JOURNAL OF BACTERIOLOGY AND VIROLOGY ISSN:1598-2467(Print) 2093-0429(Online)

Watermelon (Citrullus lanatus) is a tropical fruit that is rich in saponin—a known antimicrobial agent in specific concentrations. Salmonella spp. are an enteric, gram- negative bacteria that are major foodborne pathogens and typically isolated in clinical laboratories using Salmonella-Shigella agar. By contrast, Pseudomonas spp., which are nonenteric, gram-negative bacteria are known to contaminate laboratory isolates of Salmonella spp. Xylose lysine deoxycholate agar is widely used when isolating enteric bacteria in clinical and research laboratories. However, despite its use and efficiency, it produces false-positive results because nonenteric bacteria, particularly Pseudomonas spp., are able to grow when the enteric bacteria, Salmonella spp., decarboxylates lysine, resulting in an alkaline environment for its growth. Hence, because watermelon (C. lanatus) rinds are agricultural wastes, we explored the use of saponin as an alternative to the deoxycholate component—the inhibitory agent of the traditional Xylose lysine deoxycholate (XLD) agar—to obtain the aforementioned bacterial isolates. Standard operating protocols were applied with regard to bacterial load and dilution procedures, component substitution and adjustment, and the spread-plate technique. In addition, statistical analyses—including one-way analysis of variance and Tukey’s post-hoc test—were employed to consolidate and interpret the experimental findings. Treatment with 2.5% saponin resulted in preferential growth of Salmonella spp. and effective inhibition of Pseudomonas spp. at an initial bacterial load of 1 × 10⁸ CFU/mL. By contrast, 1% saponin showed inconsistent effects on both bacterial isolates across all test samples, whereas 5% saponin led to the complete inhibition of both Salmonella and Pseudomonas spp. Overall, these findings suggest that saponin derived from watermelon rinds has potential a promising alternative inhibitory agent for the selective isolation of enteric pathogen Salmonella spp.