DipStreak™: UriSelect 3/MacConkey (Cat.no.BD-912) is a convenient device for transporting urine samples as well as for isolating, enumerating and identifying specific bacteria in urine. A unique streaking mechanism permits the isolation of single colonies even when the original bacterial population of the sample was as high as 108 organisms per milliliter. DipStreak™: UriSelect 3/MacConkey (Cat.no.BD-912) is intended for use in both physicians' office laboratories and clinical laboratories.
DipStreak™: UriSelect 3/MacConkey (Cat.no.BD-912) comprises a plastic paddle with two types of agar attached back-to-back, housed in a closed transparent plastic tube. A ring with elongated prongs is attached to the end of the paddle so that there are prongs on each side of the slide. The ends of the prongs are dipped into the urine sample. Upon re-insertion into the plastic tube, the prongs are prevented from moving and the agar surfaces are inoculated with bacteria as the paddle passes over the prongs. The result is a series of streaks of decreasing bacterial concentration which permits isolation of single colonies even when the original bacterial population of the sample was as high as 108 organisms per milliliter.
Following combination of agars is used in manufacturing of DipStreak™ Cat.no.BD-912: UriSelect 3 agar and MacConkey agar.
UriSelect 3 medium (Bio-Rad Laboratories, USA) is a non-selective agar medium composed of a rich nutrient base containing three peptones, which sustains growth of all urinary tract pathogens; chromogenic substrates for detection of the bacterial enzymes ß-glucuronidase and ß-glucosidase; and tryptophan for detection of tryptophanase activity (indole production) and of tryptophan deaminase activity (TDA).
MacConkey agar is a selective and differential medium for detection of coliform organisms and enteric pathogens. The concentration of bile salts in this medium is relatively low in comparison with other enteric plating medium; therefore selectivity for gram-negative bacteria is not as great as in some other formulations. Crystal violet inhibits gram-positive microorganisms, especially enterococci and staphylococci. Differentiation of enteric micro-organisms is achieved by the combination of lactose and the neutral red indicator. Colorless or pink to red colonies are produced depending upon the ability of the isolate to ferment lactose.
READING THE RESULTS
Remove the paddle from the tube. Observe the bacterial growth on both agars. Count and compare the number of bacteria growing on the UriSelect 3 to the chart or to theguidelines for enumeration of bacteria. DipStreak™: UriSelect 3/MacConkey (Cat.no.BD-912) is capable of detecting bacteria in urine at concentrations as low as 5x102 CFU/ml. The colony density chart allows the reporting of colony counts to the nearest power of 10. If bacterial content is above 106 CFU/ml, no single colony can be isolated because of confluent growth. A standard quantitative urine culture should be performed.
Interpretation of Results
In the past, the presence of > 105 CFU/ml was regarded as a positive result while < 104 CFU/ml was negative. From 104 to 105 CFU/ml were considered borderline cases, which called for repeat examination. Today, the trend is for specific medical departments such as Urology, Nephrology and Pediatrics to report down to 103 CFU/ml. Use the attached colony density chart for reporting results. If bacterial growth yields three or more different kinds of colonies, this is most likely due to contamination. The test should be repeated.
Principle of identification with UriSelect 3 medium:
E. coli can be identified based on demonstration of the activity of two enzymes: ß-glucuronidase and tryptophanase. ß-glucuronidase cleaves the chromogenic substrate contained in the medium, causing the colonies to turn purple. Proteus are characterised by tryptophan deaminase activity; Proteus mirabilis is indole negative. Group D streptococci produce a b-glucosidase (esculinase) which cleaves the second chromogenic substrate contained in the medium causing the colonies to turn blue.
Interpretation of Results for UriSelect3 (incubation 18-24 hr at 35oC±2 oC)
IDENTIFICATION USING A CONVENTIONAL PROCEDURE
2.0 or 3.0
0.5 - 1.0
Bright frank blue
1.0 or 2.0
Store either at 15-25oC or 2-8oC until expiry date indicated on the label.
Store away from direct sunlight.
Colony Density Chart
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