Comparison of immunofluorescence microscopy and dilution-plating for the detection of Xanthomonas campestris pv. campestris in crucifer seeds

The correlation between immunofluorescence microscopy (IF) and dilution-plating on nutrient starch cycloheximide agar (NSCA) or NSCA with the addition of nitrofurantoin and vancomycin (NSCAA) was studied for the detection ofXanthomonas campestris pv.campestris (Xcc) in crucifer seeds. When checking 50 l of the seed extract in IF, IF and dilution-plating gave corresponding results (both positive or negative) for 45.4–56.4% of the samples tested. No differences were observed in this respect between tests using a polyclonal antiserum (PCA 94) and replicate tests using monoclonal antibodies (MCA 20H6). When 20 l of the seed extract was checked in IF, 67.3–71.3% of the samples tested were both positive or negative with dilutionplating and IF. IF negative and dilution-plating positive samples were found for 0.0–7.3% of all samples tested. The percentage of IF positive and dilution-plating negative samples ranged from 26.7–29.2 (20 l seed extract checked) to 41.8–47.3% (50 l seed extract checked). Generally, the probability of isolating Xcc increased with increasing numbers of fluorescent cells found in IF. Above 10 000 cells per ml the probability of isolating Xcc ranged from 57.1–81.8%. Increasing the extraction time from 5 min to 2.5 h shaking showed no significant increase of the number of samples found positive in IF and dilution-plating. However, when using both 5 min and 2.5 h shaking as compared to 5 min shaking only, more samples can be found positive in IF (1.0–14.5%) and dilution-plating (3.0–18.5%). Examining 1 l instead of 50 l of the sample smear, would increase the correspondence between IF and dilution-plating results up to minimally 69.1% (MCA 20H6). However, the risk of false-negative results in IF as compared to dilution-plating would also increase.