The development and validation of a bacteriological screening test for antimicrobial residues in eggs

Abstract

Microbiological screening of antimicrobial residues in eggs needs special attention because of the high level of naturally occurring inhibitors contained in eggs which often lead to false positive results. However, it was discovered that heating egg samples at 800C for 10 minutes inactivated the inhibitors. The new bacteriological screening test for antimicrobial residues in eggs which was developed during this study, contains viable spores of Geobacillus stearothermophilus which are sensitive to antimicrobial residues including beta-lactams, tetracycline’s and macrolides. The new test method was validated based on the comparison with a reference method, namely the Kundrat micro-screening four-plate test,1 and published literature of another standard reference method, the Premi®Test. A preliminary trial was conducted on 36 hens that were given therapeutic oral doses of overthe- counter antimicrobials daily for seven days with one of eleven antimicrobials based on the manufacturer’s recommendations. Eggs were collected from the hens during and after treatment and tested for the presence or absence of antimicrobial residues. Several performance criteria and minimum detection concentrations were estimated and discussed. Some agreements and differences were found between the new and the reference tests with the new test being more sensitive to beta-lactams, tetracyclines and macrolides than the Kundrat and Premi®Test on the average. The use of florfenicol and norfloxacin in laying hens is banned and therefore there are no maximum residue limits (MRL) or published Premi®Test values. For meat, the MRL is 100mg/kg. It was therefore concluded that the new screening test could be used for routine screening of antimicrobial residues in eggs. A two seasonal survey was also conducted to determine the prevalence of antimicrobial residues in commercial chicken eggs in Tshwane area of Gauteng Province, South Africa. Although the season did not impact statistically on the antibiotic residues found in the eggs; eggs sold at the roadside (informal businesses), certain egg brands, and those sold at lower prices were found to be more likely to contain antibiotic residues than those obtained from formal outlets and at higher prices. Copyright