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Antimicrobial Agents and Chemotherapy, June 2007, p. 2192-2194, Vol. 51, No. 6
0066-4804/07/$08.00+0     doi:10.1128/AAC.00139-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Longitudinally Tracking Fluoroquinolone Resistance and Its Determinants in Penicillin-Susceptible and -Nonsusceptible Streptococcus pneumoniae Isolates in Hong Kong, 2000 to 2005

Margaret Ip,^* Shirley S. L. Chau, Fang Chi, Edmund S. C. Cheuk, Helen Ma, Raymond W. M. Lai, and Paul K. Chan

Department of Microbiology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong

Received 30 January 2007/ Returned for modification 21 February 2007/ Accepted 12 March 2007

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Of 1,388 Streptococcus pneumoniae isolates collected from 2000 to 2005, 10.5% had a ciprofloxacin MIC of 4.0 µg/ml and 1.6% (range, 0.8% to 4.3% per year) had a levofloxacin MIC of 4.0 µg/ml. Molecular characterization indicated that fluoroquinolone resistance occurred independently in our prevalent Spain^23F-1 clone, expressing serotypes 23F, 19F, and 14. Rates of resistance to levofloxacin in S. pneumoniae have remained stable at a Hong Kong hospital.

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Drug-resistant Streptococcus pneumoniae is a major concern in Hong Kong. The rate of fluoroquinolone resistance in S. pneumoniae has been reported as one of highest in the world, with a levofloxacin resistance rate (MIC of 4 µg/ml) of 13.3% (7). Among invasive isolates, a resistance rate of 3.8% was documented, although it increased to 15.2% in penicillin-resistant S. pneumoniae isolates (8). These strains have been linked to penicillin-nonsusceptible pneumococcal (PNSP) clones (8, 16). In Canada, the rate of ciprofloxacin resistance in S. pneumoniae increased from <1% in 1997 to 4.2% in 2005, with a rise in the levofloxacin resistance rate to 1.1% (1). In the United States, levofloxacin resistance increased from 0.6% to 1.3% (18). Among those strains with a levofloxacin MIC of 2 µg/ml, first-step mutations were present in 16.2% of isolates recovered from nursing home residents, compared to 6.5% of isolates recovered from others (17). Such strains have been considered precursors to fully resistant strains (3, 21). Treatment failure with fluoroquinolones due to selection with second-step mutation has been reported (5, 14).

At the Prince of Wales Hospital (PWH), a 1,350-bed teaching hospital in Hong Kong, we longitudinally tracked the prevalence of fluoroquinolone resistance in S. pneumoniae isolates for the period 2000 to 2005. The clonal relationships of S. pneumoniae isolates with a ciprofloxacin MIC of 4 µg/ml as determined by serotyping and pulsed-field gel electrophoresis (PFGE) and the mutations at the quinolone resistance-determining region (QRDR) of the gyrA, gyrB, parC, and parE genes were studied.

Bacterial isolates and susceptibility tests. A total of 1,388 nonduplicate S. pneumoniae isolates from the blood and sputa of patients admitted to the PWH were examined. The MICs of penicillin, cefotaxime, ciprofloxacin, and levofloxacin were determined by the broth dilution method described by the Clinical and Laboratory Standards Institute (2).

PFGE and serotyping. One hundred eight isolates with ciprofloxacin MICs of 4 µg/ml were examined by PFGE as described previously (6, 11). Serotyping was performed using Pneumotest antisera (Statens Serum Institute, Copenhagen, Denmark). Representative isolates of the Spain^23F-1 clone which produced the indistinguishable PFGE pattern type A were included. Other PFGE types and subtypes were assigned as done by Tenover et al. (23).

Analysis of fluoroquinolone resistance genes. The mutations in the QRDRs of the gyrA, gyrB, parC, and parE genes were examined by PCR-restriction fragment-single-strand conformational polymorphism analysis as described previously (13). Some of these amino acid substitutions have previously been published (13).

The overall levofloxacin resistance rate was 1.6% (range, 0.8% to 4.3%) (Table 1). The mean percentage of isolates with a ciprofloxacin MIC of 4.0 µg/ml was 10.5% (range, 7.4% to 24.5%). Between 2001 and 2005, 55.6% (720/1,294) of isolates were PNSP (range, 49.2% to 64.5% over the 5 years). The percentages of isolates for which ciprofloxacin MICs were 4.0 µg/ml and which were penicillin susceptible and penicillin nonsusceptible were 10% (58/579) and 11.2% (91/809), respectively, while the rates of levofloxacin resistance were 0.5% and 2.2%, respectively. The MIC₅₀ and MIC₉₀ of levofloxacin for both penicillin-susceptible and PNSP isolates were 1.0 and 2.0 µg/ml, respectively. The MIC₅₀ of ciprofloxacin was 1.0 µg/ml for penicillin-susceptible isolates but 2.0 µg/ml for PNSP isolates, while the MIC₉₀ for both was 4.0 µg/ml.

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TABLE 1. Percentages of fluoroquinolone resistance in Streptococcus pneumoniae isolates at the PWH from 2000 to 2005

The characteristics of S. pneumoniae strains with ciprofloxacin MICs of 4.0 µg/ml are listed in Table 2 . The most common serotype was 23F (35 isolates), followed by 19F (25 isolates). Other serotypes included 4, 6B, 6 (non-B), 8, 9, 14, 17, 19 (non-F), and 22, and the remainder were nontypeable. The most common PFGE pattern type, A, which was indistinguishable from that of the isolates of the Spain^23F-1 clone, was present in 46 of 108 isolates. The majority of these isolates expressed capsular type 23F or 19F, were penicillin nonsusceptible and either susceptible or intermediate to cefotaxime, and had levofloxacin MICs ranging from 0.5 to >8.0 µg/ml. All but three isolates with an amino acid substitution in the gyrA gene (S81F) had the PFGE type A pattern. These three isolates had one or more mutations in the parC gene (S79F, D83N, and/or K137N) and a mutation in the parE gene (I460V). Of the 39 remaining isolates that had a PFGE type A pattern and that did not have an amino acid substitution in GyrA, 36 already had an amino acid substitution at ParC (K137N). Overall, 7.2% (63/881) of S. pneumoniae isolates collected from 2000 to 2003 with ciprofloxacin MICs of 4.0 µg/ml had one or more amino acid substitutions at GyrA and/or ParC, while 6.0% (52/870) had an amino acid substitution at ParC alone. Besides PFGE type A, PFGE patterns were mostly distinct and diverse (PFGE patterns not shown). These isolates mainly had a single ParE amino acid substitution (I460V).

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TABLE 2. Characteristics of S. pneumoniae isolates with ciprofloxacin MICs of 4.0 µg/ml in relation to their PFGE types, serotypes, antibiotic susceptibilities, and amino acid substitutions at QRDR of fluoroquinolone resistance determinants

PNSP infection rates rose rapidly in Hong Kong in the 1990s (10, 12) and accounted for 61% of all S. pneumoniae infections by 2000 (7). A gradual reduction in PNSP infection since 2003 has been noted. The pneumococcal conjugate vaccine was not made available in Hong Kong until mid-2006, and thus the decrease in infection rates was unlikely due to its effects. Levofloxacin resistance rates remained low at the PWH, contrary to the high rates previously reported in other Hong Kong hospitals (7). Levofloxacin-resistant S. pneumoniae has been associated with nosocomial origins, older patients (75 years old) with chronic obstructive airway disease, nursing home residents, and prior fluoroquinolone treatment (9). A study of the susceptibility of community S. pneumoniae isolates from Hong Kong in 2003 yielded 0% resistance to respiratory fluoroquinolones (15), indicating that levofloxacin-resistant S. pneumoniae strains are likely confined to specific risk groups. Various local dose regimens using 100-mg levofloxacin tablets might have partially contributed to high levofloxacin resistance rates.

The majority of levofloxacin-resistant S. pneumoniae strains originated from the Spain^23F-1 clone, but they expressed various capsular types (8, 16). Penicillin-susceptible fluoroquinolone-resistant strains were also noted. With the heterogeneity of the mutations at the QRDRs, it is likely that fluoroquinolone resistance developed independently rather than from the dissemination of a fluoroquinolone-resistant variant (7). An estimated rate of mutation to fluoroquinolone resistance in Hong Kong was 2.9% (2 in 70 strains) (16) and appeared compatible with our setting. However, levofloxacin-susceptible isolates of the Spain^23F-1 clone with a ParC amino acid substitution (K137N) and a ParE amino acid substitution (I460V) were not infrequently seen in this study and may require only a second-step mutation to become resistant (4, 22). These strains may not be detected readily with current susceptibility breakpoints for respiratory fluoroquinolones (19, 20) and will necessitate detection of their mutations. The administration of fluoroquinolone in adequate doses to achieve high area under the concentration-time curve/MIC breakpoints for levofloxacin (200) and moxifloxacin (400) has been suggested to ensure the eradication of S. pneumoniae (3).

Fluoroquinolone resistance is likely to have occurred independently in our prevalent Spain^23F-1-23F, -19F, and -14 clones. Rates of resistance to levofloxacin in S. pneumoniae isolates have remained stable at a Hong Kong teaching hospital during the last 6 years.

 
This study was supported by an Earmarked Grant (CUHK 4432/03 M) from the Research Grants Council, Hong Kong, Special Administrative Region, People's Republic of China.

 
* Corresponding author. Mailing address: Department of Microbiology, The Chinese University of Hong Kong, Prince of Wales Hospital, Hong Kong. Phone: (852) 2632 1265. Fax: (852) 2647 3227. E-mail: margaretip{at}cuhk.edu.hk

Published ahead of print on 19 March 2007.

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Antimicrobial Agents and Chemotherapy, June 2007, p. 2192-2194, Vol. 51, No. 6
0066-4804/07/$08.00+0     doi:10.1128/AAC.00139-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ip, M. Articles by Chan, P. K. Search for Related Content PubMed PubMed Citation Articles by Ip, M. Articles by Chan, P. K.