Bacterial cultures:
Microbiologists work hard to isolate a microbe from a mixed culture, grow the isolate in a pure culture, and then perform tests to identify it. A culture is made up of one or more types of organisms grown under artificial conditions that are suitable for microbial growth. A mixed culture contains more than one type of microorganism. A pure culture contains a single type of microorganism. Cultures may be grown on or in a variety of different mediums containing resources for growth:
Identification of the microorganism requires a pure culture. The diagnostic process begins with biochemical tests, stains, cultures, and recording the results, which are then compared to a standard database of test results. There is often a presumptive or preliminary identification, followed by a confirmation. Tests run on mixed cultures can lead to misidentification, therefore, subcultures are necessary to isolate a pure culture. This is the foundation for diagnosis of infectious disease and the causative organism.
- a carbon source
- a nitrogen source
- a sulfur source
- other sources
Identification of the microorganism requires a pure culture. The diagnostic process begins with biochemical tests, stains, cultures, and recording the results, which are then compared to a standard database of test results. There is often a presumptive or preliminary identification, followed by a confirmation. Tests run on mixed cultures can lead to misidentification, therefore, subcultures are necessary to isolate a pure culture. This is the foundation for diagnosis of infectious disease and the causative organism.
Plated media (agar in a Petri dish) is shown in this mage of Brain Heart Infusion (BHIA) Agar. 1.5-2% agar in the dish serves as the solidifying agent. Nutrients are dissolved in it. Agar plates are used to isolate organisms from mixed cultures. https://upload.wikimedia.org/wikipedia/commons/6/6a/Agar_Plate.jpg?_=20050507054610
The use of aseptic technique during the culturing process is key to preventing contamination. https://upload.wikimedia.org/wikipedia/commons/9/94/Aseptic_transfer_from_broth.png?_=20210708154355
TSI agar slants are examples of agar slants that have nutrients that make them both selective and differential. The surface is streaked at an angle. https://upload.wikimedia.org/wikipedia/commons/thumb/e/ea/Agar_tsi.JPG/960px-Agar_tsi.JPG?_=20090114205218
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This tubed media is an example of broth media. This is Granada broth, in which a swab is diffusing into. GBS broth is used for isolated Group B Streptococci, and is transferred to plated media at 24 hours. https://upload.wikimedia.org/wikipedia/commons/thumb/5/5a/GBS_Granada_broth.jpg/120px-GBS_Granada_broth.jpg?_=20150521180921
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These are examples of stab media tubed media, which is stabbed with microorganisms using a sterile inoculating needle. https://upload.wikimedia.org/wikipedia/commons/thumb/c/cd/Stab_cultures.jpg/500px-Stab_cultures.jpg?_=20161118161648
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This is an example of a stab culture media designed to reveal motility (2nd tube) of bacteria such as Listeria and others. https://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Cdc_stab_culture.png/960px-Cdc_stab_culture.png?_=20180408230107
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Bacteriological inoculating loops and needles
Plastic inoculation loops are only designed for single, one-time use, and discarded in the biological waste bin afterwards. Metal loops are designed to be sterilized in the Bacti-Cinerator between use, and may be used over and over again and sterilized countless times for reuse.
A traditional metal bacteriological inoculation loop;https://upload.wikimedia.org/wikipedia/commons/thumb/5/51/Inoculation_loop_rot45.jpg/500px-Inoculation_loop_rot45.jpg?_=20191026013921
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A sterile plastic inoculation loop (large);https://upload.wikimedia.org/wikipedia/commons/thumb/8/83/Inoculation_loop-plastic_big.jpg/330px-Inoculation_loop-plastic_big.jpg?_=20120313184548
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A sterile plastic inoculation loop (left/blue/large), small plastic inoculation loop (middle) and small classic metal inoculation loop (right);https://upload.wikimedia.org/wikipedia/commons/thumb/b/bb/Inoculation_loops-metal_and_plastic.jpg/960px-Inoculation_loops-metal_and_plastic.jpg?_=20120313184609
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A large (blue) and small (white) inoculation loop;https://upload.wikimedia.org/wikipedia/commons/thumb/3/35/Inoculation_loops-plastic_big_and_small.jpg/500px-Inoculation_loops-plastic_big_and_small.jpg?_=20120313184626
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A large, classic metal inoculation loop;https://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/Inoculation_loop-metal_wire.jpg/1920px-Inoculation_loop-metal_wire.jpg?_=20120313184543
A small plastic inoculation loop;https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Inoculation_loop-plastic_small.jpg/1920px-Inoculation_loop-plastic_small.jpg?_=20120313184553
https://upload.wikimedia.org/wikipedia/commons/thumb/e/ed/Platinum_loop.jpg/1920px-Platinum_loop.jpg?_=20060928122327
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The Bacti-Cinerator, used to flame the inoculation loops and needles (metal ones only) in order to sterilize them prior to streaking the agar plates with microorganisms
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Inoculation sticks are shown in this image, and they are helpful to use to touch the top of a colony to transfer it to another plate or slide;https://upload.wikimedia.org/wikipedia/commons/thumb/4/40/Inoculation_sticks.jpg/1920px-Inoculation_sticks.jpg?_=20120313184636
Basic inoculating tools;https://upload.wikimedia.org/wikipedia/commons/9/9f/Basic_inoculating_tools.jpg?_=20161118161646
https://upload.wikimedia.org/wikipedia/commons/4/4f/%D0%9F%D1%80%D0%BE%D0%BA%D0%B0%D0%BB%D0%B8%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5_%D0%B1%D0%B0%D0%BA%D1%82%D0%B5%D1%80%D0%B8%D0%BE%D0%BB%D0%BE%D0%B3%D0%B8%D1%87%D0%B5%D1%81%D0%BA%D0%BE%D0%B9_%D0%BF%D0%B5%D1%82%D0%BB%D0%B8.jpg
streak plate methods of isolation:
The identification of an unknown microbe begins with a pure culture of the microbe in question. The streak plate method produces individual colonies on the agar plate, and a portion of an isolated colony can then be transferred to a sterile medium to obtain a pure culture or subculture, which may be to another agar plate, slant, or liquid medium.
If there are two or more species, the microbial culture is said to be "mixed", or a mixed culture. A pure culture contains just one single species. Isolating an individual species from a mixed culture is the first step in identification of the microorganism. A common technique used in the microbiology lab for this purpose is the isolation technique and the streak plate.
Always assume the culture is mixed when plating, and streak the bacterial sample over the surface of the plated agar medium. During the streaking process, the cell density decreases. This eventually leads to individual cells being deposited on the agar surface. Cells that have been isolated well will grow into bacterial colonies of only the original cell type. Some colonies form from individual cells, whereas others form from pairs, chains, clusters of cells, and are otherwise known as colony-forming units (CFUs).
A common streak plate technique is the quadrant method. Some labs use 3 streaks, others use 4 (most common), and some use 5 (least common). Streaking the plate for isolation is often performed on selective media to feed the bacteria and encourage growth of various types of organisms while inhibiting the growth of others. Some selective media contain pH indicators or color indicators to aid in identification and differentiate between others, and these are known as selective and differential media.
In the 4 quadrant streak method, either use a plastic disposable loop, or a metal loop that is flamed between streaks. Hold the plate comfortably in one hand at an angle, and streak with the edge of the loop with the other hand, being gentle and careful not to cut or tear the agar. The first quadrant is a very tight streak back-and-forth. Grabbing from quadrant 1, begin a slightly looser back-and-forth streak into quadrant 2. Turn the plate, grab from quadrant 2, and make an even looser back-and-forth streak in quadrant 3. Turn the plate once more, grab from quadrant 3, and make a very loose streak to isolate colonies in quadrant 4.
If there are two or more species, the microbial culture is said to be "mixed", or a mixed culture. A pure culture contains just one single species. Isolating an individual species from a mixed culture is the first step in identification of the microorganism. A common technique used in the microbiology lab for this purpose is the isolation technique and the streak plate.
Always assume the culture is mixed when plating, and streak the bacterial sample over the surface of the plated agar medium. During the streaking process, the cell density decreases. This eventually leads to individual cells being deposited on the agar surface. Cells that have been isolated well will grow into bacterial colonies of only the original cell type. Some colonies form from individual cells, whereas others form from pairs, chains, clusters of cells, and are otherwise known as colony-forming units (CFUs).
A common streak plate technique is the quadrant method. Some labs use 3 streaks, others use 4 (most common), and some use 5 (least common). Streaking the plate for isolation is often performed on selective media to feed the bacteria and encourage growth of various types of organisms while inhibiting the growth of others. Some selective media contain pH indicators or color indicators to aid in identification and differentiate between others, and these are known as selective and differential media.
In the 4 quadrant streak method, either use a plastic disposable loop, or a metal loop that is flamed between streaks. Hold the plate comfortably in one hand at an angle, and streak with the edge of the loop with the other hand, being gentle and careful not to cut or tear the agar. The first quadrant is a very tight streak back-and-forth. Grabbing from quadrant 1, begin a slightly looser back-and-forth streak into quadrant 2. Turn the plate, grab from quadrant 2, and make an even looser back-and-forth streak in quadrant 3. Turn the plate once more, grab from quadrant 3, and make a very loose streak to isolate colonies in quadrant 4.
4 quadrant streak plate method for pure colony isolation
https://upload.wikimedia.org/wikipedia/commons/thumb/4/42/Petridish.jpg/500px-Petridish.jpg?_=20251027101706
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https://upload.wikimedia.org/wikipedia/commons/5/55/Streak_plates.png?_=20051129001914
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Streaking Quadrant 1 (Growth in quadrant 1 indicates light growth)
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Streaking Quadrant 2 (Growth in quadrant 2 indicates moderate growth)
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Streaking Quadrant 3 (Growth in quadrant 3 indicates heavy growth)
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Streaking Quadrant 4 (Growth in quadrant 4 indicates heavy growth)
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https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Bacterial_growth_on_blood_agar.jpg/960px-Bacterial_growth_on_blood_agar.jpg?_=20230902084814
streak plate method of isolation:
As microbiologists, we streak plates for bacterial or fungal isolation. This is performed on a wide variety of different media. The goal is to obtain pure cultures for further testing. Individual microbial species in a mixed culture need to be isolated and cultivated as pure cultures before correct identification and further testing can be carried out. The most common and least expensive means of isolating organisms in a mixed culture is to use the streak plate method.
The streak plate method of isolation means to spread the microbes on plated agar media so that the individual cells or colony forming units (CFUs) can become isolated and grow into individual, pure colonies. The method most often used in the clinical laboratory is the quadrant method. This uses either the three-streak or four-streak methods shown above. The four-streak method is more common. It involves four individual streaks The best results are obtained when the loop is sterilized or flamed between streaks when using a stainless steel inoculating loop. The plate is turned between streaks. The first quadrant is a tight streak, and the most growth is here. Density decreases in the four-streak pattern. Isolation is first obtained in the third or fourth-streak most of the time. This is where the purest, youngest bacteria are found. Cells from the individual, pure colonies may be transferred to sterile media to start pure cultures. If it is a subculture, test the individual colonies in the last quadrant.
Streaking the agar plate for isolation is performed on selective media so that certain types of microbes are encouraged to grow, while others are inhibited. Some selective media contain color indicators or pH indicators to show the differences between organisms. These media types are both selective and differential because they "differentiate" between organisms.
The streak plate method of isolation means to spread the microbes on plated agar media so that the individual cells or colony forming units (CFUs) can become isolated and grow into individual, pure colonies. The method most often used in the clinical laboratory is the quadrant method. This uses either the three-streak or four-streak methods shown above. The four-streak method is more common. It involves four individual streaks The best results are obtained when the loop is sterilized or flamed between streaks when using a stainless steel inoculating loop. The plate is turned between streaks. The first quadrant is a tight streak, and the most growth is here. Density decreases in the four-streak pattern. Isolation is first obtained in the third or fourth-streak most of the time. This is where the purest, youngest bacteria are found. Cells from the individual, pure colonies may be transferred to sterile media to start pure cultures. If it is a subculture, test the individual colonies in the last quadrant.
Streaking the agar plate for isolation is performed on selective media so that certain types of microbes are encouraged to grow, while others are inhibited. Some selective media contain color indicators or pH indicators to show the differences between organisms. These media types are both selective and differential because they "differentiate" between organisms.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Streak_plate_with_Serratia_marcescens1.jpg/640px-Streak_plate_with_Serratia_marcescens1.jpg
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https://upload.wikimedia.org/wikipedia/commons/thumb/0/06/Streak_plate_with_Serratia_marcescens2.jpg/640px-Streak_plate_with_Serratia_marcescens2.jpg
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https://upload.wikimedia.org/wikipedia/commons/thumb/7/7d/Phaffia_rhodozyma_on_PDA_plate_02.jpg/640px-Phaffia_rhodozyma_on_PDA_plate_02.jpg
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https://upload.wikimedia.org/wikipedia/commons/thumb/9/97/Petri_dish_1.jpg/640px-Petri_dish_1.jpg
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aseptic technique
The use of aseptic technique is so that contamination is reduced or eliminated when plating organisms. It insures that microbes are being handled in a safe manner to prevent contamination of the handler or others in the laboratory. And it means cleaning up after yourself so that no contamination remains after you have worked with bacteria or other microbes. It means being very careful with how you handle cultures, bacteria, and loops, treating the loops as sterile, and being careful not to touch anything with your loop prior to inoculation of the media, whether it be transferring a broth culture to a plate for streaking, or inoculating an isolated colony from a plate to another plate for purity and/or isolation. It may also mean inoculating many tubes of media and agar plates from a stock culture to identify a bacterium. It is crucial that you make sure that only your desired organism is successfully transferred during the inoculation process for successful and accurate identification.
Flaming the loop means to sterilize your loop or inoculating needle until is becomes bright red. The entire wire must be heated, but be careful not to insert the handle inside the incinerator or it might melt.
A pure culture is desired in order to isolate bacteria for identification and susceptibility studies. Methods for obtaining pure cultures are available by using aseptic technique and by using the streak plate method in order to dilute the bacterial cells in a sample to an end point where a single cell divides giving rise to a pure colony. One colony is referred to as a Colony Forming Unit (CFU). The colony is the identical progeny of the original cell and can be picked up and used for further study. The most isolated colony should be used when subculturing because any genetic mutations, such as antibiotic resistance, will have been gained by the most isolated colony (colonies).
Flaming the loop means to sterilize your loop or inoculating needle until is becomes bright red. The entire wire must be heated, but be careful not to insert the handle inside the incinerator or it might melt.
A pure culture is desired in order to isolate bacteria for identification and susceptibility studies. Methods for obtaining pure cultures are available by using aseptic technique and by using the streak plate method in order to dilute the bacterial cells in a sample to an end point where a single cell divides giving rise to a pure colony. One colony is referred to as a Colony Forming Unit (CFU). The colony is the identical progeny of the original cell and can be picked up and used for further study. The most isolated colony should be used when subculturing because any genetic mutations, such as antibiotic resistance, will have been gained by the most isolated colony (colonies).
- Disinfect your work area to kill any microbes that may be present. This process destroys vegetative cells and any viruses, but it may not destroy endospores. This is what the autoclave process is for. Cavicide wipes are used to wipe off countertop surfaces, along with bleach and 70% alcohol.
- Loops and needles need to be sterilized before transferring any culture when using metal loops or inoculating needles to burn and destroy any contaminating organisms that might be present. If using plastic loops, ensure that they have not touched any surfaces or your lab coat prior to inoculation, and that the bottom of the bag or case that the loops are in is properly sealed. If you accidentally drop a loop prior to inoculation, throw it away in the biological waste bin and do not use it to inoculate your media. Take out a new loop.
- Prior to inserting a cooled and sterilized loop or needle into a culture tube, once the cap is removed, flame the mouth of the tube. Inoculate your tube, reflame the moth, and recap it. Most tube media needs to have loosened caps prior to incubation.
- After the inoculation is complete, you need to resterilize your loop or needle to destroy the organisms you just inoculated into your media. Return the loop or needle to its storage place and never place it down on the countertop surface.
- Loops are used to inoculate or streak Petri dish plates. The plate cover is raised and held at an angle over the plate to protect the surface from any contamination in the air, such as dust and bacterial particles. Never talk on the plates, as spit and organisms from your mouth may contaminate the cultures. Take care not to gouge or disturb the surface of the agar with the loop.
- When all work for the day is complete, treat your work area with disinfectant and empty the biological waste bins and autoclave any bags and sharps containers that are full and ready for autoclaving.
urine cultures and some sterile cultures: semiquantitative method
The semiquantitative method is a quick and easy way to semiquantitate the concentration of bacteria in a urine or sterile body fluid sample. Urine culture is used to isolate, identify and quantitate bacteria that cause urinary tract infections. The media used are selective and differential media.
In the semiquantitative CFU counting method, it rapidly results in countable plates without needing a serial dilution. The loop that is used to streak the plates is a volumetric loop, either plastic disposable or flamed metal loop, calibrated to hold 0.001 mL or 0.01 mL of urine. A loopful of urine is transferred to a BAP as an initial single streak down the diameter of the plate. Turn the plate 90 degrees and streak back-and-forth across the line in a zigzag pattern. This evenly disperses the bacteria over the entire plate. After incubation, the colonies are counted and recorded in CFU's/mL. To determine the CFU/mL, divide the number of colonies on the plate by the volume of the loop.
OCD = CFU/loop volume
Example: OCD = 75 CFU/0.001 mL
OCD = 7.5 x 104 CFU/mL
STEPS:
1. Streak a known volume of urine on an agar plate, incubate, and perform a colony count.
2. This is accomplished by using a volumetric inoculating loop (sterilized metal loop or disposable plastic loop) calibrated to hold a specific volume of urine, preferably 0.001 mL or 0.01 mL.
3. Carefully transfer a loopful to the agar plate by making a single streak down the plate (1).
4. Turn the plate 90 degrees and streak across the original line in a zig-zag motion to evenly disperse the bacteria across the agar at right angles (2).
*NOTE: I have worked in a couple of labs that used (3) to streak at right angles again to create a "lawn" or "mat" of growth. Each laboratory is different. Most just use (1) and (2). Check your laboratory's SOP.
5. Incubate for 18-24 hours and check at regular intervals.
6. Record your counts in CFU/mL.
OCD = CFU/loop volume
OCD = 150 CFU/0.001 mL
OCD = 1.5 x 10 to the fifth power CFU/mL
(1 colony = about 1,000 cells)
Approximately 150,000 CFU/mL
In the semiquantitative CFU counting method, it rapidly results in countable plates without needing a serial dilution. The loop that is used to streak the plates is a volumetric loop, either plastic disposable or flamed metal loop, calibrated to hold 0.001 mL or 0.01 mL of urine. A loopful of urine is transferred to a BAP as an initial single streak down the diameter of the plate. Turn the plate 90 degrees and streak back-and-forth across the line in a zigzag pattern. This evenly disperses the bacteria over the entire plate. After incubation, the colonies are counted and recorded in CFU's/mL. To determine the CFU/mL, divide the number of colonies on the plate by the volume of the loop.
OCD = CFU/loop volume
Example: OCD = 75 CFU/0.001 mL
OCD = 7.5 x 104 CFU/mL
STEPS:
1. Streak a known volume of urine on an agar plate, incubate, and perform a colony count.
2. This is accomplished by using a volumetric inoculating loop (sterilized metal loop or disposable plastic loop) calibrated to hold a specific volume of urine, preferably 0.001 mL or 0.01 mL.
3. Carefully transfer a loopful to the agar plate by making a single streak down the plate (1).
4. Turn the plate 90 degrees and streak across the original line in a zig-zag motion to evenly disperse the bacteria across the agar at right angles (2).
*NOTE: I have worked in a couple of labs that used (3) to streak at right angles again to create a "lawn" or "mat" of growth. Each laboratory is different. Most just use (1) and (2). Check your laboratory's SOP.
5. Incubate for 18-24 hours and check at regular intervals.
6. Record your counts in CFU/mL.
OCD = CFU/loop volume
OCD = 150 CFU/0.001 mL
OCD = 1.5 x 10 to the fifth power CFU/mL
(1 colony = about 1,000 cells)
Approximately 150,000 CFU/mL
using the semiquantitative method for streaking urine plates:
https://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/Urine_sample_for_Culture_and_Sensitivity.jpg/960px-Urine_sample_for_Culture_and_Sensitivity.jpg?_=20220101081314
https://upload.wikimedia.org/wikipedia/commons/thumb/9/93/Medical_Support_Squadron_provide_for_Icemen_111115-F-HA566-036.jpg/640px-Medical_Support_Squadron_provide_for_Icemen_111115-F-HA566-036.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Laboratory_test_130220-F-TQ704-080.jpg/1920px-Laboratory_test_130220-F-TQ704-080.jpg?_=20150601171703
http://2.bp.blogspot.com/_IgmDxAy-TC8/S69YGvjSKPI/AAAAAAAABgI/-lPz29YC9zA/s1600/streak.JPG
visual results:
Growth of E. coli from urine on a urine biplate agar plate (5% blood agar plate and macconkey agar)
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2e/National_Lab_Week_130410-F-TT327-015.jpg/640px-National_Lab_Week_130410-F-TT327-015.jpg
Growth of urine bacteria on macconkey agar:
https://upload.wikimedia.org/wikipedia/commons/thumb/f/fd/Klebsiella_growth_on_MacConkey_agar.jpg/640px-Klebsiella_growth_on_MacConkey_agar.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/a/ae/Crescita_batterica_su_agar.jpg/640px-Crescita_batterica_su_agar.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2f/Raoultella_ornithinolytica_on_CLED_agar_of_urine_culture.jpg/640px-Raoultella_ornithinolytica_on_CLED_agar_of_urine_culture.jpg
https://upload.wikimedia.org/wikipedia/commons/thumb/b/b1/Klebsiella_oxytoca_on_CLED_Agar_of_Urine_Culture.jpg/640px-Klebsiella_oxytoca_on_CLED_Agar_of_Urine_Culture.jpg
Bacterial lawn: A streak plate method designed to create a bacterial lawn of growth for the testing of antibiotics
https://upload.wikimedia.org/wikipedia/commons/a/ab/Antibiotic_sensitivity_and_resistance.jpg
Incubation in a microbiological incubator after plating cultures:
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