Chapter 5 - Control of Microbial Growth PDF

Chapter 5 Control of Microbial Growth Terminology Sterilization – absolute removal or destruction of ALL microorganisms and viruses Disinfection – destruction of most microbes including pathogens Does not kill all microbes ex. Chemical disinfectants Antisepsis (Antiseptic) –...

Chapter 5 Control of Microbial Growth Terminology Sterilization – absolute removal or destruction of ALL microorganisms and viruses Disinfection – destruction of most microbes including pathogens Does not kill all microbes ex. Chemical disinfectants Antisepsis (Antiseptic) – Disinfection of skin or living tissue Normally using less harsh treatments (chemicals) Degerming – Removal of MOST microbes from a limited area ex. Washing hands with soap and water Sanitization – Lowering microbial counts to a safe level to meet public health standards. -cidal vs -static An agent that kills microbes has the suffix –cide An agent that stops the growth of microbes –static or –stasis ex. A bacteriocidal agent kills bacteria A bacteriostatic agent stops bacteria from reproducing Asepsis (aseptic) – an area is free of significant contamination Modern surgical techniques are aseptic to minimize infection. Using Heat to Destroy Microbes Kills microbes by destroying enzymes – bacteriocidal (germicidal) Can be used several ways: Boiling for 5 minutes kills: most pathogenic bacteria – but not all endospores most viruses This is disinfection, not sterilization! Pasteurization Sufficient to eliminate pathogens and to lower microbial numbers to slow spoilage Classic treatment = mild heating to 63°C for 30 minutes HTST (High temperature short time) = 72°C for 15 seconds UHT (Ultra High Temperature) = 140°C for 3 seconds Destroys all microorganisms that can grow under normal storage conditions. Autoclave – employs pressure (15 psi) and steam (121°C) Kills all endospores in about 15 minutes Best method for sterilization Used for: Culture media Hospital instruments Hospital materials that can withstand heat and moisture Requires that all solid surfaces are exposed to steam All liquids must reach a temperature of 121°C. Dry heat sterilzation – several methods: Direct flaming – used in microbiology laboratory to sterilize inoculation needles and loops Metallic instruments must be red-hot Incineration – effective way to dispose of contaminated materials Hot air sterilization – drying oven Materials should stay at 170°C for 2 hours Used for dry materials that are not temperature sensitive ex. Glassware, some dry chemicals. Filtration Vacuum used to force liquid through filter with very small pores – 0.1 mm, 0.22 mm or 0.45 mm Pores are too small for bacteria to pass Used to sterilize temperature sensitive liquids Antibiotics Enzymes Some Vaccines How large are viruses? Air filtration High-efficiency particulate air (HEPA) filters Used in some operating rooms to remove all microbes larger than 0.3 mm. Low temperatures Refrigeration – slows or stops microbial growth Bacteriostatic (or germistatic) Freezing – stops microbial growth Only a small portion of microbes will die Bacteriostatic High pressure Liquid suspensions treated with high pressure May kill many bacterial cells – bacteriocidal Does not kill endospores Not sterilization Desiccation Removal of water (drying) Slows or stops growth – but usually does not kill microbes – bacteriostatic. Osmotic pressure High concentrations of salts and sugars – hypertonic environment Can kill some bacteria – bacteriocidal or bacteriostatic Molds and yeasts are more resistant Radiation Destroys DNA X-rays and Gamma rays penetrate materials UV and high energy electron beams – used on surfaces Microwaves Do not kill microorganisms directly They heat water High temperatures can be used to disinfect materials. Chemical used to destroy microbes Most can only be used for disinfection – not sterilization Disk diffusion method Disk of filter paper soaked in chemical disinfectant Placed on agar plate that has already been inoculated with bacteria The plate is incubated until bacteria grow Zone of clearing determines effectiveness. Types of disinfectants and antiseptics Phenolic compounds (Phenols) Disrupt plasma membranes, denature proteins – bacteriocidal Very effective, remain active long after application Suitable for disinfection of soiled surfaces (ex. saliva, pus and feces) Often mixed with other compounds to reduce toxicity Examples: Original formulations of Lysol – for household disinfection Hexachlorophene – useful against bacteria that cause skin infections in newborns Can be used to disinfect surfaces in nurseries Caution must be used - causes neurological disorders Triclosan - used in some personal hygiene products – cosmetics and deodorant soaps. Biguanides Chlorhexidine – attacks plasma membrane – bacteriocidal To control microbes on skin and mucous membranes Used in surgical hand scrubs Can persist for up to 6 hours Only kills some viruses, does not destroy endospores. Halogens Destroy proteins and membranes Used as disinfectants and antiseptics: Chlorine Forms hypochlorous acid in water – strong oxidizing agent – denatures enzymes Examples: Household bleach (sodium hypochlorite) Excellent disinfectant Chlorine dioxide – sterilizing gas Iodine Frequently used as an antiseptic. Examples: Tincture of iodine – in an alcohol solution Iodophore – linked to carrier molecule so that is released slowly over time ex. Betadine – used as antiseptic on skin. Alcohols Denature proteins and disrupt membranes Kills bacteria and fungi Does not kill endospores and some viruses Acts quickly and evaporates Often used to disinfect surfaces in a lab, or to degerm skin Does not usually suffice as an adequate antiseptic but often combined with other chemicals to enhance effectiveness Two types used: Ethanol – 70% most effective Isopropanol – rubbing alcohol. Heavy metals Metals such as copper, silver, zinc and mercury can inhibit microbes Silver Nitrate (1%) – used as: Eye drops for new-born infants, to protect against gonorrheal Centers for Disease Control and Prevention's Public Health Image Library opthalmia Silver sulfadiazine: Topical cream for burns Silver ions incorporated into bandages and catheters to slow growth of microbes Copper sulfate – used to inhibit algae in reservoirs Mercuric chloride – used in paint to control mildew (toxic!) Zinc chloride – used as antiseptic in mouthwashes. Regular Soaps and detergents Do not work as disinfectants or antiseptics Important in mechanical removal of microbes Soap breaks up oily deposits and film (ex. on skin) Water can then wash these deposits away along with microbes Works to degerm skin Quartenary ammonium compounds (Quats) Cationic detergents – Kill most bacteria, fungi and some viruses do not kill endospores do not kill some Gram negative bacteria Strongly antimicrobial, colorless, tasteless, odorless and non-toxic at low concentrations ex. Cepecol – used as a mouthwash Some Gram negative bacteria not only survive – but grow well on Quats Ex. Pseudomonas aeruginosa – cause of burn infections. Gaseous chemosterilizers Chemical gasses used for sterilization Ethylene oxide Denatures proteins – germicidal Kills all microbes – including endospores and viruses Requires 4 – 18 hours of exposure Highly penetrating – passes through plastics Used to sterilize disposable hospital equipment and lab supplies Examples: Plastic tubing Disposable swabs. Oxidizing agents Oxidize and denature proteins Useful against anaerobic organisms Hydrogen peroxide – common antiseptic for wounds Ozone – used with chlorine to disinfect water Benzoyl peroxide – used as antiseptic on skin kills anaerobic bacteria living in tissues ex. Bacteria that cause acne.