HOW BUGS COMBAT DRUGS: ANTIMICROBIAL RESISTANCE MECHANISMS IN THE MODERN ERA. Romney Humphries, PhD D(ABMM)

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1 HOW BUGS COMBAT DRUGS: ANTIMICROBIAL RESISTANCE MECHANISMS IN THE MODERN ERA Romney Humphries, PhD D(ABMM) Section Chief, UCLA Clinical Microbiology Los Angeles CA

2 WHAT WE WILL COVER: 1. Review the mechanisms of carbapenem resistance among Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter baumannii 2. Identify the mechanism for vancomycin, daptomycin and linezolid resistance among staphylococci and enterococci 3. Review performance of antimicrobial test systems at detecting newer resistance mechanisms 4. Discuss newer antimicrobials with activity against these resistant pathogens and strategies for testing and reporting these

3 Carbapenem Resistance in GNR

4 CARBAPENEM ACTIVITY IN GNRs

5 REVIEW: MECHANISMS OF BETA-LACTAM RESISTANCE IN GNR mutational resistance acquired resistance

6 % Resistance CARBAPENEM RESISTANCE IN GNR 40 UCLA, 2014, 1 isolate per patient* Enterobacteriaceae (n=7614) K. pneumoniae (n=957) P. aeruginosa (n=744) A. baumannii (n=55) 0 Ertapenem Meropenem Imipenem *note % R increases by 10% if each isolate counted for Psa

7 SO WHY ALL THE STINK ABOUT CRE? Before 2000: Mostly β-lactamase + porin loss Extended spectrum β-lactamases (ESBLs) & AmpC enzymes Did not increase over time After 2000, CRE incidence increased 2001: 1% of Klebsiella, Enterobacter, and E. coli reported to NHSN (NNIS) were NS to carbapenems 2011: 4% of Enterobacter, E. coli NS to carbapenems 10% of Klebsiella sp. NS to carbapenems Much of this increase appears to be due to the spread of carbapenemase-producing CRE Rates vary region to region in the US 7

8 CARBAPENEMASES Class Examples Produced by: Notes A ESBLs KPC carbapenemases Enterobacteriaceae, P. aeruginosa, A.baumannii Most inhibited by clavulanic acid Usually plasmidmediated (not SME) B SME carbapenemases Metallo-β-lactamases (MBL) (NDM, VIM, IMP, GIM, SPM, etc) S. marcescens P. aeruginosa Enterobacteriaceae Acinetobacter S. maltophilia Inhibited by EDTA Do not hydrolyze aztreonam C D AmpC OXA carbapenemases (e.g. OXA-48, -181, - 232) Enterobacteriaceae P. aeruginosa Acinetobacter baumannii Enterobacteriaceae Inducible in some genera Not inhibited by clavulanic acid Hydrolyze carbapenems to some degree Adapted from Queenan & Bush Clin Microbiol Rev. 20:440. Bush & Jacoby AAC. 54:969; Bush, K Ann NY Acad Sci 1277:84. 8 Slide from Janet Hindler

9 WHY ARE CRE IMPORTANT? Clinically important Often resistant to multiple classes of antibiotics (on plasmids) Pan-resistant CRE have been described (several cases at UCLA) Associated with high mortality rates (up to 70%) 1 May be > 50% in ICU patients Combination therapy appears to improve outcomes UCLA Data, n=90 CRE (2013) Antimicrobial Carbapenems Meropenem Imipenem Ertapenem Cephalosporins Ceftriaxone Ceftazidime Cefepime % susceptible Piperacillin-Tazobactam 0 Aminoglycosides Gentamicin Amikacin Tobramycin Fluoroquinolones Ciprofloxacin Levofloxacin Trimethoprimsulfamethoxazole 22 9

10 WHAT IS A CRE? - Any Enterobacteriaceae or just Klebsiella pneumoniae? - Resistant to any carbapenem, all carbapenems, select carbapenems? - Resistant vs. not-susceptible? - Must have a carbapenemase? Be a CPO? - Is KPC? 10

11 WHAT IS A CRE? Current CDC Surveillance Definition Enterobacteriaceae that are: Resistant to one or more of the following: doripenem, ertapenem, meropenem, or imipenem* OR Production of a carbapenemase detected by PCR, MHT, Carba- NP, metallo-beta-lactamase test UCLA Clinical Definition Enterobacteriaceae that are: Intermediate OR resistant to one or more of the following: doripenem, meropenem, or imipenem* *Proteus/Providencia/Morganella exceptions for imipenem Remember! Not all CRE have a carbapenemase 11

12 A NOTE ABOUT BREAKPOINTS Carbapenem Breakpoints for the Enterobacteriaceae 2009 (OLD) Breakpoints Post-2010 (CURRENT) Breakpoints S I R S I R Test for carbapenemase routinely if screen positive No need to test for carbapenemase routinely CLSI M100-S25. Table 2A.

13 CARBAPENEMASE TESTS Test for suspected carbapenemase production in Enterobacteriaceae, P. aeruginosa, Acinetobacter spp. When to do these: For epidemiological or infection control purposes. NOTE: no change in interpretation of carbapenem susceptibility results is required for Carba-NP /MHT positive isolates. Such testing is not currently recommended for routine use. p. 120 M100 S25

14 Carbapenemase tests: M100 Tables 3B and 3C. MHT Carba NP Molecular Use Enterobacteriaceae Enterobacteriaceae P. aeruginosa Acinetobacter Enterobacteriaceae P. aeruginosa Acinetobacter Strengths Simple Rapid Determines type of carbapenemase Limitations Some false pos (eg, ESBL/ampC + porin) Special fresh reagents Special reagents Some false neg (eg NDM) Enterobacteriaceae only Some invalid results False neg for OXAtype carbapenemase Specific to targeted gene CLSI M100-S25.

15 Modified Hodge Test (MHT) for Carbapenemases #1 meropenem disk Good for: KPC (usually) OXA-48-like No good for: NDM-1 and other MBLs #2 #3 Lawn of indicator E. coli Carbapenemase diffuses into agar, hydrolyzes meropenem, and E. coli no longer inhibited appears as blip (test strains #1 & #2 positive)

16 CARBA NP TEST Isolated colonies (lyse) Hydrolyse imipenem if carbapenemase present Change in ph of indicator (red to yellow/orange) Rapid <2h Good for: KPC, MBLs No good for: OXA-type C pases Nordmann et al Emerg Infect Dis. 18:1503. Tijet et al Antimicrob Agents Chemother. 57:4578. Vasoo et al J Clin Microbiol. 51:3092. Dortet et al J Med Microbiol. 63:772. Dortet et al Antimicrob Agents Chemother. 58:2441. NO imipenem + imipenem

17 CARBA NP TEST MATERIALS/REAGENTS Reagent prep takes time! Solutions to prepare: 10 mm Zinc sulfate heptahydrate Phenol red solution 0.1 N NaOH Carba NP Solution A (phenol red + zinc solutions) Carba NP Solution B (Carba NP Solution A + imipenem)

18 UTILITY OF CARBAPENEMASE TESTS: AN OUTBREAK STORY Shaun Yang and Peera Hemarajata UCLA uses current CLSI carbapenem BP, no routine carbapenemase detection performed Hospital epidemiology and laboratory noticed uptick in number of CRE cases Traditional epidemiological tools cannot identify source UCLA postdocs perform whole genome sequencing on three CRE from UCLA patients Questionable if unit-based transmission PFGE results inconclusive Two isolates are KPC but one is an OXA-232!!

19 OXA-232 PHENOTYPE N e g K P C O XA48 O XA232 N e g K P C O XA232 Carba-NP Negative MHT Positive Ertapenem Meropenem Imipenem >16 μg/ml >16 μg/ml 2 μg/ml - Second report in US - Missed by old PCR because gene is new, our primers didn t anneal - Screen all CRE for this phenotype - Confirm with LDT PCR for OXA Hemarajata Yang and Humphries 2015 AAC In press

20 Number of Patients with CRE UTILITY OF CARBAPENEMASE TESTS 34 isolates January 2014 January 2015 tested 6 KPC NDM-1/SME OXA-232 None Identify source patient (happened to be patient whose isolate was sequenced by NGS) 7 additional patients with OXA -232 isolates identified (all identical by rep- PCR) Duodenoscope transmission (ERCP) 0 Role for routine determination of CRE mechanism? Courtesy of: Shaun Yang P. Hemarajata

21 CARBAPENEM RESISTANT PSEUDOMONAS AERUGINOSA AmpC + porin mutations + efflux P. aeruginosa is a SPICE (M-SPACE) bug AmpC gene is on chromosome and can become de-repressed P. aeruginosa is not as permeable to antimicrobials as are the Enterobacteriaceae Antimicrobials (and nutrients) pass through outer membrane porin channels Carbapenems preferentially enter via the OprD porin Loss / mutation of OprD can lead to carbapenem resitance Pseudomonas aeruginosa expresses several efflux pumps hyperexpression of these results in carbapenem resistance Most common = AmpC overexpression + OprD loss 1 1. Castenheira et al AAC :6844

22 PSEUDOMONAS AERUGINOSA CLSI BREAKPOINTS (MIC µg/ml) 1 Agent Old Breakpoints Current Breakpoints Susc Int Res Susc Int Res Doripenem None (FDA 2 S) Imipenem Meropenem CLSI also revised corresponding disk diffusion breakpoints M100-S25 Table 2B-1.

23 CARBAPENEM RESISTANT ACINETOBACTER BAUMANNII Carbapenemases Harbor OXA-class D carbapenemases carbapenem hydrolyzing class D beta-lactamase (CDHL) Chromosomal or on plasmid Acquire via transposons Often not expressed When expressed = elevated carbapenem MIC, but do not touch cephalosporins Combined with porin los or overexpression of efflux systems= carbapenem resistance Also may acquire other classes of carbapenemases 1. Potron et al 2015 IJAA 45:568

24 ACINETOBACTER BAUMANNII CLSI BREAKPOINTS (MIC µg/ml) 1 Agent Old Breakpoints Current Breakpoints Susc Int Res Susc Int Res Doripenem None (FDA 2 S) Imipenem Meropenem CLSI also revised corresponding disk diffusion breakpoints M100-S25 Table 2B-1.

25 PLEASE DON T FORGET! IMIPENEM MEROPENEM Lesho et al 2005 CID 41:758

26 WHAT CAN WE DO WHEN WE ISOLATE THESE CARBAPENEM-R ORGANISMS? ALWAYS report unexpected resistance even if performing cascade reporting 1 Develop a protocol to address isolates that are confirmed as resistant to all agents on routine test panels Either by testing additional agents in-house or at a reference laboratory 2 1 report broad-spectrum agents only when isolate is resistant to narrower-spectrum agents 2 Now a CAP requirement - MIC M100-S25. Instructions for Use

27 SUPPLEMENTAL DRUGS TO CONSIDER FOR AST OF CR GNR Antimicrobial Agent Enterobacteriaceae P. aeruginosa A. baumannii Minocycline Yes No Yes Tigecycline 2 Yes (ex. Pro/Prov/Morg) No??? Colistin (or polymyxin B) Yes 1 Yes Yes Fosfomycin Yes (E. coli, others?) Urine No Ceftazidimeavibactam Yes Yes No Ceftolozane tazobactam Yes Yes No 1 No CLSI breakpoints; EUCAST = 2 µg/ml S, >2 µg/ml R 2 Not on urine isolates

28 SPECIMEN: BLOOD DIAGNOSIS: OSTEOMYELITIS ACINETOBACTER BAUMANNII MIC (µg/ml) amikacin amp-sulbactam cefepime ceftazidime ciprofloxacin gentamicin imipenem meropenem piper-tazo tobramycin trimeth-sulfa >32 R >32 R >32 R >32 R >2 R >10 R >8 R >8 R 128/4 R >10 R >4/76 R What else to test/report?

29 CLSI M100-S25. App. B2. Intrinsic Resistance.

30 Table 1A. Agents to Consider for Testing and Reporting Acinetobacter spp. Note: Where is colistin, tigecycline? CLSI M100-S24.

31 ACINETOBACTER SPP. MINOCYCLINE PO, IV formulation FDA approved for Acinetobacter infections Considerably more active than doxycycline and tetracycline against A. baumannii 67% of MDR isolates are S 1 Recent studies demonstrating clinical efficacy Bishburg et al Infect Dis Clin Pract. 22:26. Jankowski et al Infect Dis Clin Pract. 20:184. Option for multidrug resistant strains 1 Hawser et al ICAAC. Abstract #C

32 Minocycline ACINETOBACTER BAUMANNII MINOCYCLINE VS. TETRACYCLINE (N=5478) 1 Tetracycline Susceptible ( 4 µg/ml) Intermediate (8 µg/ml) Resistant (>8 µg/ml) Resistant (>8 µg/ml) 0 (0%) Intermediate (8 µg/ml) 1 (<0.1%) Susceptible ( 4 µg/ml) 2000 (36.5%) 1 SENTRY surveillance program

33 TESTING MINOCYCLINE VS. ACINETOBACTER If tetracycline S, can predict S If tetracycline R, need to test Etest not well studied 133 isolates evaluated in one study disk generated 53.4% minor errors vs. BMD Etest 18% minor errors vs. BMD Can use if result is I by Etest or disk, discuss with pharmacy/ physician, these may actually be S Akers et al AAC 53:2683

34 COLISTIN / POLYMYXIN B Introduced 1959; IV and inhaled only Toxicity and other issues Drug of last resort Spectrum: active against most but not all MDR P. aeruginosa, A. baumannii, other GNR Resistance can emerge Proteus/Providencia, Serratia, Burkholderia cepacia intrinsically resistant

35 COLISTIN TESTING CLSI breakpoints for A. baumannii, P. aeruginosa UNDER REVIEW! No well-defined method for testing 1 Disk exist ( wise to confirm S with MIC) Etest exists ( RUO, DO NOT perform well 2 ) Trek Sensititre panel ( RUO, works well vs BMD/AD 2 ) 1 Hindler and Humphries 2013 JCM 51: Humphries 2015 Pharmacotherapy 35:22

36 COLISTIN / POLYMYXIN B CLSI M100-S25 Organism MIC (µg/ml) Zone (mm) Susc Int Res Susc Int Res Acinetobacter spp. 2-4 none Pseudomonas aeruginosa Enterobacteriaceae None provided 3 1 colistin; 2 polymyxin B; 3 some use EUCAST: MIC Susc 2 S, >2 R Yes, I realize there are disk breakpoints but please don t test this way!

37 TIGECYCLINE IV route Spectrum: broad (G+ and G- aerobes&anaerobes) Often active against CRE Poor activity - Proteus / Providencia / Morganella Not for urinary tract infections No CLSI breakpoints Drug manufacturer has not come to CLSI FDA breakpoints Use / report without comment Pseudomonas aeruginosa

38 TIGECYCLINE AND ACINETOBACTER? Clinical experience is limited, especially for VAP Some suggest poor outcomes: VAP Blood stream infection Poor eradication Peleg et al. JAC :128 Karageorgopoulos et al JAC :45 Anthony et al CID :567 Lee et al CID Eur J Clin Microbiol Infect Dis :1211 Emergence of resistance while on therapy noted Wyeth 2009 update to product insert Generally not used unless only option

39 TIGECYCLINE AND ACINETOBACTER BAUMANNII No breakpoints, no test methods FDA cleared Susceptibility by method for 1,105 A. baumannii Method % S % I %R BMD MicroScan Vitek Etest Marchaim et al JCM 52: % Major errors (false resistance)

40 SUPPLEMENTAL DRUGS TO CONSIDER FOR AST OF MDR GNR Antimicrobial Agent Colistin (or polymyxin B) Automated System? Disk? Etest? No Yes, but poor RUO, poor Minocycline Some Yes Tigecycline Yes Yes Fosfomycin No Yes Yes, performance? Yes, performance? Yes, performance? Note: Etest often available in RUO or IVD form May be IVD for organisms other than the one you want to test check PI!) Works well for some things, less so for others MUST verify perfomance!

41 How about testing new antimicrobials?

42 CEFTAZIDIME AVIBACTAM 56 YO woman, history of renal transplant Returns to UCLA with bacterial sepsis KPC-producing K. pneumoniae isolated from blood: Deciding on use of ceftazidime-avibactam for compassionate use Ceftazidime avibactam resistant! First report of this in a KPC-producer 1 No MBL in isolate, looks like a combination of: KPC hyperexpression Porin loss Efflux CZA: 19mm need to be able to test these antimicrobials! 1 Humphries, Yang, Hemarajata, Miller, Hindler and Gregson 2015 AAC In press

43 PORIN MUTATION Wild type K. pneumoniae OmpK36 Avycaz-R K. pneumoniae OmpK36

44 ANTIMICROBIALS APPROVED IN PAST 3 YEARS Generic Name Trade name Avycaz Notes Beta-lactam / beta-lactamase inhibitor combo Treatment of CRE (excluding MBLs*) Dalbavancin Dalvance Lipoglycopeptide Treatment of skin infections (2 doses) Oritavancin Orbactiv Lipoglycopeptide Treatment of skin infections (1 dose) Sivextro Tedizolid Oxazolidinone Treatment of skin infections Ceftazidimeavibactam Ceftolozanetazobactam Zerbaxa Beta-lactam / beta-lactamase inhibitor combo Treatment of UTI, IAI (ESBL activity) Telavancin Vibativ Lipoglycopeptide Treatment of pneumonia FDA AST? No Yes!* No No No No * FDA cleared on Trek panel (custom only)

45 WHAT TO DO? Is there a surrogate agent that can be used? Dalbavancin vancomycin 1 Oritavancin vancomycin (probably predicts S) 2 Tedizolid linezolid (predicts S) 3 What is the baseline susceptibility for the agent? RUO disks / Etest available? (how are you going to report?) Ceftolozane-tazobactam Ceftazidime-avibactam Tedizolid Can the company test in cases of suspect rx. failure? Does the resistance mechanism make a difference? ( eg: MBL and ceftazidime-avibactam) 1. Jones et al ICAAC D-875b 2. Jones et al Antimicrob Agents Chemother 59: Zurenko et al Annals Clin Microbiol Antimicrob 13:46

46 Vancomycin, daptomycin, linezolid

47 VANCOMYCIN AND S. AUREUS S. aureus - Vancomycin MIC Interpretive Criteria ( g/ml)* S I R Vancomycin MIC of 2µg/ml associated with poorer outcomes 1 IDSA: clinical response vs. MIC should be used to determine continued use of vancomycin 2 Etest MIC > than BMD 3 VISA See-saw effect with oxacillin often seen Thickened cell wall Phenotype unstable Atypical colony appearance May be assoc. with elevated daptomycin MIC VRSA Acquired vana from E. faecalis RARE (12 in USA) Typical colony morphology * Important notes on vancomycin testing! Disk diffusion unreliable! Must incubate MIC test a full 24 hr 1. vanhal 2012 CID 54: Liu 2011 CID 52:1 3. Prakash 2008 AAC 52:

48 Vancomycin MIC (N=101 MRSA) Etest MICs > Reference Broth Microdilution and Agar Dilution Prakash et al Antimicrob Agents Chemother. 52:4528. See also Hsu et al Intl J Antimicrob Agents. 32:378. Sader et al Antimicrob Agents Chemother. 53:3162.

49 VISA FROM UCLA PATIENT BLOOD CULTURE Drug 11/20 12/17 12/21 12/23 daptomycin 0.25 S 0.25 S 0.25 S 4 NS oxacillin >16 R >16 R >16 R 4 R vancomycin 0.5 S 1 S 1 S 4 I colony types Hem cream Wk hem white Wk hem Cream, ppt Non-hem White Time to Detection 0.5 day 0.5 day 1 day 4.9 days NS, not susceptible VSSA (non VISA) VISA

50 Resistance due to: VISA Thickened cell wall # reported in USA: Less than 100 Detection: Colony appearance: Often difficult; phenotype unstable Atypical for S. aureus Cell wall Vancomycin susceptible Vancomycin intermediate

51 48 hour growth on blood agar plate VISA Marlowe, et al J Clin Microbiol. 39:2637. MRSA (not VISA)

52 CHARACTERISTICS SOMETIMES OBSERVED FOR VISA Colony morphology may be atypical for S. aureus (some pinpoint) Delayed growth in broth (e.g., blood culture) Weak / delayed coagulase reaction After several subcultures: Colony morphology becomes typical for S. aureus Vancomycin MIC decreases and isolate becomes vancomycin susceptible MICs for daptomycin increase MICs for -lactams decrease

53 % daptomycin S DAPTOMYCIN SUSCEPTIBILITY BY VANCOMYCIN RESISTANCE MECHANISM VSSA VISA VRSA hvisa Humphries Pollett and Sakoulas 2013 CMR 26:759

54 VRSA Resistance due to: Acquisition of vana gene from vancomycin-r Enterococcus (VRE) # reported in USA: 13 Detection: Colony appearance: Resistance Most standard methods will detect Typical S. aureus May loose meca; daptomycin MIC not affected

55 TRANSFER OF VANA FROM VRE TO S. AUREUS vana Ferber, D Science 302:1488.

56 VANCOMYCIN AND ENTEROCOCCUS

57 ENTEROCOCCUS AND VANCOMYCIN Gene Species Vancomycin MIC vana vanb vanc vand E. faecium E. faecalis E. faecalis E. faecium E. casseliflavus E. gallinarum E. faecium E. faecalis E. avium E. gallinarum E. raffinosus Teicoplanin MIC Infection Control? >128 (R) >16 Important 8-64 (I-R) <1 (S) Important 2-16 (S-R) <1 (S) Not important (R) 4-64 (S-R) Not important vane E. faecalis 8-32 (I) 16 (I) Not important vang E. faecalis 8 (I) <1 (S) Not important Vancomycin S I R MIC (ug/ml)

58 DAPTOMYCIN MODE OF ACTION Disruption of cell membrane curvature K+ release Membrane depolarization Death Daptomycin Cell membrane phospholipids Ion channel formation Daptomycin Binds PG Oligomerizes & inserts into membrane Generates ion-conduction structure Humphries, Pollett and Sakoulas Clin Microbiol Rev Submitted Dependent on Ca2+ Conformational changes Increases exposure of hydrophobic moieties Mediate anionic-anionic interaction with membrane 58

59 DAPTOMYCIN NON-SUSCEPTIBILITY IN ENTEROCOCCUS Wild-type distribution of daptomycin MICs in Enterococcus non-susceptible susceptible E. faecium E. faecalis Staphylococcus Only 2 non-susceptible (NS) isolates in clinical trials 59

60 Mean Daptomycin MIC (ug/ml) DAPTOMYCIN NS ENTEROCOCCI (DNSE) 10.5% of E. faecium at UCLA are daptomycin-ns daptomycin exposed Occurs in both patients treated with, and naïve to daptomycin MIC is higher if occurs in context of daptomycin treatment p=0.05 daptomycin naïve 60

61 RESISTANCE MECHANISM? PHENOTYPE: Daptomycin NS isolates have a increased net surface charge vs. susceptible isolates Daptomycin NS isolates make more cell septa Have a more fluid or very rigid membrane Goldilocks effect GENOTYPE: 1) Cell envelope response system mutations Example: LiaFSR three-component regulatory system 2) Phospholipid biosynthesis pathway mutations Exampes: cardiolipin synthetase (cls) Glycerophosphodiesterase (gdpd) Humphries Pollett and Sakoulas 2013 CMR 26:759

62 HOW TO DETECT DAPTOMYCIN RESISTANCE? Performance of automated systems is poor ~ 10% major errors reported for MicroScan system 2-6 % major errors by BD Phoenix and biomerieux Vitek2 Few studies have included NS Enterococci, and recognition of NS isolates may be poor (eg, biomerieux system only detect 50% of our isolates as NS) FDA issues

63 LINEZOLID MECHANISM OF ACTION Bacteriostatic activity by protein synthesis inhibition Reversibly binds and blocks the ribosomal the A site in the peptidyl transferase center (PTC) Prevents placement of aminoacyl-trna Binding site does overlap with those of other PTC inhibitors, interaction is specific enough to allow linezolid activity even if R Leach et al., 2007 Mol Cell

64 % of isolates tested LINEZOLID RESISTANCE AMONG CONS FROM SURVEILLANCE STUDIES LEADER UCLA Resistance in S. aureus, enterococci RARE LEADER, USA Linezolid Experience and Accurate Determination of Resistance non-duplicate isolates from bacterial pneumonia and acute skin and skin structure infections UCLA data represent 1 isolate per patient, all isolates from blood. 64

65 RESISTANCE MECHANISMS AMONG LRS STUDIED TO DATE 23S Mutation CoNS S. aureus G2576U 60% 64% Other 20% 6 % None 12% 30% Rare 1) rrna gene redundancy 2) Not transferable Linezolid Resistant Staphylococci L3 / L4 Mutation CoNS S. aureus L3 22.3% 20% L4 54.3% 0% 3) Fitness cost CoNS S. aureus cfr 15.9% 5% Gu et al., 2013 JAC

66 LABORATORY TESTING CONSIDERATIONS Detection of Linezolid Susceptibility for 50 staphylococci (n=15 linezolid R) by the Clinical Laboratory Method % false susceptibility (VME)* Disk Diffusion Etest Siemens Microscan BD Phoenix biomerieux Vitek * Compared to referenced broth microdilution % false resistance (ME)* Linezolid Interpretive Criteria (Breakpoints) for the staphylococci S I R MIC (µg/ml) 4-8 * Resistant isolates must be confirmed by an MIC method DD (mm)* Tenover et al. J Clin Microbiol 45:

67 SOME PARTING THOUGHTS This stuff is tricky! Even EASY things aren t that easy (how sure are you that your MSSA is really MSSA? 1 ) Not many studies in the literature independently assess the performance of automated systems Does your system have a limitation for these? Etest the solution to all that ails you (but it s good for some things) 1 Tenover and Tickler 2015 Clinical Microbiology Newsletter 37:79-84

68 Thank you! questions?