Control Of Microbial Growth Lecture Notes PDF

Control of Microbial Growth A Glimpse of History Approaches to control Selecting an antimicrobial procedure Physical methods Chemical methods Preservation of perishable products  History of microbial control Joseph Lister revolutionized surgery & developed effective m...

Control of Microbial Growth A Glimpse of History Approaches to control Selecting an antimicrobial procedure Physical methods Chemical methods Preservation of perishable products  History of microbial control Joseph Lister revolutionized surgery & developed effective methods that prevent surgical wound becoming infected. Initially he used carbolic acid to control wound infections & later developed the methods of sterilizing instruments. Various microbes are responsible for many health problems and food spoilage; thus, effective control of microbes is needed on priority basis. Various methods of controlling microbial growth on inanimate objects & some body surfaces will be discussed. Most of the methods discussed here are non-selective in that they adversely effect all forms of life.  Approaches to control Physical methods: include heat, irradiation, filtration and mechanical removal (washing). Chemical methods: use antimicrobial chemicals. Some processes may use a combination of both physical and chemical methods. These methods are of varying levels of effectiveness against different organisms. The method chosen for microbial control depends on the circumstances and degree of control required.  Principles of control Sterilization: – process of removing or destroying all microorganisms and viruses; an absolute term. – Sterile item is absolutely free of microbes, endospores and viruses. – Can be achieved through filtration, heat, chemicals and irradiation. – The term sterile does not encompass prions as they are not destroyed by standard sterilization procedure. – Combustion is the basic sure method used to destroy prions.  Principles of control Disinfection: implies use of antimicrobial chemicals. – process that eliminates most or all pathogens. – Unlike sterilization, some viable microbes may exist. – Disinfectants are used on inanimate objects & surfaces. – Disinfectants are biocides (means to kill life). – Typically used to target microscopic organisms including bacteria, endospores, fungi & viruses. Thus, they are often called germicides. – Bactericidal: means kill bacteria. – Antiseptics: disinfectants formulated (non-toxic enough) for use on skin.  Principles of control Pasteurization: – Brief heat treatment used to kill pathogens and reduce organisms that cause food spoilage. – Food and inanimate objects can be pasteurized. Decontamination: – Treatment to reduce pathogens to level considered safe to handle. – Washing, use of heat or disinfectants. Sanitization: – Implies a process that substantially reduced microbial population that meets accepted health standards. – This does not indicate a specific level of control. Preservation: – Process used to delay spoilage of perishable items – Often includes the addition of microbial growth- inhibiting ingredients/chemicals (bacteriostatic).  Situational considerations Control measures required on routine basis in daily life may not be sufficient for situations such as hospitals, microbiology labs, foods and food production labs. Daily life: washing & scrubbing with soaps and detergents, cooking foods, cleaning surfaces and refrigeration. Simple hand washing with water and soap is the single most important step in preventing the spread of many infectious diseases. Hospitals: Danger of healthcare-associated infections (HAIs) that are hospital acquired and opportunistic infections. Thus, strict and rigorous microbes control measures are required. Operating rooms in hospitals are important places where instruments are used for invasive procedure so they must be sterile. Prions are new concerns for hospitals.  Principles of control Microbiology labs: routinely work with microbes so require rigorous microbes control methods & aseptic techniques. All media & instruments should be sterile, and materials used for microbial work should be discarded properly. Food & food production facilities: Heat treatment, irradiation & use of chemicals are routinely used. Surfaces should be kept free of microbes to avoid contamination. Water treatment facilities: water should be free of pathogenic microbes. Chlorine is traditionally used to disinfect water; however, it may form compounds called disinfection by-products (DBPs) which may have some long-term health risks. Some pathogens such as Cryptosporidium parvum oocysts are resistant to traditional disinfection procedures. Water facilities should minimize both DBPs & C. parvum oocysts.  Selecting an antimicrobial procedure No single procedure fulfills all the desired parameters; therefore, no single, ideal, multipurpose, non-toxic method exists. Choice of an antimicrobial method depends on : 1. Type of Microbes: Products contaminated with microorganisms more resistance to killing require a more rigorous heat or chemical treatment e.g. – Bacterial endospores: Bacillus and Clostridium. They require extreme heat or chemical treatment. – Protozoan cysts & oocysts: e.g., C. parvum & Giardia lamblia are of main concern in water treatment. Unlike endospores, they are rapidly destroyed by boiling. – Mycobacterium species: The presence of waxy wall make them resistant to many chemical treatments so stronger and more toxic disinfectants are required.  Selection of an antimicrobial procedure 1. Type of Microbes (contd). - Pseudomonas species: Resistant to common disinfectants and sometime even grow in them. - Non-enveloped or naked viruses: such as polio are more resistant to disinfectants while enveloped viruses (e.g., HIV) are more sensitive to heat and disinfectants. 2. Numbers of Microorganisms: – The time required by a procedure to kill a population of microbes is dictated in part by the number of organisms initially present. – More time is needed to kill a larger population than a small population. – Washing or scrubbing minimizes the time needed to sterilize and disinfect a product.  Selection of an antimicrobial procedure 2. Numbers of Microorganisms present (cont’d): ‘Decimal reduction time’ or ‘D value’ is the time required for killing 90% of a population of bacteria under specific conditions. 3. Environmental conditions: Temp, pH and presence of fats and other organic materials strongly influence microbial death rates. Presence of dirt, grease and other organic material can interfere with the heat penetration and action of disinfectants (thoroughly clean before disinfection).  Selection of an antimicrobial procedure cont 4. Risk for Infection: – Selection of germicidal procedure is decided based up on the potential risk of items transmitting infectious agents. – Critical, semi-critical, and noncritical items are defined based on their varied invasiveness in the health setting. – Critical items: Needles, scalpels & biopsy forceps. – Semi-critical items: Endoscopes & endotracheal tubes. – Non-critical items: Stethoscopes, blood pressure cuffs. – Items posing greatest threat of disease transmission should be treated with more rigorous sterilization methods. 5. Composition of the item: – Some sterilization and disinfections procedure are not suitable for certain type of materials e.g., many plastics are heat-sensitive.  Physical methods used to destroy or remove microorganisms and viruses Heat treatment is one of the most useful methods as it is reliable, safe, relatively fast and inexpensive. Moist heat: destroys organisms by irreversibly coagulating their proteins e.g., boiling, pasteurization and pressurized steam. 1. Boiling: – Boiling (100°C at sea level) for 10 minutes kills most of the microbes and viruses except endospores.  Using heat to destroy microbes 2. Pasteurization: method developed by Louis Pasteur. – A brief heat treatment that significantly reduces the number of food spoilage bacteria & pathogens. – Does not sterilize. – Used to pasteurize milk, juices, wine etc. It increases shelf life & protects consumers from many infections. Methods of pasteurization – High-temperature-short-time (HTST) method. These days, most pasteurization protocols use this method. 72°C for 15 seconds for pasteurization of milk. 82°C for 20 seconds for pasteurization ice cream. – Ultra-high-temperature (UHT) method (heating to 140°C and holding it for few seconds and then rapidly cooled) e.g., milk and boxed juices.  Using heat to destroy microbes and viruses 3. Sterilization using pressurized steam (Autoclaving) – ‘Pressure cooker’ and ‘Autoclave’ heat water in an enclosed vessel & achieve temperature above 100°C because of increase in vessel pressure beyond atmospheric pressure. – Generally used for items that can be penetrated by steam & withstand the heat and moisture e.g., surgical instruments, microbiological media, reusable glass ware & other supplies. Using heat to destroy microbes and viruses Autoclaving procedure is used as the first line of defense against microbial infections in the hospitals. Also used to sterilize microbial cultures and other biohazards before disposal. Typical conditions used for autoclaving are 15 psi pressure and 121°C for 15 minutes. Longer time periods are required to sterilize large This destroys endospores volumes. and all bacteria. For effective autoclaving, Pressure itself does not play steam should enter the any direct role in killing. items & should displace air. Using heat to destroy microbes and viruses Flash autoclaving/sterilization: When rapid sterilization is required, increase the temperature for a shorter time. Some heat sensitive indicators are used to ensure proper autoclaving e.g., tapes (change in color-black), biological indicators (Geobacillus stearothermophilus). Autoclaving at a temp of 132°C for 1 hour is thought to destroy prions. The commercial canning procedure uses pressurized steam. The process is designed to ensure that endospores of Clostridium botulinum are destroyed. Using heat to destroy microbes and viruses Dry heat: – Burn the cell constituents or irreversibly denature proteins e.g., dry heat oven, flaming and incineration. – Dry heat is not as efficient as wet heat in killing microbes and thus needs much longer time & higher temp to kill organisms. Using other physical methods to remove or destroy microbes Some heat-sensitive material can not be sterilized by heat so other methods including filtration, irradiation and high-pressure treatment are employed. Filtration of fluids: Filtration is commonly used to sterilize heat-sensitive fluids (e.g., culture media, human blood products, etc.) Two types of filters are in general use: 1. Membrane filters or microfilters: Contain pores of a defined size. Depending on size, may clear bacteria only or bacteria and viruses. Filters with pore size of 0.2 micrometers are commonly used to remove bacteria. Using other physical methods to remove or destroy microbes 2. Depth filters: Trap material including bacteria within thick porous filtration material e.g., cellulose fibers. Filtration of air: – Special filters called high-efficiency particulate air (HEPA) filters remove from air almost all microbes that have a diameter of greater than 0.3 µm. – They are used in specialized hospital rooms, biological safety cabinets and laminar flow hoods.  Irradiation Radiation 1. Ionizing radiation: e.g., gamma rays, X-rays etc. – cause biological harm both directly by destroying DNA or damaging cytoplasmic membranes and indirectly by producing reactive molecules such as superoxide and hydroxyl free radicals. – Bacterial endospores are the most radiation-resistant microbial forms, whereas Gram-ve bacteria such as Salmonella & Pseudomonas species are among the most susceptible. – Used extensively to sterilize heat-sensitive material i.e., medical equipments, disposable surgical supplies, drugs, foods, fruits, vegetables & meat. – Radiation can be carried out even after packaging.  Irradiation 2. Ultraviolet radiation (Non-ionizing radiation) – UV light in wavelength of ~220-300nm destroys microbes by directly damaging DNA. – Actively dividing microbes are most easily killed, whereas bacterial endospores are the most UV-resistant. – UV is used extensively to destroy microbes in the air and drinking water and to disinfect surfaces. – UV light is most effective when used at a close range. However, care should be taken in its use as it can damage skin & eyes & promote the development of skin cancers. Microwaves – Do not affect microorganisms directly but they can kill them by heat they generate in the item. – Microbes usually survive microwave cooking as the food heats unevenly. Using other physical methods to remove or destroy microbes High pressure processing – recently introduced for pasteurizing commercial food products. – Pressures up to 130,000 psi is thought to destroy microorganisms by denaturing proteins and altering cell permeability. – Products treated with high pressure processes retain the color and flavor of fresh foods.  Next Lecture Read remaining chapter 5