Page 9 - RVC Clinical Connections - Spring 2025
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Pathology
CLINICAL MICROBIOLOGY IN AN ERA OF
ANTIMICROBIAL RESISTANCE
Sian Frosini, Lecturer in Veterinary Clinical Microbiology
ntimicrobial resistance (AMR) is an This quantitative assessment of
established, and growing, concern susceptibility allows a more nuanced
A for One Health. In companion approach to interpretation. This is especially
animals, the emergence of multidrug- useful in the case of MDR-pathogens,
resistant (MDR) bacteria challenges where antimicrobial options may not be
successful antimicrobial treatment of clinical veterinary licensed, and interpretation may
infections. not be based on animal species-specific
Few antimicrobials remain effective clinical breakpoints.
for bacteria such as methicillin-resistant This further interpretation of MIC is
Staphylococcus pseudintermedius (MRSP) made not through comparison of the
or extended spectrum beta-lactamase- numerical value, but by assessing how far
producing (ESBL-) Enterobacterales. from the clinical breakpoint the exact MIC
Clinical prevalence of these pathogens determined for the pathogen is. At this point,
has increased over time worldwide, consideration of other host/disease factors
however literature reporting the positive (such as site of infection, health status,
impact of national veterinary antimicrobial age, animal species) and drug factors
stewardship interventions has placed a (administration frequency, route, cost) will
spotlight on the role of stewardship within Interpreting Kirby-Bauer disk diffusion plates be key parts of the decision-making process.
clinical practice, extending to diagnostic as a part of Antimicrobial Susceptibility Testing Our Diagnostic Microbiology Laboratory
stewardship as a key responsibility of clinical methods, in the RVC diagnostic microbiology combines broth microdilution with disk
microbiology laboratories. laboratory diffusion testing to provide a broad range
The role of the RVC’s Diagnostic Following overnight incubation, the of antimicrobial interpretations, tailored
Microbiology Laboratory is crucial both for inhibitory zones around the disks, where towards the specific bacterial species.
accurate bacterial species identification bacterial growth has been stopped, are For MDR-bacteria, such as MRSP and
and, critically, in characterising antimicrobial measured and interpreted using clinical ESBL-Enterobacterales, an extended
susceptibility patterns. Results from the breakpoints to determine whether the susceptibility panel is run as standard
laboratory are key for both individual case- antimicrobial is likely to be effective (termed including (where appropriate for host and
level therapeutic decision-making, and to ‘susceptible’). This provides a useful guide bacterial species) antimicrobials such as
enable epidemiological tracing of MDR- to selecting an antimicrobial that is likely rifampicin, nitrofurantoin and fosfomycin.
pathogens within veterinary medicine. to be clinically effective, but cannot inform Furthermore, phenotypic screening
To characterise resistance profiles, the clinician of the exact concentration of methods confirm MDR isolates of concern
antimicrobial susceptibility testing (AST) is antimicrobial needed for therapy. for Infection Prevention and Control (IPC)
performed using two key methods, with the At the RVC’s Diagnostic Microbiology protocols, and highlights if concerning
results interpreted using clinical breakpoints. Laboratory, the VITEK-2 (BioMérieux) is patterns of antimicrobial resistance have
Breakpoints consider microbiological, used to run a different method for AST: been identified on the clinical report.
pharmacokinetic and pharmacodynamic automated broth microdilution assays. Our microbiology team continues
factors relating to the predicted interaction These give a quantitative assessment of to adapt: annually reviewing clinical
between bacteria and the antimicrobials antimicrobial activity against a pathogen. breakpoints; investigating new opportunities
at the site of infection, as well as the host Broth microdilution incubates the pathogen for AMR detection; instigating specific
animal’s metabolism of the antimicrobial. in known dilutions of antimicrobial and screening during suspected outbreaks
Taking into account these complex identifies the concentration needed to inhibit where MDR-pathogen spread is suspected
factors, veterinary-specific breakpoints, bacterial growth, termed the ‘Minimum within a clinical environment; and monitoring
including those for companion animals, Inhibitory Concentration’ (MIC). resistance phenotypes to highlight to IPC
horses and farm animal species, are teams where high risk transmission-type
available for some specific pathogen-drug events could be occurring.
combinations, and the diagnostic laboratory Our Diagnostic Microbiology service is
supplements these with human-derived available to clinicians from RVC hospitals
breakpoints where necessary. To correctly and all other clients from external veterinary
apply these breakpoints, it is vital that the practices, hospitals and institutions for
pathogen is accurately identified and gold- samples submitted via the RVC Pathology
standard methods implemented for AST. & Diagnostic Laboratory Service.
Traditional microbiology utilises the
Kirby-Bauer disk diffusion method for AST. VITEK-2 (BioMerieux) is used in the RVC For the Diagnostic Pathology Service,
In this, filter paper disks impregnated with diagnostic microbiology laboratory to perform please call:01707 666 208
antimicrobials are applied to the surface of automated broth microdilution for Antimicrobial Email:
diagnosticlabs@rvc.ac.uk
agar spread with bacteria. Susceptibility Testing
Spring 2025 9
