Microbiological detection of 10 quinolone antibiotic residues and its application to artificially contaminated poultry samples

To assess if microbiological inhibition tests for detection of antibiotic residues are suitable for routine screening for quinolone residues, the limit of detection (LOD) of 10 different quinolones and fluoroquinolones was determined. Two media were tested, one at pH 6 and the other at pH 8, each seeded with one of the following test strains: Bacillus subtilis, Escherichia coli or Bacillus cereus. LODs of the 10 substances were highest on plates seeded with B. cereus, intended for selective detection of tetracycline residues. The pattern of zones on the other four plates differed for the targeted quinolones: flumequine and oxolinic acid were detected at lower concentrations at pH 6, while the LODs of ciprofloxacin, enrofloxacin, danofloxacin, marbofloxacin, sarafloxacin and norfloxacin were lower at pH 8. Nine of the 10 quinolones were detected more easily with E. coli, but the LOD of difloxacin was lower with B. subtilis. Finally, the three most sensitive media were selected and fluid from chicken meat, spiked with eight quinolones near maximum residue limits (MRL), analysed on each plate. The plate seeded with E. coli at pH 8 detected five of eight quinolones at levels of interest, but an additional E. coli plate at pH 6 was necessary for detection of flumequine in species other than poultry and fish. None of the plates detected oxolinic acid and difloxacin at MRLs in muscle tissue.