The dipslides estimate counts in CFU per mL. Drinking water is usually measured on a per 100 mL basis. The lowest dipslide number of 1000 CFU per mL which is equivalent to 100,000 CFU per 100 mL. This is well above the limit for swimming (200 CFU per 100 mL) and drinking water (< 1 CFU per 100 mL). Drinking water should be tested using EPA-accepted test methods such as pour plate and MPN.
If a dipslide is used as a substitute for a pour plate method, it is important to take the surface area of the dipslide into account. The surface area of the dipslide is 0.1 mL. A hard count of 1 colony is 1 per 0.1 mL or 10 CFU per mL. This is 1000 CFU per 100 mL – still 5x above the swimming limit and well above the drinking water limit. Note: The presence of a colony on a dipslide after incubation would be a FAIL for drinking water. The absence of a colony should NOT be considered a PASS. Due to the limited sample volume (or contact area) it is quite possible that water above the drinking water limit might not grow a colony on the dipslide.
The dipslides should be used to estimate gross contamination.
No, you need to “swab” the agar surface with the sample. Growth should include HPC (heterotrophic plate counts), as well as coliform counts.
No. Microslides can be set out at room temperature, or placed in a warm area such as the top of a refrigerator to “incubate”. Growth will still develop if Microslides are not incubated, it will simply take more time (at least 48 hours). Yogurt incubators or bottle warmers can also be used to speed up growth.
There are two techniques for sampling viscous fluids.
TECHNIQUE 1
1. Measure a 1mL sample using a sterile graduated cylinder.
2. Either pipette or carefully pour (using aseptic technique) the sample onto the paddle. Tilt the paddle at a 15-degree angle, allowing the sample to flow by first impacting the “top” of the paddle, then flowing down towards the “bottom” (opposite side) of the paddle. A Thinly-applied film should be made. The surface area of paddle is approximately 10 square centimeters.
3. Insert the paddle back into the vial and incubate in an UPRIGHT position. (There should not be sample dripping downward, but even so, it should not harm the agar growing surface.)
4. Counts are expressed as CFU/cm3.
TECHNIQUE 2
1. Use a sterile cotton swab (or other sterile transfer device such as a loop) to collect material to be sampled.
2a. Non-Quantitative Application: Apply sample to agar paddle surface as a thin film using aseptic technique.
2b. Quantitative Application: A known amount (1mL) of sample is aseptically applied to the top edge of the paddle, then spread using a loop (or similar spatula-type device) over the remaining 10 square centimeter paddle surface.
3. Insert the paddle back into the vial and incubate in an UPRIGHT position.
4. Counts are expressed as CFU/cm3 (when a 1mL sample is applied).
If you are incubating at room temperature, we recommend an incubation period of 36 hours. If you are incubating at 86-90 degrees Fahrenheit (30 degrees Celsius), we recommend an incubation period of 24 hours. It is acceptable to incubate Microslides for 48 hours or more, however, keep in mind that the longer the Microslides are left to incubate, the larger the colonies may appear on the agar surface. When large numbers of microorganisms are present, an extended incubation period may result in Microslides that are difficult to interpret due to confluent growth.
Although you should determine what is an acceptable level of bacteria for your unique system, a general guide would be less than 105-106 CFU/ml for cooling water and less than 106-107 CFU/ml for metalworking fluid.
No, the Microslide is not acceptable to use.
The print on a Microslide vial is as follows: XXX/XXX BBE: YY-MM-DD XXXXXX. Where XXX/XXX is the media on side one /side two of the paddle, BBE is the expiration date, and the last set of numbers is the batch number. Side one of the paddle is marked by a laser indentation line on the tip of the paddle.
The shelf-life of Microslides is 6-9 months if properly stored. Store out of direct sunlight at room temperature (do no refrigerate).
The full surface area of the agar (10 square centimeters) should be covered.
The Nutrient-TTC agar color is normally light yellow when the agar is cast. After testing, during the incubation phase, the agar may change to a light green color. This color change is a result of either a microbial-induced or chemically-induced pH change in the media. This color change alone does not indicate the presence of microorganisms. Development of red spots or other growth on the agar are an indication of microorganisms.
Yes. Nutrient-TTC (NUT), Tryptic Soy (TSA), and MacConkey (MAC) agars can all be used for marine waters. Testing is essentially identical as for freshwater.