Antibiotic resistance and β-lactamase activity in Azospirillum

Abstract

Sixty-three Azospirillum strains isolated from different plants and locations were examined for antibiotic resistance. All showed high-level resistance to trimethoprim (MIC ⩾ 1024μg ml −1), and β-lactam antibiotics. Depending on the β-lactam used, 60.3 to 93.6% of the strains were resistant to ⩾512μg ml −1. Lower-level resistance occurred with aminoglycosides; neomycin and gentamycin (32 μ g ml −1) inhibited growth of all strains as did streptomycin (256 μg ml −1) and spectinomycin (512 μg ml −1). Chloramphenicol and tetracycline inhibited the growth of 98.4% of the strains at concentrations of 8 μg ml −1. The β-lactamase activity of 14 penicillin-resistant strains (MIC ⩾ 128–1024μg ml −1) was studied and found generally to be associated with the presence of an inducible enzyme. Little correlation was, however, found between the MIC value and that of the induced or uninduced enzyme levels. Antibiotic resistance and high β-lactamase activity could be advantageous to some Azospirillum for multiplication in the rhizosphere and root infection.