1. What happens if more than 300 colonies are observed on the Compact Dry plate? What should be done?
* With colonies that can be observed
If visible colonies are present, a plate with more than 300 colonies should not be counted, because this may lead to underestimating the colony count due to overlapping.
* No colonies observed
- If the sample contains a microbial population that is too high (≥ 300), there may be no isolated colonies to observe, so it is reported as "Too Numerous To Count (TNTC)". TNTC growth on the Compact Dry plate appears as a color change of the fabric pad, similar to the corresponding positive test result (for example: red for Compact Dry TC, blue-green for Compact Dry YM).
- If any of the above cases occurs, the sample needs to be diluted further to reach the dilution level at which the count can be made (within the range of 1-300 colonies).
2. What happens if 1.0 mL of sample does not diffuse completely and evenly over the plate because of viscosity?
* High microbial load
Samples with high viscosity should be diluted appropriately. This will result in a dilution of ≥ 10⁻², which is less viscous and diffuses easily over the Compact Dry plate's pad.
* Low microbial load
Take 1.0 mL of sample already diluted at the 10⁻¹ ratio, then drop it evenly onto at least 5 different spots on the Compact Dry plate.
3. How can a heavily pigmented sample be handled without affecting the standard dilution?
* High microbial load
As with a high-viscosity sample, a heavily pigmented sample can also be diluted until the pigmentation fades.
* Low microbial load
For a sample with a low microbial load and heavy pigmentation, the dilution can be increased (for example: 10⁻²), but the sample volume also needs to be adjusted. This can be done by filtering 10 mL of the diluted sample through a membrane filter to retain the required sensitivity.
4. Can Compact Dry plates be stored at cold temperatures?
Yes, Compact Dry plates can be stored at cold temperatures. However, the plates must be brought back to room temperature before use.
5. What is the detection limit of the Compact Dry plate?
The recommended limit is 1-300 colonies; otherwise, the dilution needs to be adjusted.
6. Does the pH of the sample affect the interpretation of results?
No, the interpretation of results is based only on the pigmentation and enzymatic activity of the target microorganism. However, pH can affect the recovery of pH-sensitive microorganisms. To avoid this, adjust the pH of the sample by adding 1N NaOH or 1N HCl (per the BAM protocol) if the sample is too acidic or too alkaline.
7. Results from the Compact Dry Salmonella plate are considered presumptive positives — what should be done to confirm the presence of Salmonella?
To confirm a presumptive Salmonella positive, pick an individual colony from the Compact Dry Salmonella plate. Stab-and-streak onto TSIA (Triple Sugar Iron Agar) without heating, and streak onto LIA (Lysine Iron Agar). Incubate TSIA and LIA at 35°C for 24 hours.
- On TSIA, Salmonella typically produces an alkaline (red) slant and an acid (yellow) butt, with or without H₂S production (blackening of the agar).
- On LIA, Salmonella produces an alkaline (purple) reaction in the tube butt. Most Salmonella strains produce H₂S on LIA.
- In addition, positive results from TSIA or LIA need to be checked further with a latex agglutination test.
- Suspect Salmonella colonies will be transferred from the TSIA/LIA medium to a latex agglutination test kit with 1 drop of saline to emulsify the colony. Observe the agglutination reaction automatically within the first two minutes. Then, if needed, add 1 drop of latex to continue observing the reaction. Agglutination confirms the presence of Salmonella. This reaction is produced by latex particles coated with multivalent antibodies against the various Salmonella antigens.
8. What is the difference between the Compact Dry Total Viable Count (TC) plate and the Compact Dry Heterotrophic Plate Count (AQ) plate?
- Both of these Compact Dry plates are used to measure the total number of microorganisms but are designed for different sample types.
- The Compact Dry TC plate is used to determine the total microorganisms in most food, water, environmental, and air samples.
- The Compact Dry AQ plate is designed for heterotrophic microorganisms — all microorganisms that need organic matter to grow. Compact Dry AQ is commonly used in water, food, soil, and air samples, and complies with national standards, for example: the Philippines Ministry of Health regulation 2017-0010.
9. What is the difference between the Compact Dry Total Viable Count (TC) plate and the Compact Dry Total Count for Tea (TCC) plate?
Both the Compact Dry TC plate and the Compact Dry TCC plate are used to measure the total microorganisms. However, natural tea products contain catechin, which inhibits microbial growth. The Compact Dry TCC plate contains a catechin neutralizer to reduce this antibacterial effect. Therefore, to test the total microorganisms in tea products, the Compact Dry TCC plate should be used instead of the Compact Dry TC plate.