Presentation is loading. Please wait.

Presentation is loading. Please wait.

Antimicrobial Susceptibility Testing – Part II

Similar presentations


Presentation on theme: "Antimicrobial Susceptibility Testing – Part II"— Presentation transcript:

1 Antimicrobial Susceptibility Testing – Part II
Karen Honeycutt, M.Ed., MT(ASCP)SM CLS 418 Clinical Microbiology Student Laboratory Session Microbiology

2 MIC: Microdilution Commercial Antibiotic at various concentrations
Antimicrobial Susceptibility Testing - MIC MIC: Microdilution Commercial Antibiotic at various concentrations Inoculate with standardized inoculum MIC Interpret (NCCLS) Microbiology

3 MIC: Microdilution Antimicrobial Susceptibility Testing - MIC MIC
Growth control Sterility control MIC ug/ml Ampicillin 8.0 16.0 32.0 >32.0 Piperacillin 64.0 128.0 Aztreonam <8.0 Microbiology

4 Antimicrobial Susceptibility Testing - MIC
MIC: Agar Dilution 24 hr inc. R = resistant control S = susceptible control A, B, C, D = isolates of S. aureus R = resistant control S = susceptible control A = resistant (MRSA) B, C, D = susceptible isolates of S. aureus Microbiology

5 Detection of MRSA Methicillin “R” S. aureus or multi-drug “R”
Antimicrobial Susceptibility Testing - MRSA Detection of MRSA Methicillin “R” S. aureus or multi-drug “R” Beta-lactamase susceptible penicillins: penicillin, ampicillin 95% of all S. aureus “R” to these penicillins Beta-lactamase resistant penicillins oxacillin, methicillin, nafcillin drug of choice to treat Staphylococcus infections Oxacillin is the representative drug tested for the penicillinase “R” penicillins Microbiology

6 Detection of MRSA If resistant to oxacillin, then MRSA
Antimicrobial Susceptibility Testing - MRSA Detection of MRSA If resistant to oxacillin, then MRSA MRSA is resistant to all penicillinase-resistant penicillins oxacillin, methicillin, nafcillin Also resistant to all beta-lactam antibiotics Drug of choice becomes vancomycin Microbiology

7 Detection of MRSA Heteroresistant colonies “R” strains grow best at:
Antimicrobial Susceptibility Testing - MRSA Detection of MRSA Heteroresistant colonies within one colony “S” & “R” strains “R” strains grow best at: 33 to 35° C 2-4% NaCl in media pH Use direct inoculum (for all Staphs) - don’t grow up to turbid suspension Microbiology

8 Detection of Vancomycin “R” Enterococci
Antimicrobial Susceptibility Testing - VRE Detection of Vancomycin “R” Enterococci Vancomycin “R” Enterococcus sp. (E. faecium) Very difficult to treat No standard treatment protocol Usually confirmed by 2 methods (MIC, agar dilution, K-B) Incubate a full 24 hours before interpretation is “S” (refer to manufacturer’s protocol) Microbiology

9 Susceptibility Testing: Enterococcus sp.
Antimicrobial Susceptibility Testing Susceptibility Testing: Enterococcus sp. Systemic infections (blood, body fluids, wounds, etc.) treat with cell wall active antibiotic and aminoglycoside Synergistic break down cell wall aminoglycoside to ribosome Microbiology

10 Susceptibility Testing: Enterococcus sp.
Antimicrobial Susceptibility Testing Susceptibility Testing: Enterococcus sp. Synergy Screen Test synergy with high level aminoglycoside gentamicin 500 ug/ml streptomycin 1000 ug/ml If “S” to penicillin and high level aminoglycoside then should be synergistic Penicillin “S” Streptomycin Synergy “S” Gentamicin Synergy “S” Microbiology

11 Susceptibility Testing: Enterococcus sp.
Antimicrobial Susceptibility Testing - VRE Susceptibility Testing: Enterococcus sp. Penicillin “S” Strep Syn “S” Gent Syn “R” Penicillin synergistic w/ Streptomycin Penicillin “R” Vancomycin “S” Strep Syn “S” Gent Syn “S” Vancomycin synergistic w/Streptomycin & Gentamcin Penicillin “R” Vancomycin “S” Strep Syn “R” Gent Syn “R” No synergistic response Microbiology

12 Streptococcus pneumoniae – screen for penicillin susceptibility
Antimicrobial Susceptibility Testing – S. pneumoniae Streptococcus pneumoniae – screen for penicillin susceptibility Detect resistance to penicillin Screen: K-B with oxacillin on MHA w/5% sheep blood < 20 mm zone = presumptive evidence of resistance to penicillin Microbiology

13 Antimicrobial Susceptibility Testing
Beta-lactamase Test Nitrocefin (yellow) changes color (red) when beta-lactam ring hydrolyzed Nitrocefin disc most sensitive method Reaction time varies Microbiology

14 Beta-lactamase Test Haemophilus influenzae Anaerobes (ID)
Antimicrobial Susceptibility Testing Beta-lactamase Test Haemophilus influenzae Anaerobes (ID) Staphylococcus sp. (if test “S” to penicillin, ampicillin) Microbiology

15 Detection of specific resistance mechanisms
Antimicrobial Susceptibility Testing Detection of specific resistance mechanisms MRSA: Detection of PBP2a – rapid latex agglutination test detects the altered protein encoded for by the mecA gene Genotypic methods – detection of genes or plasmids encoding for resistance at the molecular level Microbiology

16 Predictable Patterns If Gram-positive bacteria, then “S” to vancomycin
Antimicrobial Susceptibility Testing - Patterns Predictable Patterns If Gram-positive bacteria, then “S” to vancomycin If beta-hemolytic Streptococci, then “S” to penicillin Pg 7 of notes Microbiology


Download ppt "Antimicrobial Susceptibility Testing – Part II"

Similar presentations


Ads by Google