Comparison of a gene probe with classical methods for detecting 2,4-dichlorophenoxyacetic acid (2,4-D)-biodegrading bacteria in natural waters

Colony hybridizations with a gene probe for enumeration of 2,4-dichlorophenoxy-acetic acid (2,4-D)-degrading bacteria were compared with classical enrichment and radiolabel most-probable-number (MPN) assay methods. Two natural water samples (rivers) and raw sewage were tested by each method. UV scans of enrichment cultures revealed 2,4-D degradation with raw sewage occurred in 4–11 days, 4–>22 days with Mary's River water, and 5–>22 days with Willamette River water. [¹⁴C]-2,4-D MPN analysis, measuring release of¹⁴CO₂, yielded estimates of bacteria per milliliter able to degrade 2,4-D. Raw sewage estimates were 1.4 × 10⁵ 2,4-D degraders/ml, Mary's River >1.6 × 10⁵/ml, and Willamette River water 1.6 × 10⁴/ml. Activities noted by UV scan enrichment data supported these results.Autoradiograms of colony blots were also used to estimate numbers of 2,4-D-degrading bacteria. These estimates were also supported by the UV scan data from enrichment cultures. Raw sewage gave counts between 5 × 10⁴ and 2.9 × 10⁵ 2,4-D-degrading bacteria/ml, which correlates well with the estimates obtained by¹⁴C-MPN analyses. River waters, both much lower in total bacterial counts and organic carbon than raw sewage, yielded fewer 2,4-D-degrading bacteria than estimated by¹⁴C-MPN. Media composition and cometabolism may account for discrepancies in estimates for 2,4-D-degrading bacteria observed when colony blot and¹⁴C-MPN analyses were compared.Replica plating made it possible to test for 2,4-D biodegradation from colonies reactive in autoradiograms. Five of 12 (42%) colonies reacting in the colony hybridization exhibited biodegradation activities. Nonreactive colonies failed to degrade 2,4-D.