the multiple paths to heteroresistance and intermediate resistance to vancomycin in staphylococcus...

3
EDITORIAL COMMENTARY The Multiple Paths to Heteroresistance and Intermediate Resistance to Vancomycin in Staphylococcus aureus Stan Deresinski Department of Medicine, Stanford University, Redwood City, California (See the major article by Vidaillac et al on pages 6774.) Keywords. vancomycin; intermediate resistance; heteroresistance; Staphylocococcus aureus; mechanisms A comprehensive understanding of the mechanisms by which Staphylococcus aureus achieves intermediate levels of resistance to vancomycin could foster the development of improved diagnostic tests and therapeutic agents, with po- tentially improved clinical outcomes. Vidaillac et al, in this issue of the Journal [1], used well-characterized successive S. aureus clinical isolates that had ac- quired serial mutations together with reduced susceptibility during vancomy- cin treatment in order to compare the in vivo evolution of resistance in these iso- lates to that seen in vitro [2]. They rea- soned that the patients isolates, all recovered from blood, may not have been fully representative of organisms present at tissue site(s) of infection. Using an in vitro pharmacokinetic/pharmacodynam- ic (PK/PD) model with simulated vegeta- tions, they were able to observe phenotypic changes, including reduced viability, loss of pigmentation, and changes in colony size and hemolytic activity, prior to the identication of the detection of resistant mutants. With the appearance of the latter, daptomycin minimal inhibitory concentrations (MICs) rose, along with those of vancomycin, and this was accom- panied by thickened cell walls and re- duced autolysis, decreased surface anionic charge, and susceptibility to cationic pep- tides. Whole-genome sequencing demon- strated that the vancomycin-intermediate S. aureus (VISA) phenotype was acquired via at least 3 different genetic pathways and that, in one of the 2 cases, the pathway differed from that seen in vivo. High-level resistance of S. aureus to vancomycin, resulting from horizontal transfer of transposon Tn 1546 carrying the vanA operon from Enterococcus species, is well understood but rare [3]. In contrast, intermediate resistance is more common, more complex, and less well understood. VISA and vancomycin- heteroresistant S. aureus (hVISA; hetero- resistance to vancomycin is considered a transitional state to intermediate resis- tance) exhibit a number of typical pheno- typic characteristics, including cell wall and septal thickening, abnormal cell divi- sion, reduced cell wall turnover and au- tolysis, reduced hemolytic activity, and reduced virulence in animal models [4]. The vancomycin heteroresistance and intermediate resistance phenotypes are believed to result from altered cell enve- lope charge and diminished penetration of the drug through the thickened cell wall. The excess D-ala-D-ala targets in the cell wall act as a molecular sink, impair- ing vancomycin from accessing its penta- peptide target [5, 6]. Mutations associated with the in- termediate resistance phenotype have been identied in VraSR (vancomycin resistanceassociated sensor/regulator), GraSR (glycopeptide resistanceassociated sensor/regulator), WalKR (also known as YycGFand VicKR), as well as rpoB and others. VraSR, GraSR, and WalKR are each 2-component regulatory systems (TCS). TCS are ubiquitous among bacte- ria, allowing them to sense and respond to their environment [7]. They possess a cell membrane histidine kinase acting as a sensor/transducer that responds to its external signal by autophosphorylation in the presence of adenosine triphos- phate. The phosphoryl group is then transferred to a response regulator, gen- erally a DNA-binding protein that regu- lates transcription. S. aureus encodes 16 such signal transduction systems in its chromosome, and SCCmec encodes an additional one [7, 8]. VraSR had previously been found to be upregulated in hVISA strain Mu3 and VISA strain Mu50 [8, 9]. It acts under the regulation of yvqF as an on-off switch of Received and accepted 1 March 2013; electronically pub- lished 28 March 2013. Correspondence: Stan Deresinski, MD, Department of Medicine, Stanford University, 2900 Whipple Ave, Ste 115, Redwood City, CA 94062 [[email protected]]. The Journal of Infectious Diseases 2013;208:79 © The Author 2013. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail: journals. [email protected]. DOI: 10.1093/infdis/jit136 EDITORIAL COMMENTARY JID 2013:208 (1 July) 7 at Georgia State University Libraries / Acquisitions on October 31, 2013 http://jid.oxfordjournals.org/ Downloaded from at Georgia State University Libraries / Acquisitions on October 31, 2013 http://jid.oxfordjournals.org/ Downloaded from at Georgia State University Libraries / Acquisitions on October 31, 2013 http://jid.oxfordjournals.org/ Downloaded from

Upload: s

Post on 20-Dec-2016

213 views

Category:

Documents


0 download

TRANSCRIPT

E D I T O R I A L C O M M E N T A R Y

The Multiple Paths to Heteroresistance andIntermediate Resistance to Vancomycin inStaphylococcus aureus

Stan Deresinski

Department of Medicine, Stanford University, Redwood City, California

(See the major article by Vidaillac et al on pages 67–74.)

Keywords. vancomycin; intermediate resistance; heteroresistance; Staphylocococcus aureus; mechanisms

A comprehensive understanding of themechanisms by which Staphylococcusaureus achieves intermediate levels ofresistance to vancomycin could foster thedevelopment of improved diagnostictests and therapeutic agents, with po-tentially improved clinical outcomes.Vidaillac et al, in this issue of the Journal[1], used well-characterized successiveS. aureus clinical isolates that had ac-quired serial mutations together withreduced susceptibility during vancomy-cin treatment in order to compare the invivo evolution of resistance in these iso-lates to that seen in vitro [2]. They rea-soned that the patient’s isolates, allrecovered from blood, may not have beenfully representative of organisms presentat tissue site(s) of infection. Using an invitro pharmacokinetic/pharmacodynam-ic (PK/PD) model with simulated vegeta-tions, theywere able to observephenotypicchanges, including reduced viability, lossof pigmentation, and changes in colony

size and hemolytic activity, prior to theidentification of the detection of resistantmutants. With the appearance of thelatter, daptomycin minimal inhibitoryconcentrations (MICs) rose, along withthose of vancomycin, and this was accom-panied by thickened cell walls and re-duced autolysis, decreased surface anioniccharge, and susceptibility to cationic pep-tides. Whole-genome sequencing demon-strated that the vancomycin-intermediateS. aureus (VISA) phenotype was acquiredvia at least 3 different genetic pathwaysand that, in one of the 2 cases, the pathwaydiffered from that seen in vivo.High-level resistance of S. aureus to

vancomycin, resulting from horizontaltransfer of transposon Tn 1546 carryingthe vanA operon from Enterococcusspecies, is well understood but rare [3].In contrast, intermediate resistance ismore common, more complex, and lesswell understood. VISA and vancomycin-heteroresistant S. aureus (hVISA; hetero-resistance to vancomycin is considered atransitional state to intermediate resis-tance) exhibit a number of typical pheno-typic characteristics, including cell walland septal thickening, abnormal cell divi-sion, reduced cell wall turnover and au-tolysis, reduced hemolytic activity, andreduced virulence in animal models [4].The vancomycin heteroresistance andintermediate resistance phenotypes are

believed to result from altered cell enve-lope charge and diminished penetrationof the drug through the thickened cellwall. The excess D-ala-D-ala targets in thecell wall act as a molecular sink, impair-ing vancomycin from accessing its penta-peptide target [5, 6].

Mutations associated with the in-termediate resistance phenotype havebeen identified in VraSR (vancomycinresistance–associated sensor/regulator),GraSR (glycopeptide resistance–associatedsensor/regulator), WalKR (also known as“YycGF” and “VicKR”), as well as rpoBand others. VraSR, GraSR, and WalKRare each 2-component regulatory systems(TCS). TCS are ubiquitous among bacte-ria, allowing them to sense and respondto their environment [7]. They possess acell membrane histidine kinase acting asa sensor/transducer that responds to itsexternal signal by autophosphorylationin the presence of adenosine triphos-phate. The phosphoryl group is thentransferred to a response regulator, gen-erally a DNA-binding protein that regu-lates transcription. S. aureus encodes 16such signal transduction systems in itschromosome, and SCCmec encodes anadditional one [7, 8].

VraSR had previously been found tobe upregulated in hVISA strain Mu3 andVISA strain Mu50 [8, 9]. It acts under theregulation of yvqF as an on-off switch of

Received and accepted 1 March 2013; electronically pub-lished 28 March 2013.

Correspondence: Stan Deresinski, MD, Department ofMedicine, Stanford University, 2900 Whipple Ave, Ste 115,Redwood City, CA 94062 [[email protected]].

The Journal of Infectious Diseases 2013;208:7–9© The Author 2013. Published by Oxford University Presson behalf of the Infectious Diseases Society of America. Allrights reserved. For Permissions, please e-mail: [email protected]: 10.1093/infdis/jit136

EDITORIAL COMMENTARY • JID 2013:208 (1 July) • 7

at Georgia State U

niversity Libraries / A

cquisitions on October 31, 2013

http://jid.oxfordjournals.org/D

ownloaded from

at G

eorgia State University L

ibraries / Acquisitions on O

ctober 31, 2013http://jid.oxfordjournals.org/

Dow

nloaded from

at Georgia State U

niversity Libraries / A

cquisitions on October 31, 2013

http://jid.oxfordjournals.org/D

ownloaded from

a large cell wall–stress stimulon that re-sponds to cell wall–active antibiotics,among other triggers [10]. Of note is thatoverexpression of vraSR is also associatedwith reduced susceptibility of S. aureus todaptomycin [11], consistent with the ob-servation of a frequent correlationbetween vancomycin and daptomycinMICs [5].

Mutations in GraSR were first identi-fied in association with reduced suscepti-bility to vancomycin, but they are alsoimportant in the resistance of S. aureusto cationic antimicrobial peptides [12,13].GraSR controls genes involved in thestress response and also controls the ex-pression of several virulence genes. Inaddition, it has significant regulatoryover-lap with the WalKR TCS [13]. WalKR(also known as “YtxGF”) controls cellwall degradation and turnover (whichleads to activation of the innate immuneresponse, with counterbalancing evasionthrough control of the SaeSR regulon).WalKR, which controls a number of viru-lence genes, is the only TCS demonstrat-ed to be essential for the viability ofS. aureus [14, 15]. Mutations in WalK orWalR are associated with the intermedi-ate resistance phenotype, as well as withreduced susceptibility to daptomycin andattenuated virulence [15].

rpoB, another relevant gene, albeit onethat does not encode a TCS, encodes theβ subunit of RNA polymerase. A muta-tion in rpoB (different from those muta-tions causing rifamycin resistance) isassociated with the intermediate resis-tance phenotype, as well as with reducedsusceptibility to daptomycin [16]. Thishas been associated with increased ex-pression of the dlt operon, which elevatesthe surface membrane charge and mayaccount for cross-resistance of vancomy-cin and daptomycin, which are function-ally cationic molecules (the anionicdaptomycin acts like a cation in the pres-ence of Ca++) [6].

Mutations in ≥1 of these 3 TCS or inrpoB can be identified in almost all VISAand hVISA. Sequencing of candidate lociof 22 VISA and 9 hVISA clinical isolates

identified a great diversity of nonsynony-mous single-nucleotide polymorphisms(SNPs), with an association with clonalbackground [17]. Mutations in GraR,GraS, and WalR were detected only inVISA isolates. The MIC was correlatedwith the number of SNPs in an individu-al isolate.A large number of additional SNPs

have been observed in hVISA and VISAstrains, but the evidence for a causal roleis generally lacking. Exceptions includemutations in clpP, a regulatory proteasethat is reported to be responsible for theraised vancomycin MIC in a laboratory-derived VISA strain and that appears toact together with WalK mutations toelevate the MIC to a greater extent thanthat seen with each individually [18].ClpP has been found to be necessary forstress tolerance and biofilm formation inActinobacillus pleuropneumoniae [19].Mutation in the gene encoding PP2Cphosphatase has also been associatedwith reduced susceptibility to both van-comycin and daptomycin in a USA300clinical isolate [20]. Mutations in thegene encoding serine-threonine phos-phatase, stp-1, which is involved in regu-lation of cell wall metabolism andvirulence, have also been associated withreduced vancomycin susceptibility [21].While the emergence of hVISA and

VISA is generally assumed to be theresult of exposure to vancomycin, hVISAwas present in Japan prior to the intro-duction of vancomycin or teicoplanintherapy in that country [22]. This mayhave been the result of exposure to othercell envelope–active antibiotics, such asβ-lactams [23], that are known to affect anumber of TCS, but it can also be specu-lated that it resulted from natural selec-tion of mutants by other molecules thatactivate stress mechanisms. The environ-ment of the nares, a preferred S. aureusniche, contains cationic antimicrobialpeptides (CAMPs) [24]. CAMPs inducethe VraSR regulon in S. aureus [25], con-sistent with previous evidence that thenatural CAMP, LL-37, strongly activatesLiaRS, a homolog of VraSR in Bacillus

subtilis [26]. In addition to the presenceof CAMPs in nasal secretions, colonizingS. aureus coexist in the nares with otherbacteria that may produce bacteriocins,which can also cause cell envelope stress inbacteria [27]. These stressful environmentalconditions may have set the conditions forthe natural selection leading to the pres-ence of hVISA in the absence of prior ex-posure to vancomycin.

These observations and their interrela-tionships are incomplete, yet remarkablycomplex, reflecting the remarkable adap-tive ability of S. aureus.

The ability to observe the evolutionarysteps in the development of reduced van-comycin susceptibility in vitro, as dem-onstrated with the PK/PDmethod used byVidaillac et al [1], is an important tech-nological step toward increased under-standing of this resistance phenotype [1].The evidence developed by these andother investigators clearly demonstratesthat the emergence of hVISA and VISAcan occur via a plethora of pathways. Asa consequence of these polygenetic routesto reduced vancomycin activity, there isno signature mutation whose absencecan reliably inform the clinician that anorganism is fully susceptible to vancomy-cin. The clinician is further hampered bythe fact that vancomycin MIC determina-tions in S. aureus are method dependent,making even the phenotypic identifica-tion of such isolates potentially problem-atic [28].

Note

Potential conflict of interest. S. D. is on sci-entific advisory boards of Pfizer, RibX, Merck,and Cerexa.The author has submitted the ICMJE Form for

Disclosure of Potential Conflicts of Interest.Conflicts that the editors consider relevant to thecontent of the manuscript have been disclosed.

References

1. Vidaillac C, Gardete S, Tewhey R, et al. Al-ternative mutational pathways to intermedi-ate resistance to vancomycin in methicillin-resistant Staphylococcus aureus. J Infect Dis2013. In this issue.

8 • JID 2013:208 (1 July) • EDITORIAL COMMENTARY

2. Mwangi MM, Wu SW, Zhou Y, et al. Track-ing the in vivo evolution of multidrug resis-tance in Staphylococcus aureus by whole-genome sequencing. Proc Natl Acad Sci U SA 2007; 104:9451–6.

3. Périchon B, Courvalin P. VanA-type vanco-mycin-resistant Staphylococcus aureus. An-timicrob Agents Chemother 2009; 53:4580–7.

4. Sieradzki K, Tomasz A. Alterations of cellwall structure and metabolism accompanyreduced susceptibility to vancomycin in anisogenic series of clinical isolates of Staphylo-coccus aureus. J Bacteriol 2003; 185:7103–10.

5. Cafiso V, Bertuccio T, Spina D, et al. Modu-lating activity of vancomycin and daptomy-cin on the expression of autolysis cell-wallturnover and membrane charge genes inhVISA and VISA strains. PLoS One 2012; 7:e29573.

6. Cui L, Iwamoto A, Lian JQ, et al. Novelmechanism of antibiotic resistance originat-ing in vancomycin-intermediate Staphylo-coccus aureus. AntimicrobAgents Chemother2006; 50:428–38.

7. Capra EJ, Laub MT. Evolution of two-com-ponent signal transduction systems. AnnuRev Microbiol 2012; 66:325–47.

8. Kuroda M, Ohta T, Uchiyama I, et al. Wholegenome sequencing of meticillin-resistantStaphylococcus aureus. Lancet 2001; 357:1225–40.

9. Matsuo M, Hishinuma T, Katayama Y, et al.Mutation of RNA polymerase beta subunit(rpoB) promotes hVISA-to-VISA phenotyp-ic conversion of strain Mu3. AntimicrobAgents Chemother 2011; 55:4188–95.

10. Gardete S, Kim C, Hartmann BM, et al.Genetic pathway in acquisition and loss ofvancomycin resistance in a methicillin resis-tant Staphylococcus aureus (MRSA) strain ofclonal type USA300. PLoS Pathog. 2012; 8:e1002505.

11. Cui L, Isii T, Fukuda M, Ochiai T, et al.An RpoB mutation confers dual heteroresist-ance to daptomycin and vancomycin in

Staphylococcus aureus. Antimicrob AgentsChemother 2010; 54:5222–33.

12. Mehta S, Cuirolo AX, Plata KB, et al.VraSR two-component regulatory systemcontributes to mprF-mediated decreased sus-ceptibility to daptomycin in in vivo-selectedclinical strains of methicillin-resistant Staph-ylococcus aureus. Antimicrob Agents Che-mother 2012; 56:92–102.

13. Falord M, Mäder U, Hiron A, et al. Investiga-tion of the Staphylococcus aureus GraSRregulon reveals novel links to virulence, stressresponse and cell wall signal transductionpathways. PLoS One 2011; 6:e21323.

14. Delauné A, Dubrac S, Blanchet C, et al. TheWalKR system controls major staphylococcalvirulence genes and is involved in triggeringthe host inflammatory response. InfectImmun 2012; 80:3438–53.

15. Howden BP, McEvoy CR, Allen DL, et al.Evolution of multidrug resistance duringStaphylococcus aureus infection involvesmutation of the essential two componentregulator WalKR. PLoS Pathog 2011; 7:e1002359.

16. Watanabe Y, Cui L, Katayama Y, et al.Impact of rpoBmutations on reduced vanco-mycin susceptibility in Staphylococcusaureus. J Clin Microbiol 2011; 49:2680–4.

17. Hafer C, Lin Y, Kornblum J, et al. Contribu-tion of selected gene mutations to resistancein clinical isolates of vancomycin-intermedi-ate Staphylococcus aureus. AntimicrobAgents Chemother 2012; 56:5845–51.

18. Shoji M, Cui L, Iizuka R, et al. walK and clpPmutations confer reduced vancomycin sus-ceptibility in Staphylococcus aureus. Antimi-crob Agents Chemother 2011; 55:3870–81.

19. Xie F, Zhang Y, Li G, et al. The ClpP ProteaseIs Required for the Stress Tolerance andBiofilm Formation in Actinobacillus pleuro-pneumoniae. PLoS One. 2013; 8:e53600.

20. Passalacqua KD, Satola SW, Crispell EK,Read TD. A mutation in the PP2C phospha-tase gene in a Staphylococcus aureus USA300clinical isolate with a reduced susceptibility

to vancomycin and daptomycin. AntimicrobAgents Chemother 2012; 56:5212–23.

21. Cameron DR, Ward DV, Kostoulias X, et al.Serine/threonine phosphatase Stp1 contrib-utes to reduced susceptibility to vancomycinand virulence in Staphylococcus aureus. JInfect Dis. 2012; 205:1677–87.

22. Yamakawa J, Aminaka M, Okuzumi K, et al.Heterogeneously vancomycin-intermediateStaphylococcus aureus (hVISA) emergedbefore the clinical introduction of vancomy-cin in Japan: a retrospective study. J InfectChemother 2012; 18:406–9.

23. Katayama Y, Murakami-Kuroda H, Cui L,Hiramatsu K. Selection of heterogeneousvancomycin-intermediate Staphylococcusaureus by imipenem. Antimicrob AgentsChemother 2009; 53:3190–6.

24. Cole AM, Dewan P, Ganz T. Innate antimi-crobial activity of nasal secretions. InfectImmun 1999; 67:3267–75.

25. Pietiäinen M, François P, Hyyryläinen HL,et al. Transcriptome analysis of theresponses of Staphylococcus aureus to anti-microbial peptides and characterizationof the roles of vraDE and vraSR in antimicro-bial resistance. BMC Genomics 2009; 10:429.

26. Pietiäinen M, Gardemeister M, Mecklin M,et al. Cationic antimicrobial peptides elicita complex stress response in Bacillussubtilis that involves ECF-type sigma factorsand two-component signal transductionsystems. Microbiology 2005; 151(Pt 5):1577–92.

27. Martínez B, Zomer AL, Rodríguez A, et al.Cell envelope stress induced by the bacterio-cin Lcn972 is sensed by the Lactococcal two-component system CesSR. Mol Microbiol2007; 64:473–86.

28. van Hal SJ, Barbagiannakos T, Jones M, et al.Methicillin-resistant Staphylococcus aureusvancomycin susceptibility testing: methodol-ogy correlations, temporal trends and clonalpatterns. J Antimicrob Chemother 2011; 66:2284–7.

EDITORIAL COMMENTARY • JID 2013:208 (1 July) • 9