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Microbiology Testing FAQs — Methods & Media | HiMedia USA
How much does 500g of PDA make?
500g of Potato Dextrose Agar powder makes approximately 12.8 liters of prepared medium (39g per liter). Volume calculations: In 90mm plates (20mL/plate): 500g yields ~640 plates. In bottles (200mL): 500g yields ~64 bottles. Cost analysis for cannabis testing facility testing 100 samples/day with 3 plates per sample (300 plates/day): 500g supplies 2 days. Monthly need (6,000 plates) = 4.7kg PDA powder. RTU plates may be more cost-effective for this volume due to labor savings.
What is LST Broth?
Selective lactose broth containing lauryl sulfate inhibiting gram-positives. Used for presumptive coliform test in water/food. Gas in Durham tube = presumptive coliforms.
What is m-Enterococcus Agar used for?
m-Enterococcus Agar is selective medium for enumeration of enterococci in water by membrane filtration per EPA Method 1600. Enterococci are fecal indicators more stable in seawater than E. coli. Contains: (1) Sodium azide - inhibits gram-negatives, (2) Triphenyltetrazolium chloride (TTC) - reduced to red formazan by enterococci. Enterococci produce red/pink colonies. Used for: (1) Marine/estuarine water quality, (2) Recreational beach water, (3) Wastewater monitoring. EPA recommends enterococci for saltwater beaches. Filter water → place on m-Enterococcus → incubate 48h at 41°C.
What is TGE Agar used for?
Tryptone Glucose Extract (TGE) Agar is general-purpose medium for plate counts of bacteria in milk and dairy products per Standard Methods for Dairy Products. Similar to PCA but optimized for dairy. Contains tryptone, glucose, beef extract. Used for: (1) Standard plate count in milk, (2) Dairy product testing, (3) Cultured dairy products, (4) Ice cream testing. Supports good recovery of psychrotrophic bacteria from refrigerated dairy. Incubate 32°C for 48 hours (dairy standard). Provides accurate counts with minimal interference from dairy matrix.
Why are Bile-Tolerant Gram-Negative bacteria tested in cannabis?
BTGN testing in cannabis detects potential fecal contamination and enteric pathogens (E. coli, Salmonella, other coliforms). Bile tolerance indicates organisms from intestinal tract. Concerns: (1) Inhalation of contaminated cannabis may cause respiratory infections, (2) High BTGN indicates poor cultivation practices (contaminated Water, soil, handling), (3) Immunocompromised medical cannabis users at high risk. State regulations mandate BTGN testing: California, Colorado, Massachusetts require BTGN <1,000 CFU/g for flower, stricter limits for concentrates/edibles. VRBG is standard method for BTGN enumeration.
What is VRBG Agar used for?
Violet Red Bile Glucose (VRBG) Agar is used for enumeration of Bile-Tolerant Gram-Negative (BTGN) bacteria in cannabis and food products. Contains crystal violet and bile salts to selectively inhibit gram-positive bacteria while allowing gram-negative organisms to grow. In cannabis testing, BTGN count is a regulatory requirement in many states (limits typically <1,000 CFU/g for flower). Also used for Enterobacteriaceae count in food testing per ISO 21528. Purple/red colonies indicate BTGN presence. Incubate 24 hours at 35-37°C.
What is Biotin Assay Medium used for?
Biotin Assay Medium is used for microbiological determination of biotin (Vitamin B7) content in pharmaceutical products, supplements, and food. Uses Lactobacillus plantarum which requires biotin for growth. Procedure: (1) Add biotin standards and samples to medium, (2) Inoculate with L. plantarum, (3) Incubate 16-24 hours, (4) Measure turbidity, (5) Compare sample turbidity to standard curve, (6) Calculate biotin concentration. Growth is proportional to biotin content. USP method for biotin assay. Similar media available for other vitamin assays (folate, niacin, B12).
What is MRS Broth used for?
MRS (deMan, Rogosa, and Sharpe) Broth is the standard medium for cultivation and enumeration of Lactobacillus and other lactic acid bacteria. Rich formulation with peptones, beef extract, yeast extract, glucose, and Tween 80. Lower pH (6.2-6.6) favors lactobacilli. Used for: (1) Probiotic product enumeration (CFU/capsule), (2) Fermented food analysis (yogurt, kimchi, sauerkraut), (3) Beer/wine microbiology, (4) Lactobacillus stock cultures. MRS Agar (with agar added) for plate counts. Incubate anaerobically at 35-37°C for 48-72 hours.
What is Rogosa SL Agar used for?
Rogosa SL Agar is a selective medium for isolation and enumeration of Lactobacillus species from clinical, food, dairy, and probiotic products. Contains: (1) Low pH (5.4) - inhibits most bacteria except Lactobacillus, (2) Sodium acetate - selective agent, (3) High nutrient content. Lactobacillus colonies appear as small white/cream colonies. Used for: (1) Yogurt and fermented food QC, (2) Probiotic product testing, (3) Oral/vaginal lactobacilli in clinical samples, (4) Beer/wine spoilage detection. Incubate anaerobically or in CO2 at 35-37°C for 48-72 hours.
Why is BHI used for blood culture bottles?
BHI is used in blood culture systems because: (1) Rich nutrient content supports fastidious bacteria causing septicemia (Streptococcus, Listeria, Haemophilus), (2) Supports organisms stressed by patient antibiotic therapy, (3) Contains growth factors (NAD, hemin) supporting auxotrophic organisms, (4) Low glucose (2 g/L) prevents acid production that would inhibit sensitive organisms, (5) Supports anaerobes when supplemented. Modern automated blood culture systems (BACTEC, BacT/Alert) use BHI-based media. Superior recovery compared to simpler media like TSB.
What is Brain Heart Infusion Broth used for?
Brain Heart Infusion (BHI) Broth is a highly nutritious general-purpose medium supporting growth of fastidious and non-fastidious organisms. Contains brain and heart infusion providing rich nutrients, peptones, and growth factors. Used for: (1) Cultivation of streptococci, pneumococci, meningococci, (2) Blood culture bottle enrichment, (3) Anaerobe cultivation (with supplements), (4) Sterility testing, (5) Preparation of bacterial suspensions for testing, (6) Revival of freeze-dried cultures. One of the most nutrient-rich media available. Supports wider variety of organisms than TSB or Nutrient Broth.
What is RCM used for?
Reinforced Clostridial Medium (RCM) is used for cultivation of Clostridium species and other anaerobes. Contains: (1) Peptones and beef extract - rich nutrients, (2) Yeast extract - growth factors, (3) Glucose - energy, (4) Sodium acetate and cysteine - reduce oxygen, (5) Starch and agar - O2 removal. Used for: (1) Clostridium perfringens enumeration in food, (2) C. difficile cultivation, (3) General anaerobe cultivation, (4) Spore studies. Can be used as broth or solidified with extra agar. Incubate anaerobically at 35-37°C.