Lecture 7: MICR 433 2025 Spring - Germicidal Chemicals

Using chemicals to destroy microorganisms and viruses Chemicals which can be used to disinfect or sterilize are called germicidal chemicals. They react irreversibly with vital proteins, DNA, cytoplasmic membranes or viral envelops. Their mechanisms of action are often poorly u...

Using chemicals to destroy microorganisms and viruses Chemicals which can be used to disinfect or sterilize are called germicidal chemicals. They react irreversibly with vital proteins, DNA, cytoplasmic membranes or viral envelops. Their mechanisms of action are often poorly understood. They are generally less reliable than heat. Chemicals are suitable for treating large surfaces and many heat sensitive items. Some can be used as antiseptic. Some chemicals are weakly germicidal but have bacteriostatic activity and can be used as preservatives.  Selecting the appropriate germicide It is a complex decision. Some points to be considered 1. Toxicity: Germicides are at least somewhat toxic to humans and the environment. Risk versus benefit should be weighed. To encourage the use of less toxic options, the EPA allows manufacturers to place a specially designed ‘Safer Choice’ label on products considered the least hazardous.  Selecting the appropriate germicide 2. Activity in the presence of organic matter: Hypochlorite (bleach) is readily inactivated by organic matter; however, phenolics can tolerate the presence of some organic matter. 3. Compatibility with material being treated: Some items like electrical equipment can not tolerate liquid disinfectants so use gaseous alternatives. Hypochlorite is corrosive and can damage some metal and rubber. 4. Residue: Some leave a residue which is toxic or corrosive. Sterilized items must be rinsed with sterile water.  Selecting the appropriate germicide 5. Cost and availability: Some are less expensive and readily available e.g., hypochlorite. Some are expensive and are not readily available e.g., Ethylene oxide. 6. Storage and stability: Phenolics and iodophores are available as concentrated stock solutions. But some have two components and once mixed have limited shelf life. 7. Environmental risk: Some retain their antimicrobial activity after use & can interfere with sewage treatment system. They should be neutralized before disposal.  Potency of germicidal chemicals Formulations usually contain ≥ one antimicrobial agent. Food and Drug Administration (FDA)regulates chemicals that can be used to process medical devices while Environmental Protection Agency (EPA) regulates disinfectants.  Potency of germicidal chemicals Germicides are grouped according to their potency. 1. Sterilants (also called sporocides): Destroy all microbes including endospores & viruses & can be used on heat- sensitive instruments. 2. High-level disinfectants: Destroy all viruses and vegetative cells, but not endospores. 3. Intermediate-level disinfectants: Kill all vegetative cells including Mycobacteria, fungi, most viruses but not all. Do not kill endospores. 4. Low-level disinfectants: Removes vegetative bacteria fungi, and enveloped viruses. – Not mycobacteria, naked viruses or endospores. Intermediate-level and low-level disinfectants are also called general purpose disinfectants.  Classes of germicidal Chemicals Alcohols: – Aqueous solutions of 60-80% ethanol/isopropanol rapidly kill vegetative bacteria & fungi but do not reliably destroy endospores & naked viruses. – Coagulate enzymes & proteins & damage lipid membranes. – Proteins are more soluble & denature more easily in alcohol mixed with water & thus aqueous solutions (70% ethanol) are more effective than pure alcohols. – Used as antiseptics to degerm skin. – CDC recommends the use of alcohol-based hand sanitizers for health personnel. More effective than soap & convenient to use. – Used as disinfectants for treating instruments & surfaces. – Relatively non-toxic, inexpensive, no residue, evaporate fast so less contact time. – Used for preparing tinctures which are more effective.  Classes of germicidal Chemicals Aldehydes: – Include glutaraldehyde, orthophthalaldehyde (OPA) & formaldehyde. Glutaraldehyde: – Destroy microbes and viruses by inactivating proteins and nucleic acids. – A 2% solution of alkaline glutaraldehyde is one of the most widely used liquid sterilants for treating heat-sensitive medical items. – It destroys all forms of microbial life including endospores and viruses.  Classes of germicidal Chemicals Orthophthalaldehyde (OPA): - new type of disinfectant being studied as alternative to glutaraldehyde. Formaldehyde: - used as a gas or an aqueous 37% solution called formalin. – an extremely effective germicide which kills all forms of life within minutes. – used to kill bacteria and inactivates viruses for use as a vaccine. – used to preserve biological specimens. – irritating vapors and suspected carcinogen.  Classes of germicidal Chemicals Biguanides (group of chemicals): – Chlorhexidine (most effective) extensively used in antiseptic products (skin creams, disinfectants and mouth wash). – low toxicity and can kill a wide range of microbes including vegetative bacteria, fungi and some enveloped viruses by reacting with proteins. Ethylene oxide: – is an extremely useful gaseous sterilizing agent and destroy all microbes including endospores and viruses. – A special chamber is used to sterilize items with ethylene oxide. It is carcinogenic in nature.  Classes of germicidal Chemicals Halogens: – Include chlorine and iodine and work by oxidizing proteins and other cell components. Chlorine: – Destroy all types of microorganisms and viruses. – Chlorine releasing compounds such as sodium hypochlorite are used to disinfect waste liquids, swimming pool water, instruments and surfaces. – At much lower concentration used to disinfect drinking water. – Properly chlorinated drinking water contains ~ 0.5 parts per million (ppm) chlorine. – Conc. used in water are not effective against Cryptosporidium parvum oocysts & Giardia lamblia cysts. – Household bleach is 5.25% sodium hypochlorite (used at 1:100  Classes of germicidal Chemicals Chlorine dioxide (ClO2) – Is a strong oxidizing agent and is increasingly being used as a disinfectant and sterilant. – Used to treat drinking water, wastewater, and swimming pools. – Unlike chlorine, it does not react with organic compounds to form trihalomethanes or other toxic chlorinated products. – Liquid CI02 decompose readily so generated on-site. Iodine: – unlike chlorine, does not reliably kill endospores but used as disinfectant. – Tincture of iodine or iodophores are used as disinfectants or antiseptics. – Some Pseudomonas species survive in the concentrated solutions of iodophores.  Classes of germicidal Chemicals Metal compounds: – Too toxic to human tissues to be used medically. – Silver is one of the few metals still used as disinfectant. – Silver sulfadiazine cream are used topically to prevent infection of second- and third-degree burns. – Some silver containing bandages are in use. Ozone (O3): – is a powerful oxidizing agent. – increasingly used as an alternative to chlorine for disinfecting drinking water and wastewater.  Classes of germicidal Chemicals Peroxygens: – H2O2 and peracetic acid are powerful oxidizing agents and can be used as sterilants. – Readily biogradable and less toxic. – H2O2 is less effective as antiseptic as broken down by catalase. – Peracetic acid is more potent germicide than H2O2 and is widely used to disinfect and sterilize medical devices.  Classes of germicidal Chemicals Phenolic compounds (Phenolics) – Phenol (Carbolic acid) is important historically as one of the earliest disinfectants. But its use now is limited. – include triclosan and hexachlorophene which are relatively non-toxic. – are the active ingredients in Lysol. – destroy cytoplasmic membranes & denature proteins. – wide range of activity and reasonable cost. – remains effective even in the presence of detergents and organic contaminants. – Triclosan is used in a variety of personal care products such as toothpaste, lotions and antibacterial soaps. – Hexachlorophene is highly effective against Staphylococcus aureus but is of limited use because neurological damage.  Classes of germicidal Chemicals Quaternary Ammonium Compounds (Quats) – Cationic (+vely charged) detergents that are relatively non- toxic & used to disinfect food preparation surfaces. – Inactivated by anionic soaps and detergents. – Widely used to disinfect inanimate items and for preserving non-food substances. – They react with membranes and destroy vegetative bacteria and enveloped viruses. – Not effective against endospores, mycobacteria and naked viruses. – Pseudomonas resist the effects of quats and can grow in them.  Preservation of perishable products Preventing or slowing the growth of microbes extends the shelf-life of various products and can be achieved using various methods 1. Use of chemical preservatives – Non-food items use germicides i.e., mouth washes contain a quaternary ammonium compound. – Food items use weak acids (i.e., benzoic, sorbic & propionic acids). Low pH inhibits the growth of most bacteria. – Nitrates/nitrites are typically used to preserve meats (inhibit the germination of endospores and growth of Clostridium botulinum). – Nitrates and nitrites may pose a hazard because of their conversion to nitrosoamines which are potential carcinogens.  Preservation of perishable products 2. Low-temperature storage – Slows enzymatic reactions. Some fruits and vegetables are stored in cold storage for months. – Freezing is another important means of preserving foods and other products. – It stops all microbiological activity. Freezing forms ice crystals that damage microbial cells.  Preservation of perishable products 3. Reducing water availability – Adding salt or sugar: high solute environment causes ‘plasmolysis’ & prevent growth of microbes. – Staphylococcus aureus (food poisoning organism) can grow under very high salt concentrations. – Drying food (dessication, lyophilization): Drying stops the growth of microbes but does not kill them. – Number of Salmonella cases are sometime reported when dried eggs or their products are used.  Next Lecture Read chapter 20: Antimicrobial medications.