Table 1.

Interpretation of the results of plate counting

Observed resultUsual interpretationAlternative interpretations
A colony is formedA viable cell gave rise to the colonyAt least one viable cell gave rise to the colony—but it may have been two or more cells coinciding at the same place on the plate or a clump of cells that contained at least one viable individual
No colony is formedThere were no viable cells in the sample(i) The growth medium and/or incubation conditions were incorrect
(ii) The cells were damaged/stressed and therefore unable to grow on solid medium
(iii) The population density was low and therefore cell-cell communication could not take place, resulting in no observable growth
(iv) Insufficient time was allowed for visible colony development in slowly growing cells