Estimate the number of rod-shaped bacteria from the mounted sample
used in 29[10] (yeasts and spores), but before examination allow
the sample to stand not less than 15 minutes after mounting. Employ
a magnification of about 500, which may be obtained by the use of
an 8 mm. Zeiss apochromatic objective with X18 Zeiss compensating
ocular with draw-tube not extended, or an 8 mm. Spencer apochromatic
objective with X20 Spencer compensating ocular and a tube length of
190, or their equivalents.[11]
Count and record the number of bacteria having a length greater than
one and one-half times their width in an area consisting of five of
the small size squares. Count five such areas, preferably one from
near each corner of the ruled portion of the slide and one from near
the center. Determine the total number of the rod-shaped bacteria per
area in the five areas and multiply by 480,000. This gives the number
of this type of bacteria per cc. If a dilution of 1 part of the sample
with 8 parts of water instead of 1 part of the sample with 2 parts of
water is used in making up the sample, then the total count obtained
as above must be multiplied by 1,440,000. Omit the micrococcus type of
bacteria in making the count. Thus far it has proved impracticable to
count the micrococci present, as they are likely to be confused with
other bodies frequently present in such products.
[10] This number refers to the section as given in the Methods of
Analysis of the Association of Official Agricultural Chemists.
[11] The 4 mm. achromatic objective and the 10X ocular as given in the
list of apparatus may also be used to secure this magnification.