGraw-Hill Higher Education.