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Membrane filters

Filtration can also be used to remove microbes from liquid samples using membrane filtration . Membrane filters for liquids function similarly to HEPA filters for air. Typically, membrane filters that are used to remove bacteria have an effective pore size of 0.2 µm, smaller than the average size of a bacterium (1 µm), but filters with smaller pore sizes are available for more specific needs. Membrane filtration is useful for removing bacteria from various types of heat-sensitive solutions used in the laboratory, such as antibiotic solutions and vitamin solutions. Large volumes of culture media may also be filter sterilized rather than autoclaved to protect heat-sensitive components. Often when filtering small volumes, syringe filter s are used, but vacuum filter s are typically used for filtering larger volumes ( [link] ).

a) Photo of 2 chambers separated by a filter; a tube runs from below the filter to a device. B) A photo of a syringe with a filter on the end.
Membrane filters come in a variety of sizes, depending on the volume of solution being filtered. (a) Larger volumes are filtered in units like these. The solution is drawn through the filter by connecting the unit to a vacuum. (b) Smaller volumes are often filtered using syringe filters, which are units that fit on the end of a syringe. In this case, the solution is pushed through by depressing the syringe’s plunger. (credit a, b: modification of work by Brian Forster)
  • Would membrane filtration with a 0.2-µm filter likely remove viruses from a solution? Explain.
  • Name at least two common uses of HEPA filtration in clinical or laboratory settings.

[link] and [link] summarize the physical methods of control discussed in this section.

A table titled physical methods of control; 4 columns – method, conditions, mode of action, and examples of use. Groupings are: heat, cold, pressure, desiccation, radiation, sonication, and filtration. Heat. Boiling, 100 °C at sea level, Denatures proteins and alters membranes; usese Cooking, personal use, preparing certain laboratory media. Dry-heat oven, 170 °C for 2 hours, Denatures proteins and alters membranes, dehydration, desiccation; uses Sterilization of heat-stable medical and laboratory equipment and glassware. Incineration, Exposure to flame,Destroy by burning, Flaming loop, microincinerator. Autoclave, Typical settings: 121 °C for 15–40 minutes at 15 psi, Denatures proteins and alters membranes, Sterilization of microbiological media, heat-stable medical and laboratory equipment, and other heat-stable items. Pasteurization, 72 °C for 15 seconds (HTST) or 138 °C for ≥ 2 seconds (UHT), Denatures proteins and alters membranes, Prevents spoilage of milk, apple juice, honey, and other ingestible liquids. Cold. Refrigeration, 0 °C to 7 °C, Inhibits metabolism (slows or arrests cell division), Preservation of food or laboratory materials (solutions, cultures). Freezing, Below −2 °C, Stops metabolism, may kill microbes, Long-term storage of food, laboratory cultures, or medical specimens. Pressure. High-pressure processing, Exposure to pressures of 100–800 MPa, Denatures proteins and can cause cell lysis Preservation of food, Hyberbaric oxygen therapy. Inhalation of pure oxygen at a pressure of 1–3 atm, Inhibits metabolism and growth of anaerobic microbes, Treatment of certain infections (e.g., gas gangrene). Dessication. Simple desiccation, Drying, Inhibits metabolism, Dried fruits, jerky. Reduce water activity, Addition of salt or water Inhibits metabolism and can cause lysis, Salted meats and fish, honey, jams and jellies. Lyophilization, Rapid freezing under vacuum, Inhibits metabolism Preservation of food, laboratory cultures, or reagents. Radiation. Ionizing radiation, Exposure to X-rays or gamma rays, Alters molecular structures, introduces double-strand breaks into DNA, Sterilization of spices and heat-sensitive laboratory and medical items; used for food sterilization in Europe but not widely accepted in US. Nonionizing radiation, Exposure to ultraviolet light, Introduces thymine dimers, leading to mutations, Surface sterilization of laboratory materials, water purification. Sonication, Exposure to ultrasonic waves, Cavitation (formation of empty space) disrupts cells, lysing them, Laboratory research to lyse cells; cleaning jewelry, lenses, and equipment. Filtration. HEPA filtration, Use of HEPA filter with 0.3-µm pore size Physically removes microbes from air, Laboratory biological safety cabinets, operating rooms, isolation units, heating and air conditioning systems, vacuum cleaners. Membrane filtration Use of membrane filter with 0.2-µm or smaller pore size, Physically removes microbes from liquid solutions, Removal of bacteria from heat-sensitive solutions like vitamins, antibiotics, and media with heat-sensitive components.
See alt text for previous figure. This figure is a continuation of a 2 part figure, the contents of which are described in full in the alt text for the previous figure.

Key concepts and summary

  • Heat is a widely used and highly effective method for controlling microbial growth.
  • Dry-heat sterilization protocols are used commonly in aseptic techniques in the laboratory. However, moist-heat sterilization is typically the more effective protocol because it penetrates cells better than dry heat does.
  • Pasteurization is used to kill pathogens and reduce the number of microbes that cause food spoilage. High-temperature, short-time pasteurization is commonly used to pasteurize milk that will be refrigerated; ultra-high temperature pasteurization can be used to pasteurize milk for long-term storage without refrigeration.
  • Refrigeration slows microbial growth; freezing stops growth, killing some organisms. Laboratory and medical specimens may be frozen on dry ice or at ultra-low temperatures for storage and transport.
  • High-pressure processing can be used to kill microbes in food. Hyperbaric oxygen therapy to increase oxygen saturation has also been used to treat certain infections.
  • Desiccation has long been used to preserve foods and is accelerated through the addition of salt or sugar, which decrease water activity in foods.
  • Lyophilization combines cold exposure and desiccation for the long-term storage of foods and laboratory materials, but microbes remain and can be rehydrated.
  • Ionizing radiation , including gamma irradiation, is an effective way to sterilize heat-sensitive and packaged materials. Nonionizing radiation , like ultraviolet light, is unable to penetrate surfaces but is useful for surface sterilization.
  • HEPA filtration is commonly used in hospital ventilation systems and biological safety cabinets in laboratories to prevent transmission of airborne microbes. Membrane filtration is commonly used to remove bacteria from heat-sensitive solutions.

Fill in the blank

In an autoclave, the application of pressure to ________ is increased to allow the steam to achieve temperatures above the boiling point of water.


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Ionizing radiation can penetrate surfaces, but nonionizing radiation cannot.


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Moist-heat sterilization protocols require the use of higher temperatures for longer periods of time than do dry-heat sterilization protocols do.


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Short answer

What is the advantage of HTST pasteurization compared with sterilization? What is an advantage of UHT treatment?

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How does the addition of salt or sugar help preserve food?

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Which is more effective at killing microbes: autoclaving or freezing? Explain.

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Questions & Answers

deffination of staining
Bhavanimangali Reply
with the aid of a well labeled diagram describe the conducting system
Maridad Reply
what is cellular immunity
namugenyi Reply
Cellular Immunity. -Lymphocytes act against target cell. -Acts directly by killing infected cells.
What are NK cells
Natural killer cells
what are Antigen determinant
cellular immunity is the state where the lymphocytes destroy the infected or targeted cell
any examples of oedema
introduction of microbial diversity-1
Bhavanimangali Reply
List the type of micro organism arround us and how they can be seen and with what kind of instrument
clinton Reply
how is the arrangements of bacteria in bacilli
Vaidah Reply
Provide some examples of bacterial structures that might be used as antibiotic targets and explain why.
Coccobacilli, Club-Shaped bacilli, Bacilli with rounded ends, Fuilform bacilli, Bacilli with ends square.
three main antibiotic targets in bacteria: The cell wall or membranes that surrounds the bacterial cell. The machineries that make the nucleic acids DNA and RNA. The machinery that produce proteins
The bacterial cell wall. Protein production. and DNA synthesis. Why, this is because most drugs (antibiotics) affects the cell wall of the bacteria, which makes the bacteria weak or susceptible in human body.
UV rays affecting the..
Mali Reply
what is microbiology
Baba Reply
Microbiology is the study of microorganisms, which are prokaryotic and eukaryotic cell which includes bacteria, fungi, viruses and pathogenic protozoa.
Microbiology is the branch of Life science which deals with scientific study of many Microorganisms.
what is types of microbiology
Immunology, Bacteriology, Virology, Mycology, Algology etc
Virology, Immunology, Bacteriology, Algology, Mycology, Protoozology etc
and what is mycology
Immunology, Serology, Virology, Microbial Genetics, Parasitology, Bacteriology, Mycology, Molecular, Cell Biology, Agricultural, Water,Soil, Food Industrial ,Pharmaceutical, Applied, Environmental, Clinical, Medical,Marine Microbiology, Microbial Systematics, Etc, are & many types of Microbiology.
study of fungi is called mycology
Mycology is the branch of Microbiology which deals with scientific study of Fungi.
Study of microorganisms,which we can't see with our naked eye is called microbiology
Mycology is the scientific study of Fungi.
virology is the study of viruses
what is microbiology? microbiology is the study of small microorganisms that we can not with our naked eyes.
what is taxonomical classification of microbiology
The algae, protozoa, slime moulds, fungi, bacteria, archaea and viruses ,are taxonomic classification of Microorganisms
We have Bacteria, Archaea, Protozoa, Algae, Fungi, Viruses.
microbiology is the study of microbes too small to be seen by naked eyes
microbiology is a branch of biology which deals with study of smallest living microrganisms such as bacteria protozoa fungi and viruses
microbiology is the study of microorganisms which can't be seen by our naked eyes
Micro - Minute Bio - Life Logus - Study
what is the meaning of antimicrobial susceptibility testing
Devshree Reply
seven gram positive bacteria
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seven examples of gram negative bacteria
Physical conditions that would enable selective Isolation of staphylococcus epidermis
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Nutritional requirements that would enable selective Isolation of staphylococcus epidermis
Nutritional requirements that would enable selective Isolation of staphylococcus epidermis
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Digestion of food is completed in __
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Small Intestine
large inteatine
small intestine
Small intestine
small intestine
small intestine specific in illum
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Raviha Reply
numerical and molecular taxanomy
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difference btwn hausteria and appesorium

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