The Biobrio 13(3 & 4), 2026

Antibiotic resistance study of Azotobacter species isolated from agriculture field of Mahisi Block, Saharsa

Shikha Singh & Shikha Choudhury

ABSTRACT:

Healthy agricultural soil depends heavily on beneficial microbes that replenish nutrients naturally. In this study, five distinct bacterial isolates belonging to the genus Azotobacter were recovered from the root zones (rhizosphere) of crop fields across Mahisi block in Saharsa district, Bihar. Using nitrogen-free Ashby's mannitol medium, colonies were isolated and systematically analyzed for their colony appearance, microscopic features, and biochemical behavior in line with standard diagnostic keys. All five isolates proved to be Gram-negative, oval-shaped cells producing smooth, glistening colonies. Isolate 02 produced a distinct yellow pigment, whereas Isolates 01, 03, 04, and 05 formed creamy white, opaque colonies. Because farmlands are increasingly exposed to chemical treatments and livestock runoff, we screened each isolate against five common antibiotics: Amoxicillin (30 mcg), Azithromycin (15 mcg), Ciprofloxacin (5 mcg), Doxycycline (30 mcg), and Erythromycin (15 mcg) using the disc diffusion method. All five strains demonstrated complete resistance to Erythromycin, showing no inhibition whatsoever. Conversely, they proved sensitive to Ciprofloxacin, Doxycycline, Amoxicillin, and Azithromycin, producing clear zones of inhibition. These results provide practical insights into the baseline hardiness and natural antibiotic responses of native soil bacteria, helping identify resilient candidate strains for sustainable biofertilizer development.

Keywords:    

Azotobacter chroococcum, Rhizosphere, Ashby's medium, Antibiotic resistance, Biofertilizer, Zone of inhibition.

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