EUCAST expert rules in AST: Gram positives

EUCAST expert rules in AST: Gram positives BSAC User Day 11 October 2012 Trevor Winstanley

What is an expert? An expert is a man who has made all the mistakes that can be made, in a narrow field. Niels Bohr

An expert knows all the answers - if you ask the right questions. Levi Strauss

Make three correct guesses consecutively and you will establish a reputation as an expert. Laurence J. Peter

An expert is somebody who is more than 50 miles from home, has no responsibility for implementing the advice he gives, and shows slides. Edwin Meese

Expert rules 

Describe an action to be taken 





Assist clinical microbiologists 



based on current clinical and/or microbiological evidence in response to AST results interpretation of AST results

Contribute to QA 

highlight anomalous or unlikely results

www.eucast.org

EUCAST expert rules 

The EUCAST expert rules in AST are divided into: 

intrinsic resistances



exceptional phenotypes



interpretive rules

Intrinsic resistance  



Opposed to acquired or mutational resistance Characteristic of all (or almost all) representatives of the species The drug is, in effect, clinically useless 



AST is normally unnecessary

“Susceptible” results should be viewed with caution  

an error in identification or susceptibility testing? if S is confirmed, the drug should preferably not be used

Intrinsic resistance

Intrinsic resistance

e.g.Enterococci

Rule 4 intrinsic resistance

Rule 4 intrinsic resistance (1) 

Gram-positive bacteria 



Staphylococci 



fusidic acid, fosfomycin, novobiocin

S. cohnii, S. xylosus 



ceftazidime

S. saprophyticus 



aztreonam, temocillin, polymixin B / colistin, nalidixic acid

novobiocin

S. capitis 

fosfomycin


Streptococci, enterococci 



Enterococci 



cephalosporins, erythromycin, sulphonamides

E. faecalis, E. gallinarum, E. casseliflavus 



fusidic acid, aminoglycosides (low-level)

clindamycin, quinupristin-dalfopristin

E. gallinarum, E. casseliflavus 

vancomycin


Corynebacterium spp. 



Listeria monocytogenes 



cephalosporins

Leuconostoc spp., Pediococcus spp., some Lactobacillus spp. 



fosfomycin

vancomycin, teicoplanin

Clostridium ramosum, C. innocuum 

vancomycin

Exceptional phenotypes  

Not yet reported or very rare An error in identification or susceptibility testing?  





further study reference laboratory

Resistance may develop and increase over time Local or national differences 

very rare in one hospital, area or country, more common in another

Rules 6 & 7 exceptional phenotypes

Exceptional phenotypes (1) 

Staphylococci, JK coryneforms, S. pneumoniae, ß-haemolytic streptococci 



S. aureus, JK coryneforms, S. pneumoniae, ß-haemolytic streptococci 



R to teicoplanin

S. pneumoniae 



R to vancomycin, linezolid, quinupristin-dalfopristin, daptomycin, tigecycline

R to imipenem, meropenem, rifampicin

ß-haemolytic streptococci 

R to penicillin, cephalosporins

Exceptional phenotypes (2) 

Enterococci 



E. faecalis, E. gallinarum, E. casseliflavus, E. avium 



S to quinupristin-dalfopristin (+/- R to ampicillin)

E. faecium 



R to linezolid, daptomycin, tigecycline; R teicoplanin but not vancomycin

R to quinupristin-dalfopristin (+/- S to ampicillin)

C. difficile 

R to metronidazole or vancomycin

Interpretive reading of the antibiogram 1.

To establish the susceptibility phenotype

2.

To infer the potential resistance mechanism

3.

To predict a previously defined phenotype from the resistance mechanisms

Courvalin P, ASM News, 1992; Livermore DM et al. J Antimicrob Chemother 2001; 48 (Suppl 1): 87-102; Cantón R. Enferm Infecc Microbiol Clin 2010; 28:375-385

e.g. Staphylococci

Rule

12.3

TOB

KAN

GEN

GEN

TOB

KAN

AMI

STR ARB

R

R

R

R

R

S

APH(3’)1-3

ANT(4’)(4”)-1

APH(2’) AAC(6)

√

Grade

[B]

Evidence grades 

A. Clinical evidence 



B. Evidence is weak and based only on a few case reports or on experimental models 



reporting the test result as susceptible leads to clinical failures

reporting the test result as susceptible may lead to clinical failures

C. Microbiological data 

clinical use of the agent should be discouraged

Rule 8.2 interpretive rules ß-lactam agents 

IF Staphylococcus spp. R to benzylpenicillin or IF ßlactamase is detected (testing discouraged) THEN report as R to all penicillins except isoxazolyl-penicillins and ß-lactamase inhibitor combinations [C]

Technically difficult and prevalence > 90% in most countries.


IF Staphylococcus spp. R to isoxazolyl-penicillins (oxacillin, cefoxitin, mecA, PBP2a) THEN report as R to all ß-lactams except ceftaraline and ceftobiprole (low affinity for PBP2a) [A]


IF ß-haemolytic streptococci (groups A, B, C, G) S to benzylpenicillin THEN report S to aminopenicillins, cephalosporins and carbapenems [C]

© Nathan Reading

Rare isolates of group B streptococci have benzylpenicillin MICs up to 1 mg/L.


IF S. pneumoniae R by the oxacillin disk screening test THEN determine MIC of benzylpenicillin, aminopenicillins, cephalosporins and carbapenems [B]

Production of mosaic PBPs leads to various patterns of ß-lactam resistance. Report as interpreted by MIC. © Nathan Reading


IF viridans group streptococci R to benzylpenicillin THEN determine MIC of aminopenicillins and cefotaxime / ceftriaxone and report as interpreted [C]

Production of mosaic PBPs leads to various patterns of ß-lactam resistance. Results cannot be inferred from benzylpenicillin results.


IF Enterococcus spp. R to ampicillin THEN report as R to ureidopenicillins and carbapenems [C]

Alterations in PBP5 leads to decreased affinity for ß-lactams.

© Nathan Reading

Rare ß-lactamase-producing isolates have been reported in a few countries (not UK).

Rule 11.1 interpretive rules MLS antibiotics 

Report azithromycin, clarithromycin and roxithromycin as per erythromycin result [C]

Erythromycin is the class representative for 14- and 15membered ring macrolides.

Production of an efflux pump. Ribosomal methylase encoded by erm genes (MLSB phenotype).

Interpretive rules MLS antibiotics Rule

Organisms

ERY

CLI

Inducible MLSB

CLI

Grade

11.2

Staphylococcus spp.

R

S

-

S

[B]

11.3

Streptococcus spp.

R

S

-

S

[C]

M phenotype. Probably expressing efflux pumps (msr, or mef(A) in streptococci) – clindamycin is not a substrate. Risk for selection of mutants resistant to clindamycin is no greater than that for erythromycin-susceptible isolates. © Nathan Reading

Interpretive rules MLS antibiotics Rule

Organisms

ERY

CLI

Inducible MLSB

CLI

11.2

Staphylococcus spp.

R

S

+

R

11.3

Streptococcus spp.

R

S

+

Peptostreptococcus spp.

>8

Bacteroides spp.

> 32

11.4

S

or

CLI

Grade

S + warning

[B]

S + warning

[C]

R

Ribosomal erm methylase genes confer inducible MLSB phenotype (iMLSB).

© Nathan Reading

“Clinical failure during treatment with clindamycin may occur by selection of constitutively resistant mutants. Use of clindamycin is probably best avoided in severe infections”.

[C]

iMLSB resistant isolates: clindamycin therapy failures in staphylococci No. of patients treated with clindamycin

No. of failures

No. of MLSB constitutive isolates selected

2

2

2/2

McGehee (1968)

3

2

1/3

Rao (2000)

2

2

1/2

Frank (2001)

3

1

1/3

Drinkovic (2002)

1

1

1/1

Siberry (2003)

1

1

1/1

Levin (2005)

12

9

7 / 12

Reference

Interpretive rules MLS antibiotics 

Rule 11.5. IF Staphylococcus spp. R to clindamycin THEN report a warning than bactericidal activity of quinupristin-dalfopristin is reduced [C]

Constitutive MLSB resistance phenotype (cMLSB). Cross-resistance to streptogramin B-type factor. Conflicting data from experimental endocarditis models using Q-D vs. constitutive MLSB resistant isolates. © Nathan Reading

Rule 12 interpretive rules for aminoglycosides and staphylococci

Predicted loss of synergy with ß-lactams and glycopeptides in staphylococci Rule

TOB

12.3

GEN

GEN

TOB

KAN

AMI

R

R

R

R

R

R

R

R

>8

12.1 12.2

KAN

R R

R

STR ARB

S

APH(3’)1-3

ANT(4’)(4”)-1

APH(2’) AAC(6)

√

√

or

√

[C]

√

or

√

[C]

√

[B]

or

Grade

Rule 12 interpretive rules for aminoglycosides and enterococci /streptococci

Predicted loss of synergy with ß-lactams and glycopeptides in enterococcia and streptococcib Rule

GEN

KAN

GEN

TOB

KAN

AMI

R

> 512

12.4

R

> 512

12.5

12.6

STR

> 128

STR ARB

R

R

R

APH(3’)-1-3

S

APH(2’) AAC(6)

Grade

[A]a

√ or √

[C]b

√

R

R

ANT(6)

Other enzymes / ribosomal mutation

or

√ √

[B]a [C]b [A]a [C]b

Fluoroquinolone resistance 

Selection and transmission of spontaneous chromosomal mutants 

Increased levels bacterial membrane pumps 



remove fluoroquinolones from the cell

Changes in target enzymes  

reduce fluoroquinolone binding DNA gyrase; Topoisomerase IV  

S. aureus (especially MRSA) S. pneumoniae (in community and in multidrug resistant clones)

DNA gyrase and topoisomerase IV Fluoroquinolones (Q) block DNA replication Resistance results from mutations in QRDR

separates interlocked DNA strands

ParC Ξ GrlA in staphylococci

alters DNA supercoiling

Rule 13 interpretive rules for fluoroquinolones

Risk of resistance development during fluoroquinolone therapy – first-step mutation Rule

13.1

13.3

Organisms

Staphylococcus spp.

Streptococcus pneumoniae

OFL

R

R

CIP

or

or

R

R

LEV

MOX

and S or

S

and or

S

S

Acquistion of

Grade

… at least one target mutation in grlA (Ξ parC)

[C]

… at least one target mutation in e.g. parC (parE) More reliably detected using norfloxacin

[C]

Complete / partial resistance to all fluoroquinolones – combined mutations Rule

Organisms

13.2

Staphylococcus spp.

13.4

Streptococcus pneumoniae

LEV MOX

Acquistion of

Grade

R

or

R

… combined mutations in grlA (Ξ parC) and gyrA

[C]

R

or

R

… combined mutations in e.g. parC and gyrA

[B]