Natural Co‐Infection With <i>Streptococcus agalactiae</i> and <i>Klebsiella pneumoniae</i> in Cultured Nile Tilapia: Assessing Plant‐Based Therapeutics as Sustainable Alternatives to Antibiotics

ABSTRACT Nile tilapia ( Oreochromis niloticus ), a key species in global freshwater aquaculture and a major contributor to Bangladesh's fish production, is increasingly affected by bacterial infections that compromise farm productivity. This study investigated natural co‐infection of bacterial pathogens in diseased tilapia and assessed the effectiveness of both conventional commercial antibiotics and plant‐derived extracts. Clinical and post‐mortem examination, phenotypic and biochemical characterisation, molecular diagnostics, histopathological examination and antibiotic susceptibility testing confirmed Streptococcus agalactiae and Klebsiella pneumoniae as the causative pathogens. Notably, two hallmark clinical manifestations of streptococcal infection—meningoencephalitis and panophthalmitis—were observed. Molecular identification through 16S rRNA gene sequencing (GenBank accessions PP967716, PP967717 and PP967718) revealed high similarity with reference strains—99.86% for S. agalactiae (NR_040821) and 99.78% for K. pneumoniae (NR_114506). Infected fish showed severe histopathological lesions in the brain, gills, spleen, kidney and liver. Antibiogram analysis indicated multidrug resistance among the isolates, while acetonic extracts of six medicinal plants— Elaeocarpus serratus (olive fruit), Phyllanthus emblica (emblica fruit), Psidium guajava (guava leaf), Allium sativum (garlic), Syzygium aromaticum (clove) and Nigella sativa (black cumin)—exhibited strong antibacterial activity. A treatment trial against co‐infection demonstrated that olive fruit extract significantly improved cumulative percent survival ( p &lt; 0.05) compared to amoxicillin and showed comparable efficacy to tetracycline. Kaplan–Meier survival analysis revealed low survival in fish treated with amoxicillin (16.7 ± 1.93%), while tetracycline (83.3 ± 1.93%) and olive extract (80 ± 1.91%) resulted in markedly higher survival ( p &lt; 0.05). This study provides the first report of natural co‐infection by S. agalactiae and K. pneumoniae in tilapia farms of Sylhet, Bangladesh, and highlights the limitations of conventional antibiotics due to resistance. The findings underscore the promising potential of plant‐based therapeutics, particularly olive extract, as sustainable alternatives for the management of bacterial diseases in aquaculture.