Conventional Plate Count Method Research Articles

Detection of Active Yeast Cells ( Saccharomyces cerevisiae ) in Frozen Dough Sections

A new method based on fluorescence microscopy was developed to detect active yeast cells in cryosections of wheat dough. The sections were stained with 4',6-diamidino-2-phenylindole (DAPI) and counterstained with Evans blue. The active yeast cells in the sections appeared brilliant yellow and were readily distinguished from the red dough matrix. The dead cells allowed penetration of the Evans blue through the cell membrane, which interfered with the DAPI staining and caused the dead cells to blend into the red environment. The number of active yeast cells in fermenting dough sections containing different proportions of living and dead yeast cells correlated well with the gas-forming capability of the yeast in the dough but not with the results of the conventional plate count method. The new method allows the study of yeast activity not only during the different stages of frozen dough processing but also during the fermentation of doughs.

Comparative analysis of Redigel and Aerobic Plate Count methods for viable cell counts of selected foods*

The Redigel system uses a calcium pectate gel in place of agar in an alternative method for determining aerobic colony counts in food. The system was compared with the conventional Aerobic Plate Count method for the determination of each food being tested. The correlation coefficient for each food product ranged from 0.10 (wheat flour) to 1.00 (pecans) with an overall correlation of 0.964. The Redigel system can be used as a conventional alternative method for the numeration of bacteria from foods.

Gas-liquid chromatographic method for the estimation of coliforms in milk

A method for the rapid determination of the number of viable coliforms in milk or other food materials is described. The method is based on the gas-liquid chromatographic (GLC) determination of metabolic CO 2 in equilibrium with the head space of sealed vials, inoculated with aliquots of the sample under examination. The determination of the CO 2 concentration allows the calculation of initial cell level on the basis of a calibration curve. The calibration curve was prepared by plotting the logarithm of the initial colony forming units versus the minimal detection time (MDT) for CO 2 production, determined by GLC. The examination of 24 samples of raw milk showed that this procedure can provide the same information as conventional plate count methods. In addition, the GLC method includes a saving in time and costs.

Comparison of a Dry Medium Culture Plate (Petrifilm SM Plates) Method to the Aerobic Plate Count Method for Enumeration of Mesophilic Aerobic Colony-Forming Units in Fresh Ground Beef

Mesophilic aerobic microbial populations in fresh ground beef were enumerated with a new system, Petrifilm™ SM Plates (PSM), and with the conventional aerobic plate count (APC) method using standard methods agar (SMA). Total colony-forming units were determined in 119 fresh ground beef samples (29 extra-lean, 30 lean and 60 regular) purchased at nine different retail markets over a period of 6 wk. Linear regression analysis of PSM vs. APC counts gave a slope of 0.963, an intercept of −0.027, and a correlation coefficient of 0.951. Mean log10 counts on PSM were 5.86 compared to 6.11 on SMA (P<0.01) or a mean log10 difference of −0.25. These analyses indicate that the Petrifilm SM method would be a possible alternative for the aerobic plate count method.

Turbidimetric measurement as a rapid method for the determination of the bacteriological quality of minced meat

A prototype rapid automated turbidimetric apparatus was used to record the growth of pure cultures of Escherichia coli, Staphylococcus aureus and Streptococcus faecalis to evaluate instrument performance. The turbidimetric measurements were applied in experiments concerned with the determination of the bacteriological quality of fresh minced meat in terms of total counts. As a method of automated routine examinations of minced meat, it showed great promise. A correlation coefficient of 0.98 was established between the conventional plate count method and the turbidimetric method. Samples could be processed within 24 h.

Liquid scintillation alpha spectrometry techniques

Accurate, quantitative determinations of alpha emitting nuclides by conventional plate counting methods are difficult, because of sample self-absorption problems in counting and because of non-reproducible losses in conventional sample separation methods. Liquid scintillation alpha spectrometry offers an attractive alternative with no sample self-absorption or geometry problems and with 100% counting efficiency. Sample preparation may include extraction of the alpha emitter of interest by a specific organic phase-soluble compound directly into the liquid scintillation counting medium. Detection electronics use energy and pulse-shape discrimination, to yield alpha spectra without beta and gamma background interference. Specific procedures have been developed for gross alpha, uranium, plutonium, thorium and polonium assay. Possibilities for a large number of other applications exist. Accuracy and reproducibility are typically in the 1% range. Backgrounds of the order of 0.01 cpm are readily achievable. The paper will present an overview of liquid scintillation alpha counting techniques and some of the results achieved for specific applications.

Enumeration of Microorganisms by Their Dynamic ac Conductance Patterns

Bacterial growth affects the ac conductance of a two-terminal measuring cell containing growth media. The dynamic characteristics of the recorded conductance provide quick information about the microorganisms' initial concentration and their relative growth parameters. A simplified electrobacteriological model is described which explains the changes in the medium conductivity. The mathematical model was verified experimentally. This model establishes the use of conductance measurement as a rapid method to assess bacterial concentrations. High correlation existed between the conductance method and the conventional plate count method.

Viability of Heterotrophic Bacteria in Freshwater

The viabilities of planktonic bacteria from nine freshwaters and one raw sewage were compared. Viabilities estimated from microcolony formation in slide culture (18·4–48·7%) were higher than a conventional plate count method (1·8–14·9%). Microcolony viability was estimated in four ways; the one recommended was the ratio of the microcolony count and total count of microcolonies and single cells after 24 or 72 h incubation. Bacterial viability strongly correlated with the percentage of bacteria &lt;0·2 μm3 in size and the concentration of soluble carbohydrate in the water.

MICROTITER METHOD FOR ENUMERATING VIABLE BACTERIA IN MILK

A rapid microtiter method was investigated as a means of evaluating viable bacterial cells in milk samples, using the conventional Standard Plate Count method as a comparison. Statistical analyses showed high correlation between the two methods. The advantages of this new method are: (a) the ability to analyze many samples simultaneously and (b) savings of time, space, material, and labor. The practical application of this new method in dairy products was shown by a study of bacterial growth during refrigerated storage of milk.

Plate-dilution frequency technique for assay of microbial ecology.

The plate-dilution frequency technique described facilitates simultaneous enumeration of a wide range of physiologically different microorganisms in complex systems with a precision comparable to dilution tube (most probable number) methods. Replicate microsamples are inoculated from each member of a dilution series onto areas delineated on plates of prepoured solid media; the plates are incubated, and the occurrence of growth or specific biochemical transformation is recorded for each inoculated area. Microbial enumeration is accomplished by reference to appropriate tables. Details of the experimental procedures are described, and tables are presented from which microbial numbers with 95% confidence limits can be obtained and compared for significant difference directly for 10-fold and 4-fold dilution series. Results of experiments in which microbial populations were estimated simultaneously by the plate-dilution frequency and conventional plate count methods are compared. The potential of the technique for broad-spectrum microbial assay is also discussed.

Plate-Dilution Frequency Technique for Assay of Microbial Ecology1

The plate-dilution frequency technique described facilitates simultaneous enumeration of a wide range of physiologically different microorganisms in complex systems with a precision comparable to dilution tube (most probable number) methods. Replicate microsamples are inoculated from each member of a dilution series onto areas delineated on plates of prepoured solid media; the plates are incubated, and the occurrence of growth or specific biochemical transformation is recorded for each inoculated area. Microbial enumeration is accomplished by reference to appropriate tables. Details of the experimental procedures are described, and tables are presented from which microbial numbers with 95% confidence limits can be obtained and compared for significant difference directly for 10-fold and 4-fold dilution series. Results of experiments in which microbial populations were estimated simultaneously by the plate-dilution frequency and conventional plate count methods are compared. The potential of the technique for broad-spectrum microbial assay is also discussed.