susceptibilidad en uk

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Journal of Antimicrobial Chemotherapy (2000) 46, 931940

Introduction

Gram-positive bacteria show increasing resistance to manyantibiotics, complicating antimicrobial therapy. Particularproblems include methicillin resistance in staphylococci,glycopeptide resistance in enterococci, penicillin andmacrolide resistance in pneumococci and, to a much morelimited extent, the emergence of glycopeptide-intermedi-ate Staphylococcus aureus. Against this background thereis an agreed need for more effective surveillance of resist-ance. This can be undertaken by gathering routine suscep-tibility test results generated by hospital laboratories,1,2

by central testing of collected isolates,35

or by sentinellaboratories, which test isolates according to a strictly con-trolled protocol.68 Controlled sentinel surveys have theadvantage, compared with the routine collection of data,that the isolates are tested against a standard set of anti-biotics and to a defined protocol. Compared with centraltesting, they minimize transport costs and central work-load.

In this study, 25 sentinel laboratories across the UK wereselected to determine the prevalence of antibiotic resistanceamong Gram-positive pathogens. The organisms surveyedcomprised S. aureus, coagulase-negative staphylococci

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Susceptibility of Gram-positive cocci from 25 UK hospitals to

antimicrobial agents including linezolid

C. J. Henwooda*, D. M. Livermorea, A. P. Johnsona, D. Jamesa, M. Warnera, A. Gardinerb and the

Linezolid Study Group

aAntibiotic Resistance Monitoring and Reference Laboratory, Central Public Health Laboratory,Colindale Avenue, London NW9 5HT; bPharmacia Corporation, Davy Avenue, Knowlhill,

Milton Keynes MK5 8PH, UK

The prevalence of antibiotic resistance amongst Gram-positive cocci from 25 UK hospitals

was studied over an 8 month period in 1999. A total of 3770 isolates were tested by the sentinellaboratories using the Etest; these bacteria comprised 1000 pneumococci, 1005 Staphylo-coccus aureus, 769 coagulase-negative staphylococci (CNS) and 996 enterococci. To ensurequality, 10% of the isolates were retested centrally, as were any found to express unusual resist-ance patterns. The prevalence of penicillin-resistant Streptococcus pneumoniae, vancomycin-resistant enterococci and methicillin-resistant S. aureus (MRSA) varied widely amongst thesentinel laboratories. The resistance rates to methicillin among S. aureusand CNS were 19.2and 38.9%, respectively, with MRSA rates in individual sentinel sites ranging from 0 to 43%.No glycopeptide resistance was seen in S. aureus, but 6.5% of CNS isolates were teicoplaninresistant and 0.5% were vancomycin resistant. Vancomycin resistance was much more frequentamong Enterococcus faecium (24.1%) than E. faecalis(0.5%) (P< 0.05), with most resistantisolates carrying vanA. The rate of penicillin resistance in pneumococci was 8.9%, and this

resistance was predominantly intermediate (7.9%), with only six hospitals reporting isolateswith high level resistance. The prevalence of erythromycin resistance among pneumococciwas 12.3%, with the majority of resistant isolates having the macrolide efflux mechanismmediated by mefE. All the organisms tested were susceptible to linezolid with MICs in the range0.124 mg/L. The modal MICs of linezolid were 1 mg/L for CNS and pneumococci, and 2 mg/L for

S. aureusand enterococci. Linezolid was the most potent agent tested against Gram-positivecocci, including multiresistant strains, and as such may prove a valuable therapeutic option forthe management of Gram-positive infections in hospitals.

*Corresponding author. Tel:44-20-8200-4400, ext. 4282; Fax: 44-20-8358-3292; E-mail: [email protected]

2000 The British Society for Antimicrobial Chemotherapy

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C. J. Henwood et al.

(CNS), enterococci and pneumococci. The isolates weretested by each sentinel laboratory against a standard set ofreference antibiotics and also linezolid, which is the firstoxazolidinone agent to be licensed. Oxazolidinones inhibitthe initiation of protein synthesis9 and linezolid shows

considerable promise in treating severe infections causedby Gram-positive pathogens.10,11

Materials and methods

Study design

Twenty-five sentinel laboratories with a wide geographicaldistribution in the UK were asked to collect and testconsecutive clinically significant isolates (defined by anintention to treat), comprising: 40 pneumococci, 40 entero-

cocci, 30 CNS and 30 S. aureus. Each laboratory was alsorequested to collect and test an additional 10 methicillin-resistant S. aureus (MRSA) isolates. Duplicate isolatesfrom the same infection episode in the same patient wereexcluded. The laboratory completed a case record form foreach isolate, giving the patients age, gender, clinical diag-nosis and the site of isolation.

Identification of bacteria by sentinel laboratories

Staphylococci were identified by Grams stain and coagu-lase reaction; pneumococci on the basis of optochin suscepti-

bility as tested on horse blood agar and enterococci with theAPI20-STREP or ATB32-STREP system (bioMrieux, LaBalme les Grottes, France) or, in the case of one labora-tory, using the BBL Crystal GP system (Becton Dickinson,Oxford, UK). CNS were not identified to species level.

Susceptibility testing by sentinel laboratories

Isolates were grown overnight on a non-selective mediumand resuspended in 0.9% MuellerHinton broth (Oxoid,Basingstoke, UK) to the density of a 0.5 McFarland stan-dard, except for mucoid pneumococci, which were resus-

pended to the density of a 1.0 McFarland standard. Thesuspensions were inoculated on to 15 cm diameter plates ofMuellerHinton agar (Oxoid). This medium was supple-mented with 5% lysed horse blood for pneumococci.Oxacillin was tested separately against staphylococci on9 cm plates with 2% NaCl added to the agar. Etest strips(Cambridge Diagnostic Services, Cambridge, UK) wereapplied to the plates and the results were read according tothe manufacturers directions. Staphylococci and entero-cocci were incubated in ambient air at 37C, whereas pneu-mococci were incubated in 5% CO2 at 3537C. MICresults were read after 24 h incubation. Oxacillin, vanco-mycin and teicoplanin results for staphylococci were re-read after 48 h, with these latter results taken as definitive.

Quality assurance

Proficiency in testing was established by asking centres totest six reference strains from the Antibiotic ResistanceMonitoring and Reference Laboratory (ARMRL) at thestart of the study and to include these with each batch ofisolates tested. These comprised S. aureus ATCC 29213,Streptococcus pneumoniae ATCC 49619 and Enterococcusfaecalis ATCC 29212 together with one multi-resistantisolate of each of these species. The sentinel laboratorieswere asked to test these strains according to the studyprotocol and to return the results. The identity of the con-trol strains was not known by the sentinel laboratories.Testing of clinical isolates was commenced only after acentre was obtaining MICs within one dilution of the MICrange designated for the control strains by the ARMRL.

Further quality control (QC) was achieved by the sen-tinel laboratories sending every 10th isolate collected to

the ARMRL for retesting. Isolates with unusual resist-ances were also collected for retesting. These comprisedany isolates with vancomycin or linezolid MICs 4 mg/L,any S. aureus with teicoplanin MICs 4 mg/L, any entero-cocci identified as a species other than E. faecalis, and anystaphylococci where routine and Etest results for oxacillindisagreed.

Re-identification at the ARMRL

Identification of enterococcal species other than E. faecaliswas undertaken using published methods.12,13

Susceptibility testing at the ARMRL

MIC determinations at the ARMRL were undertakenusing an agar incorporation method, except for trova-floxacin, where the Etest was used. MuellerHinton agar(Oxoid) was employed, supplemented with 5% lysed horseblood (Tissue Culture Services, Buckingham, UK) forpneumococci. Inocula of 104 cfu/spot were delivered with amultipoint inoculator. Incubation was as described abovefor the Etest. MICs were defined as the lowest drug con-centrations to prevent growth completely and isolates were

defined as sensitive or resistant based on NCCLS criteria.

14

All antimicrobial powders for determining MICs wereobtained from Sigma (Poole, UK), with the exceptionof ciprofloxacin (Bayer, Newbury, UK) and linezolid(Pharmacia Corporation, Milton Keynes, UK).

Molecular studies

All enterococci with vancomycin MICs 4 mg/L werecollected by the ARMRL and the presence of vanA andvanB genes was determined by PCR.15 Sentinel labora-tories were also asked to send in erythromycin-resistantpneumococci for the detection of the mefE16,17 and ermB18

genes by PCR. An Escherichia coli control strain with the

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Antimicrobial susceptibility of Gram-positive cocci in the UK

cloned ermB gene was kindly supplied by Dr MarilynRoberts from the University of Washington (Seattle, WA,USA) and an S. pneumoniae control strain with mefE byDr Virginia Shortridge from Abbott Laboratories (AbbottPark, IL, USA).

Statistical analyses

Statistical analysis was performed using the chi-squaredtest with Yates correction, and with a Pvalue0.05 indi-cating significance.

Results

Bacteria isolated

The sentinel laboratories collected and tested 3770 isolates.These comprised 1000 S. pneumoniae; 1005 S. aureus; 769CNS and 996 enterococci. The enterococci comprised 875E. faecalis, 108 E. faecium and 13 isolates belonging toother species including Enterococcus gallinarum, Entero-coccus avium and Enterococcus raffinosus. Among theS. aureus strains, 755 were consecutive isolates and 250were collected specifically as MRSA. A total of 940 isolateswere retested at the ARMRL.

Quality assurance

Two types of QC were built into the study. In the first

instance, centres were asked to test six reference strainsbefore commencing the study. These organisms comprisedthree susceptible ATCC strains and three resistantARMRL strains. The sentinel laboratories results fellwithin the ARMRL QC ranges for 90.5% and 86.7% oftests with the ATCC and ARMRL strains, respectively(Table I). Even when a sentinel laboratorys MIC resultfell outside the ARMRL MIC ranges, the deviation wascommonly only one dilution. There was 98% agreementbetween the ARMRL and the sentinel laboratories withregard to the susceptibility categorization. Categorizationagreement was maintained for S. aureus strain ARU 30300

with gentamicin, penicillin and oxacillin and for S. aureusATCC 29213 tested with erythromycin, although manysentinel laboratories obtained MIC values outside theARMRLs control range in these cases. These differenceswere not confined to a single laboratory (Table I). In thecase ofS. pneumoniae ARU 3489, where the QC rangesspanned the intermediate/resistant border, both intermedi-ate and resistant categories were accepted as correct.

As a second validation, 10% of all isolates collected wereretested centrally. For pneumococci (n 101), no differ-ence in susceptibility categories was seen for any retestedbacterium, except for one isolate found resistant to trova-floxacin by the source laboratory but deemed susceptibleon retesting. Testing for S. aureus likewise was generally

accurate, with only two S. aureus isolates identifiedincorrectly as MRSA by the sentinel laboratories. For CNS,however, MICs of oxacillin determined at the ARMRLwere often two or more dilutions higher than the Etestresults obtained by the sentinel laboratories. This did not

change resistance categorization relative to the NCCLSbreakpoint14 (MIC 0.25 mg/L) but would have resultedin under-reporting of oxacillin resistance with the BSACbreakpoint (2 mg/L). Most (95%) isolates referred asE. faecalis were found to have been identified correctlywhen retested by PCR at the ARMRL. All non-E. faecalisenterococci were sought by the ARMRL, since reports19

led us to anticipate problems with their identification usingAPI20-STREP strips. Of the 124 isolates received as non-E. faecalis, 28 (22.7%) were found to have been identifiedincorrectly. Among 106 isolates received as E. faecium,six were re-identified as E. faecalis, E. gallinarum or

E. raffinosus whereas 100 were confirmed as E. faecium. Of15 isolates received as E. gallinarum, 10 were E. faecium.

Isolates received by the ARMRL as unusually

resistant

Sentinel laboratories were also asked to refer in all isolateswith atypical resistances. Two MRSA isolates were reportedinitially as resistant to vancomycin with MICs of 8 mg/L,but were found susceptible on retesting (MIC 4 mg/L). Twocentres initially reported linezolid resistance in enterococci(a total of eight isolates) and S. aureus (a total of 12

isolates). On retesting at the ARMRL, the linezolid MICfor all these 20 isolates was 4 mg/L. All enterococci (n 38)received as vancomycin resistant were confirmed as such.

Prevalence of resistance

S. aureus. A total of 755 S. aureus isolates were tested by thesentinel laboratories as consecutive isolates, and a further250 as confirmed MRSA. Skin and wound infections werethe most frequent sources (63.2%), followed by respiratorytract infections (10.2%) and blood (7.6%). The prevalenceof MRSA amongst the consecutive isolates was 19.2%, but

this rose to 29% (114/392) if out-patient isolates wereexcluded. MRSA rates for individual hospitals varied from0 to 43%. Most methicillin-susceptible S. aureus (MSSA)isolates were resistant to penicillin (84.2%), but only 8.2%were resistant to erythromycin and 3.4% to ciprofloxacin.In contrast, 87.9% of MRSA isolates were resistant toerythromycin and 94.7% were resistant to ciprofloxacin.Resistance to gentamicin was also more prevalent amongMRSA than MSSA but remained infrequent (8.8%).

No glycopeptide-intermediate S. aureus isolates werereported, but vancomycin MICs for 5% of MRSA isolateswere on the breakpoint at 4 mg/L. Linezolid MIC rangeswere similar for MSSA and MRSA, with most MIC valuesbetween 0.5 and 4 mg/L, although one MSSA (not retested

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Table I. Susceptibility testing of control strains by 25 sentinel laboratories

Antimicrobial ARMRL QC MICNo. of labs reporting strains as

%Strain agent rangesa (mg/L) Interpretation S I R Q

S. pneumoniae linezolid 0.52 S 25 ATCC 49619 penicillin 0.251 I 2 23

erythromycin 0.030.125 S 25

cefotaxime 0.030.125 S 25 trovafloxacin 0.060.25 S 25 vancomycin 0.120.5 S 25

S. pneumoniae linezolid 0.52 S 25 ARU 3489 penicillin 14 I/R 5 20

erythromycin 416 R 25cefotaxime 14 I/R 2 2 21trovafloxacin 0.060.5 S 25 vancomycin 0.251 S 25

S. aureus linezolid 14 S 25 ATCC 29213 ciprofloxacin 0.120.5 S 25

gentamicin 0.121 S 25 oxacillin 0.120.5 S 25

penicillin 0.252 R 4 21erythromycin 0.251 S 25 teicoplanin 0.251 S 25 vancomycin 0.52 S 25

S. aureus linezolid 14 S 25 ARU 30300 ciprofloxacin >32 R 25

gentamicin 0.251 S 25 oxacillin 128>256 R 25penicillin >32 R 25erythromycin >256 R 25teicoplanin 0.52 S 25 vancomycin 0.54 S 24 1

E. faecalis linezolid 14 S 25 ATCC 29212 ampicillin 0.52 S 25

gentamicin 432 S 25 teicoplanin 0.060.25 S 25 vancomycin 14 S 24 1

E. faecalis linezolid 14 S 25 ARU 5187 ampicillin 64>256 R 25

gentamicin 256>1024 R 25teicoplanin 1664 R 7 18vancomycin >256 R 25

aRanges of MIC results were obtained from NCCLS (2000) publications for ATCC strains14 or were as determined by ARMRL for ARU strains.S, sensitive; I, intermediate; R, resistant.


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at the ARMRL) was reported with a linezolid MIC of 0.064mg/L. Modal MICs of linezolid for MRSA and MSSA wereboth 2 mg/L. The MIC90 for MRSA was 4 mg/L, comparedwith 2 mg/L for MSSA. This difference, however, reflectedonly a slightly higher proportion of MRSA for which the

MIC was 4 mg/L and the MIC95 values for both MRSA andMSSA were 4 mg/L.

Coagulase-negative staphylococci. MIC distributions andresistance rates for CNS are shown in Table II. Isolatesfrom blood or line infections accounted for 52.7% of thetotal infections. Based on the NCCLS breakpoint of 0.25mg/L, 68.9% of the CNS were oxacillin resistant, whereasonly 39% were resistant with respect to the BSAC break-point of 2 mg/L.

Teicoplanin-resistant CNS were collected at 10 hos-pitals, with local prevalence rates ranging up to 26%. These

isolates were not examined to determine whether thephenomenon was present in a single species. Four isolatesfrom different hospitals also showed reduced vancomycinsusceptibility, with MICs of 8 mg/L. Resistance to cipro-floxacin, erythromycin and gentamicin was frequent amongCNS, with prevalence rates exceeding 25%. All isolateswere susceptible to linezolid, with MICs from 0.5 to 4 mg/Land a modal value of 1 mg/L.

Enterococci. Resistance rates and MIC distributions forthe enterococci are shown in Table III. E. faecalis andE. faecium accounted for 87.9 and 10.8% of the entero-coccal isolates, respectively, after re-identification at the

ARMRL. E. faecalis isolates were mostly from urinesamples (70.5%), and were divided equally between thosefrom hospital patients and those from community patients.By contrast, virtually all the E. faecium isolates (99/108)were from hospitalized patients and many (36%) werefrom blood. About 91% of the confirmed E. faecium iso-lates were resistant to ampicillin (MIC 8 mg/L), as weresingle isolates ofE. raffinosus, E. avium and E. gallinarum.Eight E. faecium isolates were sensitive to ampicillin, withMICs 8 mg/L as confirmed at the ARMRL. One of thesewas vancomycin resistant.

Thirty-eight vancomycin-resistant enterococci (VRE)

were received from 15 hospitals. Many (31%) were E.faecium from blood and only four were E. faecalis, three ofthem from urine samples from hospitalized patients andone from a burn patient in an intensive care unit. High-levelgentamicin resistance (MIC 500 mg/L) was also moreprevalent in E. faecium (38.9%) than in E. faecalis (24.2%;P 0.05, 2 test). No resistance to linezolid was found inany of the enterococci, and MICs ranged from 1 to 4 mg/Lfor most isolates.

PCR-based identification of Van determinants wasundertaken on all 38 VRE. Twenty-six, including the fourresistant E. faecalis isolates, carried the vanA gene. Thesecharacteristically had high-level resistance to vancomycin(MIC 256 mg/L). The vanB gene was present in five

isolates and was characteristically associated with lower-level vancomycin resistance (MIC 64 mg/L) and suscep-tibility to teicoplanin (MIC 4 mg/L). The seven remainingVRE were E. gallinarum isolates with low-level resistanceto vancomycin (MICs 816 mg/L) but were susceptible

to teicoplanin. This behaviour corresponds to the VanCphenotype, which is characteristic of this species. Theseisolates did not give PCR products with primers for vanAor vanB.

Pneumococci. A total of 1000 S. pneumoniae isolates weretested by the sentinel laboratories, but two duplicates wereidentified from the case record forms received and wereexcluded. Pneumococci were most often isolated from therespiratory tract (53%), eyes (16.5%), blood (14.7%) andears (10.8%). Overall, 8.9% of the pneumococcal isolateswere resistant to penicillin. Most resistance was intermediate

(MIC 0.121 mg/L), but 1% of the isolates had high-levelresistance (MIC 2 mg/L). Resistance rates to cefotaxime(MIC 0.5 mg/L) and erythromycin (MIC 0.5 mg/L)were 2.0 and 12.3%, respectively. No resistance to linezolidor vancomycin was found and linezolid was active over anarrow range of MICs (0.1254 mg/L), with a mode of1 mg/L.

To analyse the susceptibility profiles of pneumococcimore fully, the isolates were categorized according to theirresistance to penicillin (Table IV). Of the 911 isolatessusceptible to penicillin, 10.5% were resistant to erythro-mycin and none was resistant to any other drug tested. Of78 isolates with intermediate resistance to penicillin,

15.4% had intermediate resistance to cefotaxime (MIC 0.5 mg/L) and 29.8% were resistant to erythromycin.One penicillin-intermediate isolate from a respiratory tractinfection in a community patient was resistant to trova-floxacin, with an MIC of 4 mg/L. The nine isolates resistantto penicillin were all resistant or intermediately resistant tocefotaxime (MIC 0.5 mg/L), and five were resistant toerythromycin. These nine isolates were received from sixhospitals and were isolated from sputa (six), eyes (two) andan ear (one). No highly resistant pneumococci came fromblood, but 8.2% (12/147) of the bacteraemia isolates hadintermediate resistance to penicillin and one had inter-

mediate resistance to cefotaxime.Of the 124 pneumococci with erythromycin MICs 0.5

mg/L, only 64 isolates were available for testing for thepresence ofermB and mefE. Of these, 42 (66%) had mefE,21 (33%) had the ermB gene and one had both. All theisolates with ermB were resistant to clindamycin (MICs48 mg/L, results not shown), whereas those with mefEonly were susceptible (MICs 0.25 mg/L).

Discussion

Studies of consecutive isolates are a valuable tool formonitoring antibiotic resistance. In this study, sentinel

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laboratories tested specific antibiotics against specific bac-teria to an agreed protocol and the results were pooled. Thisstrategy requires a tight quality assurance system and toachieve this, resistant and susceptible QC strains were sentto each sentinel laboratory. Only 90.5% of the sentinel lab-

oratories MICs for the ATCC strains, and 86.7% of thosefor the multi-resistant strains, fell within the ARMRLsMIC ranges. Nevertheless, deviations were mostly smalland there was excellent (98%) agreement with respect tocategorization of susceptibilities. For the isolates referredto the ARMRL as part of the second QC validation, therewas generally good agreement between the ARMRL andsentinel laboratory categorization, but some problems wereencountered in the identification of non-E. faecalis entero-cocci by the sentinel laboratories. Specifically, E. gallinarumwas often falsely identified as E. faecium. Others19 havereported the same problem, which was overcome here by

central re-identification of all non-faecalis enterococci.The overall rate of resistance to methicillin amongS. aureus isolates was 19.2%, but rose to 29% among isolatesfrom hospitalized patients and 28% among isolates frombacteraemias. Routine laboratory reports for blood cultureisolates in England and Wales, as sent to the Public HealthLaboratory Service (PHLS),2 show the prevalence ofmethicillin resistance among S. aureus, increasing from1.7% in 1990 to 34% in 1998. The latter figure is in goodagreement with the present study, allowing that only 60S. aureus bacteraemias were represented here. The MRSAprevalence rate in the UK remains lower than some south-ern European countries but is much higher than in Scandi-

navian countries.20The prevalence of methicillin resistance amongst S.

aureus varied between the sentinel hospitals: six hospitalshad methicillin resistance rates 5%, whereas others hadprevalence rates as high as 43%. Analogous surveys byAndrews et al.4,5 have revealed similar variation, and havealso shown that rates in individual hospitals can vary widelybetween two consecutive years.

Higher resistance rates to other antibiotics were seenamongst MRSA than amongst MSSA. Specifically, mostMRSA isolates were resistant to ciprofloxacin and erythro-mycin, though resistance to gentamicin remained rare (8.8%

amongst MRSA). This profile is typical of the epidemicMRSA (EMRSA) 15 and 16 types currently prevalent inthe UK.21 None of the S. aureus isolates was resistant toteicoplanin or vancomycin but MICs of these glycopeptideswere on the breakpoint of 4 mg/L for c. 5% of the isolates.All the S. aureus isolates were susceptible to linezolid withMICs 4 mg/L.

The resistance rate to oxacillin among CNS was 68.9%based on the new NCCLS breakpoint14 (0.25 mg/L) butthis would decrease to 38.9% based upon the BSAC break-point (2 mg/L). PCR examination of CNS isolates withoxacillin MICs of 0.52 mg/L showed these isolates to lackmecA (results not shown), supporting the BSAC break-point.

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TableIII.

MICdistributionforEn

terococcusfaecalisandEnterococc

usfaecium

Organism

Antimicrobialagent

NumberofisolateswithindicatedMIC(mg/L)

(no.ofisolates)breakpoint(mg/L)

0.0

3

0.0

6

0.1

25

0.2

5

0.5

1

2

4

8

16

32

64

128

%R

E.

faecalis

ampicillin,

8

1

3

9

103

556

191

12

0

(875)

gentamicin,

500

1

1

4

7

23

195

344

68

232

24.2

linezolid,

4

1

2

149

634

89

0

teicoplanin,

8

1

11

174

391

219

64

7

4

3

1

0.5

vancomycin,

4

1

1

2

87

456

324

4

0.5

E.

faecium

ampicillin,

8

1

1

1

1

4

2

6

14

12

66

90.7

(108)

gentamicin,

500

3

11

35

13

1

45

38.9

linezolid,

4

31

67

10

0

teicoplanin,

8

1

4

17

51

10

3

5

9

6

2

20.4

vancomycin,

4

1

25

49

6

5

22

24.1

MI

C50,bold.

MI

C90,underlined.


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Table IV. Activity of antimicrobials against S. pneumoniae

Organism Antimicrobial agentNumber of isolates with indicated MIC (mg/L

(no. of isolates) breakpoint (mg/L) 0.03 0.06 0.12 0.25 0.5 1 2 4 8 16 32

Penicillin penicillin, 0.12, 1 880 30 susceptible cefotaxime, 0.5, 1 862 20 23 4 (911) erythromycin, 0.25, 0.5 97 130 451 138 17 1 2 9 11 15 14

linezolid, 4 2 13 182 563 144 7 trovafloxacin, 1, 2 32 164 493 207 14 vancomycin, 1 1 3 54 415 404 34

Penicillin penicillin, 0.12, 1 30 16 18 14 intermediate cefotaxime, 0.5, 1 15 11 10 17 13 12 (78) erythromycin, 0.25, 0.5 5 8 31 10 1 1 2 3 2 1

linezolid, 4 11 50 16 1

trovafloxacin, 1, 2 3 14 44 13 2 1 1 vancomycin, 1 1 3 34 40 Penicillin penicillin, 0.12, 1 6 3 resistant cefotaxime, 0.5, 1 1 7 1 (9) erythromycin, 0.25, 0.5 3 1 2 1 1

linezolid, 4 1 6 2 trovafloxacin, 1,2 1 4 4 vancomycin, 1 4 5

All strains penicillin, 0.12, 1 879 30 30 16 18 15 9 1 (998) cefotaxime, 0.5, 1 877 31 33 22 14 18 2

erythromycin, 0.25, 0.5 102 138 487 149 18 1 5 11 15 17 16linezolid, 4 1 13 194 620 162 8 trovafloxacin, 1,2 35 179 542 224 16 1 1

vancomycin, 1 2 3 57 454 445 37

MIC50, bold.MIC90, underlined.NT, not tested.


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E. faecalis was the predominant enterococcal species,with most isolates coming from urine specimens (70.5%),collected in almost equal proportions from hospital andcommunity patients. On the other hand, the E. faeciumisolates were predominantly from hospitalized patients,

with 36% of the isolates from bacteraemias. Vancomycinresistance was seen mostly in E. faecium (24.1%) and wasmuch rarer in E. faecalis (0.5%). VanA was the most preva-lent phenotype. The prevalence rate for vancomycin resist-ance in E. faecium agreed well with data for bacteraemiasin England and Wales in 1998 (24%), as reported to thePHLS,2 but the prevalence rate for this resistance in E. fae-calis was lower. This discrepancy perhaps reflects the highproportion of isolates from urine samples from communitypatients in the present collection and the fact that 1520%of E. faecalis bacteraemias reported to the PHLS involveampicillin-resistant organisms more likely to be E. fae-

cium.

2

The overall rates of VRE in this survey were similarto those reported by Andrews et al.5 in a 19971998 UKsurvey.

The resistance rate to penicillin amongst pneumococciwas 8.9%. This shows an increase in resistance from thatfound in previous multi-centre PHLS surveys,3 which indi-cated rates of 1.5% in 1990 and 3.9% in 1995. Penicillinresistance was mostly intermediate level, with considerablevariation in prevalence between hospitals. Past surveys inthe UK have shown similar variations in resistance ratesbetween regions,22 but an upward trend in intermediateresistance is apparent at many hospitals.5 In contrast to theUSA,23,24 differences in penicillin resistance rates were not

observed in relation to age groups or isolation sites. Resist-ance to erythromycin appears to have stabilized at around813%, with no significant differences to the rates found inthe 1995 UK survey3 (P 0.05). This conclusion is alsosupported by the routine data for pneumococci from bac-teraemia, as reported to the PHLS.2 Investigation of mech-anisms of macrolide resistance amongst the S. pneumoniaeisolates by PCR suggested that mefE was more prevalentthan ermB, but the sample size was small.

Linezolid is being developed to treat serious infectionscaused by Gram-positive bacteria, including those resistantto the antibiotics currently available. Previous studies have

shown linezolid to have promising laboratory2526

andclinical1011 activity. Its in vitro activity was confirmedhere for UK isolates. Linezolid was equally active againstmethicillin-susceptible and -resistant staphylococci, vanco-mycin-susceptible and -resistant enterococci and against allpneumococci, irrespective of their penicillin or macrolideresistance. For each group of bacteria, linezolid MICsspanned a very narrow range of dilutions, with MICs neverexceeding 4 mg/L. Together with promising clinicalresults,27 the present observations suggest that linezolidwill become a valuable agent for the treatment of seriousinfections caused by Gram-positive cocci.

Acknowledgements

We would like to thank all the staff who helped at thesentinel laboratories, also Terri Parsons and Luke Tysallfrom the ARMRL for technical assistance, and Pharmacia

Corporation for financial assistance. We would also like tothank colleagues in the Laboratory of Hospital Infectionfor identifying the enterococci, Marilyn Roberts fromWashington University for the control strain for ermB andVirginia Shortridge from Abbott Laboratories for themefE control strain. Linezolid Study Group: I. Gould, K.Milne (Aberdeen Royal Infirmary, Aberdeen), N. Kirk, S.Baillie (Ashford Hospital, Ashford), A. M. Walker, K. T.Dunkin (Bangor Hospital, Bangor), J. Watts, G. Wilson(Royal United Hospital, Bath), J. Paul, L. McCormick(Royal Sussex County Hospital, Brighton), R. C. Spencer,U. NiRiain (Bristol Royal Infirmary, Bristol), D. F. J.Brown, E. Keuleyan (Addenbrookes Hospital, Cam-bridge), A. Paull, I. Hosein (University Hospital of Wales,Cardiff), P. T. Mannion, S. B. Fraser (Countess of ChesterHospital, Chester), I. Thangkhiew, G. Ackland (Coventryand Warwickshire Hospital, Coventry), D. Bullock, S.Brown (Derbyshire Royal Infirmary, Derby), P. Chadwick,J. Elliott (Hope Hospital, Salford), M. Wilcox, A. Secker(Leeds General Infirmary, Leeds), B. Oppenheim, D.Weston (Withington Hospital, Manchester), E. McKay-Ferguson (South Cleveland Hospital, Middlesbrough), B.Das, C. Jones (Milton Keynes General Hospital, MiltonKeynes), D. Crook, D. Griffiths (John Radcliffe Hospital,Oxford), D. A. B. Dance, S. Marshall (Derriford Hospital,

Plymouth), M. Dryden (Royal Hampshire County Hos-pital), A. Sefton, M. Yuan (Royal London Hospital, Lon-don), A. Bint, C. Marshall (Royal Victoria Infirmary,Newcastle upon Tyne), R. Warren, K. Howells (RoyalShrewsbury Hospital, Shrewsbury), A. MacGowan, K.Bowker (Southmead Hospital, Bristol), A. Tuck (South-ampton General Hospital, Southampton), A. Anderson,L. Smith (York District Hospital, York).

Deceased; this paper is dedicated to his memory.

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Received 19 April 2000; returned 2 July 2000; revised 21 July 2000;

accepted 15 August 2000

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