Control of Microbial Growth Physical and Chemical methods

Control of Microbial Growth: Physical and Chemical methods of Microbial Control: Definitions: Sterilization: is a process used to achieve sterility, an absolute term meaning the absence of all viable microorganisms. Means killing of all living microorganisms, including viruses, bacterial spores, fungi, and fungi spores to achieve sterility of surgical, medical, and laboratory items. Disinfection: is a process which reduces the number of contaminating microorganisms to a level which leave microbes no longer harmful to health.

Methods of Sterilization: 1 - Heat sterilization 2 - Ionizing irradiation 3 - Filtration 4 - Sterilant gases 5 - Sterilant liquids Sterilization by Heat: Heat sterilization is divided into two types: 1 - Dry heat sterilization. 2 - Moist heat sterilization.

Moist heat sterilization: Moist heat is much more effective than dry heat because it kills microorganisms by coagulating and denaturing their enzymes and structural proteins, a process in which water participates. This type of heat sterilization will be performed by exposure to moist heat at 121˚C for 15 minutes. Moist heat sterilization requires temperatures above that of boiling water. By raising the pressure of steam in a pressure vessel called Autoclave to 1. 5 bars, water will boil at 121˚C.

a Dry heat sterilization: Dry heat is believed to kill microorganisms by causing a destructive oxidation of essential cellular components. Types of dry heat sterilization: 1 - Hot air Oven sterilization: Oven is a double walled steel chamber ( with a door) that is electrically heated and thermostatically controlled. The optimum conditions for sterilization by hot air oven will be: For 2 hours at 160˚C, one hour at 170˚C and 30 minutes at 180˚C. 2 - Red heat sterilization and flaming: In the laboratory. Inoculating wires, loops and points of forceps are sterilized by holding them in the flame of Bunsen burner until they are red hot.

a Sterilization by Irradiation: Ionizing radiation: Gamma radiation emitted from a radioactive element, usually Co 60, provides a reliable means of sterilizing plastic and other materials that would be damaged by heat. It is widely used on a commercial basis for sterilization of packaged disposable articles such as syringes and catheters. Ultraviolet radiation: Mercury vapour lamps emitting most of their radiation in the region of 250 -260 nm are bactericidal and, to a lesser extent, sporicidal.

A The energy level of UV radiation is low and its penetration power is poor. Therefore, these lamps should be used only for the disinfection of clean surfaces. Sterilization by Filtration: Aqueous liquids, including solutions of heat-labile substances, may be sterilized by forced passage through a filter of porosity small enough to retain any microorganisms contained in them.

Types of filters: 1 - Membrane filters: these membranes are manufactured from polymeric materials such as cellulose nitrate, cellulose diacetate, and polycarbonate. They are discs with a width of 13 to 293 mm in diameter and with porosities from 0. 015 to 12 μm. These are suitable for filtration of viruses, and toxins. 2 - Syringe filters: Membranes of 13 to 25 mm diameter are commonly fitted in syringe-like holders of plastic. The pore sizes of these filters are 0. 22 to 0. 45 μm. They can be used for filtration of bacteria, and bacterial toxins.

Sterilization by Gaseous processes: Ethylene oxide: This is highly penetrative, non-corrosive and microbicidal gas which is used in industry for the sterilization of single-use, heat-sensitive medical devices such as prosthetic heart valves and plastic catheters. To ensure sterility, material should be exposed to a gas concentration of 700 -1000 mg/l at 45 -60˚C and relative humidity above 70% for about 2 hours.

Low- temperature steam and formaldehyde: This combines thermal effects of steam generated at sub-atmospheric pressure ( 73 -80 ˚C) and a gaseous agent; formaldehyde. Use: For heat sensitive instruments and items of hospital equipment and for large laboratory incubators.

Disinfection: Uses: 1 - In hospital environment, for cleaning of bedpans, surfaces. also for skin disinfection before surgical operation. 2 -In microbiology laboratory, for skin disinfection, instrument disinfection, laboratory surfaces cleaning, and others. Methods of disinfection: 1 - Heat. 2 - Ultraviolet radiation 3 - Gases. 4 - Filtration 5 - Chemicals.

Disinfection by chemicals: 1 -Alcohols: Isopropanol, ethanol and industrial methylated spirits have optimal bactericidal activity in aqueous solution at a concentration of 70 -90% and have little bactericidal effect outside this range. They have limited activity against mycobacteria and are not sporicidal. Action against viruses is generally good.

n Because they are volatile, alcohols have been widely recommended as rapidly drying disinfectants for skin and surfaces. It will not make a good penetration when organic matter such as blood is contaminating surfaces. Alcohols or alcohol-based formulations with chlorhexidine or povidone iodine are good choices for hand skin disinfection.

2 -Aldehydes: Most aldehyde disinfectants are based on glutaraldehyde or formaldehyde formulations, alone or in combination. Glutaraldehyde( 2% solution) has a broad-spectrum action against vegetative bacteria, fungi and viruses, but acts more slowly against spores. It is often used for equipment such as endoscopes that can not be sterilized by heat. It is also used for disinfection of newborn incubators that may be contaminated by fungi, fungal spores, or bacterial spores.

3 -Halogens: A- Hypochlorites: These broad spectrum, inexpensive chlorine releasing disinfectants are the disinfectants of choice against viruses, including hepatitis B virus. Hypochlorites have widespread application as laboratory disinfectants on bench surfaces and for heavy spoilage such as blood contamination.

B-Iodine: like chlorine, iodine is inactivated by organic matter. The iodophors, which contain iodine complexed with an anionic detergent, or povidone iodine, a water soluble complexof iodine and polyvinyl pyrrolidone, are less irritant and cause less staining. Aqueous and alcohol-based povidone iodine preparations are widely used in skin disinfection, including disinfection before operative surgery.

4 -Phenolics: These have been widely used as general purpose environmental disinfectants in hospital and laboratory practice. They exhibit broad spectrum activity and are relatively cheap. Chloroxylenols (Dettol) is an example of phenolic disinfectant that may be used for disinfection of laboratory and hospital ground. The bis-phenol hexachlorophene has particular activity against Gram positive cocci, and used as skin disinfectant.

5 -Oxidizing agents and hydrogen peroxide: Various agents, including chlorine dioxide, peracetic acid and Hydrogen peroxide, have good antimicrobial properties but are Corrosive to skin and metals. Hydrogen peroxide is highly reactive and has limited application for the treatment of wounds. Factors influencing the performance of chemical disinfection: 1 - The concentration and stability of the agent. 2 - The number and the type of microorganisms. 3 - The temperature and p. H. 4 - The presence of organic or other interfering substances.

Chemical structure of disinfectants: chloroxylenol hexachlorophene