Antimicrobial resistance of Moraxella catarrhalis isolates in Taiwan

Background/PurposeThe prevalence of ampicillin-resistant Moraxella catarrhalis has been higher in Taiwan than in other countries, with reports of 97.7% in the 1990s. The aims of this study were to assess resistance trends for M. catarrhalis, which causes respiratory tract infections, against several classes of oral antibiotics and to compare the minimum inhibitory concentration (MIC) of antimicrobial agents against M. catarrhalis isolates between 1993–1994 and 2001–2004.MethodsClinical isolates of M. catarrhalis (n = 314) were collected from 11 large medical centers in Taiwan between 2001 and 2004. β-Lactamase production tests were performed. The MICs for 13 different oral antibiotics were calculated using the agar dilution method. Pulsed-field gel electrophoresis (PFGE) was performed for 18 randomly selected high-level ampicillin-resistant (BRO-1 β-lactamase-positive, MIC ≥ 32 μg/mL) isolates to investigate their genetic relatedness.ResultsThe overall rate of β-lactamase-producing isolates was 97.8% (307/314). All isolates were susceptible to amoxicillin + clavulanate, chloramphenicol, cefixime, ciprofloxacin, erythromycin, levofloxacin, moxifloxacin, and roxithromycin. The rate of resistance to cefaclor and cefuroxime was 8.3% and 1.3%, respectively, while no resistance was found in 1993–1994. Resistance to trimethoprim–sulfamethoxazole (SXT) and tetracycline was 18.5% and 19.8%, respectively. Comparison of 1993–1994 and 2001–2004 isolates revealed that the zone diameter for amoxicillin + clavulanate disks decreased from 43 mm in 1993–1994 to 32 mm in 2001–2004 (p < 0.001). However, MIC50 (0.25 μg/mL in both 1993–1994 and 2001–2004) and MIC90 (0.5 μg/mL in both 1993–1994 and 2001–2004) for amoxicillin + clavulanate did not differ between the study periods. The PFGE typing results demonstrate that at least two closely related BRO-1 clones are spreading in Taiwan.ConclusionThe rates of resistance to cefaclor, cefuroxime, tetracycline and SXT are now increasing in Taiwan. Molecular typing showed that at least two closely related BRO-1 clones are circulating. Although amoxicillin + clavulanate remains the antimicrobial therapy of choice for M. catarrhalis infections, continued surveillance of antimicrobial susceptibility and application of control measures against further transmission are required to inhibit the emergence of the resistant strains.