Characterization of naturally occurring atrazine-resistant isolates of the purple non-sulfur bacteria.

Six isolates of the purple non-sulfur bacteria, which upon primary isolation were naturally resistant to the herbicide atrazine, were characterized with respect to their taxonomic identity and the mechanism of their resistance. On the basis of electron microscopy, photopigment analysis, and other criteria, they were identified as strains of Rhodopseudomonas acidophila, Rhodopseudomonas palustris, or Rhodocyclus gelatinosus. These isolates exhibited degrees of atrazine resistance which ranged from 1.5 to about 4 times greater than that of cognate reference strains (American Type Culture Collection) tested. Furthermore, all of the reference strains tested were more intrinsically resistant to atrazine than was Rhodobacter sphaeroides. No unique plasmids which might encode for herbicide degradation or inactivation were found in these isolates. Resistance to the herbicide in these isolates was not the result of diminished binding of the herbicide to the L subunit of the bacterial reaction center. Differences in herbicide resistance among the various species of this group may be the result of compositional and chemical differences in the individual reaction centers. However, the increase in atrazine resistance for the isolates characterized in this study probably occurs by undefined mechanisms and not necessarily by changes in the binding of the herbicide to the L subunit of the photosynthetic reaction center.