Question

How accurate is the viable count method in predicting cell density? What factors might affect the accuracy of this method?

Answer 1

• Enumeration of bacteria is estimating the actual or approximate quantification of the bacteria in a sample.
• Viable count is a included under direct count method.
• Viable count measures the living cell that are capable of reproducing.
• The cell density are estimated as visual changes: Forming colonies colony count, membrane filter method), turbidity (turbidometric changes), spectrophotometric changes, biochemical changes (gas production acid production).
• However, viable count method is not accurate to determine total microbial cells present in the sample or even the actual number of viable cells
• Mostly, viable count will give an underestimated value of the actual count.

Reasons:

• The non-viable cells may also contain components that affect the viable cell estimation or interfere with growth of viable cells.
• This may occur due to clumping or aggregation of cells giving lesser colony count or altered turbidometric or spectrophotometric values.
• The dilution for plating may be erroneous resulting in colonies number as too few to count or too numerous to count.
• The viable cells may remain dormant (such as spores) or may not grow at certain condition, giving wrong estimates of the viable count.