Interpreting streptomycin susceptibility test results for Salmonella enterica serovar Typhimurium

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<ul><li><p>International Journal of Antimicrobial Agents 27 (2006) 538540</p><p>Interpreting streptomycin susceptibar Tall Dartin</p><p>y Collegty of Ired 7 Mar</p><p>Abstract</p><p>Resistanc used aconsensus o parisoStandards In trationgenotype fo opriate8 mg/L an ptible 2006 Else reserv</p><p>Keywords: Streptomycin; Salmonella Typhimurium; Resistance; Disk diffusion; MIC; Resistance genotype</p><p>1. Introdu</p><p>Althougsalmonellamycin is wexample, reistic phenostreptomycassociateddefinite phexercise insusceptibilthat agreemhigh, was lfor the poowidely acceof suscepti</p><p>The Clispecifies a</p><p> CorresponE-mail ad</p><p>0924-8579/$doi:10.1016/jction</p><p>h streptomycin has no role in the therapy ofinfection, testing for susceptibility to strepto-idely used as an epidemiological marker. Forsistance to streptomycin is part of the character-</p><p>type of resistance to ampicillin, chloramphenicol,in, sulphonamides and tetracyclines (ACSSuT)with Salmonella enterica serovar Typhimuriumage type DT104. External quality assessmentvolving laboratories across Europe reporting</p><p>ity test results for isolates of Salmonella foundent between laboratories, although overall quite</p><p>east satisfactory for streptomycin [1,2]. A reasonr concordance identified in the report is a lack ofpted criteria for interpretation of results of testing</p><p>bility of streptomycin against Enterobacteriaceae.nical and Laboratory Standards Institute (CLSI)disk content (10g), a method and interpretive</p><p>ding author. Tel.: +353 91 544 628.dress: Geraldine.Doran@nuigalway.ie (G. Doran).</p><p>criteria (susceptible 15 mm, intermediate 1214 mm andresistant 11 mm) for disk diffusion susceptibility testingof streptomycin against Enterobacteriaceae [3]. Unlike mostantimicrobial agents, however, there are no correspondingCLSI interpretive criteria for dilution susceptibility testing[3]. The Neo-Sensitabs disk diffusion system of Rosco Diag-nostics, used in a number of published studies, specifies a100g disk content and criteria of susceptible at 26 mm,intermediate at 2325 mm and resistant at 22 mm (Rosco,Taastrup, Denmark) [4,5]. As with the CLSI, correspondinginterpretive criteria for dilution susceptibility testing are notstated.</p><p>In their 2002 report, the US National AntimicrobialResistance Monitoring System (NARMS) performed brothmicrodilution susceptibility testing in Sensititre plates andapplied the criteria of susceptible at 32g/mL and resis-tant at 64g/mL [6]. Bywater et al. also considered thebreakpoint for resistance as 64g/mL [7]. The GlobalSalmonella Surveillance EQUAS exercise applies the crite-ria of susceptible at 8g/mL and resistant at 32g/mLbased on examination of population distribution (personalcommunication).</p><p> see front matter 2006 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved..ijantimicag.2006.03.005Salmonella enterica serovGeraldine Doran a,, Martina NiChulain b, Ni</p><p>Geraldine Corbett-Feeney a,b, Ma National Salmonella Reference Laboratory, Universit</p><p>b Department of Bacteriology, National UniversiReceived 12 January 2006; accepte</p><p>e or susceptibility of Salmonella enterica to streptomycin is widelyn the interpretation of streptomycin susceptibility test results. Comstitute (CLSI) disk diffusion method, the minimum inhibitory concenr 90 isolates of S. enterica serovar Typhimurium suggests that apprd resistant at 16 mg/L. For CLSI disk diffusion, we propose suscevier B.V. and the International Society of Chemotherapy. All rightsility test results foryphimuriumeLappe a, Colette OHare a,Cormican a,b</p><p>e Hospital, Galway, Irelandland, Galway, Ireland</p><p>ch 2006</p><p>s an epidemiological marker. However, there is no clearn of results obtained with the Clinical and Laboratory(MIC) determined by Etest and streptomycin resistanceinterpretive criteria for MIC results are susceptible at</p><p>at a zone diameter 13 mm and resistant at 10 mm.ed.</p></li><li><p>G. Doran et al. / International Journal of Antimicrobial Agents 27 (2006) 538540 539</p><p>Streptomycin resistance in Enterobacteriaceae is mostoften attributable to aminoglycoside-modifying enzymes [8].The phosphotransferases encoded on genes strA (aph(3)-Ib) and strpresent in Sreported asresistanceby Chen ebeen identiresistanceresistance iTyphimuriu</p><p>A reviewenterica indover the yefor the appour laboratrently 121For S. Ent17 mm. Thtribution fa(1214 mmOn sevenment exercor resistanwhich the iour result wEscherichiaacceptablefor a conseappropriatecriteria for</p><p>2. Materia</p><p>Ninetytant, 26 inprevious suCLSI disklates were(n = 3) andreceived as(n = 7). Thand 2003 a(n = 10), DDT132 (n =(n = 12), UDT18, DT2disk diffusstreptomycdeterminedtions using25922 waspolymeraseaadA2 [12</p><p>were those previously described [12,13]. DNA of E. coli K12was included as a negative control in all amplification reac-tions.</p><p>esults and discussion</p><p>ble 1 shows the relationship between MIC, zone diam-and genotype. All isolates with MIC 16 mg/L wereive for one or more of the three streptomycin resistanceminants. All isolates with MIC 8 mg/L were negativereptomycin resistance determinants. The diameter of theof inhibition showed an inverse relationship with MIC.e was no clear separation between the gene-positive and-negative populations by disk diffusion. However, all 42tes with zone diameter 13 mm were resistance gene-tive and all 35 isolates with a zone diameter 10 mm</p><p>resistance gene-positive. The 13 isolates with zoneeters in the range 1112 mm included 4 resistance gene-ive and 9 resistance gene-negative isolates. Althoughumbers are small, there is a suggestion of a relationshipeen tholateseas fo4 mg/32 mgnts.ese dof S</p><p>nt CLcin repectrutype pould b3 mmdetermfor isoest thaptible</p><p>1um inhin resismurium</p><p>)Nostr</p><p>41334127</p><p>1712</p><p>90total of fine negositiveB (aph(6)-Id) are reported as being commonlyalmonella [5,9]. The aadA1 (ant(3)-Ia) gene wasthe most frequently identified aminoglycoside</p><p>determinant in Salmonella of multiple serovarst al. [10]. The aadA2 (ant(3)-Ib) gene has alsofied as an important mechanism of streptomycinin Salmonella and is the basis for streptomycinn the widely disseminated multidrug-resistant S.m DT104 clonal group [11].of our routine disk susceptibility test data for S.</p><p>icated a consistent pattern of bimodal distributionars. The range of zone diameter values observedarent wild-type population of S. Typhimurium inory is 1016 mm with a mode of 14 mm (cur-4 mm is considered intermediate by the CLSI).</p><p>eritidis, the range is 1320 mm with a mode oferefore, the modal value for the wild-type dis-lls within the range categorised as intermediate) for S. Typhimurium but not for S. Enteritidis.</p><p>occasions in 3 years of external quality assess-ises, we reported as intermediate (six occasions)t (one occasion) isolates of S. Typhimurium forntended result was susceptible. On each occasionas reproduced in our laboratory, and results withcoli ATCC 25922 were consistently within the</p><p>range. This experience and the lack of any basisnsus in the literature prompted us to evaluate theness of current CLSI disk diffusion interpretivestreptomycin against S. Typhimurium.</p><p>ls and methods</p><p>isolates of S. Typhimurium comprising 40 resis-termediate and 24 susceptible isolates based onsceptibility results performed according to thediffusion criteria [3] were selected. The iso-from human (n = 56), porcine (n = 14), bovineother animal (n = 10) sources as well as isolatespart of external quality assessment programmes</p><p>e 83 routine isolates were collected during 2001nd comprised the following phage types: DT104T104b (n = 30), DT12 (n = 2), DT120 (n = 4),2), DT193 (n = 5), DT2 (n = 2), DT4 (n = 2), U302</p><p>310 (n = 4), U311 (n = 3), and one isolate each of1, DT40, DT41, DT85, DT99 and U302V. CLSI</p><p>ion testing was repeated on all isolates and thein minimum inhibitory concentration (MIC) wasby Etest according to the manufacturers instruc-MuellerHinton agar. Escherichia coli ATCCused as a control. All isolates were tested bychain reaction (PCR) for genes strA, aadA1 and</p><p>,13]. PCR primers and amplification conditions</p><p>3. R</p><p>Taeterpositdeterfor stzone</p><p>Thergeneisolanegawere</p><p>diampositthe nbetwsix iswherwas 6MICmina</p><p>Thlates)curre</p><p>tomythe swild-ria shat 1ate/inMICsuggsusce</p><p>TableMinimtomycTyphi</p><p>MIC(mg/L248163264128256Total</p><p>a Atwo gegene pe specific resistance genotype and the MIC. Forwith only aadA1, the median MIC was 16 mg/L,r 20 isolates with only aadA2 the median MICL. Eleven isolates had aadA1 and aadA2 (median/L) and two isolates had all three resistance deter-</p><p>ata show that the great majority (22 of 23 iso-. Typhimurium categorised as intermediate bySI disk diffusion interpretive criteria lack strep-sistance determinants and represent one end ofm of zone diameter values observed within theopulation. We suggest that the interpretive crite-e modified to resistant at10 mm and susceptible, with the range 1112 mm considered intermedi-</p><p>inate. In our laboratory we now determine thelates with zone diameters of 1112 mm. Our datat interpretive criteria for MIC results should beat 8 mg/L and resistant at 16 mg/L.</p><p>ibitory concentration (MIC), zone diameter range and strep-tance genotype of 90 isolates of Salmonella enterica serovar</p><p>. ofains</p><p>Zone diameterrange (mm)</p><p>No. genepositive</p><p>No. genenegative</p><p>1417 0 41217a 0 131117a 0 34</p><p>711a 12 0712a 7 067 17 06 1 06 2 0</p><p> 39 51ve isolates gave a zone diameter of 11 mm (three gene positive,</p><p>ative) and eight isolates gave a zone diameter of 12 mm (oneand seven gene negative).</p></li><li><p>540 G. Doran et al. / International Journal of Antimicrobial Agents 27 (2006) 538540</p><p>Re-evaluation of interpretive criteria for susceptibil-ity testing of streptomycin against Enterobacteriaceae hasreceived a low priority because it is of little clinical interest.However, given the epidemiological applications of the strep-tomycin resistance phenotype, this subject merits a reassess-ment by the CLSI and other agencies that determine break-point interpretive criteria. Additional studies in independentlaboratories should be performed to provide additional evi-dence to support such a re-evaluation.</p><p>References</p><p>[1] Threlfall J, Fisher I, Ward L, et al. Harmonisation of antibiotic sus-ceptibility testing for Salmonella: results of a study by 18 nationalreference laboratories within the European Union-funded Enter-NetGroup. Microb Drug Resist 1999;5:195200.</p><p>[2] Korver H, Maas HME, Mooijman KA, et al. Test results ofSalmonella typing by the NRLsSalmonella in the Member Statesof the EU and by the EnterNet Laboratories. http://www.rivm.nl/bibliotheek/rapporten/330300002.pdf [accessed 15 March 2006].</p><p>[3] Clinical and Laboratory Standards Institute. Performance standardsfor antimicrobial susceptibility testing; Fifteenth Informational Sup-plement. M100-S15. Wayne, PA: CLSI; 2005.</p><p>[4] Rosco Diagnostica A/S. Users guide: Neo-SensitabsTMsuscepti-bility testing. 17th ed. Taastrup, Denmark: Rosco DiagnosticaA/S; 2004. http://www.rosco.dk/Files/Filer/searchable/userguides/Neo-Sensitabs 17th ed Users Guide 04+07-2004.pdf [accessed 15 March2006].</p><p>[5] Madsen L, Aarestrup FM, Olsen JE. Characterisation of strep-tomycin resistance determinants in Danish isolates of SalmonellaTyphimurium. Vet Microbiol 2000;75:7382.</p><p>[6] US Food and Drug Administration. NARMS Retail Meat AnnualReport 2002. http://www.fda.gov/cvm/Introduction.htm [accessed 15March 2006].</p><p>[7] Bywater R, Deluyker H, Derover E, et al. A European survey ofantimicrobial susceptibility among zoonotic and commensal bacte-ria isolated from food-producing animals. J Antimicrob Chemother2004;54:74454.</p><p>[8] Vakulenko SB, Mobashery S. Versatility of aminoglycosidesand prospects for their future. Clin Microbiol Rev 2003;16:43050.</p><p>[9] Pezzella C, Ricci A, DiGiannatale E, Luzzi I, Carattoli A. Tetracy-cline and streptomycin resistance genes, transposons and plasmidsin Salmonella enterica isolates from animals in Italy. AntimicrobAgents Chemother 2004;48:9038.</p><p>[10] Chen S, Zhao S, White DG, et al. Characterization of multi-ple antimicrobial-resistant Salmonella serovars isolated from retailmeats. Appl Environ Microbiol 2004;70:17.</p><p>[11] Briggs CE, Fratamico PM. Molecular characterization of an antibi-otic resistance gene cluster of Salmonella typhimurium DT104.Antimicrob Agents Chemother 1999;43:8469.</p><p>[12] Maidhof H, Beatriz G, Abbas S, Elsheikha HM, Whittam TS, BeutinL. A multiresistant clone of Shiga toxin-producing Escherichia coli0118:[H16] is spread in cattle and humans over different Europeancountries. Appl Environ Microbiol 2002;68:583442.</p><p>[13] Boyd D, Cloeckaert A, Chaslus-Dancla E, Mulvey MR. Characteri-zation of variant Salmonella genomic island 1 multidrug resistanceregions from serovars Typhimurium DT104 and Agona. AntimicrobAgents Chemother 2002;46:171422.</p><p>Interpreting streptomycin susceptibility test results for Salmonella enterica serovar TyphimuriumIntroductionMaterials and methodsResults and discussionReferences</p></li></ul>