Counting of viable cluster-forming and non cluster-forming bacteria: a comparison between the drop and the spread methods

Viable cells from two different bacterial species, namely Escherichia coli and Beijerinckia derxii were enumerated by three different operators using the Drop and the Spread methods. Statistical analysis showed that counts obtained by the two methods were significantly different only for B. derxii, probably due to the influence of the extracellular slime layer that promotes the formation of cell clusters. Statistically significant differences were also observed when comparing results from the three operators.