Antibiotic Susceptibility Testing Methods
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Questions and Answers

What is the purpose of measuring the minimum inhibitory concentration (MIC) in phenotypic methods?

  • To identify the presence of antibiotic resistance genes
  • To quantify the bacterial growth rate under different conditions
  • To determine the specific antibiotic concentration needed to inhibit bacterial growth (correct)
  • To evaluate the overall health of the bacterial culture

    • How is the disk diffusion method for antibiotic susceptibility testing conducted?

  • Bacterial colonies are physically removed and placed on antibiotic-infused agar
  • Bacteria are mixed with antibiotics and then plated directly onto growth medium
  • Bacteria are spread on an agar plate, and disks containing antibiotics are placed on it (correct)
  • Bacteria are cultured in broth and disks are submerged in the solution

    • What distinguishes the Etest from other phenotypic methods?

  • It detects antibiotic resistance through visual colony morphology
  • It employs nitrocellulose strips with a continuous gradient of antibiotic (correct)
  • It directly measures RNA presence in bacteria
  • It uses colored indicators to measure bacterial growth

    • In which scenario would genotypic methods of antibiotic susceptibility testing be preferred over phenotypic methods?

    <p>When quick results are required and bacterial growth is not feasible (B)</p> Signup and view all the answers

    What is the MIC range indicating susceptibility for antibiotics when using broth microdilution?

    <p>&lt; 2 µg/mL (B)</p> Signup and view all the answers

    What is one limitation of the disk diffusion method?

    <p>It cannot differentiate between intermediate resistance and susceptibility (D)</p> Signup and view all the answers

    Which antibiotic resistance gene is associated with vancomycin-resistant Enterococcus (VRE)?

    <p>vanA (C)</p> Signup and view all the answers

    What is a common feature of automated systems used in antibiotic susceptibility testing?

    <p>They utilize multiwell plates to accommodate multiple antibiotics (A)</p> Signup and view all the answers

    What does the term 'intermediate' indicate in the context of MIC interpretation?

    <p>The antibiotic may be effective at higher doses (D)</p> Signup and view all the answers

    What was the initial antibiotic treatment modification for the patient with Staphylococcus aureus?

    <p>Narrowed coverage to cefazolin (A)</p> Signup and view all the answers

    Which organism was identified from the wound drainage in the second case study?

    <p>Pseudomonas aeruginosa (A)</p> Signup and view all the answers

    What resistance gene was detected in the second patient study using PCR?

    <p>IMP gene for beta-lactam resistance (D)</p> Signup and view all the answers

    What was the antibiotic class used for empiric coverage against Gram-negative bacteria in case study #2?

    <p>Cephalosporins (D)</p> Signup and view all the answers

    What precaution should be taken when handling a patient with a multi-drug resistant organism?

    <p>Contact precautions (B)</p> Signup and view all the answers

    How long was the first patient treated for Staphylococcus aureus infection?

    <p>12 weeks (B)</p> Signup and view all the answers

    Which specific antibiotic was added to enhance biofilm penetration at the infected hardware site?

    <p>Rifampin (C)</p> Signup and view all the answers

    What clinical issue did the patient in case study #2 experience despite multiple antibiotic courses?

    <p>Wound discharge oozing (D)</p> Signup and view all the answers

    What was the Gram-staining result for the organism from the first case study?

    <p>Gram-positive cocci in clusters (D)</p> Signup and view all the answers

    What type of organism was identified as a Washington state Notifiable condition in case study #2?

    <p>Imipenem-resistant Pseudomonas aeruginosa (A)</p> Signup and view all the answers

    What is a challenge associated with genotypic methods of antibiotic susceptibility testing?

    <p>They do not accurately predict phenotypic resistance. (B)</p> Signup and view all the answers

    What key factor affects the choice of method for antibiotic susceptibility testing?

    <p>Turnaround time, accuracy, and cost. (D)</p> Signup and view all the answers

    Which statement accurately describes the role of microbiology laboratories in antibiotic treatment planning?

    <p>They provide historical susceptibility data to guide initial empiric therapy. (C)</p> Signup and view all the answers

    Which of the following is an important consideration in therapeutic drug monitoring?

    <p>Drug interactions and toxic side effects. (D)</p> Signup and view all the answers

    Why are standardization and strict quality control standards important in AST?

    <p>They ensure reproducibility between labs and methods. (B)</p> Signup and view all the answers

    What does a positive result for PBP2a indicate in antibiotic susceptibility testing?

    <p>The organism is resistant to methicillin (MRSA). (C)</p> Signup and view all the answers

    In what scenario might antibiotics not be necessary for the treatment of an infection?

    <p>When the patient does not have a bacterial infection. (C)</p> Signup and view all the answers

    What is a primary reason for initiating therapy with broad-spectrum antibiotics?

    <p>When the causative organism remains unidentified during initial evaluation. (C)</p> Signup and view all the answers

    Which of the following factors does NOT play a role in determining the zone size interpretation criteria for AST?

    <p>The cost of the antibiotics. (A)</p> Signup and view all the answers

    What is the importance of local antibiograms in the selection of empiric therapy?

    <p>They track historical susceptibility and guide treatment decisions. (A)</p> Signup and view all the answers

    What is a consequence of overtreatment with antibiotics?

    <p>Development of resistant infections (C)</p> Signup and view all the answers

    Which of the following is NOT a major driver of antimicrobial resistance?

    <p>Access to clean drinking water (D)</p> Signup and view all the answers

    What does antimicrobial stewardship aim to improve?

    <p>Safety and outcomes for individual patients (D)</p> Signup and view all the answers

    What is a recommended strategy for addressing antibiotic resistance?

    <p>De-escalating antibiotics when appropriate (C)</p> Signup and view all the answers

    Which factor can contribute to the complexity of drug discovery in antibiotics?

    <p>Regulatory obstacles and low financial incentives (C)</p> Signup and view all the answers

    What is an example of a policy that aims to mitigate antimicrobial resistance?

    <p>Implementing formal stewardship programs (C)</p> Signup and view all the answers

    Which of the following is a key component of infection prevention and control programs?

    <p>Tracking patient-to-patient transmissions (D)</p> Signup and view all the answers

    What might the future hold for antibiotic development?

    <p>Obstacles such as climate change impacting development (D)</p> Signup and view all the answers

    Which of the following describes a common misconception about antibiotic prescriptions?

    <p>All infections require antibiotic treatment (A)</p> Signup and view all the answers

    What is a key consideration in deciding if antibiotics are needed?

    <p>Determining if it is a bacterial infection (A)</p> Signup and view all the answers

    Flashcards

    Staphylococcus aureus

    A Gram-positive bacterium known for causing infections.

    MSSA

    Methicillin-Sensitive Staphylococcus aureus, responsive to methicillin.

    IV Antibiotics

    Intravenous medications used to treat infections.

    Antibiotic Stewardship

    Optimizing antibiotic use to combat resistance.

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    Cefazolin

    A cephalosporin antibiotic used for Gram-positive infections.

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    Pseudomonas aeruginosa

    A multi-drug resistant bacteria often found in wound infections.

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    Contact Precautions

    Infection control measures to prevent spread.

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    IMP Gene

    A gene indicating resistance to certain antibiotics.

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    Multi-Drug Resistant Organism

    Bacteria resistant to multiple antibiotics.

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    Gram-positive cocci

    Bacteria that retain the crystal violet dye and appear spherical.

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    Antibiotic susceptibility testing (AST)

    A method to determine which antibiotics can effectively treat specific bacterial infections.

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    Genotypic methods

    Techniques that identify genetic resistance markers in bacteria.

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    Phenotypic resistance

    Resistance observable in bacteria based on their visible characteristics and behavior.

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    Minimum inhibitory concentration (MIC)

    Lowest concentration of an antibiotic that prevents bacterial growth.

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    Lateral flow immunochromatographic assay

    An antibody-based test for rapid detection of bacterial proteins.

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    Standardization in AST

    Ensuring consistent testing conditions and criteria across laboratories.

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    Therapeutic drug monitoring

    The measurement of drug levels to ensure effective treatment and prevent toxicity.

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    De-escalation of therapy

    Reducing antibiotic spectrum once the pathogen is identified.

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    Local antibiograms

    Reports detailing antibiotic susceptibility patterns in specific local pathogens.

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    Source control

    The process of removing or addressing the infection’s source, such as draining an abscess.

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    Broth Microdilution

    A method where doubling dilutions of antibiotic are added to wells with bacteria to determine MIC.

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    Disk Diffusion

    A method where antibiotic-soaked disks are placed on a bacterial lawn to observe inhibition zones.

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    Zone of Inhibition

    The clear area around antibiotic disks where bacteria do not grow, indicating susceptibility.

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    Gradient Strips (Etests)

    Strips with a gradient of antibiotic placed on bacteria to determine MIC based on growth inhibition.

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    PCR

    A technique to amplify DNA segments to detect resistant genes in bacteria.

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    Interpretation of Results

    Assessing susceptibility (S), intermediate (I), or resistance (R) based on MIC or zone size.

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    Antimicrobial Stewardship

    A program that promotes the rational use of antibiotics to improve patient outcomes and reduce resistance.

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    Drivers of Antimicrobial Resistance

    Factors contributing to the increased prevalence of resistant bacteria, including misuse of antibiotics.

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    Hospital Infection Control

    Programs aiming to prevent healthcare-associated infections through protocols and safety measures.

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    Symptoms of Spinal Osteomyelitis

    Symptoms often include back pain, fever, and neurological issues post-surgery.

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    Common Misdiagnoses

    Conditions such as viral infections, malignancies, or autoimmune issues that can mimic bacterial infections.

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    Overtreatment with Antibiotics

    Excessive use of antibiotics that disrupts normal flora and promotes infection by resistant organisms.

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    CDC Core Elements of Stewardship

    Guidelines for hospitals on improving antibiotic use through education, tracking, and interventions.

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    VRE and MRSA Screening

    Testing for vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus in healthcare settings.

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    Advanced Diagnostic Technology

    New technologies that enhance the identification and management of antimicrobial infections.

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    Phage Therapy

    A treatment approach using bacteriophages to target and destroy specific bacteria.

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    Study Notes

    Antibiotic Susceptibility Testing (AST) Methods

    • Phenotypic Methods: These methods involve observing the growth of the organism when exposed to antibiotics.
      • Broth Microdilution: Doubling dilutions of an antibiotic are placed in individual wells of a microtiter plate. A standard bacterial inoculum is added to each well. The plate is incubated and examined for growth. The minimum inhibitory concentration (MIC) is the lowest antibiotic concentration that inhibits growth. The MIC is interpreted according to organism-specific and drug-specific criteria, typically categorized as Susceptible (S), Intermediate (I), or Resistant (R).
        • Automated, high-throughput formats assist with clinical testing by using multiwell plates with indicators for bacterial growth. Instruments automate the reading process.
      • Gradient Strips (Etests): Bacteria form a lawn on an agar plate. Strips containing a gradient of the antibiotic are placed on the plate. The minimum inhibitory concentration (MIC) is determined by where the elliptical zone of growth inhibition intersects with the strip.
      • Disk Diffusion: Bacteria form a lawn on the plate. Antibiotic-containing disks are placed on the lawn. The antibiotic diffuses into the agar. The zone of inhibition is measured. A larger zone indicates susceptibility to the antibiotic. Conversely, a small or nonexistent zone suggests resistance.

    Genotypic Methods

    • These methods directly identify resistance genes.
      • Polymerase Chain Reaction (PCR) with sequencing: PCR amplifies specific resistance genes. Sequencing identifies the presence or absence of these genes. This method correlates well with phenotypic results (observed susceptibility). This assists with identifying resistances like vancomycin-resistant Enterococcus (VRE) – vanA or vanB genes, methicillin-resistant Staphylococcus aureus (MRSA) – mecA gene, or rifampin-resistant Mycobacterium tuberculosis – rpoB gene mutations.
      • Genotypic methods sometimes struggle to predict MIC (minimum inhibitory concentration) and phenotypic resistance. Gram-negative bacteria, in particular, often have several complex resistance mechanisms, including alterations in membrane permeability, making genotype-phenotype correlation less straightforward.

    Other Methods

    • Antibody-Based Detection: Rapid detection of resistance factors, like PBP2a from Staphylococcus aureus (mecA gene product), can be performed via lateral flow immunoassays.
    • Advanced Diagnostics: Digital microscopy combined with machine learning may correlate bacterial growth patterns with susceptibility or resistance characteristics.

    Selecting the Appropriate Method

    • Factors to Consider: Turnaround time, throughput, accuracy, cost, and desired information (e.g., MIC).

    Standardization

    • Critical for Accurate AST: The FDA, CLSI (Clinical and Laboratory Standards Institute), and the European Committee on Antimicrobial Susceptibility Testing determine standards for interpretative MIC and zone size criteria. Reproducibility between labs requires strict quality control standards and clearly defined guidelines for media, drug concentration, incubation, and other variables. Ranges are developed through pharmacodynamics, site of infection, clinical outcome studies, and distributions of wild-type MICs.

    Additional Concepts

    • Antibiotic Misuse: Inappropriate prescribing and incomplete adherence, agricultural overuse, and climate change negatively impact efficacy.
    • Antibiotic Stewardship: Rational and judicious use of antibiotics is essential to prevent resistance, improve patient outcomes, and maintain antibiotic effectiveness in the clinical setting and society. Infection prevention and control programs track hospital-associated infections and follow standard precautions to prevent transmission. Local antibiograms can guide prescribing, tracking historical susceptibility, but require caution due to differences in patients, and should be combined with appropriate data.
    • Drivers of Resistance: Resistance arises through environmental factors such as waste products from healthcare and manufacturing, misuse/overuse in agriculture and hospitals, poor water/sanitation conditions, and others.
    • Treatment Considerations: Broad-spectrum antibiotics are frequently used initially in unsure situations. Further treatment is targeted by laboratory results, de-escalating and elevating coverage as needed. Source control interventions may be necessary along with treatment to help with outcomes of infections. Consider alternative therapies or no antibiotic use, if warranted. Treatment might involve therapeutic drug monitoring, interactions with other drugs, side effects, route of administration, allergies, and infections involving multiple organisms/sites, especially for conditions within (and outside) the body as certain drugs are effective only at certain sites such as brain or lung treatment.

    Case Studies (Examples)

    • Case Study 1: A patient with spinal osteomyelitis was treated with broad-spectrum antibiotics initially, then modified to target the specific bacterial pathogen (Staphylococcus aureus) identified by laboratory testing.
    • Case Study 2: A patient with a multi-drug resistant Pseudomonas aeruginosa infection required targeted therapy, including susceptibility testing, as well as possibly contact precautions and other standard infection control practices.

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    Description

    Explore the various phenotypic methods used in antibiotic susceptibility testing. This quiz covers techniques such as broth microdilution and gradient strips (Etests), detailing their processes and significance in determining the efficacy of antibiotics. Test your knowledge on how these methods impact clinical testing and bacterial treatment strategies.

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