An improved direct viable count for the enumeration of bacteria in milk

The direct viable count (DVC), a microscopic method for the enumeration of viable bacteria, was modified by replacing nalidixic acid with ciprofloxacin. This modification made it possible to apply this method to a variety of Gram-negative and Gram-positive bacteria which was not previously possible. Of the four antibiotics tested (nalidixic acid, novobiocin, ciprofloxacin and mitomycin C), ciprofloxacin and mitomycin C were the only ones effective for use in the DVC with all of the bacteria tested. In addition, ciprofloxacin could be used at a single concentration (1 microgram/ml) while adjustments were necessary with the other antibiotics when examining bacteria from different genera and, in some instances, from different species. The use of ciprofloxacin in the DVC resulted in viable cells that had elongated by 5-11 times their original length. We conclude that the modified DVC will be useful in growth and survival studies of bacterial pathogens and spoilage organisms in milk and other foods.