Many techniques exist for the measurement of microbial growth in terms of assessing microbial cell numbers and microbial cell mass. These methods as stipulated in Table 1 helps the microbiologists to elucidate the doubling (generation) time of a microbial cell. As explained in Table 1, these techniques also allow the microbiologist to assess the actual amount or number of both viable and non-viable (dead) cells present in the growth medium. However, the type of measurement technique to be employed is usually dependent on the type of analysis the microbiologist seeks to undertake.
Table 1: Techniques of enumerating bacterial cell numbers and cell mass
|S/NO||CELL NUMBERS||CELL MASS|
|1.||Direct microscopic counts. This is done using the microscope & improved Neubauer counting chamber device. Both dead cells & viable cells are counted.||Measurement of chemical parameters of microbial cells (for example, nucleic acids & protein molecules).|
|2.||Plate counts. It involves the plating out or spreading of serially diluted bacterial culture on media plate. Viable cells are mainly counted using this method. This method gives the colony forming unit (CFU/ml) value of the counted bacteria.||Measurement of physical parameters of microbial cells (for example, dry weight of bacteria).|
|3.||Electronic counting machines. This method is used to count eukaryotes & other large organisms including yeasts, protozoa & algae. Electronic counters count both dead cells & living cells.||Measurement of chemical activity of microbial cells (for example, CO2 & O2 production or utilization).|
|4.||Membrane filtration technique. This method involve the direct count of microbial cells growing on specialized filters known as membrane filters. Membrane filters has pores small enough to hold bacteria cells from a solid culture media or any solution that is passed through it.||Measurement of bacterial turbidity. Microbial growth can be evaluated by assessing their growth in a broth medium. This technique allows microbiologists to determine microbial mass by measuring the intensity of light directed or absorbed by a turbid growth of bacterial cells in a broth culture medium. A spectrophotometer is used to measure this growth; and growth readings are taken in terms of absorbance which is extrapolated and taken as the actual cell mass of the organism. Spectrophotometer specifically measures the optical density (OD) or turbidity of the growing bacterial cells in a broth culture medium. Increase in the turbidity of the broth culture indicates increase of the microbial cell mass in the growth medium, and this is proportional to the number of microorganism present in the growth vessel (both viable and dead cells).|
SYNOPSIS OF MICROBIAL COUNT
VIABLE CELL COUNT
Viable cell count gives an estimate of the total number of living cells present in a given volume of a sample. Viable cell count can be determined by automated machines and with the use of counting chambers such as the haemocytometer (Figure 1) in the microbiology laboratory. Haemocytometers are routinely used in the laboratory to determine viable cell count from samples and suspension of microbial cells in the laboratory.
Colony counts which involve the enumeration of the number of colonies on particular solid culture media can also be used to determine the viable counts of microbial cells. Pour plate methods and spread plate techniques are some colony counting techniques employed in the enumeration of microbial viable cell count. The technique of determining the optical density (OD) value of a given sample or microbial suspension in the microbiology laboratory is generally known as turbidimetry. Turbidimetry is the technique used to estimate the actual concentration of cells in a given suspension by passing a beam of light through the suspension (suspended in a cuvette). The intensity of the transmitted light passing through the suspended fluid is compared to a standard control in order to determine the OD value of the cells in the liquid. Optical density (OD) is the measure of the amount of light absorbed by a bacterial suspension when evaluated in a spectrophotometric cuvette or blank. The OD value of a given suspension or sample is evaluated using specialized type of instruments such as the spectrophotometer and turbidimeters which gives reading as absorbance. Absorbance reading is synonymous to optical density (OD) value.
TOTAL CELL COUNT
Total cell count is an estimate of the total number of both living and dead cells in a given volume of a sample. It can be determined by direct counting methods using microscopy and other electronic instruments such as the coulter counter and turbidometric technique. Total cell counting techniques can be used to count bacteria, fungi (for example yeasts) and bacterial spores excluding viruses and viroids. Viruses are mainly counted using the electron microscope.
OPTICAL DENSITY (OD)
Optical density (OD) is a measure of the decrease in transmitted light when light is passed through a microbial suspension. It is usually expressed mathematically as OD=log10 (Io/I). “Io” is the intensity of the incident light while “I” is the intensity of the transmitted light. OD values can be obtained from a spectrophotometer; and it gives an estimate of the number cells present in a given microbial suspension. Spectrophotometer is an instrument used to measure how much a chemical substance or microbial cell absorbs light when a beam of light is passed through the sample. It helps to determine the concentration of a solute substance in solution that is capable of absorbing light in the ultraviolet and visible range. The technique of using the spectrophotometer to measure how much a chemical substance or microbial cell suspension absorbs light when a beam of light is passed through the sample is known as spectrophotometry. Table 1 summarizes some of the basic techniques of enumerating bacterial cell numbers and cell mass.
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