Additional tests for testing for Antimicrobial resistance

1. Supplemental testing methods
Screening agar
Agar contains known conc. of antibiotic
Growth on agar indicates resistance
Oxacillin screening agar: 6 g/ml oxacillin
Screening of staphylococci
Vancomycin screening agar: 6 g/ml vanco
Screening of enterococci and staphylococci

3. Supplemental testing methods
Predictor drugs
Staphylococci R to Oxacillin =
R to penicillins, cephalosporins, and imipenem
High level gentimicin R in enterococci =
R to all currently available aminoglycosides
Ampicillin R in enterococci =
R to all penicillin derivatives and imipenem

4. Direct detection of resistance mechanisms
Beta-lactamase (phenotypic)
Chromogenic substrate incorporated into disk
- color change in presence of enzyme
Usefulness is limited:
Penicillin R in Neisseria gonorrhea
Ampicillin R in Haemophilus influenzae
Penicillin R in anaerobes

5. Direct detection of resistance mechanisms
Extended spectrum beta-lactamase
Mutations in plasmid-encoded beta-lactamases
- hydrolyze extended spectrum cephalosporins
and aztreonam
- more than 100 types have been identified
- isolates are often resistant to other classes
Interpretive criteria available for:
- E. coli, K. pneumoniae, K. oxytoca, P. mirabilis

6. Direct detection of resistance mechanisms
Extended spectrum beta-lactamase
Screen with aztreonam or cefpodoxime
R = requires confirmatory testing
Confirmatory testing:
Ceftazidime v. ceftaz + clavulanic acid
Cefotaxime v. cefotax + clavulanic acid
KB: >/= 5 mm increase w/ BLI
MIC: >/= 3-fold decr in MIC w/ BLI

7. With clavulanic
acid
Without clavulanic
acid

8. Direct detection of resistance mechanisms
Oxacillin R due to PBP2a (phenotypic)
Latex agglutination test to detect altered
PBP in staphylococci
Presence confers resistance to Oxacillin
Depends on expression of protein

9. Direct detection of resistance mechanisms
Oxacillin R due to PBP2a (genotypic)
PCR to detect mecA gene in staphylococci
Positive not dependent on expression, a positive
result simply indicates presence

10. Direct detection of resistance mechanisms
Inducible clindamycin resistance (D test)
Resistance to macrolides can occur through:
efflux (msrA)
ribosome alteration (erm)
Erythro R msrA or inducible erm
Clinda S
Erythro R constitutive erm
Clinda R

11. L: Erythro, R: Clinda
No resistance Efflux
Inducible erm Constitutive erm

12. Laboratory strategies for testing
Goals of effective strategies include:
Relevance
Accuracy
Communication

13. Laboratory strategies for testing
Criteria used for assessing relevance:
Clinical significance of isolate
Predictability of susceptibility against drugs
of choice
Availability of reliable standardized methods
Selection of appropriate agents

14. Laboratory strategies for testing
Clinical significance
Abundance in direct smear
Ability to cause disease in that body site
Colonizer or pathogen?
Body site of isolation

15. Laboratory strategies for testing
Predictability of susceptibility
Testing not required when susceptibility is
predictable
Pen S in beta-hemolytic streptococci
Ceph S in GC
Clinical requirements can result in exceptions

16. Laboratory strategies for testing
Availability of standardized methods
Testing cannot be performed if standardized
method does not exist
Method and interpretive guidelines required –
CLSI guide
Info available for most pathogenic bacteria
Fungi, Nocardia, AFB (Mycobacterium)

17. Laboratory strategies for testing
Selection of agents
Previously discussed criteria:
Organism ID or group
Acquired resistance patterns
Testing method used
Site of infection
Formulary

18. Laboratory strategies for testing
Communication
Prompt and thorough review of results
Prompt resolution of unusual results
Augment susceptibility reports with messages
that help clarify and explain potential
therapeutic problems not necessarily
evident by data alone

19. Antibiograms:
The hospital antibiogram is a periodic summary
of antimicrobial susceptibilities of local bacterial
isolates submitted to the hospital's clinical
microbiology laboratory. Antibiograms are often
used by clinicians to assess local susceptibility
rates, as an aid in selecting empiric antibiotic
therapy, and in monitoring resistance trends over
time within an institution.

20. Antibiograms:
Antibiograms can also used to compare
susceptibility rates across institutions and
track resistance trends. These trends can then
be used to develop the appropriate formulary
for the hospital.