Bacterial culture is the gold standard for diagnosis of urinary tract infection (UTI), and culture should preferably be combined with antimicrobial susceptibility testing (AS1) to guide treatment.¹ Antibiotics included in AST panels typically include: (i) Drugs that are used by veterinarians for treatment of infections, (ii) Surrogate antibiotics that are not used for treatment but can predict susceptibility to other drugs (e.g., ampicillin as surrogate for amoxicillin), and (iii) Antibiotics used as indicators of important resistance phenotypes. Escherichia coli is the most common pathogen in canine and feline UTI constituting more than 50% of clinical isolates. A particularly important resistant variant of E. coli produces the enzyme extended-spectrum beta-lactamase (ESBL), which degrades most beta-lactams. ESBL-producing bacteria are often multidrug-resistant and therefore difficult to treat using the antibiotics available in veterinary medicine. Furthermore, these bacteria are nosocomial pathogens that tend to spread in the clinic environment, and they have zoonotic potential.² In order to recognize these bacteria, the clinician should look for resistance to 3rd generation cephalosporins whenever E. coli or other bacteria belonging to the Enterobacteriaceae family are detected. Drugs like cefpodoxime and cefotaxime are particularly good indicators of ESBL-­producing bacteria. Staphylococci are less commonly involved in UTI in pets, but when they occur, the clinician should look for resistance to oxacillin and cefoxitin, as these drugs are indicators for detection of methicillin-resistant Staphylococcus pseudintermedius (MRSP) and methicillin-resistant Staphylococcus aureus (MRSA), respectively.

Apart from understanding the role of antibiotics used for AST, clinicians should have a basic insight into methodology and criteria for interpretation of results. This is particularly true when AST is done in-house in the veterinary practice, but also for being able to analyse critically results reported by an external diagnostic laboratory. For AST, dilution and diffusion methods are most commonly used. For the former, a bacterium is exposed to two-fold dilutions of a drug in broth or on an agar plate. Upon incubation, antibiotic susceptibility is measured as the minimum inhibitory concentration (MIC), which is the antibiotic concentration that inhibits visible bacterial growth. For the diffusion method, tablets or discs containing antibiotics are placed on an agar surface with a bacterial inoculum. Antibiotic susceptibility is then measured after incubation as the diameter of the inhibition zone around each disc/tablet. Dilution testing is generally more precise, but it is relatively expensive and offers limited flexibility, as commercial test panels have a fixed composition of antibiotics. On the contrary, the diffusion methodology is cheap and flexible, as different disks may be added depending on the type of bacterium (gram-negative/-positive), local availability of drugs, etc. This is also the reason why disk diffusion is the preferred AST method for in-house use by veterinarians. However, despite the apparent simplicity, AST should only be done by trained personnel according to a proper standard specifying methodology and interpretation of results. Failure to do so may lead to inaccurate results and consequently a risk of drug misuse or treatment failure.³

There are several national and international standards available for AST, but currently only one widely accepted standard for testing bacterial isolates of veterinary origin. This guideline from the Clinical Laboratory Standards Institute⁴ specifies various methodological aspects that should always be followed such as (i) incubation time and temperature, (ii) type and amount of agar and broth medium, (iii) preparation of bacterial inoculum, and (iv) use of bacterial reference strains for quality control. The CLSI document also specifies clinical breakpoints (CBPs) for standard interpretation of results. CBPs are cut-off values used to classify bacteria as susceptible, intermediate, or resistant based on MICs from dilution methods, or zone inhibition diameters from diffusion methods. CBPs are defined with the help of mathematical modelling using MIC distributions of the target bacterium and pharmacokinetics and pharmacodynamics of the antibiotic. CBPs often vary between animal and bacterial species, and they may vary between organ systems. For example, for canine skin and soft tissue infections E. coli is susceptible to ampicillin if the MIC is ≤0.25 µg/ml, whereas the corresponding breakpoint for E. coli in canine UTI is ≤8 µg/ml. This large difference is due to ampicillin being concentrated in the bladder upon excretion. If the person interpreting the MIC is not aware of this difference - or not aware of sample origin - the bacterium may be falsely classified as resistant or susceptible with potential impact on treatment outcome.

Once AST is done and interpreted according to standards, the veterinarian should select an appropriate antibiotic from the list of drugs to which the target bacterium is susceptible. Selecting the right drug can be complicated, as the veterinarian should try to avoid broad-spectrum drugs, prioritize drugs licensed for the infection and patient, prioritize drugs that concentrate at the site of infection, avoid drugs with potential side effects, and consider the optimal administration route, dose, and length of treatment.

In conclusion, AST is useful for guiding antimicrobial choice and for detection of important resistance phenotypes such as ESBLs. However, veterinarians should strongly consider using external laboratories for AST unless they stick meticulously to approved standards specifying methodology and criteria for interpretation. Even after a properly performed AST, it may be challenging to choose an antibiotic, and it is therefore recommended that veterinarians consult microbiologists or national treatment guidelines when in doubt.

References

1.  Guardabassi L, Prescott JF. Antimicrobial stewardship in small animal veterinary practice: from theory to practice. Veterinary Clinics of North America: Small Animal Practice. 2015;45(2):361–376.

2.  Wieler LH, Ewers C, Guenther S, Walther B, Lubke-Becker A. Methicillin­ resistant staphylococci (MRS/and extended-spectrum beta-lactamases (ESBL)-producing Enterobactenaceae in companion animals: nosocomial infections as one reason for the rising prevalence of these potential zoonotic pathogens in clinical samples. International Journal of Medical Microbiology. 2011;301(8):635–641.

3.  Weese JS, Giguere S, Guardabassi L, Morley PS, Papich M, Ricciuto DR, Sykes JE. ACVIM consensus statement on therapeutic antimicrobial use in animals and antimicrobial resistance. Journal of Veterinary Internal Medicine. 2015;29(2):487–498.

4.  Clinical Laboratory Standards Institute (CLSI). Performance standards for antimicrobial disk and dilution susceptibility tests for bacteria isolated from animals; second informational supplement. CLSI document VET01S2. Wayne: CLSI; 2013.