Medium composition and quality are the guarantee of medicine
The selection of drug susceptibility test media has certain principles and requirements, and is not changed based on personal experience or preference. In order to make clinical trials and health care service levels have consistent standards, many countries or institutions have formulated different reference standards. Due to differences in reference standards, the requirements for drug susceptibility media are also different. Our country's drug susceptibility test result judgment standards refer to the US CLSI standard document, which requires that except for special flora, the medium used in K-B method must be M-H medium; the British reference standard BSAC requires the medium to be ISO medium, while M-H nutrient agar is only suitable for staphylococcal drug susceptibility testing. Therefore, in our work, we must strictly refer to CLSI and choose the appropriate and correct drug susceptibility medium according to the needs of different flora.
mediumThe influencing factors for drug susceptibility results are as follows:
1. pH: Changes in pH can cause the diameter of the inhibition zone to increase or decrease.
2. The medium contains substances that can antagonize the activity of bacterial drugs, such as calcium and magnesium ions, which can reduce the antibacterial activity of aminoglycosides.
3. The thickness of the medium: the standard thickness is 4mm, the thickness is > 4mm, the bacteriostatic circle will become smaller, and if it is < 4mm, it will become smaller.
4. The humidity of the culture medium: Before using the drug dish, it is usually balanced at room temperature for half an hour to dry the surface of the culture medium. Excessive humidity in the culture medium is not conducive to the growth of bacteria or the dilution of antibacterial drugs.
5. Hardness of the medium: Excessive hardness is not conducive to the diffusion of antimicrobials in the agar, which can lead to a smaller diameter of the bacteriostatic ring.
6. Nutritional conditions: Poor nutritional conditions are not conducive to the growth of bacteria, and the diameter of the bacteriostatic circle becomes larger; conversely, too good cultivation and nutritional conditions will reduce the diameter of the bacteriostatic circle.
After explaining these principles and principles, I analyzed the issues in their dialogue one by one:
According to CLSI, most bacterial susceptibility test agar is standard M-H agar. M-H agar basically does not contain substances that inhibit the antibacterial activity of sulfonamides and tetracyclines, and can meet the growth needs of most bacteria. Enterococcus has low nutritional requirements and a fast growth rate. Therefore, M-H agar (MHA) can be selected instead of blood dishes. For most bacteria, MHA does not need to be added to sheep blood. Proteins in blood or serum bind to antibacterial molecules to varying degrees, which affects the activity of antibacterial drugs. For example, certain components in sheep blood can inhibit the antibacterial activity of sulfonamides and tetracyclines, and can also affect the drug susceptibility results of neonatal mycin, nafcilin and cephalospora to Enterococcus.
Caustic bacteria with high nutritional requirements such as Haemolytic Streptococcus and Streptococcus pneumoniae need to use M-H agar + 5% defibrillated goat blood medium, also known as M-H drug susceptibility test blood agar plate (MHBA). Although MHBA looks similar to ordinary blood plates for isolating bacteria or Columbia blood plates (CNA), the formula and industry standards (such as pH, humidity, etc.) are different, and cannot be used instead of each other. In order to compare the drug susceptibility results of Streptococcus pneumoniae ATCC49619 on MHBA and CNA media, we did drug susceptibility tests with MHBA and CNA respectively under other conditions. The results found that the drug susceptibility results of CNA showed that the diameter of the bacteriostatic circle was too large, and the bacterial growth was poor, resulting in "false sensitivity" results.
Therefore, as a qualified microbiological examiner, we should always abide by the operating procedures, keep in mind the standard requirements, read the rules and principles, and avoid ambiguity and speciousness in order to ensure the accuracy of the results.
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