5.5. Using Chemicals to Destroy Microorganisms and Viruses  Germicidal Chemicals Used to disinfect and sterilize Used to treat large surfaces and heat sensitive items Chemicals react with cell components Less reliable than heat Toxic or Non-toxic  Potency of Germicidal Chemical Formulations 1)Sterilants Destroy all microorganisms…including endospores Heat-sensitive critical instruments 2)High-level disinfectants Destroy viruses, vegetative cells Do not reliably kill endospores Semi-critical instruments 3)Intermediate-level disinfectants Destroy vegetative bacteria, mycobacteria, fungi, and most viruses Disinfect non-critical instruments 4)Low-level disinfectants Destroy fungi, vegetative bacteria except mycobacteria, and enveloped viruses Do not kill endospores, naked viruses Disinfect furniture, floors, walls Operating Room: Chemical Treatment of Soldier

5.5. Using Chemicals to Destroy Microorganisms and Viruses  Selecting the Appropriate Germicidal Chemical 1)Toxicity benefits must be weighed against risk of use 2)Activity in presence of organic matter Many germicides inactivated 3)Compatibility with material being treated Liquids cannot be used on electrical equipment…use gases 4)Residues: can be toxic or corrosive 5)Cost and availability Less $ = readily available 6)Storage and stability Concentrated stock decreases storage space 7)Environmental risk Agent may need to be neutralized before disposal

Classes of Germicidal Chemicals

 Alcohols 60–80% aqueous solutions of ethyl or isopropyl alcohol Kills vegetative bacteria and fungi Not reliable against endospores, some naked viruses Commonly used as antiseptic and disinfectant Limitations Evaporates quickly, limiting contact time Can damage rubber, some plastics, and others  Aldehydes Glutaraldehyde, formaldehyde, and orthophthalaldehyde (OPA) Inactivates proteins and nucleic acids 2% alkaline glutaraldehyde common sterilant Immersion for 10–12 hours kills all microbial life Formaldehyde Effective germicide that kills most microbes quickly Used to kill bacteria and inactivate viruses for vaccines Used to preserve specimens

Classes of Germicidal Chemicals  Biguanides Chlorhexidine most effective Extensive in antiseptics Stays on skin, mucous membranes Relatively low toxicity Destroys vegetative bacteria, fungi, some enveloped viruses Common in many products: skin cream, mouthwash  Ethylene oxide Gaseous sterilant Destroys microbes including endospores and viruses Reacts with proteins Penetrates fabrics, equipment, implantable devices Pacemakers, artificial hips Useful in sterilizing heat- or moisture-sensitive items Applied in special chamber resembling autoclave Limitations: mutagenic and potentially carcinogenic

Classes of Germicidal Chemicals  Halogens oxidize proteins, cellular components  Chlorine: Destroys all microorganisms and viruses Used as disinfectant…E.g., swimming pools, water, waste liquids Caustic to skin and mucous membranes 1:100 dilution of household bleach effective Very low levels disinfect drinking water…E.g., 2 30 min Cryptosporidium oocysts, Giardia cysts survive  Iodine: Kills vegetative cells, unreliable on endospores Commonly used as iodophore Iodine slowly released from carrier molecule  Metal Compounds Combine with sulfhydryl groups of enzymes, proteins High concentrations too toxic to be used medically Silver still used as disinfectant: creams, bandages Silver nitrate eyedrops were required to prevent Neisseria gonorrhoeae infections acquired during birth Argyria – colloidal silver

Classes of Germicidal Chemicals  Ozone O 3 : unstable form of oxygen Decomposes quickly, so generated on-site Powerful oxidizing agent Used as alternative to chlorine Disinfectant for drinking and wastewater  Peroxygens Powerful oxidizers used as sterilants Readily biodegradable, no residue Less toxic than ethylene oxide, glutaraldehyde Hydrogen peroxide: effectiveness depends on surface….inanimate or living Aerobic cells produce enzyme catalase –Breaks down H 2 O 2 to O 2, H 2 O More effective on inanimate object Peracetic acid: more potent than H 2 O 2 Effective on organic material Useful on wide range of material

Classes of Germicidal Chemicals  Phenolic Compounds (Phenolics) Phenol one of earliest disinfectants Has unpleasant odor, irritates skin Phenolics kill most vegetative bacteria Mycobacterium at high concentrations Not reliable on all virus groups Destroy cytoplasmic membranes, denature proteins Wide activity range, reasonable cost, remain effective in presence of detergents and organic contaminants Leave antimicrobial residue Some sufficiently non-toxic; used in soaps, lotions Triclosan, hexachlorophene  Quaternary Ammonium Compounds (Quats) Cationic (positively charged) detergents Nontoxic, used to disinfect food preparation surfaces Charged hydrophilic and uncharged hydrophobic regions Reduces surface tension of liquids Aids in removal of dirt, organic matter, organisms Most household soaps, detergents are anionic But positive charge of quats attracts them to negative charge of cell surface Reacts with membrane Destroys vegetative bacteria and enveloped viruses Not effective on endospores, mycobacteria, naked viruses

5.6. Preservation of Perishable Products  Chemical preservatives Food preservatives must be non-toxic for safe ingestion Weak organic acids (benzoic, sorbic, propionic) Inhibit metabolism, alter cell membrane function Control molds and bacteria in foods and cosmetics Nitrate and nitrite used in processed meats Inhibit endospore germination and vegetative cell growth –Stops growth of Clostridium botulinum –Higher concentrations give meats pink color Shown to be carcinogenic—form nitrosamines

5.6. Preservation of Perishable Products  Low-Temperature Storage Refrigeration inhibits growth of pathogens and spoilage organisms by slowing or stopping enzyme reactions Psychrotrophs, psychrophilic organisms can still grow Freezing preserves by stopping all microbial growth Some microbial cells killed by ice crystal formation, but many survive and can grow once thawed

5.6. Preservation of Perishable Products  Reducing Available Water Accomplished by salting, adding sugar, or drying food Addition of salt, sugar increases environmental solutes Causes cellular plasmolysis (water exits bacterial cells) Some bacteria grow in high salt environments Staphylococcus aureus Drying often supplemented by salting Lyophilization (freeze drying) foods Coffee, milk, meats, fruits, vegetables Drying stops microbial growth but does not reliably kill Numerous cases of salmonellosis from dried eggs

12 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 05 Control of Microbial Growth  Covers methods to:  Destroy  Remove  Inhibit  Microbial growth on:  Inanimate objects  Some body surfaces  Methods can damage all forms of life  Antibiotics and antimicrobial medications target only microbes…CH20