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Sian Frosini, Lecturer in Veterinary Clinical Microbiology    

Antimicrobial resistance (AMR) is an established, and growing, concern for One Health. In companion animals, the emergence of multidrug-resistant (MDR) bacteria challenges successful antimicrobial treatment of clinical infections.

Few antimicrobials remain effective for bacteria such as methicillin-resistant Staphylococcus pseudintermedius (MRSP) or extended spectrum beta-lactamase-producing (ESBL-) Enterobacterales.

Clinical prevalence of these pathogens has increased over time worldwide, however initial data supporting the positive impact of national antimicrobial stewardship interventions on reducing the prevalence of MRSP in Germany has been recently reported by Professor Anette Loeffler from the RVC (Loeffler et al., 2024). A spotlight has therefore been placed on the role of stewardship within clinical practice, and this extends to the concept of diagnostic stewardship as a key responsibility of clinical microbiology laboratories.

The role of the RVC’s Diagnostic Microbiology Laboratory is crucial both for accurate bacterial species identification and, critically, in characterising antimicrobial susceptibility patterns. Results from the laboratory are key for both individual case-level therapeutic decision-making, and to enable epidemiological tracing of MDR-pathogens within veterinary medicine.

To characterise resistance profiles, antimicrobial susceptibility testing (AST) is performed using two key methods, with the results interpreted using clinical breakpoints. Breakpoints consider microbiological, pharmacokinetic and pharmacodynamic factors relating to the predicted interaction between bacteria and the antimicrobials at the site of infection, as well as the host animal’s metabolism of the antimicrobial.

Taking into account these complex factors, veterinary-specific breakpoints, including those for companion animals, horses and farm animal species, are available for some specific pathogen-drug combinations, and the diagnostic laboratory supplements these with human-derived breakpoints where necessary. To correctly apply these breakpoints, it is vital that the pathogen is accurately identified and gold-standard methods implemented for AST.

Traditional microbiology utilises the Kirby-Bauer disk diffusion method for AST. In this, filter paper disks impregnated with antimicrobials are applied to the surface of agar spread with bacteria.

Following overnight incubation, the inhibitory zones around the disks, where bacterial growth has been stopped, are measured and interpreted using clinical breakpoints to determine whether the antimicrobial is likely to be effective (termed ‘susceptible’). This provides a useful guide to selecting an antimicrobial that is likely to be clinically effective, but cannot inform the clinician of the exact concentration of antimicrobial needed for therapy.

At the RVC’s Diagnostic Microbiology Laboratory, the VITEK-2 (BioMérieux) is used to run a different method for AST: automated broth microdilution assays. These give a quantitative assessment of antimicrobial activity against a pathogen. Broth microdilution incubates the pathogen in known dilutions of antimicrobial and identifies the concentration needed to inhibit bacterial growth, termed the ‘Minimum Inhibitory Concentration’ (MIC).

This quantitative assessment of susceptibility allows a more nuanced approach to interpretation. This is especially useful in the case of MDR-pathogens, where antimicrobial options may not be veterinary licensed, and interpretation may not be based on animal species-specific clinical breakpoints.

This further interpretation of MIC is made not through comparison of the numerical value, but by assessing how far from the clinical breakpoint the exact MIC determined for the pathogen is. At this point, consideration of other host/disease factors (such as site of infection, health status, age, animal species) and drug factors (administration frequency, route, cost) will be key parts of the decision-making process.

Our Diagnostic Microbiology Laboratory combines broth microdilution with disk diffusion testing to provide a broad range of antimicrobial interpretations, tailored towards the specific bacterial species. For MDR-bacteria, such as MRSP and ESBL-Enterobacterales, an extended susceptibility panel is run as standard including (where appropriate for host and bacterial species) antimicrobials such as rifampicin, nitrofurantoin and fosfomycin.

Furthermore, phenotypic screening methods confirm MDR isolates of concern for Infection Prevention and Control (IPC) protocols, and highlights if concerning patterns of antimicrobial resistance have been identified on the clinical report.

The microbiology team, overseen by myself, continues to adapt: annually reviewing clinical breakpoints; investigating new opportunities for AMR detection; instigating specific screening during suspected outbreaks where MDR-pathogen spread is suspected within a clinical environment; and monitoring resistance phenotypes to highlight to IPC teams where high risk transmission-type events could be occurring.

Our Diagnostic Microbiology service is available to clinicians from RVC hospitals and all other clients from external veterinary practices, hospitals and institutions for samples submitted via the RVC Pathology & Diagnostic Laboratory Service.

Reference: 

Loeffler A, Beever L, Chang YM, Klein B, Kostka V, Meyer C, Müller E, Weis J, Wildermuth B, Fishwick J, Lloyd DH. ''. Vet Rec. 2024 Mar-13 30;194(7):e3714. doi: 10.1002/vetr.3714. Epub 2023 Dec 15. PMID: 38100180.