The Enumeration of Bacteria by the Microscopic Method

Determination of leucocyte concentrations in cow's milk with a Coulter counter

A technique for determining the concentration of leucocytes in cow's milk is described which is more rapid and accurate as well as much less tedious than conventional microscope methods. The leucocytes are isolated from the fat globules of comparable size by a novel centrifuging procedure. Sizing of the leucocytes with a Coulter electronic counter shows that their volumes range from about 45 to 1770 µm3but a partially overlapping distribution of other particulate matter prevents the accurate determination of the total number of cells present. The numbers counted in truncated leucocyte distributions, however, correlate linearly with total cell counts determined with an improved microscope method up to 6 × 106cells/ml of milk.

Counting total cell numbers microscopically on stained films

Two main sources of error in the conventional stained film cell count method, i.e. errors from imperfect spreading of the suspension and uneven distribution of cells in the film, have been eliminated by counting all the cells in the film. Staining solution and sodium alginate were added to the suspension before spreading over the slide. With a microsyringe, 20-microliters suspension was spread in two bands of ca 1.7 mm x 60 mm each. Then total cells could be counted easily in the x 100 magnification field by moving the slide in one direction. Results were satisfactory. Coefficients of variation were 2.6 and 1.1% in two separate yeast determinations each with five replicates. For counting higher magnifications, a suitable image analyser for automating the method is believed worthy of study.

Enhanced staining of bacterial flagella using aged mordant in the silver stain

Intensity of bacterial flagella staining using a modified silver stain was increased by aging the mordant for one week at room temperature. The use of aged mordant increased the apparent diameters of stained flagella and resulted in a darker stain. The mordant remained stable for at least four months at room temperature. The staining protocol presented allows application to liquid or solid cultures.

An improved technique for counting leucocytes in milk by the direct microscopic method

An improved microscopic method for counting leucocytes in milk has been developed, which results in acceptable levels of reproducibility. To overcome the lack of uniform cell distribution within a film a stratified sampling of the total film area is examined, which prevents the counting of any one area twice. The mean between-film coefficient of variation (S.D./x X 100) for all milk samples examined which contained more than 350,000 leucocytes/ml. was less than 10%.