Assessment of microbiological and residual antibiotics status in milk sold in Abeokuta, Ogun State. Nigeria

This study investigated the prevalence and concentration of antibiotic residues in twenty brands of milk. Determination and quantification of antibiotic residues was done using High Performance Liquid Chromatography. Aerobic plate count was carried out and the associated bacteria isolated. Identification of bacteria was done using standard microbiological methods. Molecular characterization and identification of bacterial isolates was done using 16S rRNA gene sequencing method. Antibiotic sensitivity was carried out on bacterial isolates using disc diffusion method. Plasmid profile of drug resistant isolates was done using alkaline analysis method. Comparison of means was done using Analysis of Variance. Antibiotic residues were detected in 10 (50%) of the 20 milk brands analyzed. The residual levels of tetracycline, oxytetracycline and chlortetracycline ranged between 5ng/kg and 1569ng/kg while none of the samples had doxycycline residues. The prevalence of tetracycline residues in evaporated milk brands and powdered milk samples were 100 and 23 % respectively. The aerobic plate counts ranged from 2.5 × 102 to 6.5 × 102 CFU∕mL for evaporated milk and 2.5 × 101 to 6.0 × 101 CFU∕mL for powdered milk. A total of ten bacterial species were isolated and identified. Susceptibility result showed that 95% of the isolates were sensitive to erythromycin, cefuroxime and gentamycin; 12.5% were resistant to tetracycline and ampicillin, 10% were resistant to ofloxacin and 7.5% to doxycycline and penicillin. Five of the bacterial isolates were resistant to more than one class of antibiotics. Resistant isolates subjected to plasmid profiling had detectable plasmids with estimated sizes between 120bp and 1000bp. The obtained results provide evidence that the presence of antibiotic residues in evaporated and powdered milk is an indication that the public is exposed to the harmful effects of the residues.