Path Micro Exam 2: Antimicrobial Testing
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38 terms
Terms | Definitions |
|---|---|
 What three important purposes do standardization serve? | 1) Optimize growth conditions so that inhibition of growth can be attributed to the antimicrobial agent and not other factors 2) Optimize conditions for antimicrobial integrity and activity so that failure to inhibit growth is attributed to the resistance mechanisms 3) Maintain reproducibility and consistency of results so that the same organism will produce same profile for all labs that perform the test |
| What is CLSI? | - Clinical and Laboratory Standards Institute - A nonprofit organization that sets recommendations, guidelines, or standards for all areas of the laboratory to improve quality of care |
| What components of antimicrobial susceptibility testing are standardized and controlled? | - Inoculum size (density) - Medium (pH, Cation concentration, Blood, serum components, Thymidine content) - Incubation (Atmosphere, Temperature, Duration - Antimicrobial concentrations - Quality control |
| What important factors that play key roles in patient outcome, are not taken into account by susceptibility testing? | - Antibiotic diffusion in tissues, host cells - Serum protein binding of antimicrobials - Drug interactions - Patients innate defenses, immune system - Multiple simultaneous illnesses - Virulence, pathogenicity of etiology - Site and severity of infection |
| What three general methods are used to detect and evaluate antimicrobial susceptibility? | - Direct measurement of antimicrobials against an isolate - Detection of a specific resistance mechanism in an isolate - Measurement of complex antimicrobial-isolate-interactions |
| Deciding which method to utilize to detect and evaluate antimicrobial susceptibility depends on what factors? | - Clinical need - Accuracy - Convenience |
| Direct measurement of antimicrobial activity is accomplished by... | - Conventional susceptibility testing methods such as broth dilution, agar dilution, and disk diffusion - Commercial susceptibility testing systems - Special screens and indicator tests |
| What are the two most important requirements for appropriate inoculum preparation? | - Use of a pure culture - A standardized inoculum |
| Standardized inoculum | 3-10 colonies from the primary plate (no mixed cultures), log phase (18-24h after incubation), incubate inoculated broth for four hours, standardized to match 0.5 McFarland (1.5x10⁸ CFU/mL) |
| 0.5 McFarland turbidity standard | - Composed of 1% sulfuric acid and 1.175% barium chloride - Provides an optical density comparable to the density of a bacterial suspension of 1.5x10⁸ colony forming units per mL |
| Antimicrobial battery or panel | - The antimicrobial agents that are chosed for testing against a particular bacterial isolate - The content and the application of a battery or panel is determined by the CLSI recommendations, pharmacy, and antimicrobial availability |
| Broth Dilution Susceptibility Testing Conditions | Test medium: Mueller-Hinton Inoculum size: 5x10⁵ CFU/mL Incubation cond: 35°C; air (except N. meningitidis. which needs 5-7% CO₂) Incubation duration: 20-24 hr |
| Minimum inhibitory concentration (MIC) | The lowest concentration of a particular antibiotic that visually inhibits the growth of bacteria |
| Interpretive categories of susceptibility tests | Sensitive (S) Intermediate (I) Resistant (R) |
| Microdilution | A dilution in which the total broth volume is 0.05 to 0.1 mL |
| Macrodilution | A dilution in which the broth volumes are 1 mL or greater |
| Agar Dilution Susceptibility Testing Conditions | Test medium: Mueller-Hinton agar (except N. gonorrhoeae which needs GC agar or MIC) Inoculum size: 1x10⁴CFU/mL Incubation cond: 35°C; air (except N. meningitidis which needs 5-7% CO₂) Incubation duration: 20-24 hr |
| What is a major advantage of agar dilution method vs. broth dilution method when testing N. gonorrhoeae? | The agar dilution method provides a means for determining MICs for N. gonorrhoeae which does not grow sufficiently in broth to be tested by broth dilutions |
| Disk Diffusion Method | - A method used to determine microbial sensitivity to antimicrobial agents in which antibiotic disks are placed on an inoculated Petri dish, incubated, and observed for inhibition of growth. - Zones of inhibition around each disk is measured in mm |
| Disk Diffusion Susceptibility Testing Conditions | Test medium: Mueller-Hinton agar Inoculum size: Swab from 1.5x10⁸ CFU/mL Incubation cond: 35°C; air (except N. gonorrhoeae, S. pheumoniae, and H. influenze which needs 5-7% CO₂) Incubation duration: 16-18 hr |
| Etest (AB BIODISK) | - A method that combines disk diffusion with MIC data - Uses plastic strips in which one side of strip has antimicrobial agent concentration gradient and other side has numeric scale that indicates drug concentration |
| Oxicillin agar screen | Detects staphylococcal resistance to penicillinase-resistant penicillins |
| Vancomycin agar screen | Detects enterococcal resistance to vancomycin |
| Aminoglycoside screen | Detects enterococcal high-level resistance to aminoglycosides that would compromise synergy with a cell wall-active agent such as ampicillin or vancomycin |
| Oxicillin disk screen | Detects streptococcus pneumoniae resistance to penicillin |
| "D" test | Differentiates clindamycin resistance in Staphyllococcus aureus |
| Beta-Lactamase Detection | - Chromogenic cehalosporinase test - A phenotypic method - Color development: positive - Useful for N. gonorrhoeae, Haemophilus influenza, staphyllococci |
| MBC | - Lowest concentration of drug that achieves 99.9% killing |
| Synergy | Activity of drug combination is greater than the activity of a single drug |
| Antagonism | Combined activity of is less than the activity of a single drug |
| Gentamicin class of antimicrobial | An aminoglycoside |
| Aminoglycoside mechanism of action | - Inhibits protein synthesis by binding to 30S ribosomal subunit which interferes with translation of mRNA - Bacteriostatic (inhibtis growth) - Used for both gram (+) and (-) spp - Anaerobic bacteria cannot take these agents up intracellularly so they are not inhibited by aminoglycosides. |
| Beta-lactam mechanism of action | - The four-membered nitrogen beta-lactam ring targets and Inhibits cell wall synthesis by binding enzymes involved in peptidoglycan production (binds to PBP's) - Popular due to bacterialcidal action (kills bacteria) and lack of toxicity to humans - Used for both gram positive and negative spp |
| Penicillin and ampicillin class of antimicrobial | Beta-lactam |
| Vancomycin and bacitracin class of antimicrobial | Glycopeptides |
| Glycopeptide mechanism of action | - Binds to the precursors of cell wall synthesis and block accesss of the building blocks to the transpeptidase (doesnt bind to PBP's) - Essentially inhibits bacterial cell wall synthesis - Cannot penetrate the outer membrane of gram negative spp - Used for gram positive antimicrobial therapy - Vanomycin most common (ingested) - Bacitracin is only topical (ointment) due to toxicity |
| Fluroquinolones | - Derivative of nalidixic acid - Interferes with DNA gyrase, which is essential DNA replication and transcription (bactericidal) - Broad spectrum of activity that includes Gram (+) and (-) - Gets chelated out when used with antacids |
| Rifampin | - Interferes with the production of mRNA - Does not penetrate outer membrane of Gram (-) so only used with Gram (+) |
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