Evaluation of the Bacillus calidolactis plate for post screening assay of β-lactam antimicrobial residues in Kenyan dairies

As part of efforts to establish a control program for antimicrobial residues in local Kenyan dairies, a low cost Bacillus calidolactis test plate was evaluated after laboratory standardization. Test parameters studied were: the practicability of the test handling, the test sensitivity in terms of limits of detection (LODs) compared to codex alimentarius maximum residue limits (MRLs) and repeatability within two different laboratories. The observed LODs were: penicillin G (2 μg/kg), ampicillin (2 μg/kg), amoxicillin (2 μg/kg), oxacillin (15 μg/kg), cefalexin (50 μg/kg), cephapirin (30 μg/kg) and ceftiofur (50 μg/kg). These levels were lower than established Codex Alimentarius MRLs. The agreement between the two laboratories was calculated to be 0.83 (83%) with a determined proportion of kappa (κ) ranging between 0.61 and 0.80κ which corresponds to a good agreement strength. The standard error se (k)=0.052 and the confidence intervals at 95% for κ=0.51–0.71 for the two laboratories. The study suggests that raw milk exhibiting inhibition diameters of <19 mm (p<0.05) in the B.calidolactis test plate fulfills the codex alimentarius MRL for the tested β-lactams at their various detection levels. It was concluded that the plate test could be a useful and affordable post screening assay for commonly used β-lactam antimicrobial in the dairy sector within low-income countries.