Comparative study of the Postantibiotic Effect of Penicillin G, Azithromycin and Danofloxacin on Staphylococcus aureus isolates from bovine intramammary infection

MARCHETTI, LAURA; MONCADA CÁRDENAS, ALEJANDRO; GORTARI CASTILLO, LIHUEL; BULDAIN, DANIEL; JULCA LOZANO, KAREN; DI FILIPPO, JULIA; MESTORINO, NORA

Conferencia; Tenth International Conference on Antimicrobial Agents in Veterinary Medicine (AAVM); 2020

Antimicrobial Agents in Veterinary Medicine (AAVM)

A comparative study of the antimicrobial activity vs. time (lethality curves) and postantibiotic effect (PAE) of penicillin G (PenG), azithromycin (AZT) and danofloxacin (DAF) was carried out against Staphylococcus aureus (n=6) isolated from lactating dairy cows with subclinical mastitis. S. aureus ATCC 29213 was used as control strain. Lethality curves were determined in fresh Mueller-Hinton broth (MHB) alone and in broth enriched with 40% milk. Antimicrobials were added in concentrations higher than the MICs (PenG at 4xMIC, AZT at 5xMIC and DAF at 10xMIC). Then, bacterial counts were performed every hour. PAEs were determined by exposing for 2 hours 1.5. x 107 cfu/mL of S. aureus in logarithmic growth phase at different concentrations of drugs. The antimicrobial agent was removed by dilution with fresh Mueller-Hinton broth. Bacterial counts were determined every hour along 6 hours. The PAE was measured as the difference in time required to increase 1 Log10 cfu/mL for test and control cultures after drug removal. PAE >0.5 hours was considered significant. MICs of PenG, AZT and DAF in MHB and milk were 0.25, 1, 1 and 0.25, 8, 1 µg/mL, respectively. PAEs obtained in MHB and milk were: PenG 0.42-1.54 h, AZT 2.30-1.80 h, and DAF 2.11-1.14. respectively. PenG induced the lowest PAE tested. AZT and DAF showed longer PAE in broth but it was affected in presence of milk. We observed a moderate PAE with PenG in MHB, but this effect slightly increases its persistence in milk, in which additionally to the compositional differences we found a lower pH (7.4 vs 6.5, MHB vs milk). The knowledge of the duration of PAE could be an important clinical tool for optimizing current dosing schedules in mastitis treatment by S. aureus. There is a general lack of this information for antimicrobials and important pathogens in veterinary medicine. It provides information about antibiotics action that cannot be derived from standard in vivo sensitivity tests. The effects found in vitro testing may be nullified by a variety of in vivo factors (pH for example) that are intrinsic to the drug, the host, and the strain.