Stability of frozen stock solutions of beta-lactam antibiotics, cephalosporins, tetracyclines and quinolones used in antibiotic residue screening and antibiotic susceptibility testing.

The stability of frozen stock solutions of antibiotics belonging to three different families was evaluated using an agar diffusion test, with Bacillus subtilis as a test strain. Diameters of inhibition zones were measured at monthly intervals during 6 months, and the decline in active substance was calculated. Penicillin and amoxicillin lost nearly half of their potency, the cephalosporins ceftiofur and cefapirin one quarter, but ampicillin was more stable. The quinolones flumequine, enrofloxacin and marbofloxacin were relatively stable; the loss of activity was less than 10% after 6 months of preservation at -20 degrees C. This was also the case for doxycycline and chlortetracycline, while oxytetracycline and tetracycline lost about 25% of their potency. When used in microbiology, i.e. for residue testing or for determination of minimum inhibitory concentrations, a diminution of activity less than 25% will not be noticed. For these applications, the four tetracyclines and three quinolones tested can be kept for 6 months at -20 degrees C, while the beta-lactam antibiotics should be discarded after 3 months. Standard stock solutions of beta-lactam antibiotics and cephalosporins should preferably be used the same day when they are intended for quantitative residue analysis.