Bacteria replicate by binary fission, with one cell dividing into two cells. The time required for one cell to divide into two cells is called the generation time or doubling time. The generation time of a bacterium in culture can be as little as 20 minutes for a fast-growing bacterium such as E. coli or as long as 24 hours for a slow-growing bacterium such as Mycobacterium tuberculosis.
If bacteria are in a balanced growth state, with enough nutrients and no toxic products present, the increase
in bacterial numbers is proportional to the increase in other bacterial properties, such as mass, protein content, and nucleic acid content. Thus measurement of any of these properties can be used as an indication of bacterial growth. When the growth of a bacterial culture is plotted during balanced growth, the resulting
curve shows four phases of growth: (1) a lag phase, during which bacteria are preparing to divide, (2) a log
phase, during which bacteria numbers increase logarithmically, (3) a stationary phase, in which nutrients are becoming limited and the numbers of bacteria remain constant (although viability may decrease), and (4) a death phase, when the number of nonviable bacterial cells exceeds the number of viable cells. An example of such a growth curve is shown in Figure 1-8.
Determination of Cell Numbers
In the diagnostic laboratory the number of bacterial cells present is determined in one of three ways:
Direct counting under the microscope: This method can be used to estimate the number of bacteria present in a specimen. It does not distinguish between live and dead cells Direct plate count: By growing dilutions of broth cultures on agar plates, one can determine the number of colony-forming units per milliliter (CFU/mL). This provides a count of viable cells only. This method is used in determining the bacterial cell
count in urine cultures
Density measurement: The density (referred to as cloudiness or turbidity) of a bacterial broth culture in log phase can be correlated to CFU/mL of the culture. This method is used to prepare a standard inoculum for antimicrobial susceptibility testing.