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Microbiology Testing FAQs — Methods & Media | HiMedia USA
How does crystal violet prevent Proteus swarming?
Proteus species (P. mirabilis, P. vulgaris) produce urease and exhibit characteristic swarming motility on standard agar - rapidly spreading thin film that can cover entire plate obscuring other colonies. Crystal violet inhibits this swarming by: (1) Interfering with flagellar function, (2) Partially inhibiting growth rate, (3) Disrupting cell membrane processes. On MacConkey with increased crystal violet, Proteus produces discrete non-swarming colonies instead of spreading growth. This allows identification of other organisms (E. coli, Salmonella, Klebsiella) that would otherwise be obscured. Critical for clinical urine cultures where Proteus common.
What is MacConkey with Crystal Violet?
MacConkey Agar with Crystal Violet has increased crystal violet concentration for enhanced selectivity against gram-positive bacteria and swarming of Proteus species. Standard MacConkey contains 1mg/L crystal violet; this formulation has higher amounts. Benefits: (1) Better inhibition of gram-positives (especially from heavily contaminated samples), (2) Prevents Proteus swarming (which can obscure other colonies), (3) Cleaner plates for colony picking. Use when: heavy gram-positive contamination expected, Proteus swarming is problem, pharmaceutical/clinical samples. May slightly reduce recovery of some sensitive gram-negatives.
When should I use MacConkey No.3 vs regular MacConkey?
MacConkey No.3 (less selective) - use for: clinical specimens (especially with antibiotic therapy), pharmaceutical EM, stressed organisms, maximum recovery priority. Regular MacConkey (more selective) - use for: food testing, water coliforms, routine screening, when selectivity needed against gram-positives. No.3 may allow some gram-positive breakthrough but recovers more gram-negatives. For regulatory testing, check method specifications - many require standard MacConkey. No.3 beneficial when recovery of all gram-negatives more important than complete inhibition of gram-positives.
What is MacConkey Agar No.3?
MacConkey Agar No.3 is modified formulation with reduced crystal violet and bile salt concentrations compared to standard MacConkey. This less selective version improves recovery of stressed or injured gram-negative bacteria from clinical specimens and pharmaceutical samples. Still differentiates lactose fermenters (pink) from non-fermenters (colorless). Preferred when: (1) Testing clinical specimens with potential antibiotic therapy, (2) Pharmaceutical environmental monitoring (less harsh on environmental strains), (3) Water samples with stressed organisms. Use standard MacConkey for food/routine testing.
What is HiCrome UTI Agar?
HiCrome UTI Agar is chromogenic medium for rapid identification of urinary tract infection (UTI) pathogens from urine specimens. Different organisms produce characteristic colony colors: E. coli (pink/magenta), Klebsiella/Enterobacter (blue), Proteus (brown), Enterococcus (turquoise), Staphylococcus (white/cream), Pseudomonas (cream/colorless), Candida (white). Allows simultaneous detection and differentiation in mixed cultures. Enables presumptive identification in 24 hours vs 48-72 hours with traditional methods. Used in clinical microbiology for urine culture. Reduces need for biochemical confirmation in many cases.
What is TAMC testing in cannabis?
Total Aerobic Microbial Count (TAMC) is required testing in cannabis regulations. Measures total viable aerobic bacteria in product. Method: (1) Homogenize 1g cannabis in 9mL diluent, (2) Serial dilutions, (3) Plate on Tryptone Soya Agar (TSA), (4) Incubate 30-35°C for 48-72 hours, (5) Count all colonies, report CFU/g. State limits vary: California flower <100,000 CFU/g, concentrates <10,000 CFU/g. High TAMC indicates contamination or poor cultivation practices. TSA is standard medium for TAMC - general purpose, supports diverse organisms.
What is Slanetz and Bartley Medium used for?
Slanetz and Bartley Medium is selective medium for enumeration of fecal streptococci/enterococci in water by membrane filtration per ISO 7899-2 and EPA Method 1600. Contains sodium azide and crystal violet inhibiting gram-negative bacteria. Enterococci produce maroon/dark red colonies after 44 hours at 35-37°C. Used for: (1) Recreational water monitoring, (2) Drinking water testing, (3) Fecal pollution indicators. Enterococci more stable in seawater than E. coli, better indicator for marine recreational water. Confirm colonies on Bile Esculin Azide Agar.
What is Lactose Broth used for?
Lactose Broth is used for presumptive coliform testing in food and water by Multiple Tube (MPN) method. Contains: lactose (fermentable sugar), peptone (nutrients), Durham tube (inverted tube to trap gas). Procedure: (1) Inoculate dilutions into lactose broth tubes, (2) Incubate 24-48 hours at 35°C, (3) Gas production (visible in Durham tube) = presumptive positive for coliforms, (4) Confirm positives with Brilliant Green Bile Broth or EMB/MacConkey. MPN tables used to estimate coliform count from pattern of positive tubes. Classic water testing method, largely replaced by membrane filtration.
What is CIN Agar used for?
CIN Agar is selective differential medium for isolation of Yersinia enterocolitica from food and clinical specimens. Contains three antibiotics: Cefsulodin - inhibits Pseudomonas, Irgasan (triclosan) - inhibits gram-positives, Novobiocin - inhibits Proteus. Y. enterocolitica produces bull's-eye colonies (dark red center with transparent border) due to mannitol fermentation. Cold enrichment (4°C for 3 weeks) prior to plating enhances recovery from food. Incubate 24-48 hours at 28-30°C (not 35-37°C). Used for outbreak investigations, pork testing. The composition and performance criteria of this medium are as per the specifications laid down in ISO 10273:2017(E)
Why is SS Agar only used for clinical specimens?
SS Agar is highly selective due to high bile salt and brilliant green concentrations. Good for clinical specimens (where Salmonella/Shigella are primary targets and heavy contamination with normal flora). TOO inhibitory for food testing: (1) May suppress stressed or injured Salmonella from food processing, (2) May inhibit Salmonella serovars, (3) Not FDA BAM approved for food testing. For food → use XLD or HE Agar (less inhibitory, better Salmonella recovery). For stool/rectal swabs → SS Agar excellent choice. Choose media selectivity based on sample type.
What is SS Agar used for?
Salmonella-Shigella (SS) Agar is a highly selective differential medium for isolation of Salmonella and Shigella from clinical specimens (stool, rectal swabs). Contains: (1) Bile salts and brilliant green - strongly inhibit gram-positives and most coliforms, (2) Thiosulfate and ferric citrate - detect H2S, (3) Lactose - differentiate fermenters. Salmonella: colorless/transparent colonies with black centers. Shigella: colorless colonies without black centers. Lactose fermenters (coliforms): pink/red colonies but largely inhibited. Too inhibitory for food samples - use XLD or HE instead.
What is HiCrome MRSA Agar used for?
HiCrome MRSA Agar is selective chromogenic medium for detection of methicillin-resistant Staphylococcus aureus (MRSA) from nasal swabs and other clinical specimens. Contains: (1) Cefoxitin - selects for methicillin resistance, (2) Chromogenic substrate - produces distinctive green/teal colonies for S. aureus. Only MRSA grows. MRSA screening applications: (1) Hospital admission surveillance, (2) Healthcare worker screening, (3) Contact tracing in outbreaks, (4) ICU/high-risk unit monitoring. Direct plating method - results in 18-24 hours vs 3-4 days for traditional culture. Critical for infection prevention.
What is HiCrome ESBL Agar used for?
HiCrome ESBL Agar is selective chromogenic medium for detection of Extended-Spectrum Beta-Lactamase (ESBL) producing Enterobacteriaceae. Contains cefpodoxime (3rd generation cephalosporin) - only ESBL-producing organisms resistant to it can grow. Different organisms produce different colors: E. coli (pink), Klebsiella (blue), Enterobacter (blue-green). Used for: (1) ESBL surveillance screening, (2) Infection control in hospitals, (3) Outbreak investigations, (4) Rectal/stool screening for ESBL carriers. Growing public health concern - ESBL organisms resistant to many antibiotics, important for infection prevention programs.
What is Bile Esculin Agar used for?
Bile Esculin Agar tests esculin hydrolysis in presence of bile - characteristic of Group D Streptococcus and Enterococcus. Contains: (1) Bile (4%) - inhibits most bacteria, (2) Esculin - hydrolyzed by Group D Strep/Enterococcus, (3) Ferric citrate - reacts with esculetin (esculin breakdown product) producing black color. Positive: growth + black (or brown halo). Used for: (1) Enterococcus identification, (2) Probiotic culture verification (E. faecium common probiotic), (3) Group D Strep testing. Inoculate slant or stab, incubate 48h at 35°C. For differential isolation and presumptive identification of group D Streptococci in food and other products.
What is Cetrimide Agar used for?
Cetrimide Agar is a selective medium for isolation of Pseudomonas aeruginosa from clinical, pharmaceutical, and water samples. Cetrimide (quaternary ammonium compound) selectively inhibits most bacteria except Pseudomonas species. P. aeruginosa produces: (1) Green-blue pigment (pyocyanin), (2) Fluorescent yellow-green pigment under UV (pyoverdin), (3) Sweet grape-like odor. Used for: (1) Burn wound infections, (2) Pharmaceutical water testing, (3) USP <61> identification, (4) Environmental monitoring. Incubate 24-48 hours at 35-37°C. View under UV (365nm) to confirm fluorescence. In accordance with the microbial limit testing by harmonized methodology of USP/EP/BP/JP/IP.
What is Nutrient Broth used for?
Nutrient Broth is simple liquid medium for cultivation of non-fastidious bacteria. Contains beef extract and peptone in water. Used for: (1) General bacterial cultivation, (2) Biomass production for testing, (3) Inoculum preparation, (4) Enrichment (non-selective), (5) Maintaining stock cultures, (6) Teaching laboratories. Less nutrient-rich than TSB or BHI. Sufficient for robust organisms (E. coli, Bacillus, Pseudomonas) but may not support fastidious organisms. Incubate at organism's optimal temperature with shaking if desired for aeration.
When should I use SDA vs PDA for fungal cultivation?
Both support fungi, but different applications: SDA (pH 5.6, high dextrose) - preferred for pathogenic fungi in clinical mycology, better for yeast isolation, pharmaceutical USP testing, defined formulation for consistency. PDA (pH 5.6, potato infusion) - preferred for plant pathogenic fungi, food mycology, general environmental molds, complex nutrients support sporulation. For clinical specimens → SDA. For cannabis/food TYMC → PDA. For pharmaceutical microbial limits → either acceptable but SDA more standardized. Both should be supplemented with antibacterial agents when testing heavily contaminated samples.
What is Sabouraud Dextrose Agar used for?
Sabouraud Dextrose Agar (SDA) is the standard medium for cultivation of yeasts and molds, particularly pathogenic fungi. Low pH (5.6) and high dextrose content favor fungal growth while inhibiting most bacteria. Used for: (1) Clinical mycology - fungal infections, (2) Pharmaceutical microbial limits (USP <61>/<62>), (3) Environmental monitoring for fungi, (4) Food mycology, (5) Antifungal susceptibility testing. Add chloramphenicol (antibacterial) when bacterial contamination expected. Incubate 25-28°C for 5-7 days (extend to 30 days for slow-growing dimorphic fungi).
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 Mueller-Hinton Agar used for?
Mueller-Hinton (MH) Agar is the standard medium for antimicrobial susceptibility testing (AST) by disk diffusion (Kirby-Bauer) method per CLSI guidelines. Formulation optimized for: (1) Reproducible results, (2) Consistent diffusion of antibiotics, (3) Minimal interference with antibiotics, (4) pH 7.2-7.4 (critical for some antibiotics). MH supports most clinically significant bacteria (not fastidious). For fastidious organisms, use MH + 5% sheep blood (for Streptococcus) or MH + blood + NAD/hemin (for Haemophilus). Plate depth must be 4mm (25mL per 100mm plate).
What is Todd Hewitt Broth?
Enriched broth for streptococci, especially Group B Strep screening. Used in prenatal GBS screening (vaginal-rectal swabs). Incubate with antibiotics for selective GBS enrichment. For cultivation of group A haemolytic Streptococci used for serological studies.
What is Heart Infusion Broth?
Enriched broth for fastidious organisms, particularly streptococci and pneumococci. Simpler than BHI. Used for blood culture systems, bacterial cultivation, preparing inocula. For cultivation of a wide variety of fastidious organisms.
What is Bordet-Gengou Agar used for?
Bordet-Gengou Agar is selective medium for Bordetella pertussis isolation from nasopharyngeal swabs. Contains potato infusion, glycerol, and sheep blood. B. pertussis produces small, smooth, pearl-like colonies ("mercury drops"). Incubate 3-7 days in humidified incubator. Largely replaced by Regan-Lowe charcoal agar or PCR, but still used in some labs. For detection and isolation of Bordetella pertussis and Bordetella parapertussis.
What is Charcoal Agar?
Charcoal Agar contains activated charcoal absorbing toxic metabolites and free radicals, improving recovery of fastidious organisms. Used for Bordetella pertussis (whooping cough), Francisella, other fastidious pathogens. Charcoal provides dark background making colony visualization easier. Similar concept to BCYE for Legionella. For cultivation of Bordetella pertussis, for vaccine production and also for stock culture maintenance.
What is Columbia Blood Agar Base?
Columbia Blood Agar Base is enriched base medium for blood agar preparation. More nutrient-rich than standard Blood Agar Base. Contains peptones, cornstarch, and sodium chloride. When supplemented with 5-7% sheep blood, supports fastidious organisms: Streptococcus (including S. pneumoniae), Haemophilus (with X+V factors supplement), Corynebacterium, Neisseria. Shows clearer hemolysis patterns than other blood agar bases. Preferred for: (1) Clinical specimens, (2) Throat cultures, (3) Wound cultures, (4) Antimicrobial susceptibility testing. Cornstarch neutralizes toxic metabolites improving organism recovery. The composition and performance criteria of this medium are as per the specifications laid down in ISO 10272-2:2017.
What is Cooked Meat Medium used for?
Cooked Meat Medium is enriched liquid medium for cultivation of anaerobic bacteria, particularly Clostridium species. Contains: (1) Cooked meat particles - provide reducing environment and nutrients, (2) Peptone, yeast extract - growth factors, (3) Glucose - energy. The meat particles: (a) Create anaerobic conditions, (b) Provide hemin and vitamins, (c) Absorb toxic metabolites. Used for: (1) Clostridium isolation (C. perfringens, C. botulinum), (2) Anaerobe culture, (3) Sterility testing of biologicals (historical), (4) Maintaining anaerobic stock cultures. Turbidity or meat digestion indicates growth.