Presentation on theme: "Module 4 Preparation of solid media for culture and DST"— Presentation transcript:
1Module 4 Preparation of solid media for culture and DST
2Learning objectives At the end of this module you will be able to: recognize the different media for mycobacteria culture and explain their advantages/disadvantages;prepare all the reagents, including drug solutions, for preparation of media for culture and DST;prepare and dispense the culture medium;check the quality of tubes at the end of the process and store them properly;perform the sterility check.
3Content outline Description of culture media Preparation of plain culture mediaPreparation of selective and drug- containing mediaQuality of media
4Culture media Two main groups: Egg-based media Liquid media Löwenstein–Jensen mediumKudoh modified Ogawa medium (acid-buffered)Liquid mediaHerman-Kirchner liquid mediumDubos oleic acid–albumin liquid mediumMiddlebrook 7H9 liquid medium
5Culture media The ideal medium should: be economical and simple to prepare;inhibit the growth of contaminants;support luxuriant growth of small numbers of bacilli;have long shelf-life.
6Egg-based media: advantages Easy to prepare.Cheap.Support good growth of tubercle bacilli.Long shelf-life (several weeks at 4 ºC).Limited contamination during preparation.Malachite green minimizes the growth of non- mycobacterial organisms.Contamination may not cover all the surface of the medium.
7Egg-based media: disadvantages Long time (up to 8 weeks) for evident growth.Human resources, space, specific equipment needed.Possible problems in obtaining eggs of good quality, genuine inspissator.
8Specificities of egg-based media Löwenstein–Jensen (LJ):use for culture and DST;with pyruvate and without glycerol for M. bovis.Kudoh medium (acid-buffered Ogawa):no need for neutralization/centrifugation during decontamination procedure;cannot be used for DST.
9Liquid media Middlebrook 7H9 and modified versions Commercially availableAutomated systems available
10Liquid media: advantages Shorter recovery time:culture solid: 16 days for smear-positive, 29 days for smear-negative (on average);culture liquid: 8 days for smear-positive, 16 days for smear-negative (on average).Increased sensitivity compared with solid media, especially for:cerebrospinal fluidpleural fluidbiopsies.
11Liquid media: disadvantages Prone to contaminationBiosafety issueCost
12Use of combination of liquid and solid media increases sensitivity Solid vs liquid mediaSolidLiquidCost++++Contamination++Semiquantitative resultsNot possibleMorphologyAFB + coloniesAFBBiohazardIsolation timeUse of combination of liquid and solid media increases sensitivity
15General precautions Keep the environment as clean as possible. Use sterile glassware and equipment.Use high-grade chemicals and reagents unless otherwise specified.Check temperature of inspissators.Follow strict aseptic techniques when preparing media, e.g. flaming flasks and tubes.Clean and disinfect shells before breaking eggs.Do not overheat media during inspissation.Do not leave prepared media exposed to light.Do not skimp on the volume of medium.
16Choice of glasswareReusable glass tubes sealed with screw- caps and made from resistant borosilicate laboratory glass14-ml McCartney bottles5-ml bijou bottles
17Cleaning of glassware Brush glassware in hot water. Eliminate residue. Rinse repeatedly in hot water.Rinse in distilled water.
18Egg-based media Prepare mineral salt solution. Prepare malachite green solution, 2%.Note: 1 and 2 are commercially available.Homogenize whole eggs (20–25 eggs for 200 tubes).Prepare complete medium.
19Egg-based media composition ComponentsLJ, modifiedOgawaOgawa modified (Kudoh)Monopotassium dihydrophosphate (KH2PO4), anhydrous2.4 g6 g12 gMagnesium sulfate (MgSO4 ·7H2O)0.24 g–Magnesium citrate0.6 gL-Asparagine3.6 gSodium glutamate3 gDistilled water up to600 mlGlycerol (ml) or pyruvatea (g)12 ml or 7.2 g36 ml24 mlEgg homogenate1000 ml1200 mlMalachite green (2%)20 mlpH (about)6.86.4
20Egg preparation Wash eggs with soap and water. Soak eggs in 70% ethanol for 15 minutes.Filter whipped eggs through sterile gauze fabric.
22Coagulation of medium Before loading, heat the inspissator to 80 ºC. Place the bottles in a slanted position.Coagulate the medium for 45 minutes at80–85 ºC in 80% relative humidity.Do not heat further.
23Pay attention!Quality of egg-based media deteriorates when coagulation is done at too high a temperature or for too long.Discolouration of coagulated medium may be due to excessive temperature or prolonged heating time.Small holes or bubbles on the surface of the medium indicate faulty coagulation procedures.
24Quality control and storage Sterility checkIncubate the whole media batch at 35–37 ºC for 48 hours. Discard if there is any growth.StorageDate media and store in the refrigerator.Media will keep for several weeks if caps are tightly closed to prevent drying out.Slants should not be older than 2 months.
25Media log-sheet – first part Preparation: SALT SOLUTIONOperator's name:Date of preparation:ReagentsQuantity (ml or g)ManufacturerReferenceBatch numberExpiry dateMonopotassiumdihydrophosphate (KH2PO4), anhydrous.Magnesium sulfate(MgSO4 ·7H2O)Magnesium citrateL-AsparagineSodium glutamateGlycerol (ml) or pyruvate* (g)Distilled water up toAutoclave cycleDate:Operator's nameTime (min)Temperature (ºC)
26Media log-sheet – second part Preparation: Second stepPreparation date:Operator’s name:ReagentsQuantity(ml or g)ManufacturerReferenceBatch numberMalachite greenEggsNumber of eggs:Inspissator cycleDate:Operator's name:Time (min)Temperature (ºC)Sterility check
28Selective media LJ with pyruvate, without glycerol: M. bovis LJ with p-nitrobenzoic Acid (PNB):M. tuberculosis does not growFollow the procedure for preparation of culture media already described and add the selected substances (in correct dilutions) to the complete media before they are distributed into tubes and inspissated.
29Mass concentrations for drugs and critical concentration Test drugsSolventFinal mass concentration in culture medium (mg/litre)DesignationAbbrev.DilutionFor quality controlH37Rv (MTB control, strain)Critical concen-trationIsoniazidINHSterile dw0.025•0.05•0.100.2RifampicinRMPDMSO2.5•5.0•10.040.0Dihydro-streptomycinDSM0.5•1.0•2.04.0EthambutolEMB0.125•0.25•0.52.0
30Procedure Follow the procedure for preparation of culture media. Add the selected substances (in solution) to the complete media before distribution into tubes and inspissation.Follow the procedure for inspissation and all the precautions already described.
31Amount to be weighed = 1/potency DrugsConcentration is crucial for reliable DST.Use pure drug powders, not drugs for patients’ treatment.Amount to be weighed = 1/potency
32PreparationMethod 1Add 1% of drug solution to the basic culture medium. No further correction for the final total volume.Method 2Add 10% of aqueous drug solution and adjust to the final total volume of medium prepared.
33Method 1Prepare a standard batch (1620 ml) of LJ basic culture medium according to the SOP “Plain LJ media”.Isoniazid (INH) potency factor 1:Solution I: 10.0 mg INH dissolved in 50 ml distilled water (200 µg/ml)Solution II: 2.5 ml Sol. I made up to 25 ml with distilled water (20 µg/ml)Solution III: 5.0 ml Sol. II made up to 10 ml with distilled water (10 µg/ml)0. 2 µg/ml0.1 µg/ml0.05 µg/ml0.025 µg/mlMedia (ml)19819.8Solution II (ml)2–Solution III (ml)0.20.100.05Water (ml)0.15Final volume (ml)20020
34Method 2 Aqueous drug solution: 10 % of total volume (= 162 ml for 1620 ml of final volume of the batch).Subtract this volume water from the 600 ml of water used to prepare the salt solution.Mineral salt solution mlMalachite green solution mlHomogenized eggs mlTotal ml
35Method 2 Isoniazid (INH): factor 1.0 Solution I: 10.0 mg INH dissolved in 100 ml distilled water (100 µg/ml)Solution II: 1.0 ml Sol. I made up to 50 ml with distilled water (2.0 µg/ml)Solution III: 5.0 ml Sol. II made up to 10 ml with distilled water (1.0 µg/ml)0.2 µg/ml0.1 µg/ml0.05 µg/ml0.025 µg/mlMedia (ml)18018Solution II (ml)201–Solution III (ml)0.100.05Water (ml)0.15Final volume (ml)200
36Quality control – DST Slants should not be older than 4 weeks Inoculate 2 DST sets ( one with critical concentration and one with the lower concentration slants of each antibiotic) with the M. tuberculosis H37Rv strain (fully susceptible).The MIC standards for the fully susceptible H37Rv are (mg/ml): INH 0.06, RMP 4.0, DSM 2.0, EMB 0.5H37Rv should grow only on slants with lower concentration then than MICIf the batch fails the criteria, it should be discarded and a new batch should be prepared and testedReasons for failure should be discussed and examined
37Poor-quality media Bubbles Non-homogeneous dye solution Discolouration Contamination≥8 weeks old (>4 weeks for drug-containing media)Improper storage
38True and false exercise Use of a combination of liquid and solid media increases sensitivity of culture.Overheating of media during inspissation guarantees sterility of the slants.Discolouration of media is an indication of a poor-quality medium.
39Module review: take-home messages Good-quality media are essential for reliable diagnosis of tuberculosis.Liquid media shorten the recovery time but are more susceptible to contamination.For preparation of drug-containing media, pure drug powders must be used, not the drugs used for treatment of patients.
40Self-assessmentDescribe the different media used for mycobacterial culture.Describe the two options for adding drug solutions to the media to prepare DST media.How can you recognize poor-quality media?