Journal of Antimicrobial Chemotherapy (1995) 36, 833-837

Susceptibility of Streptococcus pneumoniae strains isolated in Italy topenicillin and ten other antibiotics

Anna Marchese, Eugenio A. Debbia, Andrea Arvigo, Adelaide Pesce andGian Carlo Schito

Institute of Microbiology, Medical School, University of Genoa, Viale Benedetto XV, 10,16132 Genoa, Italy

The susceptibility of 312 Streptococcus pneumoniae strains isolated during 1993 in Italyhas been studied. Overall resistance to penicillin, erythromycin and co-trimoxazole was5.5, 6 and 18% respectively. The penicillin resistant isolates (nine displaying low- andeight high-level resistance) were further tested against cefotaxime, ceftriaxone,cefepime, cefodizime, ceftazidime, imipenem, meropenem and rifampicin. Thelow-level penicillin resistant strains were inhibited by all these drugs. Among the eighthigh-level penicillin resistant pneumococci, four were resistant to one or morecephalosporins. Imipenem and rifampicin remained effective against all isolates. Incomparison with other European Mediterranean countries, penicillin resistance is aminor threat at present in Italy.

Introduction

The pandemic spread of penicillin resistance in Streptococcus pneumoniae that started twodecades ago shows an uneven geographic distribution (Appelbaum, 1992). This situationis also seen in other important pathogens of community-acquired lower respiratory tractinfections. Resistance to ampicillin, one of the major therapeutic options in theseconditions also shows marked geographical variation in Haemophilus influenzae (Kayser,Morenzoni & Santanam, 1990) and Moraxella catarrhalis (Catlin, 1990). Restrictinganalysis to S. pneumoniae in Mediterranean countries, recent published data indicate thatwhile the incidence of penicillin resistance is high in Turkey (Giir et al., 1994) and Spain(Linares et al., 1992) (about 47 and 40% respectively), it is lower in France and Greecewhere figures of 19 and 14% are reported (Goldstein et al., 1994; Kanavaki et al., 1994).Surveys in Italy have suggested that penicillin resistance is less than 5% (Cocuzza et al.,1994; Schito et al., 1994). This leaves no place for complacency since penicillin resistanceis likely to spread and to cause therapeutic concern. For this reason we have now extendedthe scope of our preliminary study (Schito et al., 1994) and have evaluated the in-vitroactivity of several antibiotics against penicillin susceptible and resistant strains.

Materials and methods

A total of 312 5. pneumoniae strains were isolated during 1993 in nine laboratoriesin central and northern Italy. The identity of the centres and their contributions to

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834 A. Marchese et at.

the survey are reported in Table I. Pneumococci from all sites were dispatched to theInstitute of Microbiology of Genoa where in-vitro antibiotic susceptibility was assessedby a microdilution assay in cation-adjusted Mueller-Hinton broth with 5% lysed horseblood, as detailed in the NCCLS guidelines (NCCLS, 1994). Penicillin, erythromycin andco-trimoxazole were tested against all isolates while ceftazidime, cefotaxime, ceftriaxone,cefepime, cefodizime, imipenem, meropenem and rifampicin activities were assayed forpenicillin resistant strains only. Breakpoints selected were those specifically proposed forS. pneumoniae (NCCLS, 1994) except for ceftazidime, cefodizime and meropenem. Forthese drugs only MIC data will be reported since reliable breakpoints for these agentsare not yet available.

Results

The contributions of S. pneumoniae isolates from participating centres were variable, withthree of them (Parma, Bergamo and Genoa) securing 70.8% of all microorganisms tested(Table I). Not surprisingly penicillin resistance was unevenly distributed in these areas.Overall resistance to penicillin was 5-5%. High-level resistance (2-6% overall) was morefrequent in microorganisms isolated from paediatric patients (four out of eight strainswere recovered from children) while intermediate susceptibility (2-9% overall) occurredpredominantly in pneumococci causing respiratory tract infections in adults (six out ofnine isolates were from the respiratory tract). Resistance rates to erythromycin andco-trimoxazole were 3 8 and 4-5% respectively. As expected, a higher proportion of thepenicillin resistant pneumococci showed reduced susceptibility to the macrolide (37-5%vs 3-8% in susceptible strains) and co-trimoxazole (87-5% vs 4-5%) and the same was true,to a lesser extent, of penicillin intermediate strains (resistance to erythromycin being22-2% and to co-trimoxazole, 250%). Overall, 92 isolates of the 312 tested (29-5%)showed resistance to one or more drugs.

The small number of high- and low-level penicillin resistant strains (17) was testedagainst eight different antibiotics. The results obtained are summarised in Table II.Ceftazidime was the least effective compound, with MICs for 90% of the penicillin

Table I. Geographic origin of the 312 S. pneumoniae strains andpatterns of penicillin susceptibility

Site (No.)

Genoa (65)Parma (88)Bergamo (68)Turin (15)Perugia (16)Novara (14)Bologna (19)Florence (17)Milan (10)

Total (312)

Penicillin susceptibility

susceptible(< 006 mg/L)

60 (92-3)84 (95-4)65 (95-5)14 (94-3)14 (87-5)12 (87-7)19 (100)17(100)10 (100)

295 (94.5)

no. (%)intermediate(01-1 mg/L) (

2(3-1)2 (2-3)1 (1-5)0(0)2 (12-5)2 (14-3)0(0)0(0)0(0)

9 (2.9)

resistant> 2 mg/L)

3 (4-6)2 (2-3)2 (30)1 (6-7)0(0)0(0)0(0)0(0)0(0)8 (2.6)

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Susceptibility of S. pneumoniae to ten antibiotics 835

Table n. In-vitro activity of eight antibiotics against 17 high- and low-level penicillinresistant S. pneumoniae strains isolated in Italy

Drug

CefotaximeCeftriaxoneCefepimeImipenemRifampicinCeftazidime4

Cefodizime*Meropenem''

MIC rangemg/L

<0125-^ts£ 0-125-4

0-125-4<0125=$0015

2-5*640-25-1

=€0125

MICJOmg/L

0-2501250125

=€0125=$0015

80-5

*S 0-125

MIC*,mg/L

10-50-250125

=€001532

10125

No. and(%) susceptible"

13(76-5)15 (88-2)16(94-1)17 (100)17 (100)

•Breakpoints suggested by NCCLS (1994) for 5. pneumoniae.'Breakpoints for these drugs against S. pneumoniae not available in the NCCLS (1994) guidelines.

resistant pneumococci of 32 mg/L. On the other hand, all other cephalosponns displayedhigher potency with MIC&JO of 1 mg/L of cefotaxime and cefodizime, 0-5 mg/L ofceftriaxone and 0-25 mg/L of cefepime. The two carbapenems showed greateractivity against all strains, with MIC&w of 0-125 mg/L. Rifampicin had the highest w/wpotency, with MIC90 values below 0-015 mg/L. While the nine low-level penicillin resistantisolates were inhibited by low concentrations of the drugs tested, the susceptibility of theresistant pneumococci is shown in Table III. Among the Italian strains, one (D89) wastotally resistant to the extended spectrum cephalosporins tested and another (D306) wasinsusceptible to ceftazidime, cefotaxime and ceftriaxone. D254 and D260 displayedintermediate susceptibility to these third generation parenteral cephalosporins. Imipenemand rifampicin inhibited all strains tested. MICs of meropenem were similar to those ofimipenem, while cefodizime MICs were lower than those of cefotaxime but higher thanthose of ceftriaxone. Four isolates out of 312 (1-3%) showed high level penicillinresistance and reduced susceptibility to extended spectrum cephalosporins.

Penicillin-resistant S. pneumoniae seems to pose a modest threat in Italy as yet.Available information (Cocuzza et al., 1994; Schito et al., 1994) suggests an overallresistance rate of about 4%. When compared with figures for Turkey, Spain, France and

Table in . In-vitro activity of eight antibiotics against high-level penicillin resistant (MIC^ 2 mg/L) S. pneumoniae strains isolated in Italy

Strain

D89D123D191D234D254D257D260D306

CTX

40-50125012510-2512

CRO

40-5012501250125012501254

CFM

40-501250-1250-2501250-250125

MICIMI

0125012501250-250125012501250125

(mg/L)RIF

00070007000700070007000700070007

CAZ

32326432

^64321616

CDZ

0-50-510-510-511

MER

=€0-125=€0125=€0125=$0125=50-125«: 0-125=€0125=€0125

CTX, Cefotaxime; CRO, ceftriaxone; CFM, cefepime; IMI, imipenem; RIF, rifampicin; CAZ,ceftazidime; CDZ, cefodizime; MER, meropenem.

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836 A. Marchese et al.

Greece (Linares etal., 1992; Goldstein etal., 1994; Gur etal., 1994; Kanavaki etal., 1994)that are three to ten-fold higher, the Italian situation is surprising and its possible originis of interest. It is notable that other primary respiratory pathogens such as H. influenzaeand M. catarrhalis show similar trends. In a European study, Kayser et al. (1990) assessedthe incidence of resistance to several antibiotics in Spanish, French and Italian H.influenzae strains and concluded that rates were higher in Spain, intermediate in Franceand low in Italy. For example, ampicillin resistance was 311, 13-4 and 40% in theserespective countries (Kayser et al., 1990). Reports for M. catarrhalis are fewer, althougha multicentre survey in Italy (Stefani et al., 1993) indicated that /Mactamase-mediatedampicillin resistance did not exceed 54-4%, a figure well below those reported elsewhere(Catlin, 1990). The conclusion can therefore be drawn that the selective pressuresoperating in these neighbouring countries must be remarkably different. Since thepatterns of antibiotic susceptibility are determined by the local drug prescribing habits,major variations in these practices seem likely. A recent survey by Halls (1993) hashighlighted the fact that Italy is the only country in Europe where over 80% ofantibiotic courses are given by injection, (about half being intramuscular). This isprevalent in both hospital and community practice. Cephalosporins are themost frequently used drugs in Italy and Spain but in Italy third generation injectablecephalosporins are more widely used than earlier agents (Halls, 1993). These practicesmay have influenced the patterns of resistance in respiratory pathogens. Highereradication rates are more likely following parenteral therapy because subinhibitoryconcentrations are sustained for shorter periods compared with oral drugs and thereforeless frequently associated with selective pressure. The preference for third generationcephalosporins may further explain why the incidence of methicillin-resistantstaphylococci and multiresistant enterococci is generally higher in this country (Vosset al., 1994).

Italian rates of resistance to penicillin, erythromycin and co-trimoxazole inS. pneumoniae are more like those of northern Europe than the Mediterranean area(Appelbaum, 1992). Prevention of penicillin resistance in S. pneumoniae seems to beassociated with at least two different approaches. Strict conservative antibiotic usage inScandinavia, the Netherlands, the United Kingdom, Belgium and Finland (Appelbaum,1992) or use of new and highly active agents as in Germany and Italy (Halls, 1993) bothapparently lead to low rates of resistance (less than 5%). Countries with differentprescribing patterns may run a higher risk, as reported in Turkey, Spain, France andGreece. Both approaches may partially preserve the value of the non-/?-lactam antibiotics,but the evolution of resistance to /Mactams may be different. In areas of the world wherepenicillins are the dominant selective force, most penicillin resistant pneumococci aresusceptible to the extended spectrum injectable cephalosporins. Conversely, oralcephalosporins promote mutants with higher MICs of parenteral third generationcephalosporins than of penicillin (Appelbaum, 1992). In this survey, the penicillinsusceptible strains were not tested against cefotaxime, ceftriaxone, and imipenem whichmay be recommended for the empiric therapy of resistant S. pneumoniae infections.Therefore it is far from reassuring to observe that among the eight high-level penicillinresistant pneumococci isolated, four expressed total or intermediate resistance to thesedrugs. This new observation may be a consequence of extensive use of parenteral thirdgeneration cephalosporins in Italy and may be a price for their clinical efficacy (Halls,

1993) and limited overall resistance. A more detailed analysis of S. pneumoniae strainscirculating in Italy is warranted in order to assess whether resistance induced by these

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Susceptibility of 5. pneumoniae to ten antibiotics 837

compounds is associated with pneumococci which are invariably penicillin resistant orwhich remain penicillin susceptible. If the latter were found to be the case, the policy ofin-vitro antibiotic susceptibility testing may have to be revised, undertaking routine testsfor reserve drugs previously assayed as second-line agents.

Acknowledgements

This investigation was supported in part by a FATMA grant (17/93.00241) from theItalian National Research Council and by Roche Italy SpA.

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(Received 13 February 1995; returned 6 April 1995; revised 16 June 1995; accepted 24 July 1995)

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