Molecular Characterization of Multi-Drug Resistant Histamine-Producing Gram-Negative Bacteria Isolated from Fermented...

Fermented Prosopis africana seeds (okpee) are naturally protein-rich but exude odors that attract houseflies. These flies can carry contaminants, including pathogens and histamine-producing bacteria. Bacterial histamine in food can cause illnesses and pseudo-hypersensitivity reactions. This study investigated multidrug-resistant, histamine-producing Gram-negative bacteria (GNB) isolated from okpee sold in Enugu Ezike, Nigeria.

Histamine content of the mesquite seed samples was determined using high-performance liquid chromatography. Bacteriological isolates from randomly sampled okpee were identified by 16S rRNA gene sequencing. Antibiotic susceptibility of the isolated bacteria was assessed using the Kirby-Bauer disk agar diffusion method. The isolates were also screened for resistant plasmids and virulence genes (Stx2 and toxA).

Histamine levels of the fermented samples averaged 1.0 × 10-4-6.2 × 10-1 mg/kg. Of 240 samples of okpee screened, 25 (10.5%) were contaminated with histamine-producing GNB, including Pseudomonas spp., Escherichia coli, E. fergusonii, and Shigella flexneri. All the isolated histamine-producing bacteria were resistant to amoxicillin, erythromycin, ampicillin, and cefuroxime, with more than 50% exhibiting resistance to the other antibiotics tested. The test isolates were found to harbor multiple resistant plasmids (size: 0.5 kbp-10 kbp) and virulence factor genes Stx2 and toxA.

This study observed a high prevalence of bacterial contaminants capable of producing histamine in okpee sold in Enugu Ezike, Nigeria. The isolates were multidrug-resistant and harbored virulent genes, posing a serious public health risk. Therefore, strengthening hygiene practices during the production and handling of okpee is necessary to prevent contamination and ensure product safety.