Influence of antimicrobial agents on bactericidal activity of bovine milk polymorphonuclear leukocytes

The influence of nine commonly used antibiotics on the respiratory burst activity of bovine milk polymorphonuclear leukocytes (PMNL) of high yielding cows was studied in vitro. Cellular oxidative activity was quantitated after preincubation with drugs at different concentrations and assayed by a PMA (12,13-phorbol myristate acetate)-induced luminol-enhanced chemiluminescence (CL) technique. All antibiotics except sulphadiazine and enrofloxacin decreased CL at the highest concentration. Enrofloxacin significantly increased CL. Oxytetracycline inhibited CL even at low doses. The decreased CL with danofloxacin and oxytetracycline was mainly induced by their color, which caused absorption of the blue light emitted by luminol. Production of superoxide radicals measured by the cytochrome c reduction assay was lowered by danofloxacin, penicillin and chloramphenicol. The decreased CL with ceftiofur was due to inhibition of myeloperoxidase (MPO) activity and to scavenging of reactive oxygen species. Interference with the MPO-H2O2-halide system was also observed with spiramycin, erythromycin and oxytetracycline, while the latter was also observed with penicillin. The stimulatory effect of enrofloxacin might be due to an improvement of the penetration of luminol into the PMNL or to a stimulation of the production of H2O2. Potentiation of the action of PMA by changing the ratio between bound and free intracellular Ca2+ might also be involved. Our results suggest that many antibiotics may affect neutrophil function at concentrations that may be found in milk immediately after intramammary treatment or at concentrations higher than those found in milk after intramammary treatment.