Nanofilters with smaller pore sizes of nm nanofiltration are used. The smaller the pore size, the lower the flow rate. To achieve higher total throughput or avoid premature blockage, pre-filters might be used to protect small pore membrane filters. Some studies have shown that prions can be removed or reduced by filtration. Membrane filters used in production processes are commonly made from materials such as mixed cellulose ester or polyethersulfone.
The filtration equipment and the filters may be purchased as pre-sterilized disposable units in sealed packaging, or must be sterilized by the user, generally by autoclaving at a temperature that does not damage the fragile filter membranes.
To ensure proper functioning of the filter, the membrane filters are integrity tested post-use or sometimes pre-use. A non-destructive integrity test assures the filter is undamaged, and is also a regulatory requirement enforced by agencies like the Food and Drug Administration, the European Medicines Agency, and others.
Privacy Policy. Skip to main content. Culturing Microorganisms. Search for:. Physical Antimicrobial Control. Heat Heat is one of the most common and easily available methods for controlling bacterial growth.
Learning Objectives Evaluate heat as an agent of microbrial control. Key Takeaways Key Points Different methods are used to achieve sterilization.
One of the most common is applying moist heat which includes autoclaving pressure cooking , boiling, and Tyndallisation. Dry heat sterilization is accomplished by conduction and is used widely for instruments. Other heat methods include flaming and incineration. Key Terms sterilization : Any process that eliminates or kills all forms of microbial life present on a surface, solution, or solid compound. Tyndallisation : Tyndallisation is the process of three successive steam treatments to achieve sterilization over the course of three days.
This works by killing vegetative cells, allowing germination of surviving spores, and killing the resulting vegetative cells before they have time to form further spores. Radiation Both non-ionizing and ionizing radiation methods are applied for sterilization.
Learning Objectives Compare non-ionizing and ionizing radiation in terms of microbe inhibition. Key Takeaways Key Points Ultraviolet light irradiation is a non-ionizing method useful only for sterilization of surfaces and some transparent objects.
Common methods of ionizing radiation are gamma rays, electron beams, X-rays, and subatomic particles. However, ionizing radiation could be a lethal health hazard if used inappropriately. Key Terms Graft-versus-host disease : A complication after tissue or organ transplant or blood transfusion if the blood was not irradiated. White blood cells of the transplanted tissue or organ the graft attack cells in the recipients body the host.
Low Temperatures Low temperatures usually inhibit or stop microbial growth and proliferation but often do not kill bacteria. Learning Objectives Identify how low temperatures are used for microbial control. Refrigeration preserves food by slowing down the growth and reproduction of microorganisms as well as the action of enzymes which cause food to rot.
Freezing kills some microorganisms by physical trauma, while sublethally injuring others which may recover to become infectious. Key Terms proliferation : The process by which an organism produces others of its kind; breeding, propagation, procreation, reproduction. Learning Objectives Explain high pressure as a antimicrobial control. Key Takeaways Key Points High pressure processing HPP , pascalization or bridgmanization, is a method of preserving and sterilizing food, in which a product is processed under very high pressure, leading to the inactivation of certain microorganisms and enzymes in the food.
Later it was discovered that using moderate pressures was more effective in eliminating spores than using higher pressures. Key Terms bridgmanization : Pascalization is also known as bridgmanization, named for physicist Percy Williams Bridgman. Desiccation Desiccation is the state of extreme dryness, or the process of extreme drying and can be used to control microbial growth. Learning Objectives Show how desiccation inhibits microbes.
Key Takeaways Key Points Microorganisms cannot grow and divide when desiccated, but can survive for certain periods of time, depending on their features. Drying is also a method for food preservation. Key Terms desiccation : the state of drying freeze-drying : Freeze-drying, also known as lyophilisation, lyophilization, or cryodesiccation, is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport.
Osmotic Pressure Osmotic pressure is the pressure which must be applied to a solution to prevent the inward flow of water across a semipermeable membrane. Learning Objectives Interpret osmotic pressure as a means of microbial control.
When a cell is placed in a hypertonic solution, water actually flows out of the cell into the surrounding solution thereby causing the cells to shrink and lose its turgidity. Hypertonic solutions are used for antimicrobial control.
Salt and sugar are used to create hypertonic environment for microorganisms and are commonly used as food preservatives. Key Terms turgidity : Turgidity turgor pressure pushes the plasma membrane against the cell wall of plant, bacteria, and fungi cells as well as those protiat cells which have cell walls.
Some bacteria specifically require an environment with a high concentration of sodium chloride. All four organisms will be tested for growth on media containing varying concentrations of NaCl : 0. Test Organisms:. Saccharomyces cerevisiae Staphylococcus aureus Escherichia coli Halobacterium salinarium. On the bottom of the plates, use a marker to divide them into 4 quadrants as shown below. Label each quadrant with one of the test microorganisms. Also label the bottom of the plates with your initials, lab section, and the NaCl concentration of the plate.
There will be a total of 4 plates. However, the growth of molds and fungi is not suppressed as efficiently as the growth of bacteria. Key Takeaways. When a cell is placed in a hypertonic solution, water actually flows out of the cell into the surrounding solution thereby causing the cells to shrink and lose its turgidity.
Hypertonic solutions are used for antimicrobial control. Salt and sugar are used to create hypertonic environment for microorganisms and are commonly used as food preservatives. Key Terms turgidity : Turgidity turgor pressure pushes the plasma membrane against the cell wall of plant, bacteria, and fungi cells as well as those protiat cells which have cell walls. The autoclave is still considered the most effective method of sterilization. Outside laboratory and clinical settings, large industrial autoclaves called retort s allow for moist-heat sterilization on a large scale.
In general, the air in the chamber of an autoclave is removed and replaced with increasing amounts of steam trapped within the enclosed chamber, resulting in increased interior pressure and temperatures above the boiling point of water.
The two main types of autoclaves differ in the way that air is removed from the chamber. In gravity displacement autoclaves , steam is introduced into the chamber from the top or sides.
Air, which is heavier than steam, sinks to the bottom of the chamber, where it is forced out through a vent. Complete displacement of air is difficult, especially in larger loads, so longer cycles may be required for such loads. In prevacuum sterilizers , air is removed completely using a high-speed vacuum before introducing steam into the chamber.
Because air is more completely eliminated, the steam can more easily penetrate wrapped items. Many autoclaves are capable of both gravity and prevacuum cycles, using the former for the decontamination of waste and sterilization of media and unwrapped glassware, and the latter for sterilization of packaged instruments. Figure 2. The length of exposure depends on the volume and nature of material being sterilized, but it is typically 20 minutes or more, with larger volumes requiring longer exposure times to ensure sufficient heat transfer to the materials being sterilized.
The steam must directly contact the liquids or dry materials being sterilized, so containers are left loosely closed and instruments are loosely wrapped in paper or foil. The key to autoclaving is that the temperature must be high enough to kill endospores to achieve complete sterilization. Figure 3. The white strips on autoclave tape left tube turn dark during a successful autoclave run right tube. Because sterilization is so important to safe medical and laboratory protocols, quality control is essential.
Autoclaves may be equipped with recorders to document the pressures and temperatures achieved during each run. Additionally, internal indicators of various types should be autoclaved along with the materials to be sterilized to ensure that the proper sterilization temperature has been reached. One common type of indicator is the use of heat-sensitive autoclave tape , which has white stripes that turn black when the appropriate temperature is achieved during a successful autoclave run.
This type of indicator is relatively inexpensive and can be used during every run. However, autoclave tape provides no indication of length of exposure, so it cannot be used as an indicator of sterility.
Another type of indicator, a biological indicator spore test, uses either a strip of paper or a liquid suspension of the endospores of Geobacillus stearothermophilus to determine whether the endospores are killed by the process. The endospores of the obligate thermophilic bacterium G. Biological spore indicators can also be used to test the effectiveness of other sterilization protocols, including ethylene oxide, dry heat, formaldehyde, gamma radiation, and hydrogen peroxide plasma sterilization using either G.
In the case of validating autoclave function, the endospores are incubated after autoclaving to ensure no viable endospores remain. Bacterial growth subsequent to endospore germination can be monitored by biological indicator spore tests that detect acid metabolites or fluorescence produced by enzymes derived from viable G. A third type of autoclave indicator is the Diack tube , a glass ampule containing a temperature-sensitive pellet that melts at the proper sterilization temperature.
Spore strips or Diack tubes are used periodically to ensure the autoclave is functioning properly. Although complete sterilization is ideal for many medical applications, it is not always practical for other applications and may also alter the quality of the product.
Boiling and autoclaving are not ideal ways to control microbial growth in many foods because these methods may ruin the consistency and other organoleptic sensory qualities of the food. Pasteurization is a form of microbial control for food that uses heat but does not render the food sterile.
Traditional pasteurization kills pathogens and reduces the number of spoilage-causing microbes while maintaining food quality. The process of pasteurization was first developed by Louis Pasteur in the s as a method for preventing the spoilage of beer and wine. Today, pasteurization is most commonly used to kill heat-sensitive pathogens in milk and other food products e.
However, because pasteurized food products are not sterile, they will eventually spoil. The methods used for milk pasteurization balance the temperature and the length of time of treatment.
UHT pasteurized milk can be stored for a long time in sealed containers without being refrigerated; however, the very high temperatures alter the proteins in the milk, causing slight changes in the taste and smell. Still, this method of pasteurization is advantageous in regions where access to refrigeration is limited. Figure 4. Just as high temperatures are effective for controlling microbial growth, exposing microbes to low temperatures can also be an easy and effective method of microbial control, with the exception of psychrophiles , which prefer cold temperatures see Temperature and Microbial Growth.
This temperature range inhibits microbial metabolism, slowing the growth of microorganisms significantly and helping preserve refrigerated products such as foods or medical supplies. Certain types of laboratory cultures can be preserved by refrigeration for later use. According to the US Department of Agriculture USDA , the only safe ways that frozen foods can be thawed are in the refrigerator, immersed in cold water changed every 30 minutes, or in the microwave, keeping the food at temperatures not conducive for bacterial growth.
Figure 5. Cultures and other medical specimens can be stored for long periods at ultra-low temperatures. Exposure to high pressure kills many microbes. In the food industry, high-pressure processing also called pascalization is used to kill bacteria, yeast, molds, parasites, and viruses in foods while maintaining food quality and extending shelf life. The application of high pressure between and MPa sea level atmospheric pressure is about 0. In clinical settings, hyperbaric oxygen therapy is sometimes used to treat infections.
In this form of therapy, a patient breathes pure oxygen at a pressure higher than normal atmospheric pressure, typically between 1 and 3 atmospheres atm. This is achieved by placing the patient in a hyperbaric chamber or by supplying the pressurized oxygen through a breathing tube.
Hyperbaric oxygen therapy helps increase oxygen saturation in tissues that become hypoxic due to infection and inflammation. Increased oxygen levels also contribute to the formation of toxic free radicals that inhibit the growth of oxygen-sensitive or anaerobic bacteria like as Clostridium perfringens , a common cause of gas gangrene. Hyperbaric oxygen therapy also seems to enhance the effectiveness of antibiotic treatments.
Unfortunately, some rare risks include oxygen toxicity and effects on delicate tissues, such as the eyes, middle ear, and lungs, which may be damaged by the increased air pressure. High pressure processing is not commonly used for disinfection or sterilization of fomites. Although the application of pressure and steam in an autoclave is effective for killing endospores, it is the high temperature achieved, and not the pressure directly, that results in endospore death.
One Monday in spring , an Ohio woman began to experience blurred, double vision; difficulty swallowing; and drooping eyelids.
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