Bacteria in urine (sediment and urine culture) in cats and dogs

What it shows

Urine in the bladder of a healthy animal is sterile. In the sediment the lab looks at spun-down urine under the microscope and grades bacteria by eye — few, moderate or many. Rods are easier to recognise than small round cocci, which are easily confused with tiny crystals, cell debris and fat droplets. False reports of bacteria are considered common in unstained samples; staining the sediment makes the result more reliable.

A urine culture works differently: urine is placed on growth media and the lab counts the colonies that grow. Quantitative culture is the gold standard for confirming a bacterial urinary infection. If something grows, a susceptibility test shows which antibiotics stop that particular bacterium in the lab. Put simply: the sediment shows what looks like bacteria; the culture proves they are alive and names them.

Higher than reference — what it may be a sign of

• Bacteria in a needle sample from the bladder — significant regardless of their number, because such urine does not touch the urethra or skin.
• Bacteria in a voided sample — a few from the end of the urethra or the genital tract are expected; large numbers, one uniform type or many white cells may still matter. Urine caught during urination may pick up bacteria from the lower urethra, skin or surroundings.
• Bacteria seen are not yet an infection. The vet also looks for white cells or blood and for urinary signs. A bladder infection usually comes with white cells, but with kidney infection or reduced immunity (for example, diabetes) bacteria can appear without them.
• Growth in culture — 1,000 or more colony-forming units per millilitre from a needle sample is usually considered clinically relevant.
• Bacteria without signs (subclinical bacteriuria) — confirmed by culture in an animal without urinary signs. It occurs in healthy animals and may go away by itself; the vet decides whether it needs any action.

Lower than reference

“None seen” and “no growth” are expected, but neither is absolute. Bacteria must be numerous to be visible: rods above about 10,000 per millilitre, cocci at least 100,000. In very dilute urine a culture can be positive while the sediment looks clean. A culture can stay negative after recent antibiotics (within about two weeks), at a very acid or alkaline pH, or with extremely many white cells. Slow growers such as Mycoplasma and Corynebacterium may not grow on routine culture.

Look at together with

• White and red cells in the sediment — bacteria plus many white cells support an infection; bacteria alone in a voided sample often mean contamination.
• Urine specific gravity — dilute urine is a risk factor for infection and hides bacteria.
• Urine pH and struvite — some bacteria (for example, Staphylococcus and Proteus) break down urea, make urine alkaline and can lead to struvite crystals, with a risk of stones.

Units and conversion

There is nothing to convert. Sediment bacteria are written as a grade — “none”, “few”, “moderate”, “many” or plus signs per high-power field (one reference laboratory reports 1+ to 4+), sometimes with the shape: rods or cocci. A culture is reported as “no growth” or as the bacterium’s name with its count in colony-forming units per millilitre (CFU/mL).

Reference range

The reference range is on your lab’s form; for the sediment usually “none” or “few”, for culture “no growth”. The collection method matters more here than for any other urine line. A needle sample from the bladder (cystocentesis) is preferred for culture: it does not pick up bacteria from the urethra. Voided cultures are hard to interpret; a positive one is best confirmed with a needle sample. Urine for culture is processed straight away or refrigerated and processed within 24 hours — contaminating bacteria multiply while urine stands.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Urinalysis: infectious agents
2. Merck Veterinary Manual — Bacterial cystitis in small animals
3. IDEXX — Urinalysis and urine culture: diagnostic update
4. IDEXX — Urine sediment guide
5. VCA Animal Hospitals — Urinalysis