Selective infection of maize roots by streptomycin-resistant Azospirillum lipoferum and other bacteria

The percentage of low-level streptomycin-resistant (20 μg/mL) bacteria in surface-sterilized or washed maize roots was more than a thousand times higher than that in soil populations. There was also a higher incidence of resistant bacteria in rhizosphere as compared with non-rhizosphere soil and bacteria isolated from maize roots were relatively tolerant to several other antibiotics. Azospirillum lipoferum was predominant in surface-sterilized roots of field-grown maize and was low-level streptomycin-resistant while most soil isolates were sensitive. Inoculation with A. brasilense isolated from wheat roots was unsuccessful in terms of establishment even when streptomycin-resistant strains were used. Unidentified causes of specific plant–bacteria affinities therefore transcend the role of antibiotic resistance in maize root infection.