What is the criteria for controlling bacterial growth? What are the methods used
ID: 17515 • Letter: W
Question
What is the criteria for controlling bacterial growth?What are the methods used to control bacterial growth?
Understanding the effectiveness of temperature, antibiotics, ultraviolet light, and chemicals like disinfectants and antiseptics.
Explanation / Answer
Main Requirements Many food-poisoning bacteria have to multiply to high numbers in food before they are likely to cause illness. The four main requirements for bacterial growth are food, moisture, warmth and time. Food The foods in which pathogens like to multiply are normally nutritious foods. These include meat, poultry, fish (particularly shellfish), cooked rice and pasta, milk products and eggs and also any foods which contain these as an ingredient such as meat pies, sandwiches, gravy, salads, etc. Pathogens will grow in both raw and cooked foods. Many raw foods, particularly meat and poultry, will contain pathogens. Most such foods are cooked before consumption and thorough cooking will kill pathogens, making the food safe to eat. The consumption of foods which have not been cooked or heat-treated, however, may lead to food-poisoning. The following foods have often been implicated in outbreaks: unpasteurised dairy products such as milk and cheese; foods containing raw eggs such as mayonnaise and certain desserts;* uncooked shellfish such as oysters and mussels. It is, of course, possible to find bacteria in cooked food. Pathogens may be re-introduced to cooked foods by cross-contamination after cooking. There is also the opportunity for those bacteria which are able to produce spores when foods are cooked to become active bacteria again if the cooked food is kept in warm temperatures (within the growth zone). Cooked foods must therefore be protected from contamination and kept at the right temperature (either hot or cold). * Note: Since raw eggs may sometimes contain food-poisoning bacteria, they should not be used as an ingredient in foods such as mousses and cold desserts which will not be cooked. It is now possible to buy and use pasteurised egg instead. This has been heat-treated to kill any pathogens. The Government advises that people vulnerable to infection such as babies, toddlers, older people, those who are already ill and pregnant women should only eat eggs that are thoroughly cooked until both the yolk and white are solid. They suggest that for other people there is little risk from eating cooked eggs. Moisture Most foods naturally contain sufficient moisture to provide bacteria with the water they need in order to grow. Where moisture has been deliberately removed (e.g. in dehydrated foods such as milk powder, soup mixes, etc.), then bacteria will not grow whilst the food remains dry, but once water is added then bacterial growth may occur once more. Warmth / Temperature Bacteria have varying requirements in terms of the range of temperatures in which they will grow. Those which grow at low temperatures (usually below 20°C) are called psychrophiles and at high temperatures (above 45°C) are thermophiles. Some spoilage bacteria fall into these categories. Most pathogens, however, like warmth and are known as mesophiles. They will grow at temperatures between 5°C and 63°C, commonly referred to as the growth or 'danger' zone and have an optimum temperature for growth of about 37°C. Listeria bacteria will grow very slowly below 5°C, but most pathogens become inactive (dormant) at low temperatures. They start to multiply more rapidly as the temperature rises. At a temperature of about 37°C (human body temperature) pathogens multiply most quickly but as the temperature continues to rise, their rate slows down and they will stop growing altogether above 63°C. However, in order to destroy bacteria, temperatures must rise further. A temperature of 70°C for 2 minutes is recommended as a means of killing pathogens during the normal cooking process. Time In ideal conditions (i.e. in moist foods at 37°C) bacteria will grow and multiply by dividing into two every 20 minutes. After 6 hours, in ideal conditions, one bacterial cell could become 131,072 bacteria. .................................................... The control of microbial growth is necessary in many practical situations, and significant advances in agriculture, medicine, and food science have been made through study of this area of microbiology. "Control of microbial growth", as used here, means to inhibit or prevent growth of microorganisms. This control is affected in two basic ways: (1) by killing microorganisms or (2) by inhibiting the growth of microorganisms. Control of growth usually involves the use of physical or chemical agents which either kill or prevent the growth of microorganisms. Agents which kill cells are called cidal agents; agents which inhibit the growth of cells (without killing them) are referred to as static agents. Thus, the term bactericidal refers to killing bacteria, and bacteriostatic refers to inhibiting the growth of bacterial cells. A bactericide kills bacteria, a fungicide kills fungi, and so on. In microbiology, sterilization refers to the complete destruction or elimination of all viable organisms in or on a substance being sterilized. There are no degrees of sterilization: an object or substance is either sterile or not. Sterilization procedures involve the use of heat, radiation or chemicals, or physical removal of cells. Methods of Sterilization Heat: most important and widely used. For sterilization one must consider the type of heat, and most importantly, the time of application and temperature to ensure destruction of all microorganisms. Endospores of bacteria are considered the most thermoduric of all cells so their destruction guarantees sterility. Incineration: burns organisms and physically destroys them. Used for needles, inoculating wires, glassware, etc. and objects not destroyed in the incineration process. Boiling: 100o for 30 minutes. Kills everything except some endospores. To kill endospores, and therefore sterilize a solution, very long (>6 hours) boiling, or intermittent boiling is required (See Table 1 below). Autoclaving (steam under pressure or pressure cooker) Autoclaving is the most effective and most efficient means of sterilization. All autoclaves operate on a time/temperature relationship. These two variables are extremely important. Higher temperatures ensure more rapid killing. The usual standard temperature/pressure employed is 121ºC/15 psi for 15 minutes. Longer times are needed for larger loads, large volumes of liquid, and more dense materials. Autoclaving is ideal for sterilizing biohazardous waste, surgical dressings, glassware, many types of microbiologic media, liquids, and many other things. However, certain items, such as plastics and certain medical instruments (e.g. fiber-optic endoscopes), cannot withstand autoclaving and should be sterilized with chemical or gas sterilants. When proper conditions and time are employed, no living organisms will survive a trip through an autoclave. Schematic diagram of a laboratory autoclave in use to sterilize microbiological culture medium. Sterilization of microbiological culture media is is often carried out with the autoclave. When microbiological media are prepared, they must be sterilized and rendered free of microbial contamination from air, glassware, hands, etc. The sterilization process is a 100% kill, and guarantees that the medium will stay sterile unless exposed to contaminants. An autoclave for use in a laboratory or hospital setting. Why is an autoclave such an effective sterilizer? The autoclave is a large pressure cooker; it operates by using steam under pressure as the sterilizing agent. High pressures enable steam to reach high temperatures, thus increasing its heat content and killing power. Most of the heating power of steam comes from its latent heat of vaporization. This is the amount of heat required to convert boiling water to steam. This amount of heat is large compared to that required to make water hot. For example, it takes 80 calories to make 1 liter of water boil, but 540 calories to convert that boiling water to steam. Therefore, steam at 100º C has almost seven times more heat than boiling water. Moist heat is thought to kill microorganisms by causing denaturation of essential proteins. Death rate is directly proportional to the concentration of microorganisms at any given time. The time required to kill a known population of microorganisms in a specific suspension at a particular temperature is referred to as thermal death time (TDT). Increasing the temperature decreases TDT, and lowering the temperature increases TDT. Processes conducted at high temperatures for short periods of time are preferred over lower temperatures for longer times. Environmental conditions also influence TDT. Increased heat causes increased toxicity of metabolic products and toxins. TDT decreases with pronounced acidic or basic pHs. However, fats and oils slow heat penetration and increase TDT. It must be remembered that thermal death times are not precise values; they measure the effectiveness and rapidity of a sterilization process. Autoclaving 121ºC/15 psi for 15 minutes exceeds the thermal death time for most organisms except some extraordinary sporeformers. Dry heat (hot air oven): basically the cooking oven. The rules of relating time and temperature apply, but dry heat is not as effective as moist heat (i.e., higher temperatures are needed for longer periods of time). For example 160o/2hours or 170o/1hour is necessary for sterilization. The dry heat oven is used for glassware, metal, and objects that won't melt. Irradiation: usually destroys or distorts nucleic acids. Ultraviolet light is commonly used to sterilize the surfaces of objects, although x-rays, gamma radiation and electron beam radiation are also used. Ultraviolet lamps are used to sterilize workspaces and tools used in microbiology laboratories and health care facilities. UV light at germicidal wavelengths (two peaks, 185 nm and 265 nm) causes adjacent thymine molecules on DNA to dimerize, thereby inhibiting DNA replication (even though the organism may not be killed outright, it will not be able to reproduce). However, since microorganisms can be shielded from ultraviolet light in fissures, cracks and shaded areas, UV lamps should only be used as a supplement to other sterilization techniques. An ultraviolet sterilization cabinet. Gamma radiation and electron beam radiation are forms of ionizing radiation used primarily in the health care industry. Gamma rays, emitted from cobalt-60, are similar in many ways to microwaves and x-rays. Gamma rays delivered during sterilization break chemical bonds by interacting with the electrons of atomic constituents. Gamma rays are highly effective in killing microorganisms and do not leave residues or have sufficient energy to impart radioactivity. Electron beam (e-beam) radiation, a form of ionizing energy, is generally characterized by low penetration and high-dose rates. E-beam irradiation is similar to gamma radiation in that it alters various chemical and molecular bonds on contact. Beams produced for e-beam sterilization are concentrated, highly-charged streams of electrons generated by the acceleration and conversion of electricity. e-beam and gamma radiation are for sterilization of items ranging from syringes to cardiothoracic devices. Filtration involves the physical removal (exclusion) of all cells in a liquid or gas. It is especially important for sterilization of solutions which would be denatured by heat (e.g. antibiotics, injectable drugs, amino acids, vitamins, etc.). Portable units can be used in the field for water purification and industrial units can be used to "pasteurize" beverages. Essentially, solutions or gases are passed through a filter of sufficient pore diameter (generally 0.22 micron) to remove the smallest known bacterial cells. This water filter for hikers and backpackers is advertised to "eliminate Giardia, Cryptosporidium and most bacteria." The filter is made from 0.3 micron pleated glass fiber with a carbon core. A typical set-up in a microbiology laboratory for filtration sterilization of medium components that would be denatured or changed by heat sterilization. The filter is placed (aseptically) on the glass platform, then the funnel is clamped and the fluid is drawn by vacuum into a previously sterilized flask. The recommended size filter that will exclude the smallest bacterial cells is 0.22 micron. Chemical and gas Chemicals used for sterilization include the gases ethylene oxide and formaldehyde, and liquids such as glutaraldehyde. Ozone, hydrogen peroxide and peracetic acid are also examples of chemical sterilization techniques are based on oxidative capabilities of the chemical. Ethylene oxide (ETO) is the most commonly used form of chemical sterilization. Due to its low boiling point of 10.4ºC at atmospheric pressure, EtO) behaves as a gas at room temperature. EtO chemically reacts with amino acids, proteins, and DNA to prevent microbial reproduction. The sterilization process is carried out in a specialized gas chamber. After sterilization, products are transferred to an aeration cell, where they remain until the gas disperses and the product is safe to handle. ETO is used for cellulose and plastics irradiation, usually in hermetically sealed packages. Ethylene oxide can be used with a wide range of plastics (e.g. petri dishes, pipettes, syringes, medical devices, etc.) and other materials without affecting their integrity. An ethylene oxide sterilization gas chamber. Ozone sterilization has been recently approved for use in the U.S. It uses oxygen that is subjected to an intense electrical field that separates oxygen molecules into atomic oxygen, which then combines with other oxygen molecules to form ozone. Ozone is used as a disinfectant for water and food. It is used in both gas and liquid forms as an antimicrobial agent in the treatment, storage and processing of foods, including meat, poultry and eggs. Many municipalities use ozone technology to purify their water and sewage. Los Angeles has one of the largest municipal ozone water treatment plants in the world. Ozone is used to disinfect swimming pools, and some companies selling bottled water use ozonated water to sterilize containers. An ozone fogger for sterilization of egg surfaces. The system reacts ozone with water vapors to create powerful oxidizing radicals. This system is totally chemical free and is effective against bacteria, viruses and hazardous microorganisms which are deposited on egg shells. An ozone sterilizer for use in the hospital or other medical environment. Low Temperature Gas Plasma (LTGP) is used as an alternative to ethylene oxide. It uses a small amount of liquid hydrogen peroxide (H2O2), which is energized with radio frequency waves into gas plasma. This leads to the generation of free radicals and other chemical species, which destroy organisms. An LTGP sterilizer that pumps vaporized H2O2 into the chamber.
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