When the amount of bacteria is very small, as in lakes and pure streams, bac-
teria can be counted by filtration methods. Here 100 milliliters of water are
passed through a thin membrane, whose pores are too small for the bacteria to
pass through. The bacteria that are retained from the filter are placed on a Petri
dish containing a pad soaked with liquid nutrient. An example of bacteria that
are grown using this method is coliform bacteria, which are indicators of fecal
pollution of food or water.
When using the direct microscopic count method, a measured volume of bac-
teria is suspended in a liquid placed inside a designated area of a microscopic slide.
For example, a 0.01-milliliter sample is spread over a marked square centimeter of
a slide, stained, and viewed under the 0.1 immersion objective lens. The area for
the viewing field is obtained. Once the number of bacteria is obtained or deter-
mined in several different fields, an average can be taken of the number of bacte-
ria per viewing field. From this data, the number of bacteria in the square
centimeter over which the sample has been spread can be calculated. Because the
area on the slide contained 0.01 milliliters of sample, the number of bacteria in
each milliliter of the suspension is the number of bacteria in the sample times 100.
Establishing Bacterial Numbers
by Indirect Methods
Not all microbial cells must be counted to establish their number. With some
types of work, estimating the turbidity is a practical way of monitoring micro-
bial growth. Turbidity is the cloudiness of a liquid or the loss of transparency
because of insoluble matter.
The instrument used to measure turbidity is a spectrophotometeror col-
orimeter. In the spectrophotometer, a beam of light is transmitted through the
bacteria that are suspended in the liquid medium to a photoelectric cell. As bac-
teria growth increases, less light will reach the photoelectric cell. The change of
light will register on the instrument’s scale as the percentage of transmission.
The amount of light striking the light-sensitive detector on the spectrophotome-
ter is inversely proportional to the number of bacteria under normal cultures: the
less light, the more bacteria.
Another indirect way of measuring bacterial growth is to measure the meta-
bolic activity of the colony. In this method it is assumed that metabolic waste
products, CO 2 (carbon dioxide) and acid, are in direct proportion to the number
CHAPTER 6 Microbial Growth and Controlling It^111