Purpose: The objective of this study was to compare the number Campylobacter spp. cells detected by the filter method to those detected by direct plating and determine if the filter method can be used as a means to estimate cellular density of an unknown Campylobacter spp. in suspension.
Methods: Overnight liquid cultures of five subtypes of Campylobacter jejuni and five of Campylobacter coli, all originally detected in chicken samples, were used for this study. Each subtype was applied to Campy-cefex agar, directly, and after filtration through a 0.45 or 0.65 µm filter. All plates were allowed to dry for 45 minutes, filters were removed, plates were incubated, and colonies were counted. The data is presented as log CFU/mL. Three replications were conducted for all 10 isolates.
Results: The mean recovery by direct plating was 8.1 log CFU/mL. Regardless of pore size, the overall mean number of Campylobacter spp. detected using the filter method was significantly less than direct plating (P < 0.05); 5.4 log CFU/mL by 0.45 µm and 5.5 with the 0.65 µm filter. When analyzed by subtype, significant differences (P< 0.05) in ability to travel through the filter were noted, ranging from 3.0 to 6.3 log CFU/mL.
Significance: The number of Campylobacter spp. cells that can pass through a filter and make a colony on underlying solid media is dependent on subtype. The filter method does not appear to be reliable for determination of actual numbers per mL of an unknown Campylobacter.