Skip to main content

Microbiology

Selection

Choosing the right media for specific specimen types and target organisms is a critical skill in clinical microbiology. The goal is to create an environment that supports the growth of potential pathogens while suppressing the growth of commensal flora, leading to accurate and timely diagnoses

Culture Media Selection: Tailoring Media to Specimen and Organism

  • What is Culture Media Selection?
    • Culture media selection is the process of choosing the appropriate type(s) of culture media to use for a specific clinical specimen based on the suspected pathogens and the characteristics of the specimen
  • Why is Culture Media Selection Important?
    • Optimal Growth Conditions: Selecting the right media ensures that the target microorganisms have the optimal growth conditions
    • Selective Isolation: Selective media can suppress the growth of normal flora, making it easier to isolate pathogens
    • Differential Identification: Differential media can help to differentiate between different types of microorganisms based on their growth characteristics
    • Accurate Results: Proper media selection improves the accuracy and reliability of culture results
    • Timely Diagnosis: Accurate and timely culture results can lead to faster diagnosis and treatment of infections
  • Factors to Consider When Selecting Culture Media
    • Specimen Type: The type of specimen (e.g., blood, urine, stool, sputum) will influence the choice of media
    • Suspected Pathogens: The suspected pathogens will determine the types of media that are most likely to support their growth
    • Normal Flora: The presence of normal flora in the specimen will influence the need for selective media
    • Patient History: The patient’s medical history, travel history, and antibiotic use may influence the choice of media
    • Laboratory Resources: The availability of specific media and the capabilities of the laboratory will also be factors

Media Selection by Specimen Type

Blood Cultures

  • Purpose: To detect and identify bacteria or fungi in the bloodstream
  • Media
    • Nutrient Broth: Tryptic Soy Broth (TSB) or Brain Heart Infusion (BHI) broth with anticoagulants (e.g., SPS)
    • Specialized Blood Culture Bottles: Contain additives to neutralize antibiotics and enhance microbial growth
  • Rationale
    • Blood is normally sterile, so any growth is significant
    • Nutrient broth supports the growth of a wide range of bacteria and fungi
    • Specialized bottles provide an optimal environment for microbial growth and detection
  • Considerations
    • Collect multiple blood cultures to increase the sensitivity of detection
    • Use aseptic technique to prevent contamination
    • Follow the manufacturer’s instructions for blood culture bottle use

Urine Cultures

  • Purpose: To detect and identify bacteria in urine, indicating a urinary tract infection (UTI)
  • Media
    • Blood Agar: For general growth and detection of hemolysis
    • MacConkey Agar: For selective growth of Gram-negative bacteria and differentiation of lactose fermenters
    • Cystine Lactose Electrolyte Deficient (CLED) Agar: An alternative to MacConkey agar that inhibits swarming of Proteus species
  • Rationale
    • Blood agar supports the growth of a wide range of bacteria
    • MacConkey agar selects for Gram-negative bacteria, which are common causes of UTIs
    • CLED agar provides good differentiation and inhibits swarming
  • Considerations
    • Collect a midstream clean catch urine sample to minimize contamination
    • Use a calibrated loop for quantitative plating
    • Interpret results based on colony count and species identification

Stool Cultures

  • Purpose: To detect and identify bacterial pathogens in stool, indicating gastroenteritis or other infections
  • Media
    • Blood Agar: For general growth and detection of hemolysis
    • MacConkey Agar: For selective growth of Gram-negative bacteria and differentiation of lactose fermenters
    • Hektoen Enteric (HE) Agar: For selective growth of Salmonella and Shigella
    • Xylose Lysine Deoxycholate (XLD) Agar: For selective growth of Salmonella and Shigella
    • Campylobacter Agar: For selective growth of Campylobacter species
    • Cefsulodin-Irgasan-Novobiocin (CIN) Agar: For selective growth of Yersinia enterocolitica
  • Rationale
    • MacConkey, HE, and XLD agars select for enteric pathogens while inhibiting normal flora
    • Campylobacter agar selects for Campylobacter species, which require specific growth conditions
    • CIN agar selects for Yersinia enterocolitica, which can cause gastroenteritis
  • Considerations
    • Collect stool samples early in the course of illness
    • Use transport media to preserve the viability of pathogens
    • Consider testing for Clostridium difficile toxins if antibiotic-associated diarrhea is suspected

Respiratory Cultures (Sputum, Throat Swabs, Nasal Swabs)

  • Purpose: To detect and identify bacterial or viral pathogens in respiratory specimens
  • Media (Sputum)
    • Blood Agar: For general growth and detection of hemolysis
    • Chocolate Agar: For growth of Haemophilus and Neisseria species
    • MacConkey Agar: For selective growth of Gram-negative bacteria
    • Selective Agar for Pseudomonas: e.g., Cetrimide agar
  • Media (Throat Swabs)
    • Blood Agar: For detection of Streptococcus pyogenes (Group A Strep)
    • Selective Strep Agar: e.g., Selective Strep Agar (SSA)
  • Rationale
    • Blood and chocolate agars support the growth of common respiratory pathogens
    • MacConkey agar selects for Gram-negative bacteria that may cause pneumonia
    • Selective media can help to isolate specific pathogens from the mixed flora of the upper respiratory tract
  • Considerations
    • Collect sputum samples from the lower respiratory tract, avoiding saliva
    • Use rapid antigen tests for common respiratory viruses and Streptococcus pyogenes
    • Consider testing for Mycobacterium tuberculosis in patients with suspected TB

Wound Cultures

  • Purpose: To detect and identify bacterial or fungal pathogens in wound specimens
  • Media
    • Blood Agar: For general growth and detection of hemolysis
    • MacConkey Agar: For selective growth of Gram-negative bacteria
    • Columbia CNA Agar with 5% Sheep Blood: For selective growth of Gram-positive bacteria
    • Thioglycollate Broth: For detection of anaerobic bacteria
    • Sabouraud Dextrose Agar: For selective growth of fungi
  • Rationale
    • Blood and MacConkey agars support the growth of common wound pathogens
    • CNA agar selects for Gram-positive bacteria, which are common in skin and soft tissue infections
    • Thioglycollate broth creates an anaerobic environment for the detection of anaerobes
    • Sabouraud dextrose agar selects for fungi
  • Considerations
    • Collect wound samples from the base of the wound after debridement
    • Use appropriate transport media to preserve the viability of pathogens
    • Consider testing for antibiotic resistance genes in Staphylococcus aureus

Cerebrospinal Fluid (CSF) Cultures

  • Purpose: To detect and identify bacterial, fungal, or viral pathogens in CSF, indicating meningitis or encephalitis
  • Media
    • Blood Agar: For general growth and detection of hemolysis
    • Chocolate Agar: For growth of Haemophilus and Neisseria species
    • Thioglycollate Broth: For detection of anaerobic bacteria
    • Sabouraud Dextrose Agar: For selective growth of fungi
  • Rationale
    • Blood and chocolate agars support the growth of common meningitis pathogens
    • Thioglycollate broth creates an anaerobic environment for the detection of anaerobes
    • Sabouraud dextrose agar selects for fungi
  • Considerations
    • Treat CSF as a STAT specimen and process immediately
    • Perform Gram stain and cell count on CSF
    • Consider testing for viral pathogens using PCR

Media Selection by Organism

Organism Preferred Media Rationale
Escherichia coli MacConkey Agar, EMB Agar Selective and differential for Gram-negative lactose fermenters
Staphylococcus aureus Mannitol Salt Agar (MSA), Blood Agar Selective for Staphylococcus, differential for mannitol fermentation; general growth and hemolysis
Streptococcus pyogenes Blood Agar Enriched for growth and differential for beta-hemolysis
Pseudomonas aeruginosa MacConkey Agar, Cetrimide Agar Selective for Gram-negative, can grow at higher temperatures; selective for Pseudomonas
Salmonella and Shigella Hektoen Enteric (HE) Agar, Xylose Lysine Deoxycholate (XLD) Agar, MacConkey Agar Selective and differential for enteric pathogens, inhibiting normal flora
Campylobacter jejuni Campylobacter Agar Selective for Campylobacter, requires specific atmospheric conditions
Neisseria gonorrhoeae Chocolate Agar, Modified Thayer-Martin Agar Enriched for growth, requires specific growth factors; selective for Neisseria
Haemophilus influenzae Chocolate Agar Enriched for growth, requires specific growth factors (hemin and NAD)
Mycobacterium tuberculosis Lowenstein-Jensen (LJ) Agar, Middlebrook 7H10 Agar Enriched and selective for Mycobacterium, contains inhibitors to suppress other bacteria
Candida albicans Sabouraud Dextrose Agar Selective for fungi, high sugar concentration inhibits bacterial growth

Key Considerations and Best Practices

  • Standard Operating Procedures (SOPs): Develop and follow detailed SOPs for media selection based on specimen type and suspected pathogens
  • Training: Ensure that all laboratory personnel are properly trained in media selection and culture techniques
  • Quality Control: Perform quality control testing on all media to ensure proper performance
  • Storage: Store media according to the manufacturer’s instructions
  • Expiration Date: Use media before the expiration date
  • Proper Technique: Use proper technique when inoculating and handling media
  • Labeling: Label all media clearly and accurately
  • Documentation: Document all media selection and culture procedures

Key Terms

  • Selective Media: Media that inhibit the growth of certain microorganisms while allowing others to grow
  • Differential Media: Media that allow different types of microorganisms to be distinguished based on their appearance or growth characteristics
  • Enriched Media: Media that contain specific nutrients or growth factors required by fastidious microorganisms
  • Fastidious: Having complex nutritional requirements
  • Normal Flora: Microorganisms that normally reside in or on the body
  • Pathogen: A microorganism that can cause disease
  • Aseptic Technique: A set of practices and procedures performed under carefully controlled conditions with the goal of minimizing contamination by pathogens
  • Quality Control: A set of procedures designed to ensure the accuracy and reliability of laboratory test results
  • Standard Operating Procedure (SOP): A detailed written instruction to achieve uniformity of the performance of a specific function