Antimicrobial Susceptibility Testing
AST STANDARDIZATION
- McFarland Turbidity Standards
- 1% H2SO4
- 1.175% BaCl2
- 1% H2SO4
- 0.5 McFarland – most commonly used
- Growth Medium
- pH – 7.2 – 7.4
- cation concentration
- blood and serum components
- thymidine content
- pH – 7.2 – 7.4
| Testing Medium for Different Organism | |||
| Organism | Media | Incubation | Notes |
| Enterobacteriace ae P. aeruginosa Enterococci | Mueller Hinton | 35°C; air | |
| Staphylococi | MH w/ 2% NaCl | MH w/ 2% NaCl | |
| Streptococcus | MHA w/ 5% Sheep’s Blood | 35°C | Not needed for S. pneumoniae |
| H. influenzae | HTM | 20 – 24 hrs | |
| N. meningitidis | MH w/ 2-5% lysed horse blood | 35°C; 5- 7% CO2; 24 hrs | |
| N. gonorrhoeae | GC agar + supplements | 35°C; 5% CO2; 24 hrs | No broth dilution |
| Anaerobes | Brucella BA w/ Hemin | Anaerobic, 35- 37°C; 48 hrs | Broth dilution: 1 x 10^6 cfu/ml Agar Dilution: 1 x 10^5 cfu/spot |
| AUTOMATED ANTIMICROBIAL SUSCEPTIBILITY SYSTEMS | |
| Vitek 2 |
|
| MicroScan WalkAway |
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| Phoenix System |
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| SUPPLEMENTAL METHODS FOR ANTIMICROBIAL RESISTANCE DETECTION | |
| Oxacillin Agar screen | For staphylococcal resistance to penicillinase-resistant penicillin |
| Vancomycin agar screen | For enterococcal resistance to vancomycin |
| Aminoglycoside screens | For acquired enterococcal high level resistance to aminoglycosides that would compromise synergy with a cell wall-active agent |
| Oxacillin disk screen | For streptococcus resistance among S. aureus resulting from efflux |
| Cefoxitin disk test 30 ug | To improve detection of oxacillin resistant CoNS |
| Aminoglycosides | For serious enterococcal infections, and acquired high-level resistance |
