The Growth Of Bacterial Cultures

Direct Measurement of Microbial Growth

A standard plate count reflects the number of viable microbes and assumes that each bacterium grows into a single colony.

Because it is impossible to say that each colony actually arose from an indivual cell (cells clump, fact of life) plate counts are reported as the number of colony-forming units (CFU) instead of the number of cells.

If the concentration of bacteria is too great the colonies will grow into each other and the plate will be uncountable.

To insure a countable plate a series of dilutions should be plated.  The serial dilutions should give at least one countable plate in the series (25-250 or 30-300, depending on preference of the individual lab).

A plate count may be done on plates prepared by either the pour plate method or the spread plate method.

Some samples may be very dilute to begin with, so a plate prepared with undiluted inoculum may still have too few colonies to be countable.   In this case the concentration of bacteria can be increased by filtering the sample.

In filtration, bacteria are retained on the surface of a membrane filter and then transferred to a culture medium to grow and subsequently be counted.

The most probable number (MPN) method can be used for microbes that will grow in a liquid medium; it is a statistical estimation.

• A dilution series to no growth is prepared and the combination of positives is used to look the most probable number up in a table (see (b) MPN Table below).
• Used for microbes that won't grow on solid media or are grown in differential liquid media for identification purposes.

In a direct microscopic count, the microbes in a measured volume of a bacterial suspension are counted with the use of a specially designed slide (a Petroff-Hausser cell counter). It can be done automatically with an electronic cell counter (Coulter counter).

Hard to count motile cells

Dead cells look the same as live cells

No incubation time is required

Estimating Bacterial Numbers by Indirect Methods

A spectrophotometer is used to determine turbidity by measuring the amount of light that passed through a suspension of cells.

An indirect way of estimating bacterial numbers is measuring the metabolic activity of the population (for example, acid production or oxygen consumption).

For filamentous organisms such as fungi, measuring dry weight is a convenient method of growth measurement.

You won't know how the measurement of turbidity with a spectrophotometer or measurement of metabolic activity or any other indirect measurement correlates to cell number unless you do a standard curve.