Most probable number method to enumerate a bioluminescent Xanthomonas campestris pv. campestris in soil

An adaptation of the most probable number (MPN) method and use of microfluor plates for monitoring a bioluminescent strain (171LIIH-7) of Xanthomonas campestris pv. campestris (Pammel) Dowson in debris-inoculated soil, is described. Microfluor plates with liquid culture medium were inoculated with a dilution series of soil extract previously infested with cabbage debris containing 171LIIH-7. Light produced by the bacteria was detected on X-ray film, and absorbance readings of the images were converted to a binary set of values for estimation of MPN by the computer program MPNES (Woomer et al., 1990). The MPN-microfluor plate system was compared with direct dilution plating on FS medium and MPN-miniplate enrichment ELISA with a pathovar-specific monoclonal antibody. The detection limit of X. c. pv. campestris with FS medium was about 104 cfu g−1 soil. The detection threshold of MPN-microfluor plates (102 bacteria g−1 non-sterile soil) was equivalent to the MPN-miniplate enrichment ELISA and 100 times more sensitive than direct dilution plating. The MPN-microfluor plate system is an easy, accurate, sensitive and inexpensive method to detect and enumerate bioliminescent X. c. pv. campestris in soil, and permits ecological studies in the soil environment that will provide the rationale for changes in crop rotations or other cultural practices.