Bacterial culture and sensitivities, what do they really mean? (Proceedings)
Treatment of bacterial infections can be difficult and frustrating. There are many different opinions for empiric antimicrobial therapy. Empiric therapeutic decisions should be based on the most likely causative organism, expected susceptibility of the bacteria, location of suspected infection, antimicrobial adverse effects, owner compliance, and pharmacokinetic properties of the drug. For example, bacterial prostatitis is often caused by gram-negative aerobic bacteria such as E. coli or Klebsiella spp. The culture may grow an E. coli susceptible to amoxicillin / clavulanate. However a therapeutic cure is unlikely due to the location of the infection. The prostate in considered a "protected" area for which many drugs, including most beta lactams, cannot achieve sufficient concentrations to clear the infection. Likewise an E. coli may be grown from a suspected urinary tract infection susceptible to enrofloxacin, but a cure is not achieved due to presence of uroliths. Therefore empiric choices which may superficially seem appropriate may be ineffective when all factors are considered.
Interpretation of a culture and sensitivity (C&S) result can be frustrating due to different methods of determination. Laboratories which follow CLSI (Clinical Laboratory Standards Institute) standards should be used for C&S as they perform the testing following specific criteria. Results from laboratories not using CLSI validated breakpoints and methods may not accurately predict clinical outcome from the susceptibility testing. The two most commonly used methods for determining antimicrobial sensitivity are disk diffusion (aka Kirby-Bauer) and broth microdilution. Kirby-Bauer susceptibility testing usually reports results just as an S, I, or R. Broth microdilution provides additional information on the susceptibility of the organism. In addition to an S, I, or R, a minimum inhibitory concentration (MIC) is also given. The MIC is the lowest antibiotic concentration that inhibited the growth of the bacteria in the culture. For example an E. coli culture may grow in cephalexin concentrations up through 4 mcg/mL, but not in cultures containing 8 mcg/mL of cephalexin and above. Therefore an MIC of 8 mcg/mL will be assigned, the minimum concentration which inhibited growth of the organism, and would also be labeled as resistant (R). The MIC's can be useful when multi-drug resistant bacteria are present as sometimes a clinical cure can still be achieved with an "R" by altering dose or route of administration.It is important to realize that the breakpoints (concentrations that receive an S, I, R) are dependent on the species (dog, cat, horse, etc.), the disease (UTI, respiratory, skin, etc.), the organism (E coli, Staphylococcus pseudintermedias, Pseudomonas aeruginosa, etc.), the drug (cephalexin, cefazolin, cefpodoxime, cefocevin), and dosage regimen: dose (22 mg/kg, 30 mg/kg, etc.), route (IV, IM, PO), duration (7, 10, 14 days, etc.) and frequency. When any of these parameters are changed, the breakpoint may no longer be valid and success may be changed. Unfortunately there are few approved breakpoints in veterinary medicine for companion animals (see table 1). Therefore a result of S, I, R for drugs and indications other than those in table 1 are often extrapolated from breakpoints from other indications, veterinary species, or humans which may or may not be applicable to our veterinary patients. For example, the breakpoint of cefpodoxime for susceptible bacteria causing canine soft tissue infections is ≤ 2 mcg/mL. If the urine from a dog with prostatitis is cultured and a Staphylococcus is isolated with an MIC of 1 mcg/mL, most labs will assign an "S" for the isolate. However a clinical cure will not occur, because the breakpoint was for dermal infection, the infection is in the prostate, and cefpodoxime does not penetrate the prostate. However, if the dog only had a UTI and the prostate was not involved, than the outcome of treatment would likely be a clinical cure.