Description
Middlebrook 7H11 Agar Base
Selective medium for the isolation and cultivation of mycobacteria.
Description
Middlebrook 7H11 Agar Base is a medium used with supplements for the selective isolation of Mycobacterium spp, especially Mycobacterium tuberculosis, from clinical specimens as well as for the cultivation of pure cultures of mycobacteria. This agar-based medium provides improved susceptibility testing compared to media based on coagulated egg (Lowenstein formulations) where drugs may be more easily inactivated.
Principle
Pancreatic digest of casein provides nitrogen, vitamins and amino acids for the growth of fastidious strains of M. tuberculosis. Glutamic acid, sodium citrate, pyridoxine, biotin and ammonium sulfate supply growth factors. Ferric ammonium citrate and magnesium sulfate are sources of trace ions. Phosphates help maintaining the pH of the medium. Malachite green is the selective agent inhibiting the contaminant microbial flora. Malachite green serves as pH indicator as well. Agar is the solidifying agent. Middlebrook 7H11 Agar Base is enriched with Glycerol Supplement (ref. 80021) and Middlebrook 7H10 (OADC) Supplement (ref. 81035) containing oleic acid, albumin, glucose, catalase and sodium chloride. Glycerol and glucose are energy sources. Sodium chloride maintains the osmotic equilibrium. Albumin protects the tubercle bacilli against toxic agents. Catalase destroys toxic peroxides that may be present in the medium. Oleic acid is a fatty acid utilized in the mechanism of mycobacteria.
Preparation
- Suspend 20.5 g of powder in 900 millilitres of deionized or distilled water containing 5 ml of glycerol
- Bring to boil and shake until completely dissolved
- Sterilize at 121°C for 15 minutes
- Cool up to 45-50°C
- Aseptically, add 100 ml of Middlebrook 7H10 (OADC) Supplement
Technique
appropriate references. Keep plates and tubes shielded from light and incubate at 35 ± 2°C for up to 8 weeks in aerobic atmosphere enriched with 5-10% carbon dioxide.
Interpretation of Results
Examine weekly for growth, pigment production and colony morphology. Carry out identification tests according to established laboratory procedures. N.B. Negative culture results do not rule-out active infection by mycobacteria. Some factors that may lead to unsuccessful cultures are: • The specimens was not representative of the infectious materials; i.e. saliva instead of sputum. • The mycobacteria were destroyed during digestion and decontamination of the specimen. • Gross contamination interfered with the growth of mycobacteria. • Proper aerobic conditions and increased CO 2 tension were not provided during incubation.
Specifications
- Form: Dehydrated medium
- Storage: 10-30°C
- Shelf life: 4 years