EDAPHIC FACTORS AS DETERMINANTS FOR THE DISTRIBUTION OF INTRINSIC ANTIBIOTIC-RESISTANCE IN A COWPEA RHIZOBIA POPULATION

A large collection of cowpea rhizobia strains was obtained fr-nm soil samples collected from either a semi-arid or a tropical rain forest at -Pa located at about the same latitude in the north-eastern region of Brazil and evaluated for their intrinsic antibiotic resistance to eigh t commercial antibiotics. The aim of this study was to correlate antib iotic resistance of native rhizobia strains to edaphic-climatic factor s as a way to establish suitable inoculants for specific areas. A larg e diversity regarding intrinsic antibiotic resistance was found, and 1 7 clusters were identified as varying from sensitive to gradually resi stant up to 500 mu g.ml(-1) of the antibiotics tested. Clustering anal ysis did not show any pattern related to the geographic region where i solates have been obtained. On the other hand. an increase in the anti biotic-resistant rhizobia population was associated with an increase in soil P and Al contents, isolates which were sensitive to spectinomy cin. ampicillin, streptomycin, chloramphenicol and tetracycline were p resent at higher rates in soils devoid of Al. Rhizobia bacteria produc ing mucus type I (fluid and capable of spreading over the solid media) were found preferentially in soil with Al concentrations up to 36 mg. kg(-1), diminishing quickly at higher levels.