Reasons for antimicrobial treatment failures and predictive value of in-vitro susceptibility testing in veterinary practice: An overview

•Antimicrobial treatment failure is an important threat.•In-vitro resistance is not the only reason for therapeutic failure.•Better understanding of possible reasons can help to avoid treatment failures.•Facts that can result in ineffective antibacterial treatment are summarized.•The communication on therapeutic failure can help to improve the antimicrobial use. The choice of the most suitable antimicrobial agent for the treatment of an animal suffering from a bacterial infection is a complex issue. The results of bacteriological diagnostics and the in-vitro antimicrobial susceptibility testing (AST) provide guidance of potentially suitable antimicrobials. However, harmonized AST methods, veterinary-specific interpretive criteria and quality control ranges, which are essential to conduct AST in-vitro and to evaluate the corresponding results lege artis, are not available for all antimicrobial compounds, bacterial pathogens, animal species and sites of infection of veterinary relevance. Moreover, the clinical benefit of an antimicrobial agent (defined as its in vivo efficacy) is not exclusively dependent on the in-vitro susceptibility of the target pathogen. Apart from the right choice of an antibacterial drug with suitable pharmacokinetic properties and an appropriate pharmaceutical formulation, the success of treatment depends substantially on its adequate use. Even if this is ensured and in-vitro susceptibility confirmed, an insufficient improvement of clinical signs might be caused by biofilm-forming bacteria, persisters, or specific physicochemical conditions at the site of infection, such as pH value, oxygen partial pressure and perfusion rate. This review summarizes relevant aspects that have an impact on the predictive value of in-vitro AST and points out factors, potentially leading to an ineffective outcome of antibacterial treatment in veterinary practice. Knowing the reasons of inadequate beneficial effects can help to understand possible discrepancies between in-vitro susceptibility and in vivo efficacy and aid in undertaking strategies for an avoidance of treatment failures.