Number of Bradyrhizobium japonicum and bacterial contaminants in commercial inoculants from Argentina

Forty-four samples of commercial Argentinian soybean inoculants manufacture d for sale during 1994-1996 using different carriers were tested for Bradyr hizobium japonicum content and bacterial contaminant numbers. Products of t en producers of such inoculants were studied. Viable bradyrhizobial counts ranged from zero to 3.5 x 10(10) cells g(-1) or mi, while the contaminant l oad varied from zero to 6.31 x 10(9) cfu g-l or mi. Only liquid inoculants were free of bacterial contaminants. Sixty-six percent of the inoculants co ntained more nonbradyrhizobial microorganisms that they did bradyrhizobia. The suitability of five culture media for the enumeration of bacterial cont aminants was evaluated at 30 C for seven days during 1996. Four of the five media performed highly similarly, but data showed that MacConkey agar medi um was less eficient to capture bacterial contaminants than the others four culture media. Gram positive nonspore forming bacteria and Bacillus spp. w ere dominant among the contaminants studied. A highly significant correlati on was obtained between the MPN of viable bradyrhizobia g(-1) and the numbe rs of bacterial contaminants under the conditions this study was carried ou t.