Louis Pasteur

S I R

S I + R

Wild type population

S I R

Where Do You Read Fuzzy Zones?

Outer edge

Inner edge

80% INHIBITION

Range: 10 to 31 mm; Median 14 (i.e., ~ 75-80% inhibition)

10

31

In 2009, CLSI clarified

its documents to interpret these zones of inhibition

using transmitted

light

19mm – 22mm NS S

Etest suggests using an

80% reduction cut-off to

determine MIC similar to

SXT testing; but this is susceptible

2 μg/ml 6 μg/ml (NS)

1 μg/ml (S)

Tenover et al., JCM 45:2917-22, 2007

Staphylococci Enterococci OVERALL

VM=Very major error, a false susceptible result * Results improved when read with transmitted light

Jevitt, L et al. JCM 2006;44:3098-104.

Broth microdilution MIC (μg/ml)

Ete

st M

IC (μ

g/m

l)

CMI data n=300

False susceptible

False resistance

Genotypic and phenotypic changes

0.125 0.25 0.5 1.0 2.0 4.0 8.0 16.0 32.0

μg/ml

heteroresistance susceptibility Clinical resistance

Published but not confirmed

Tenover, Clin Infect Dis. 46:675-5, 2008.

QC MICH N J JAPAN

232,000 x Magnification

Thick cell wall composed of cell wall subunits (D-ala-D-ala) capable of binding vancomycin.

J. Steenbergen et al. Unpublished observations

Daptomycin resistance does NOT correlate with cell wall thickness

Cell wall measurements on 55 organisms by three readers blinded to MIC results

PAGE | 29

Previous breakpoint: >15 mm susceptible

vanA-VRSA

This is a clue that a VISA strain is present

VISA

Suscept hVISA

Note scale

Subpopulation of hVISA isolates, for which MIC=8 μg/ml, are below detection level

Inoculum

hVISA on BHI

hVISA on Mueller-Hinton

Data courtesy of Vanthida Huang

Gradual increase

in vancomycin MICs during 10 weeks of therapy with vancomycin

for endocarditis

1 2 3 4 VISA control

2 McFarland, BHI agar, incubation for 48 hours

Macro Etest

Criteria: Vanco +

Teico >8 ug/ml

OR Teico Alone

>12 ug/ml

Courtesy of Anne Bolmstrom

VSSA hVISA VISA V=2 T=1.5 V=6 T=32 V=24 T=32

VSSA hVISA VISA

Criteria: GISA: TP or VA >8 μg/ml with std Etest MIC >6 μg/ml hGISA: TP or VA >8 μg/ml with std Etest MIC <4 μg/ml

PAGE | 37

PAGE | 38

View of Beta-Lactamases (2010) The Road to Carbapenem Resistance

Class A TEM, SHV,

CTX-M, KPCs others

Class B Metallo-

enzymes, VIM others

Class C AmpCs

and others

Class D OXA

ESBLS; Carba-

penemases

Some are

carba- penemases

Most are carba-

penemases

AmpC + porin change = carbapenem

resistance

Zone Diameter (mm)

Ceftazidime 14 Ceftazidime + Clav 23

Cefotaxime 20 Cefotaxime + Clav 24*

Increase in zone diameter by >5 mm with Clavulanic Acid for EITHER drug = ESBL

CAZ + CLAV

CTX + CLAV

CAZ

CTX

44% of labs under-reported resistance according to CLSI guidelines

Disk tests CAZ: 6mm CAZ+Clavulanate: 12 mm, Δ 6mm CTX: 6mm CTX+Clavulanate: 10mm, Δ 4 mm

PAGE | 45

Organism β-lactamases ESBL AmpC Cefepime MIC/disk

CRO MIC

PAGE | 46

Use ESBL testing only for infection control

PAGE | 47

Microbiology Pharmacy

ID Service

Lab IT Director

PAGE | 48

IMIPENEM MEROPENEM

*Meropenem phenotype is deduced by Vitek2; no reports for 2 isolates

PAGE | 50

Broth microdilution reference MIC was 16 μg/ml

(resistant); read inner most zone

of inhibition

Very typical of KPC

producing strains

Imipenem Etest

Breakpoint: ≤4 μg/ml=

susceptible

PAGE | 51

PAGE | 52

Inner colonies (MIC= 32 μg/ml)

Data courtesy of Vanthida Huang, Mercer University

Parent colonies (MIC=8 μg/ml)

PAGE | 53

PAGE | 54

New QC strains

+ -

PAGE | 55

PAGE | 56

CLSI breakpoints: <2, S; 4, I; >8 R EUCAST breakpoints: <2 S; >2 R

On average organisms are resistant to

7 antibiotics

Resistance among soil organisms is

common

Low resistance to Linezolid (synthetic),

High resistance to daptomycin (natural)

D’Costa, VM et al. Science 20 January 2006