pH stability during fermentation is associated with sustained antibacterial metabolite production in marine sediment...

Globally, the rise in antibiotic-resistant pathogens has underscored the urgent need for new strategies to discover antimicrobials, with emphasis on microbial producers of secondary metabolites. The influence of pH on bacterial recovery, metabolite expression, and antibacterial activity in isolates from marine sediments was evaluated in this study. Three culture media were used to isolate sediment bacteria across a pH gradient (5.0-8.0), and conventional biochemical methods were employed for putative identification of the bacterial isolates. The agar plug assay was used for primary antibacterial screening, while the disk diffusion assay of the cell-free and sonicated extracts was used for secondary screening after 7 days of submerged fermentation of the isolates at their different culture pH levels. The results revealed the predominance of Bacillus species from the different pH levels, with zones of inhibition ranging from 10.00 ± 1.00 to 47.50 ± 2.50 mm against clinical and environmental isolates. The four Bacillus species-like isolates selected for submerged fermentation showed a pH drift toward alkalinity, except for the culture initiated at pH 7.5, which remained stable. The secondary screening revealed a markedly reduced antibacterial activity for all isolates (≤9 mm) compared to primary screening, with the pH 7.5 isolate retaining the strongest inhibition. The findings suggest that pH stability during fermentation was strongly associated with sustained antibacterial activity, with isolates maintained at near-constant pH retaining significantly higher inhibitory activity than those in cultures whose pH increased during fermentation. This highlights a key consideration for the bioprospecting workflow targeting biosynthetic gene clusters and producers of secondary metabolites.

Marine environments are important reservoirs of bacteria capable of producing bioactive secondary metabolites; however, many promising antimicrobial producers identified during initial screening fail to retain activity during fermentation. This study demonstrates that pH stability during fermentation, rather than pH value alone, is a key determinant of sustained antibacterial metabolite production in marine sediment-derived Bacillus species. By linking isolation conditions, fermentation physiology, and bioactivity outcomes, the findings provide practical guidance for improving the reliability of marine bioprospecting and antimicrobial discovery pipelines. These insights are particularly relevant for efforts to recover stable antimicrobial producers from complex environmental systems.