Cell-free Supernatant from Exiguobacterium acetylicum Isolated from Water Cabbage (Pistia stratiotes) Roots Inhibits Bacillus subtilis and Escherichia coli

Introduction: The study was carried out to isolate and identify potential antibiotic-producing bacteria associated with water cabbage (Pistia stratiotes) roots collected from Pampanga River, Pampanga, Philippines. Seven (7) bacterial colonies were randomly chosen at the 10^-6 dilution factor. Antibiotic sensitivity test using agar well method revealed that only one isolate out of 7 selected colonies can inhibit the growth of the test organisms. Specifically, the isolate (namely T4) supernatant inhibited E. coli and B. subtilis but not S. aureus. T4’s ability to inhibit E. coli was comparable with that of Tetracycline (positive control). Surprisingly, its inhibition of B. subtilis is significantly higher than that of Tetracycline. 16S rRNA gene sequence analysis using NCBI Basic Local Alignment Search Tool revealed 99% similarity of the isolate (T4) with Exiguobacterium acetylicum, a gram-positive, antibiotic-producing bacterium previously isolated from an apple orchard rhizosphere.