Antibiotic Resistance in Nitrogen-Fixing Rhizobial Strains: Implications for Agriculture

Abstract

Rhizobia are biological nitrogen-fixing nonpathogenic microorganisms that make inert nitrogen available to legumes establishing symbiosis, living in nodules to promote growth. Livestock farmers use antibiotics without prescriptions to prevent losses, death and economic wastes, resulting in indiscriminate release of excess, unutilized antibiotics into the environment causing detrimental effects to soil microbes particularly rhizobia. This study evaluates the susceptibility pattern of selected rhizobia strains. The intrinsic antibiotic resistance (IAR) ability of common rhizobia strains including FA3, B574, RS15, USDA136, USDA9032, R25B, 532C, CC511, RANI22 and RAUG1 was investigated. The antibiotics used included ceftazidime, florfenicol, cefpodoxime, sulphamethoxazole, ciprofloxacin, gentamycin, kanamycin, ertapenem, tetracycline, meropenem and carbapenem. Strains showed highest resistance to carbapenem but were generally susceptible to ciprofloxacin, tetracycline, kanamycin and gentamycin. Rani 22 showed most resistant. Most strains showed no resistance to these antibiotics, but those with resistance abilities are of significance and can be used for recovering antibiotics contaminated sites, providing potential solutions for futuristic problems associated with AMR. This study suggests using a consortium of symbiotic rhizobia strains as a strategy to achieve optimal yields in antibiotic contaminated fields, since strains showed different susceptibility pattern. Also, the antibiotics produced active compounds/substances detrimental for rhizobia’s growth potentially hindering their activities as plant growth promoting bacteria, when applied as inoculant during agricultural practises. Antibiotics are therefore of major agricultural concern posing major threats by promoting poor plant growths, productivity and yields despite the use of rhizobia as plant growth enhancers.