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Microbiology - Control of Microbial Growth

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    Study Guide Microbiology – Control of Microbial Growth 1. Chemical Methods of Control Chemical methods are widely used to control microorganisms , especially on living tissues and in everyday environments. Most chemical agents do not achieve complete sterilization . Instead, they reduce microbial populations to safe levels or remove disease - causing organisms from surfaces. An ideal disinfectant or antiseptic should: • Kill microorganisms quickly • Be safe for the material or tissue being treated • Be effective under normal conditions 1.1 Factors Affecting the Choice of a Chemical Agent When selecting an antiseptic or disinfectant, several factors must be considered: • Concentration of the chemical used • Whether the agent is bactericidal (kills bacteria) or bacteriostatic (inhibits growth) • Type of material being treated • Presence of organic matter (blood, pus, dirt) • Temperature and pH of the environment • Contact time between the agent and the surface No single disinfectant works best in all situations. 1.2 Evaluation of Chemical Agents Phenol Coefficient Test The phenol coefficient (PC) test compares a chemical disinfectant with phenol.

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    Study Guide • Dilutions of the test agent are compared with phenol • Tested against Staphylococcus aureus and Salmonella typhi Interpretation: • PC > 1 → more effective than phenol • PC < 1 → less effective than phenol In - Use Test The in - use test evaluates how well a disinfectant works under actual conditions. • The agent is tested on the same type of surface used in real situations • More realistic than laboratory comparison tests

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    Study Guide 1.3 Major Groups of Chemical Control Agents Figure 1 A selection of chemical disinfectants and antiseptics . Phenol and Phenolics • Phenol was one of the first disinfectants, introduced by Joseph Lister in the 1860s • It is used as the standard reference compound Phenolics are modified forms of phenol:

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    Study Guide • Damage cell membranes • Denature proteins • Inactivate enzymes Examples: • Cresols (e.g., Lysol) • Bisphenols such as hexachlorophene (effective against Staphylococcus ) Chlorhexidine (Hibiclens) : • Used as a skin disinfectant • Effective against vegetative bacteria • Not effective against spores Halogens Iodine • Used as tincture of iodine (iodine in alcohol) • Iodophores (e.g., Betadine, Isodyne) combine iodine with organic molecules • Iodine binds to proteins and inhibits enzyme function Chlorine • Used as sodium hypochlorite (bleach) • Calcium hypochlorite is used in dairies and food facilities • Chloramines (chlorine + ammonia) are effective even with organic matter • Chlorine gas is widely used to disinfect drinking water Alcohols • Effective against bacteria and fungi • Not effective against spores • Most commonly used:

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    Study Guide o 70% ethanol o Isopropyl alcohol Advantages: • Act rapidly • Evaporate quickly • Leave no residue Common use: • Skin disinfection before injections Heavy Metals Heavy metals exert antimicrobial effects by reacting with sulfhydryl groups in proteins. Examples: • Silver (silver nitrate) o Used in newborn eyes to prevent Neisseria gonorrhoeae infection • Copper (copper sulfate) o Controls algae in pools and reservoirs • Zinc o Zinc chloride in mouthwashes o Zinc oxide as antifungal agent in paints Soaps and Detergents • Reduce surface tension • Help remove microorganisms mechanically Soaps : • Emulsify oils and dirt • Carry microbes away during washing

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    Study Guide Cationic detergents (quaternary ammonium compounds) : • Disrupt microbial cell membranes Examples: • Benzalkonium chloride (Zephiran) • Cetylpyridinium chloride (Cepacol) Aldehydes Aldehydes kill microorganisms by cross - linking proteins . • Formaldehyde o Used as formalin (37% solution) o Commonly used in embalming • Glutaraldehyde o Used to disinfect medical equipment o Requires several hours to kill bacterial spores Ethylene Oxide (ETO) • A true chemical sterilant • Destroys all microorganisms, including spores • Used in special chambers at warm temperatures • Requires long exposure and ventilation (toxic to humans) Used for sterilizing: • Plastic instruments • Petri dishes • Syringes • Artificial heart valves Propylene oxide is a related compound used similarly.

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    Study Guide Oxidizing Agents These agents kill microbes by releasing oxygen , which damages enzymes. • Hydrogen peroxide o Used on wounds and surfaces o Broken down by catalase on skin (causes bubbling) o Used to disinfect contact lenses Other oxidizers: • Benzoyl peroxide o Used in acne treatment o Inhibits anaerobic bacteria • Ozone o Used to disinfect water Food Preservatives Organic acids are used to prevent microbial growth in foods. Examples: • Sorbic acid – used in cheese and acidic foods • Benzoic acid – inhibits fungi in soft drinks and acidic foods • Calcium propionate – prevents mold growth in bread and bakery products Key Takeaway Chemical methods of control play a crucial role in reducing microbial populations. While most agents do not sterilize completely, they are effective when chosen and applied correctly. Understanding how each chemical works helps ensure safe and effective mi crobial control in healthcare, food preservation, and everyday life.

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