Measuring Bacterial growth- A number of methds has been developed to measure bacterial growth in liquid media and in solid support media. A few are discussed below:  

Microscopic count-bacterial cells can be counted easily on a “petroff-hausser counting chamber” (Figure 18.2). The chamber has a ruling to make square (1/400 mm2) of equivalent volume. A glass slide is placed (~1/50mm height) to make a chamber filled with bacterial cell suspension. Volume of each chamber is 1/20,000 mm3. This chamber can be used to observe bacteria with phase contrast microscope. For example, if each chamber has 8 bacteria then there are 8x20,000,000 or 1.6x108 bacteria/ml. A very high or low concentration of bacterial sample can not be counted accurately.

Plate count method-In this method, a defined amount of bacterial culture suspension is introduced onto solid support media to grow and give colonies. If number of colonies on solid media is too high, then serial dilution of original stock can be plated on solid media and number of colony can be counted with a colony counter. A manual colony counter has lamp at the bottom, a grid to divide the bacterial culture plate and a magnifying glass to visualize and count single colony. A plate with colony count of 30-300 can be used to determine the number of bacteria present in original stock.

Number of bacteria per ml=    Number of colonies counted on plate X dilution of sample

Figure 18.2: Different methods of bacterial counting.