The quality of culture media is the foundation of testing work in microbiology laboratories, as it is crucial to the accuracy and reliability of such tests and plays a pivotal role in microbiological laboratory testing. Neglecting quality issues in any link during the work process will lead to deviations in the scientificity and objectivity of test results. Therefore, only by strengthening the laboratory quality control of culture media, earnestly conducting the quality control work of culture media, and continuously improving and enhancing the quality control level of culture media can high-quality culture media be provided for microbiological testing laboratories.
- Appearance Control
Prepared culture media should have the corresponding color. Generally, culture media should be clear, without turbidity or precipitation. Before use, check whether the color of the culture medium has changed and whether evaporation/dehydration has occurred.
- Contamination Control
Select a portion from each batch of prepared culture media for contamination testing (sterility test). The culture medium can only be used if it is confirmed that no microbial growth occurs.
- Performance Testing
3.1 Selection of Test Strains
Test strains are a set of strains that possess stable characteristics of their representative species and can effectively demonstrate the optimal performance of specific laboratory culture media. They should be standard strains from international/national standard culture collections (such as ATCC). The selection of strains should refer to the health industry standard WS/T232-2002 Technical Specification for Quality Inspection of Commercial Microbiological Culture Media.
3.2 Quantitative Testing Method
Modified Miles-Misra Method:
- Perform a 10-fold serial dilution of the overnight culture of the test strain.
- Divide the test plates and reference plates into 4 regions and mark them.
- Starting from the highest dilution, drop one drop of the dilution into the marked regions of the test plate and the control plate respectively.
- Spread the dilution over the entire 1/4 region and incubate at 37°C for 18 hours.
- Count the colonies in the regions that are easy to count, and calculate the growth rate using the formula: Growth Rate = Total Number of Colonies on the Test Culture Medium Plate / Total Number of Colonies on the Reference Culture Medium Plate.
- For non-selective culture media, the growth rate of the target bacteria should not be less than 0.7, and such culture media should be conducive to the growth of target bacteria; for selective culture media, the growth rate of the target bacteria should not be less than 0.1.
3.3 Semi-Quantitative Testing Method
Modified Streak Inoculation Method:
- Divide the plate into 4 zones (A, B, C, and D) and streak 16 lines in total, with parallel lines spaced approximately 0.5 cm apart.
- Score 1 point for each streak with bacterial growth, 0.5 points for each streak where only half of the line shows bacterial growth, and 0 points for streaks with no bacterial growth or growth less than half of the streak. Sum the scores to obtain the Growth Index (G).
- The target bacteria should exhibit typical growth on the culture medium, while the growth of non-target bacteria should be partially inhibited or completely inhibited. The culture medium is acceptable when the Growth Index (G) of the target bacteria is greater than 6.
3.4 Qualitative Testing Method
Plate Inoculation Observation Method:
- Use an inoculating loop to take the culture of the test strain and streak parallel straight lines on the surface of the test culture medium.
- Incubate the inoculated plates according to the incubation time and temperature specified in the standard.
- The target bacteria should show good growth, with typical colony appearance, size, and morphology, while non-target bacteria should show weak growth or no growth.