Bacterial culture and antibiotic susceptibility in cats and dogs

What it shows

Two lines usually sit together on the form. Aerobic culture and identification: the sample is spread on growth media and kept in air; if colonies appear, the lab names the bacterium and, for urine, counts it. Antimicrobial susceptibility (MIC) is then done on the bacterium that grew: it is exposed to a series of concentrations of different antibiotics. The minimum inhibitory concentration (MIC) is the lowest one that stops visible growth.

Next to each antibiotic the report puts a letter. S (susceptible) — growth stops at concentrations the drug usually reaches in the blood. I (intermediate) — the MIC is close to what can be reached, so the drug may still work where it concentrates, for example in urine. R (resistant) — the usual concentrations do not stop it. The dividing lines (breakpoints) differ by drug and bacterium.

Higher than reference — what it may be a sign of

• One bacterium from a clean sample — a needle urine sample, tissue or body fluid — supports a real infection, especially with inflammation at the same site.
• Growth is not always infection. Culture cannot tell infection from colonisation or contamination. Swabs easily pick up the body’s own surface bacteria; free-catch urine, surface wound swabs and drain tubes are considered unsuitable samples for culture.
• Expected resident bacteria are usually left out of the report.
• R next to many antibiotics — the bacterium resists them in the test tube.

Lower than reference

“No growth” reflects only what grew from the sample as it reached the laboratory; it does not by itself define the state of the animal. Antibiotics started before sampling, a long or warm journey or a dried-out swab can lower the yield, and some bacteria, such as Campylobacter, have to be processed or frozen at once. The vet reads it together with cytology and how the sample was taken.

Look at together with

• Bacteria in the urine sediment and white cells — the microscope shows what looks like bacteria and inflammation; the culture proves the bacteria are alive and names them.
• Urine specific gravity — dilute urine is one of the reasons to culture even when the sediment looks clean.
• Cytology of fluid, pus or tissue — a stained smear gives a fast first hint (rods or cocci) while the culture grows.

Units and conversion

There is nothing to convert. Culture is written as “no growth” or as the name of the bacterium; for urine a count of colonies per millilitre is added. Susceptibility is a table: the antibiotic, its letter S, I or R, the MIC and often the range of concentrations tested. An MIC number cannot be compared between two antibiotics: each drug has its own breakpoint and tested range, so what matters is how far the MIC sits from that drug’s breakpoint.

Reference range

The reference range is on your lab’s form; from a site that is normally sterile the expected result is “no growth”. The susceptibility table has no reference range — each antibiotic is judged against its own breakpoint. The test is done in a tube: it cannot account for how well a drug reaches the infected place, and sites such as the brain, eyes, prostate or an abscess may behave differently. An S close to the breakpoint does not rule out resistance developing. Ideally the sample is taken before antibiotics are started, as cleanly as possible, chilled if it reaches the lab within 24 hours and frozen if the delay is longer. Choosing the antibiotic is the vet’s job.

Cats and dogs

Sources

1. Merck Veterinary Manual — Microbiology Testing for Animals (pharmacology / antimicrobials)
2. IDEXX Reference Laboratories — Microbiology FAQs: Diagnostic Principles and Standards
3. IDEXX — Microbiology guide to interpreting minimum inhibitory concentrations (MIC)
4. Merck Veterinary Manual — Pharmacotherapeutics in Bacterial Urinary Tract Infections in Animals