Molecular identification and antibiotic susceptibility of bacteria from watermelon in southwestern Nigeria

Abstract. Akindele ST, Agbolade OA, Osiyemi EO, Raufu TT, Obafebi TO, Olukoga MT, Adetokunbo KS, Olatunji MO. 2025. Molecular identification and antibiotic susceptibility of bacteria from watermelon in southwestern Nigeria. Asian J Trop Biotechnol 22: 64-70. Watermelon (Citrullus lanatus) is a widely consumed fruit in Nigeria, often eaten fresh without further processing. However, its ready-to-eat nature predisposes it to microbial contamination and potential transmission of antibiotic-resistant bacteria. This study applied integrated microbiological and molecular approaches to characterize bacterial communities associated with sliced watermelon and assess their antimicrobial susceptibility under tropical conditions. Fresh watermelon samples were collected from Oru (Ogun State) and Iseyin (Oyo State), Nigeria. Standard microbiological methods were employed to determine total viable counts, bacterial counts, and lactic acid bacteria counts. Isolates were characterized using biochemical tests, 16S rRNA gene sequencing, and phylogenetic analysis. Antimicrobial susceptibility was assessed using the disc diffusion method against commonly used antibiotics. Microbial analysis revealed higher bacterial loads in Iseyin samples than in Oru. Seven bacterial species were identified, including Lactobacillus reuteri, Bacillus mendelii, Curtobacterium flaccumfaciens, Lactobacillus helveticus, Acinetobacter lwoffii, Lactobacillus casei, and Lactobacillus vaccinostercus. Phylogenetic analysis confirmed close evolutionary relatedness with reference strains. Most isolates displayed high resistance to pefloxacin and amoxicillin but were sensitive to ceftriaxone and chloramphenicol. The dominance of Bacillus species and detection of lactic acid bacteria highlighted both spoilage and potential probiotic roles. This study provides baseline data for molecular surveillance of antimicrobial resistance in tropical environments and demonstrates how molecular identification and AST can inform food biotechnology, probiotic exploration, and safety management of fresh produce.