Bacteria are subjectively quantified in urine as: few, moderate and many. They can be detected in unstained urine sediments when in sufficient quantity.
Identification: Rod-shaped bacteria and chains of cocci are often readily identifiable. The top panel of the image at right show E.coli bacilli from a case of cystitis in a dog. However, small amorphous crystals, cellular debris, and small fat droplets can either mask or mimic cocci (see below for more information). If there is any doubt about the presence of bacteria, a Gram-stained smear of urine sediment (middle panel) should be examined. This gram stain confirms the presence of gram negative bacilli. Note that only extracellular bacteria can be visualized on an unstained urine sediment. Intracellular bacteria can only be identified by cytologic examination of a Wright's-stained smear of the urine sediment (the bottom panel demonstrates phagocytized bacteria within a neutrophil. The neutrophil's nucleus is swollen as a storage-related artifact).
Interpretation: Bacteria may be insignificant contaminants or important pathogens. Distinguishing between these possibilities relies on clinical signs, the method of urine collection, the number and types of bacteria, the presence of leukocytes in urine, the length of urine storage, and any underlying disease in the animal.
Cocci in urine
Upper panel: Low magnification view showing increased numbers of leukocytes and several struvite crystals (unstained wet prep). The leukocytes provide clear evidence of an inflammatory process; the background appears "busy", but bacteria are not reliably identifiable at this magnification.
Middle panel: High magnification view of unstained wet prep showing leukocytes and clumps and chains of bacteria (arrows). Amorphous crystals or debris, however, can have a virtually identical appearance. Use of phase contrast microscopy can help in distinguishing between the two, but examination of a gram-stained drop of the urine sediment is most reliable.
Lower panel: High magnification view of a gram-stained slide. A neutrophil and gram positive cocci arranged in clusters and short chains are shown (arrows: some organisms have partially or completely decolorized). It can be concluded that the inflammatory process is caused or complicated by bacterial infection.
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