SOLAR-CULT^Ò CLINICAL DIPSLIDES

INTRODUCTION

Dipslides are used in clinical settings to determine urine bacterial counts for patients with suspected urinary tract infections. Our slides are coated with growth media on both sides to increase surface contact with the specimen and enable the use of two different media on each slide. Use of different selective growth media permits simultaneous exposure of a urine specimen to a medium selective for gram-positive species on one side, for example, and to a medium selective for gram-negative species on the other. Since the majority of urinary infections are caused by a relatively small number of bacterial species, tentative diagnoses can be made through quantitative and qualitative interpretation of the growth of colonies on the media.

PRINCIPLE

The dipslide is simply a plastic slide coated with agar-based growth media. The slide is attached to the cap of a shatter-proof, optically clear, colorless plastic vial. After inoculation by dipping the slide in a fresh midstream urine specimen; the slide is returned to the vial which serves as a growth chamber when incubated at the appropriate temperature. After a suitable incubation period, the bacterial growth is evaluated in terms of number of colonies and other features in the diagnosis of urinary infection.

• The technique is a simple, rapid, reliable method of determining the presence or absence of urinary tract infection.

• Good correlation is demonstrable between Dipslide colony counts in urine and those made by more laborious alternative laboratory procedures.

• Point of care inoculation of the dipslides leads to decreased delay between sample collection and inoculation. This favors increased recovery of sensitive pathogens and decreased specimen overgrowth by others leading to bacterial counts that truly reflect the number of organisms in the specimen. It is known that counts made for urine specimens left at room temperature for prolonged periods are clinically suspect.

• It is usually an easy matter for the practiced eye to interpret the clinical significance of growth on the slides in terms of the presence or absence of pathogenic or contaminating organisms.

• Technician time spent on each specimen is greatly reduced.

SOLAR-CULTÒ clinical dipslides are available with a variety of growth specific media and color indicators. Two of the more popular of these are slides coated with Cystine Lactose Electrolyte-Deficient agar (CLED) on one side and a medium selective for gram-negative organisms on the other, either Eosin Methylene Blue agar (EMB) or MacConkey agar.

Formulated according to Mackay and Sandys(3), our CLED agar dipslide coating is specifically recommended for urinary specimens. In addition to supporting the growth of all common urinary pathogens, it enables good colony differentiation and clear diagnostic characteristics. The presence of prevalent contaminants such as Diptheroids, Lactobacilli and Micrococci is also clearly shown, giving a good indication of the degree of contamination. Furthermore, the electrolyte deficiency of the CLED medium prevents the swarming of Proteus species. Mixed infections which are typical of contaminated rather than infected urine, are easily diagnosed. Significant organisms in mixed cultures can be easily picked and examined in subculture.

MacConkey agar is available as a coating on two of our dipslides. This highly selective medium is suitable for the detection, identification and enumeration of coliform organisms. With the inclusion of bile salts and crystal violet, the medium gives improved differentiation between coliforms and non-lactose fermenting organisms while gram-positive cocci are completely inhibited.

EMB agar is available in combination with CLED agar on one of our dipslides. EMB agar is a versatile medium that is recommended for the detection, identification and enumeration of members of the coliform group.

As a variation of the above, Polymyxin B sulfate is added to the CLED agar coating of one of our dipslides. This inhibits the growth of most gram-negative organisms and makes the medium more selective for gram-positive organisms while allowing the EMB agar (or MacConkey Agar) side of the slide to be selective for gram-negative organisms .

After incubation, the slide is examined for evidence of bacterial growth. Using the approximation that each colony growing on the slide represents a single bacterial inoculum from the urine, a correlation can be made between the number of bacteria present in the urine and the number of colonies on the slide.

For your convenience, a color chart illustrating bacterial growth on urinary dipslides has been prepared to assist in the interpretation of results. This chart is available free of charge. Call us.

Contaminant bacteria in fresh urine are fewer than 10 ⁴ organisms per ml, whereas infecting organisms exceed 10⁵ per ml. Tests made with known bacterial suspensions of 10⁴ organisms per ml produce around 20-30 colonies on the dipslide, whereas a suspension containing 10⁵ organisms per ml will produce around 200 - 300 colonies. The dipslide, therefore, provides optimal discrimination over the range of viable bacterial counts most helpful in the diagnosis of urinary tract infections.

The bacterial content of urine is normally expressed as follows:

These figures apply to a single type of organism on an appropriate medium, such as CLED agar.

While these figures represent useful ranges of bacterial numbers, it should be emphasized that counts as low as 10⁴ organisms per ml may be significant in some cases.

Compared to other methods, the dipslide technique is reliable and inexpensive. Nevertheless, specimen contamination or overgrowth by commensal bacteria may yield false positive results, even at levels greater than 100,000 organisms per ml. When reading the results of a dipslide test, the assessment of total count and purity of growth should be taken from the C.L.E.D. medium (initially green) side and not from the EMB or MacConkey agar side. These latter media are intended to provide diagnostic assistance in the tentative identification of organisms present.