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Aims:  The aim of this work was to enrich stable mixed cultures from atrazine‐contaminated soil. The cultures were examined for their atrazine biodegradation efficiencies in comparison with J14a, a known atrazine‐degrading strain of Agrobacterium radiobacter. The cultures were also characterized to identify community structure and bacterial species present. Methods and Results:  The cultures were enriched and then stabilized in bacterial media. The stable mixed cultures and J14a were tested in a medium containing 100 μg l−1 of atrazine. For all cultures, atrazine was removed 33–51% within 7 days and the cell optical density increased from 0·05 to between 0·50 and 0·70. Four isolates designated ND1, ND2, ND3 and ND4 were purified from the mixed cultures and identified based on sequence analysis of the 16 S rRNA gene as Alcaligenes faecalis, Klebsiella ornithinolytica, Bacillus megaterium and Agrobacterium tumefaciens, respectively. An atrazine‐degrading gene, atzA, was present in ND2 and ND4. Conclusions:  The stable mixed cultures obtained could degrade atrazine. Klebsiella ornithinolytica ND2 and Ag. tumefaciens ND4 are atrazine degraders. Significance and Impact of the Study:  The novel stable mixed cultures could be used for bioremediating crop fields contaminated with atrazine. This is the first report of the atzA gene in Kl. ornithinolytica.

Atrazine biodegradation by immobilized pure and mixed cultures was examined. A pure atrazinedegrading culture, Agrobacterium radiobacter J14a (J14a), and a mixed culture (MC), isolated from an atrazine-contaminated crop field, were immobilized using phosphorylated-polyvinyl alcohol (PPVA). An existing cell immobilization procedure was modified to enhance PPVA matrix stability. The results showed that the matrices remained mechanically and chemically stable after shaking with glass beads over 15 days under various salt solutions and pH values.The immobilization process had a slight effect on cell viability. With the aid of scanning electron microscopy, a suitable microstructure of PPVA matrices for cell entrapment was observed. There were two porous layers of spherical gel matrices, the outside having an encapsulation property and the inside containing numerous pores for bacteria to occupy. J14a and MC were immobilized at three cell-to-matrix ratios of 3.5, 6.7, and 20 mg dry cells/mL matrix. The atrazine biodegradation tests were conducted in an aerobic batch system, which was inoculated with cells at 2,000 mg/L. The tests were also conducted using free (non-immobilized) J14a and MC for comparative purpose. The cell-to-matrix ratio of 3.5 mg/mL provided the highest atrazine removal efficiency of 40-50% in 120 h for both J14a and MC. The free cell systems, for both cultures, presented much lower atrazine removal efficiencies compared to the immobilized cell systems at the same level of inoculation.