UC Merced Undergraduate Research Journal

Antibiotic resistance is especially rampant among Gram-negative ESKAPE pathogens due to their structurally protective outer membrane, adaptive metabolic functions, and selective porins that make them more resistant to antibiotics than Gram-positive bacteria, which lack an outer membrane and instead possess a thick, exposed peptidoglycan layer that is more easily targeted by many antibiotics. This study investigated bacterial colonies in soil from an urban garden to identify bacteria capable of producing antibiotics effective against Gram-negative bacteria. One isolate, designated Isolate #9, demonstrated a clear zone of inhibition against Escherichia coli, a safe relative of K. pneumoniae, a Gram-negative bacterium. Morphological, biochemical, and metabolic characterization revealed that Isolate #9 is a Gram-negative bacillus with catalase activity, gelatin hydrolysis, glucose fermentation, nitrate reduction, and optimal growth at 30 °C, while lacking phospholipase, amylase, oxidase, and pigment production. Antibiotic susceptibility testing showed that the isolate is sensitive to trimethoprim and rifampin but resistant to tetracycline, penicillin, and gramicidin. Trimethoprim inhibits DNA synthesis by blocking dihydrofolate reductase, while rifampin targets RNA synthesis by binding to RNA polymerase. Although 16S rRNA sequencing produced inconclusive BLAST results, the biochemical profile suggests potential affiliation with the genus Proteus. These findings suggest that soils in high-traffic areas may harbor previously uncharacterized Gram-negative bacteria capable of producing antimicrobial properties that can combat clinically significant antibiotic-resistant Gram-negative pathogens.