Urine in the bladder of normal animals is sterile. Though bacteria from the distal urethra and/or genital tract may contaminate voided specimens, they are usually too few to see if a good mid-stream collection was obtained.

Although phagocytized bacteria cannot be seen in unstained wet mounts of urine sediment, they may found in stained smears of sediment. The lower panel at the right shows a neutrophil containing phagocytized bacteria. Notice that the nucleus in this cell is round; nuclei tend to become round as neutrophils age in urine.

Bacteriuria of clinical significance, e.g., bacterial cystitis, is usually accompanied by increased numbers of white cells (pyuria). The presence of a few bacteria without pyuria is very rarely significant of infection.