1 Pakistan Veterinary Journal ISSN: 0253-8318 (PRINT), 2074-7764 (ONLINE) Accessible at: www.pvj.com.pk The In vitro Antibacterial Activity of Florfenicol in Combination with Amoxicillin or Cefuroxime against Pathogenic Bacteria of Animal Origin Myung-Jin Choi a , Eun-Mi Lee a , Seung-Jin Lee, Md. Ahsanur Reza, Joong-Su Lee, Elias Gebru, Man-Hee Rhee and Seung-Chun Park* Laboratory of Veterinary Pharmacokinetics & Pharmacodynamics, College of Veterinary Medicine, Kyungpook National University, Daegu 702-701, Korea *Corresponding author: parksch@knu.ac.kr ARTICLE HISTORY ABSTRACT Received: Revised: Accepted: August 28, 2010 October 26, 2010 November 01, 2010 Key words: Animals Antibacterial agents Combination therapy Synergism This study evaluated the in vitro activity of florfenicol (F) in combination with amoxicillin (AM) or cefuroxime (CRM) against pathogenic bacteria of animal origin, including E. coli, S. aureus, S. cholerasuis and P. mirabilis. The MIC of AM ranged from 16 to 256 µg/ml. The MBC of AM (64 µg/ml) was four-fold higher than its MIC value (16 µg/ml) for E. coli, and similar to the MIC for the other three species. The MIC of F ranged from 8 to 16 µg/ml. The MBC values of F for E. coli, S. aureus, and S. cholerasuis were eight-fold higher than the respective MIC values, and 32-fold higher than the MIC of P. mirabilis. The MIC of CRM ranged from 8 to 128 µg/ml. The MBC of CRM was the highest (≥ 256 µg/ml), except for E. coli. The F/AM combination resulted in synergism (FIC index ≤ 0.5) for E. coli, S. aureus, and P. mirabilis and in-difference (FIC index >1) for S. cholerasuis. For F/CRM combination, synergism (E. coli and S. cholerasuis) and in-difference (S. aureus and P. mirabilis) were observed. Killing rate study showed a 1.5 - > 3 log 10 cfu/ml reduction of E. coli with F/AM compared to AM or F alone. The highest activity of the combinations was observed when F comprised at least 50% of the combination. Further studies using many bacterial isolates and various proportion of each drug would reveal the potential of a combination product containing F and AM/CRM for use in veterinary practice. ©2011 PVJ. All rights reserved To Cite This Article: Choi MJ, EM Lee, SJ Lee, MA Reza, JS Lee, E Gebru, MH Rhee and SC Park, 2011. The In vitro antibacterial activity of florfenicol in combination with amoxicillin or cefuroxime against pathogenic bacteria of animal origin. Pak Vet J, 31(x): xxx. INTRODUCTION The use of antimicrobial compounds in veterinary practice improves animal health and production. However, their use in animals, particularly for growth enhancement, has come under much scrutiny, as it has been shown to contribute to the increased prevalence of antibiotic-resistant bacteria of human significance (Mathew et al., 2007). Antibacterial drug resistance is a growing concern worldwide, with some pathogenic bacteria exhibiting resistance to virtually all available drugs (Lister, 2006). In addition to the benefits of higher efficacy or safety profiles than the individual drugs, combination therapy with two or more antimicrobial agents is considered to be a potentially effective means of minimizing the emergence rate of bacterial resistance (Eliopoulos and Moellering, 1991). In this regard, a number of antibacterial drug combinations, including amoxicillin/ clavulinic acid, ampicillin/sulbactam, trimethoprim/ sulfonamide, trimetoprim/sulfadimethoxine, and florfeni- col/tylosin have been used in veterinary area (Escudero et al., 1996; Fernández-Varón et al., 2005; Kim et al., 2008). As part of our long-term research that focuses on developing safe and effective combination products for use in veterinary area, we have evaluated a number of antibacterial drug combinations (Kim et al., 1997; Kim et al., 2008). Here in, we report the in vitro antibacterial activity of florfenicol/ amoxicillin and florfenicol/ cefuroxime combinations against pathogenic bacteria of animal origin. MATERIALS AND METHODS Bacterial strains Field isolates of Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Salmonella cholerasuis (S. cholerasuis) and Proteus mirabilis (P. RESEARCH ARTICLE