Department of microbiology microbial food technology group a diploma in quality assurance in microbiology diploma

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Methylene Blue Reduction Test

The methylene blue reduction test is based on the fact that the color imparted to milk by the addition of a dye such as methylene blue will disappear more or less quickly. The removal of the oxygen from milk and the formation of reducing substances during bacterial metabolism causes the color to disappear. The agencies responsible for the oxygen consumption are the bacteria. Though certain species of bacteria have considerably more influence than others, it is generally assumed that the greater the number of bacteria in milk, the quicker will the oxygen be consumed, and in turn the sooner will the color disappear. Thus, the time of reduction is taken as a measure of the number of organisms in milk although actually it is likely that it is more truly a measure of the total metabolic reactions proceeding at the cell surface of the bacteria.

The methylene blue reduction test has lost much of its popularity because of its low correlation with other bacterial procedures. This is true particularly in those samples which show extensive multiplication of the psychrotropic species.

Apparatus.–The necessary equipment consists of test tubes with rubber stoppers, a pipette or dipper graduated to deliver 10 ml of milk and a water bath for maintaining the samples at 35o to 37oC. The bath should contain a volume of water sufficient to heat the samples to 35o C within 10 minutes after the tubes enter the water and should have some means of protecting the samples from light during the incubation period. If a hot-air chamber is used, the samples should be heated to 35o C in a water bath since warm air would heat the milk too slowly.

The dry tablets contain methylene blue thiocyanate and may be obtained from any of the usual laboratory supply houses. They should be certified by the Commission on Standardization of Biological Stains. The solution is prepared by autoclaving or momentarily boiling 200 ml of distilled water in a light resistant (amber) stoppered flask and then adding one methylene blue tablet to the flask of hot water. The tablet should be completely dissolved before the solution is cooled. The solution may be stored in the stoppered, amber flask or an amber bottle in the dark. Fresh solution should be prepared weekly.

Procedure in Testing.–The following procedures are recommended.

(1) Sterilize all glassware and rubber stoppers either in an autoclave or in boiling water. Be sure all glassware is chemically clean.

(2) Measure 1 ml of the methylene blue thiocyanate solution into a test tube.

(3) Add 10 ml of milk and stopper.

(4) Tubes may be placed in the water bath immediately or may be stored in the refrigerator at 0o to 4o C for a more convenient time of incubation. When ready to perform the test, the temperature of the samples should be brought to 35o C within 10 minutes.

(5) When temperature reaches 36o C, slowly invert tubes a few times to assure uniform creaming. Do not shake tubes. Record this time as the beginning of the incubation period. Cover to keep out light.

(6) Check samples for decolorization after 30 minutes of incubation. Make subsequent readings at hourly intervals thereafter.

(7) After each reading, remove decolorized tubes and then slowly make one complete inversion of remaining tubes.

(8) Record reduction time in whole hours between last inversion and decolorization. For example, if the sample were still blue after L 5 hours but was decolorized (white) at the 2.5 hour reading, the methylene blue reduction time would be recorded as 2 hours. Decolorization is considered complete when four-fifths of the color has disappeared.

Classification.–The suggested classification is listed.

Class 1. Excellent, not decolorized in 8 hours.

Class 2. Good, decolorized in less than 8 hours but not less than 6 hours.

Class 3. Fair, decolorized in less than 6 hours but not less than 2 hours.

Class 4. Poor, decolorized in less than 2 hours.

Factors Affecting the Test.–Many factors affect the methylene blue reduction test and therefore the steps of operation should be uniform.

Since the oxygen content must be used up before the color disappears, any manipulation that increases the oxygen affects the test. Cold milk holds more oxygen than warm milk; pouring milk back and forth from one container to another increases the amount, and at milking time much oxygen may be absorbed.

The kind of organisms affect the rate of reduction. The coliforms appear to be the most rapidly reducing organisms, closely followed by Streptococcus lactis, some of the faecal Streptococci, and certain micrococci. Thermoduric and psychrotrophic bacteria reduce methylene blue very slowly if at all. A large number of leucocytes affect the reduction time materially.

Light hastens reduction and therefore the tests should be kept covered. The concentration of the dye should be uniform as an increased concentration lengthens the time of reduction. Increasing the incubation temperature augments the activity of the bacteria and therefore shortens the reduction time.

The creaming of the test samples causes a number of organisms to be removed from the body of the milk and brought to the surface with the rising fat. This factor causes variations in the reduction time, since the bacteria are not evenly distributed. The accuracy of the test i s increased, reduction time shortened and decolorization more uniform if the samples are periodically inverted during incubation.

The Resazurin Test

The resazurin test is conducted similar to the methylene blue reduction test with the judgement of quality based either on the color produced after a stated period of incubation or on the time required to reduce the dye to a given end-point. Numerous modifications have been proposed. The two most commonly used are the "one-hour test" and the "triple-reading test" taken after one, two, and three hours of incubation. Other modifications have value in specific applications.

The procedure for making the resazurin test is as follows: Prepare resazurin solution by dissolving one resazurin tablet (dye content/ tablet, approximately 11 mg, certified by Biological Stain Commission) in 200 ml of hot distilled water as was done in the methylene blue test. Place one ml of dye solution in a sterile test tube, then add 10 ml of sample. Stopper the tube, place in the incubator and, when the temperature reaches 36o C, invert to mix the milk and dye. Incubate at 36o C. Tubes are examined and classified at the end of an hour in the "one-hour test" or at the end of three successive hourly intervals in the "triplereading test." The following relationships of color and quality are generally accepted:

Color of Sample: Quality of Milk

1. Blue (no color change): Excellent

2, Blue to deep mauve: Good

3. Deep mauve to deep pink: Fair

4. Deep pink to whitish pink: Poor

5. White: Bad

The resazurin test may be a valuable time saving tool if properly conducted and intelligently interpreted, but should be supplemented by microscopic examination.

Results on the reliability of the resazurin tests are conflicting. One study in comparing the resazurin test with the Breed microscopic method on 235 samples found the test reliable. Other reports state that the resazurin test is an unreliable index of bacteriological quality in milk. A major criticism of the method is that the resazurin reduction time of refrigerated bottled milk at either 20o or 37o C is much too long to be of any value in evaluating bacteriological spoilage of stored milk.

Standard Methods notes that under no circumstances should results of either methylene blue or resazurin tests be reported in terms of bacterial numbers. The two dye reduction procedures are described in more detail in Chapter 15 of the Thirteenth Edition of Standard Methods compiled by the American Public Health Association.


Electrical methods employ a variety of measurements of the effects of electrical current flow within the Earth. The phenomena that can be measured include current flow, electrical potential (voltages), and electromagnetic fields. A summary of the better-known electrical methods is given below. In this set of notes we will consider only one of these methods, the DC resistivity method, in greater detail.

  • DC Resistivity - This is an active method that employs measurements of electrical potential associated with subsurface electrical current flow generated by a DC, or slowly varying AC, source. Factors that affect the measured potential, and thus can be mapped using this method, include the presence and quality of pore fluids and clays. Our discussions will focus solely on this method.

  • Induced Polarization (IP) - This is an active method that is commonly done in conjunction with DC Resistivity. It employs measurements of the transient (short-term) variations in potential as the current is initially applied or removed from the ground, or alternatively the variation in the response as the AC frequency is changed. It has been observed that when a current is applied to the ground, the ground behaves much like a capacitor, storing some of the applied current as a charge that is dissipated upon removal of the current. In this process, both capacitative and electrochemical effects are responsible. IP is commonly used to detect concentrations of clay, and electrically conductive metallic mineral grains.

  • Self Potential (SP) - This is a passive method that employs measurements of naturally occurring electrical potentials commonly associated with shallow electrical conductors, such as sulfide ore bodies. Measurable electrical potentials have also been observed in association with groundwater flow and certain biologic processes. The only equipment needed for conducting an SP survey is a high-impedance voltmeter and some means of making good electrical contact to the ground.

  • Electromagnetic (EM) - This is an active method that employs measurements of a time-varying magnetic field generated by induction through current flow within the earth. In this technique, a time-varying magnetic field is generated at the surface of the earth that produces a time-varying electrical current in the earth through induction. A receiver is deployed that compares the magnetic field produced by the current-flow in the earth to that generated at the source. EM is used for locating conductive base-metal deposits, for locating buried pipes and cables, for the detection of unexploded ordnance, and for near-surface geophysical mapping.

  • Magnetotelluric (MT) - This is a passive method that employs measurements of naturally occurring electrical currents, telluric currents, generated by magnetic induction from electrical currents in the ionosphere. This method can be used to determine electrical properties of materials at relatively great depths (down to and including the mantle) inside the Earth. In this technique, a time variation in electrical potential is measured at a base station and at survey stations. Differences in the recorded signal are used to estimate subsurface distribution of electrical resistivity.


ATP determination

ATP determination using the time stable bioluminescent luciferase assay. This assay is optimized for applications where ATP concentrations ranging from 10 nM up to 10 µM are determined. The luminescence signal is stable for at least 4 hours.

The luciferase bioluminescent assay includes thermostable firefly luciferase, D-Luciferin as substrate and appropriate buffer solutions optimized for sensitive ATP quantification.

The production of ATP is vital for muscle contraction, chemiosmotic homeostasis, and normal cellular function. Many studies have measured ATP content or qualitative changes in ATP production, but few have quantified ATP production in vivo in isolated mitochondria. Because of the importance of understanding the energy capacity of mitochondria in biology, physiology, cellular dysfunction, and ultimately, disease pathologies and normal aging, we modified a commercially available bioluminescent ATP determination assay for quantitatively measuring ATP content and rate of ATP production in isolated mitochondria. The bioluminescence assay is based on the reaction of ATP with recombinant firefly luciferase and its substrate luciferin. The stabilities of the reaction mixture as well as relevant ATP standards were quantified. The luminescent signals of the reaction mixture and a 0.5 µM ATP standard decreased linearly at rates of 2.16 and 1.39% decay/min, respectively. For a 25 µM ATP standard, the luminescent signal underwent a logarithmic decay, due to intrinsic deviations from the Beer-Lambert law. Moreover, to test the functionality of isolated mitochondria, they were incubated with 1 and 5 mM oligomycin, an inhibitor of oxidative phosphorylation. The rate of ATP production in the mitochondria declined by 34 and 83%, respectively. Due to the sensitivity and stability of the assay and methodology, we were able to quantitatively measure in vivo the effects of age and caloric restriction on the ATP content and production in isolated mitochondria from the brain and liver of young and old Fischer-344 rats. In both tissues, neither age nor caloric restriction had any significant effect on the ATP content or the rate of ATP production. This study introduces a highly sensitive, reproducible, and quick methodology for measuring ATP in isolated mitochondria.

Biological complexity emerges from different organizational levels in a highly regulated space-time coordi­nation of processes that involves the participation and orchestrated interaction of DNA, RNA, and proteins between each other and the environment. Fully understanding normal biological processes such as cell differentiation, development and aging, and pathological conditions requires integrated genomic, transcriptional, and proteomic studies (1–3), which demand the simultaneous isolation of DNA, RNA, and proteins from the same sample.

Quick and reliable methods that perform simultaneous extraction of DNA, RNA, and proteins from a single sample are ideal for the generation of matched samples that can save time and money and allow for the efficient use of small and precious biological samples. Researchers are increasingly turning away from classic RNA and protein extraction techniques, such as phenol-chloroform separation (4) or time-consuming cesium chloride gradient centrifugation, because of the hazardous chemicals used and that the methods are generally unsuited for routine use in the laboratory. Spin column technology is a simple and quick approach to extracting nucleic acids from small biological samples. Furthermore, most column-based procedures do not require the amount of hazardous chemicals that are used in traditional nucleic acid extraction procedures (5).

Recently, Morse and coworkers (5) discussed the combined extraction of RNA and proteins using RNA spin column–based technology, and Hummon et al. (6) showed an improved method for isolation and solubilization of proteins after TRIzol extraction of RNA and DNA from the same sample. However, none of these authors did a complete analysis of the proteins obtained at the level of two-dimensional (2-D) electrophoresis to compare the protein profile obtained with conven­tional methods used in proteomics studies. Here we present a methodology to simultaneously extract RNA/proteins and/or DNA, RNA, and proteins from the same sample using commercially available column-based nucleic acid extraction kits. We further compared the protein profile obtained with some of the methods dedicated to extracting proteins using 2-D electrophoresis, and we show that buffer choice is critical in the efficient extraction of proteins from these kits to allow proteomic studies.

6.Explain the Labotatory accreditation in detail.

Labotatory accreditation

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