STAPHYLOCOCCI: KEY AST CHALLENGES

Transcription

1 Romney Humphries, PhD D(ABMM) Section Chief, UCLA Clinical Microbiology Los Angeles CA STAPHYLOCOCCI: KEY AST CHALLENGES

2 THE CHALLENGES detection of penicillin resistance detection of MRSA vancomycin and S. aureus including VISA inducible clindamycin resistance in staphylococci

3 Penicillin and Staphylococcus spp. - -lactamase testing

4 SPECIMEN: BONE DIAGNOSIS: OSTEOMYELITIS STAPHYLOCOCCUS AUREUS MIC ( g/ml) clindamycin 0.5 S erythromycin 0.5 S oxacillin 0.25 S penicillin 0.06 S?? vancomycin 0.5 S Should we do anything about penicillin?

5 STAPHYLOCOCCUS SPP. PENICILLIN > 90% of staphylococci are penicillin R Penicillin rarely considered for treatment of staphylococcal infections but might be considered for infections requiring lengthy therapy (e.g., endocarditis, osteomyelitis) IF penicillin S Some Staphylococcus spp. that test S by MIC or disk diffusion may possess a β- lactamase and may fail penicillin therapy Agent MIC (µg/ml) Zone (mm) S I R S I R Penicillin

6 S. aureus S. aureus and CoNS (including S. lugdunensis) CLSI M100-S25. Table 3D.

7 INDUCED ß-LACTAMASE TEST Oxacillin (inducer) -Sub isolate to agar (e.g., BAP, MHA) -Drop ß-lactam disk (e.g., oxacillin, cefoxitin) -Incubate overnight -Test cells from periphery of zone -If β-lactamase positive (with or without induction), report penicillin R Pos Neg

8 STAPHYLOCOCCUS AUREUS DISK ZONE EDGE TEST (10 U PENICILLIN DISK AND STANDARD DISK DIFFUSION METHOD NOT VALIDATED FOR MIC PURIT Y PLATES) Fuzzy beach = β-lactamase negative Penicillin - S Sharp cliff = β-lactamase positive Penicillin - R S. aureus QC: Neg - ATCC Pos - ATCC (supplemental QC) CLSI M100-S25. Table 3D.

9 STAPHYLOCOCCUS SPP. PENICILLIN STRATEGY Organism Action S. aureus Report PEN if R; Suppress PEN if S If PEN needed, perform nitrocefin β-lactamase test If nitrocefin β-lactamase test negative, perform PEN disk zone edge test (next day) CoNS (except Report PEN if R; Suppress PEN if S S. If PEN needed, perform nitrocefin β-lactamase test lugdunensis) If nitrocefin β-lactamase test negative, perform induced nitrocefin β-lactamase test All Continue to test subsequent isolates from patient

10 SPECIMEN: BONE DIAGNOSIS: OSTEOMYELITIS STAPHYLOCOCCUS AUREUS clindamycin MIC ( g/ml) 0.5 S erythromycin 0.5 S oxacillin 0.25 S vancomycin 0.5 S Contact laboratory if penicillin results needed

11 WHAT ABOUT S. LUGDUNENSIS? Usually very susceptible to antimicrobial agents including penicillin-s Only 5-25% penicillin-r, -lactamase positive Frank et al Clin Microbiol Rev. 21:111. UCLA 40-50% R Recommended Rx: oxacillin / nafcillin or penicillin (if β-lactamase negative) Sanford Guide, 45th ed Do not suppress penicillin results!

12 S. lugdunensis and Penicillin S. lugdunensis Report PEN if R Perform nitrocefin β-lactamase test If nitrocefin β-lactamase test negative, perform induced nitrocefin β-lactamase test Up to 50% are PEN S CLSI M100-S25. Table 3D.

13 MRSA methicillin-resistant S. aureus - Oxacillin vs. cefoxitin tests - BORSA

14 ANTIMICROBIAL AGENT OPTIONS FOR MSSA INFECTIONS Betalactams Predictable activity dicloxacillin / nafcillin cephalexin / cefazolin amoxicillin-clav ampicillin-sulbactam carbapenem vancomycin dalbavacin, oritavancin, televancin daptomycin linezolid ceftaroline (β-lactam) tigecycline Need AST clindamycin trimethoprim-sulfa macrolide (e.g. clarithromycin) tetracycline fluoroquinolone (e.g. ciprofloxacin) (gentamicin) (rifampin)

15 ANTIMICROBIAL AGENT OPTIONS FOR MSSA INFECTIONS Betalactams Predictable activity dicloxacillin / nafcillin cephalexin / cefazolin amoxicillin-clav ampicillin-sulbactam carbapenem vancomycin dalbavacin, oritavancin, televancin daptomycin linezolid ceftaroline (β-lactam) tigecycline Need AST clindamycin trimethoprim-sulfa macrolide (e.g. clarithromycin) tetracycline fluoroquinolone (e.g. ciprofloxacin) (gentamicin) (rifampin)

16 TESTS FOR meca-mediated OXACILLIN RESISTANCE IN S. AUREUS Phenotypic tests - Disk diffusion or MIC Cefoxitin as a surrogate Report results for OXACILLIN, not cefoxitin Oxacillin MIC (+ 2% NaCl) Oxacillin-salt agar screen for S. aureus Detection of gene or gene product meca PBP2a Why use cefoxitin to detect MRSA? Sensitivity Specificity Oxacillin 86 % 74 % Cefoxitin 100 % 100 % Swenson et al JCM :

17 Heteroresistant MRSA Cefoxitin (R) 10 mm zone Cefoxitin detects heteroresistant meca-mediated MRSA better than oxacillin Colonies in zone (heteroresistant) Staphylococcus aureus / S. lugdunensis Agent MIC (µg/ml) Zone (mm) S I R S I R Cefoxitin 4* - 8** 22* - 21** Oxacillin 2-4 NA * Report as oxacillin susceptible ** Report as oxacillin resistant

18 DEFINITION OF MRSA (2) MRSA are those strains of S. aureus that express meca or another mechanism of methicillin resistance, such as changes in affinity of penicillin binding proteins for oxacillin (modified S. aureus [MOD-SA] strains) MRSA = S. aureus with meca and/or oxacillin MIC >2 µg/ml and/or cefoxitin R CLSI M100-S25. Table 2C.

19 OX S. AUREUS OR S. LUGDUNENSIS TESTING BOTH OX AND CX CX Resistance mechanism Relative Prevalence Report as OX: S S None Common S R R meca Common R S R meca (low level expression) Uncommon R* R S PBP changes or hyperproduction of β-lactamase (borderline MRSA) Rare R* *most commercial system software expertise to R (3) Isolates that test resistant by oxacillin minimal inhibitory concentration (MIC), cefoxitin MIC, or cefoxitin disk test should be reported as oxacillin resistant. CLSI M100-S25. Table 2C.

20 WHAT ABOUT BORDERLINE MRSA? Oxacillin MIC slightly above R breakpoint Cefoxitin S meca negative Mechanisms: Modifications in penicillin-binding proteins (PBPs) 1,2,4 (MOD-SA) Hyperproduction of blaz-encoded penicillinase Methicillinase Infrequently encountered Limited clinical information in literature re: therapy Chambers Clin Microbiol Rev. 10:781. Skinner et al J Clin Microbiol. 47:859. Croes et al Clin Microbiol Infection. 16:979. Maalej et al J Clin Microbiol. 50:3345.

21 WHY IS IT IMPORTANT TO CORRECTLY AND QUICKLY IDENTIFY MRSA AND MSSA? MRSA infections - β-lactams (except ceftaroline) ineffective MSSA infections β-lactams better than vancomycin For bacteremia Vancomycin associated with 2 3 times the risk of morbidity and mortality vs β-lactam (e.g., oxacillin and nafcillin, cefazolin) Switching from vancomycin to β-lactam inferior to starting with β-lactam McConeghy et al Clin Infect Dis. 57:1760. If you focus on ONE thing focus on getting this right!

22 WHAT S MECC? mecc 1 Human and bovine MRSA were reported in 2011 that carry a new meca homologue, mecc Human cases retrospectively identified in 1975, 1992 but most since Harbors ~68-70% genetic similarity to meca Mostly found in veterinary isolates, all in Europe, but some human cases to date in Denmark, Germany, France, UK and Spain Currently only detected by cefoxitin resistance - Typically have low oxacillin MICs - meca PCR, PBP2a tests do not work for these 1. Additional reading for mecc: Laurent et al 2012 EID 18:1465; Garcia-Alvarez et al 2011 Lancet Infect Dis 11: Petersen et al CMI :E16 3. Cartwright et al J Clin Microbiol. 51:

23 WHAT ARE CLSI RULES FOR DETECTING MRCONS? Species Rule S. lugdunensis Use oxacillin and cefoxitin breakpoints as for S. aureus S. saprophyticus Don t test; add comment to report re: appropriate therapy for S. saprophyticus UTI S. epidermidis Use CoNS oxacillin MIC and/or cefoxitin DD breakpoints - work well Other CoNS Use caution for oxacillin MICs µg/ml; cefoxitin DD testing is better! CoNS oxacillin-r more heterogeneous than S. aureus; lower breakpoint

24 REPORTING STRATEGY OXACILLIN MIC RESULTS FOR CONS* * For testing non-s. epidermidis isolates from sterile sites where CoNS is causing an infection Oxacillin MIC (µg/ml) S I R 0.25 Report Oxacillin S Negative Oxacillin MIC (µg/ml) Do meca or PBP2a or Cefoxitin disk Positive 4 Report Oxacillin R Report Oxacillin S Report Oxacillin R

25 Vancomycin and S. aureus See.. Howden et al Clin Microbiol Rev. 23:99.

26 SPECIMEN: BLOOD DIAGNOSIS: BACTEREMIA STAPHYLOCOCCUS AUREUS oxacillin penicillin vancomycin vancomycin MIC ( g/ml) >16 R R 1 S (automated) 1.5 S (Etest) What about vancomycin MIC?

27 S. AUREUS - VANCOMYCIN MIC INTERPRETIVE CRITERIA ( G/ML) Susceptible Intermediat e Resistant VSSA 1 vs 2 µg/ml VISA VRSA Vancomycin released 1950s; first resistance (VISA) 1990s! CLSI M100-S25. Table 2C.

28 CONTEMPORARY THOUGHTS ON TREATING MRSA BACTEREMIA WITH VANCOMYCIN Based on PK/PD, clinical outcome may vary if vancomycin treatment for isolates with susceptible MICs of 1 vs. 2 µg/ml Maintain trough serum concentration of mg/l Liu et al Clin Infect Dis. 52:285. Some say if vancomycin S MIC (µg/ml): <1.5 use vancomycin use alternative Recent study: No differences if MIC <1.5 or 1.5 µg/ml in risk of death; can t exclude increased mortality risk Kalil et al JAMA. 312:1552. Reproducibility of MIC = +/- 1 two-fold dilution!!!!

29 IDSA GUIDELINES SUGGEST USING VANCOMYCIN MICS TO GUIDE THERAPY AS FOLLOWS.. MIC (µg/ml) 2 (Susceptible) >2 (e.g., VISA or VRSA) IDSA Guidelines Recommendation The patient s clinical response should determine the continued use of vancomycin, independent of the MIC An alternative to vancomycin should be used Liu et al Clin Infect Dis. 52:1.

30 Vancomycin MIC (N=101 MRSA) Etest MICs > Reference Broth Microdilution and Agar Dilution MICs 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.

31 48 hour growth on blood agar plate MRSA VISA VSSA VISA VSSA Marlowe, et al J Clin Microbiol. 39:2637. Howden et al CMR. 23:99.

32 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

33 PERSISTENT MRSA BACTEREMIA PATIENT #1 DAPTOMYCIN MIC STORY Patient receives 49 days daptomycin for MRSA Presents to ED with sepsis: MRSA in blood MRSA only tested every 5 days for AST from blood Daptomycin NS MRSA recognized on hospital day 8 Daptomycin NS isolate was found in blood bottle collected in ED (but not tested because it took longer to grow) Giltner et al J Clin Microbiol. 52:357.

34 PERSISTENT MRSA BACTEREMIA PATIENT #1 DAPTOMYCIN MIC STORY Day of Hospitalization on Which Blood Culture Drawn Routine AST done on S. aureus at time of isolation? Why? 1 Yes First S. aureus isolated from this hospitalization 0 No Recovered after MRSA from Day 1 was isolated 8 Yes More than 5 days elapsed since previous AST done No Less than 5 days elapsed since previous AST done 15 Yes More than 5 days elapsed since previous AST done Daptomycin MIC (µg/ml) 0.5 S 4 NS* 2 NS 0.5 S, 1S 2 NS* 8 NS *results obtained from retrospective testing Giltner et al J Clin Microbiol. 52:357.

35 WHAT SHOULD WE DO ABOUT TESTING VANCOMYCIN (& DAPTOMYCIN) FOR MRSA? Use MIC method verified in your laboratory; report according to your SOP Will be some differences by method Avoid repeated subcultures If use multiple methods and get conflicting results must review patient response to vancomycin! Test all isolates from blood, especially if they have atypical morphotype or take a long time to grow If MIC 3 µg/ml by any method, report VISA Most likely in patients previously treated with vancomycin Physicians should be informed about issues with vancomycin MIC testing and need to continually monitor patient response to vancomycin therapy Some refs Howden et al Clin Microbiol Rev. 23:99. Liu et al Clin Infect Dis. 52:1. Van Hal et al Clin Infect Dis. 54:755. Kalil et al JAMA 312:1552.

36 CAN CONS SHOW DECREASED SUSCEPTIBILITY TO VANCOMYCIN? May show increase in MICs following exposure to vancomycin Most common in S. haemolyticus and S. epidermidis Some suggest use alternative to vancomycin for S. haemolyticus Falcone et al Diagn Microbiol Infect Dis. 57:325. Method - Organism MIC - S. aureus MIC (µg/ml) S I R 2 * MIC - CoNS CLSI M100-S25. Table 2C.

37 Drug OTHER ANTIMICROBIAL AGENTS FOR MRSA Route PO IM IV Comments ceftaroline x FDA indications for complicated skin infxns, communityacquired pneumonia β-lactam with activity against MRSA FDA indications for complicated skin infxns, bacteremia daptomycin x Disk diffusion does not work; must do MIC S breakpoint only NOT for respiratory infxns (lung surfactant inhibits drug); do not report on respiratory specimens linezolid x x FDA indications for nosocomial pneumonia, complicated skin infxns, uncomplicated skin infxns (MSSA); communityacquired pneumonia (MSSA) For disk diffusion, examine with transmitted light

38 Measure linezolid zones with transmitted light (any growth is significant) Linezolid Measuring Endpoints Light Ignore MIC trailing endpoints CLSI M07-A10. p

39 NEWER ANTIMICROBIAL AGENTS MRSA Drug Route PO IM IV Comments dalbavancin oritavancin telavancin x x x FDA indications for complicated skin infxns; S only breakpoint (FDA no CLSI). Once/week dose FDA indications for complicated skin infxns; S only breakpoint (FDA no CLSI). One dose FDA indications for complicated skin infxns; community-acquired / hospital-associated pneumonia; S only breakpoint (FDA no CLSI). Once/day dose tedizolid x x FDA indications for complicated skin infxns; S/R breakpoint for S. aureus (FDA no CLSI). Once/day dose

40 Staphylococcus spp. -Inducible clindamycin resistance

41 SPECIMEN: PUS (L BUTTOCK LESION) DIAGNOSIS: LOCALIZED ABSCESS MANY STAPHYLOCOCCUS AUREUS clindamycin S??? erythromycin R oxacillin R penicillin R vancomycin S What should we do about clindamycin results?

42 STAPHYLOCOCCUS SPP. ERYTHROMYCIN / CLINDAMYCIN Mechanism Determinant Erythro Clinda Efflux msra R S Ribosome modification erm R S* Ribosome modification erm R R constitutive msra = macrolide streptogramin resistance erm = erythromycin ribosome methylase *requires induction to show resistance

43 S. AUREUS D ZONE TEST D zone test is only for staphylococci that are: Erythromycin R and Clindamycin S or I Photo 1: inducible clindamycin R Photos 2 and 3: erythromycin R and clindamycin R (D zone test NOT needed) 1 2 3

44 SPECIMEN: PUS (L BUTTOCK LESION) DIAGNOSIS: LOCALIZED ABSCESS MANY STAPHYLOCOCCUS AUREUS clindamycin erythromycin oxacillin penicillin vancomycin R R R R S Final Report with Optional Comment This S. aureus is presumed to be clindamycin resistant based on detection of inducible clindamycin resistance. Clindamycin may still be effective in some patients.

45 ERYTHROMYCIN-R, CLINDAMYCIN-S STAPHYLOCOCCI TESTS FOR INDUCIBLE CLINDAMYCIN RESISTANCE Notes: Test applies to S. aureus and CoNS (OX-S and OX-R) Test not routinely done on CoNS as clindamycin rarely used for CoNS Clindamycin can be give orally; good option for outpatients if S

46 SUMMARY (1) An induced -lactamase test usually, but not always detects staphylococci capable of producing -lactamase; for -lactamase negative staphylococci with penicillin MICs 0.12 µg/ml or zones 29 mm, subsequent isolates on a patient should be subjected to AST. Penicillin disk zone edge test is used to detect β- lactamase in S. aureus whereas induced nitrocefin β- lactamase test is used for CoNS. Cefoxitin disk diffusion and cefoxitin MIC tests are more sensitive in detecting meca-mediated resistance in S. aureus than tests using oxacillin. If both oxacillin and cefoxitin are tested against S. aureus or S. lugdunensis and either is resistant, the isolate should be reported as oxacillin resistant.

47 SUMMARY (2) MRSA are strains that have meca and/or oxacillin MICs >2 µg/ml and/or cefoxitin resistance. Testing for S. aureus as MR or MS should be done as quickly as possible, especially on isolates from serious infections. MRSA with mecc have not yet been reported in the USA. Some suggest serious infections caused by MRSA with vancomycin susceptible MICs of 2 µg/ml may not respond as well to vancomycin therapy as MRSA with vancomycin MICs of 1 µg/ml. When considering alternative therapy to vancomycin for MRSA, patient clinical response and vancomycin MIC must be examined.

48 SUMMARY (3) Different methods may result in different vancomycin MICs for vancomycin-s S. aureus. Etest produces higher vancomycin MICs than CLSI broth microdilution reference method. Reproducibility of MIC tests is +/- 1 two-fold dilution. VISA are often slow growing and may yield a variety of colony morphologies. Tests for inducible clindamycin resistance should be done before reporting clindamycin S in erythromycin-r and clindamycin-s or I staphylococci. Oxacillin MIC tests work well for S. epidermidis but can overcall oxacillin-r in other CoNS that lack meca. The cefoxitin disk diffusion test is better.