Rapid and cost-effective method for microorganism enumeration based on miniaturization of the conventional plate-counting technique

The conventional plate-counting technique, while still routinely used in laboratories requiring micro-organism screening or enumeration, remains time-consuming and tiresome for the operator when large numbers of samples are assayed. We developed a miniaturized method based on serial tenfold dilution of samples in 96-well micro-plates followed by multiple pour-plating into 24-well micro-plates with an agar overlay procedure, providing easy colony counting and interpretation. The accuracy, sensitivity and reproducibility of this miniaturized method were tested using the conventional plate-counting technique as reference for viability measurements of Escherichia coli cells submitted to a sub-lethal thermic stress, enumeration of lactic bacteria during milk fermentation, and analysis of the total aerobic flora, coliform and Staphylococcus aureus content of spoiled food. No significant differences were found between the results obtained with our miniaturized method and the conventional one, regardless of the bacteria and media used. Providing a reduction in overall cost and handling time, this reliable miniaturized method should be useful for experiments involving large ranges of cell concentration as well as great numbers of samples and replicates.