Question

3. Which would be more accurate in terms of determining numbers of viable bacterial cells in a population: an estimation of turbidity in broth, or a standard plate count? Why? Discuss sources of inaccuracy in both approaches.

Answer 1

Plate count method is more reliable in counting viable bacterial numbers than measuring turbidity in a broth. Both dead and living cells increase turbidity as turbidity is nor dependent upon cell viability. But in plate count only living cells can give rise to a colony. So, colony obtained only from living cells and one colony is usually derived from a single cell. So counting colony numbers, we may speculate the number of living bacteria in the broth.

The drawback of plate counting methods are often it is observed that bacteria do not grow in a plate if medium or other growth conditions are not suitable. So, bacterial cells, though living, we don't get any colonies and estimation will be faulty. If during plating clumps of cells are accidentally placed in a location on plate, one colony will develop from the clump, so exact number of initial cells cannot be determined.

In turbidity method,both living cells and dead cells contribute to turbidity, so it is not possible to determine the number of living cells from turbidity measurement. Moreover waste products and cell debris also increase turbidity. These are source of inaccuracy of turbidity measurement method.