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Beta-lactamases and antibiotic susceptibilities of Moraxella catarrhalis isolates from Zagreb, Croatia

Moraxella catarrhalis a commensal of the nasopharynx has been recognized with increasing frequency as a potential pathogen in respiratory tract infections. It is an important causative agent of lower respiratory tract infections, otitis media and sinusitis. The β -lactamase production in M. catarrhalis first described in 1977 has been reported with increasing frequency in many countries of the world. However there were no reports from Croatia so far. The aim of this study was to investigate antibiotic susceptibilities and β -lactamase production in M. catarrhalis isolates from Croatia. Fifty M. catarrhalis strains were collected in University Children's Hospital in Zagreb during 1990-1991 from various clinical speciments (nasopharingeal swabs, middle ear fluids, sputum, bronchoaspirates). Antibiotic susceptibilities to wide range of antibiotics were determined by broth microdilution method according to National Committee for Clinical Laboratory Standards. Β -lactamases were detected with commercially available chromogenic cephalosporin disk β -lactamase test containing nitrocefin as the substrate. The substrate profile of the enzymes was determined by biological assay. β -lactamases were detected by disk chromogenic substrate (nitrocephin) test in all strains. Incidence of β -lactamase positive strains in the study period was 100%. The antibiotic concentration which inhibits 90% of the strains was 0.12 mg/L for azithromycin, 0.25 mg/L for eritromycin, tetracycline and amoxycilline/clavulanic acid, 1 mg/L for ceftibuten and chloramphenicol, 4 mg/L for cefadroxil, 16 mg/L for cefprozil, cefuroxime and cephalexin and 32 mg/L for amoxycilline. 80% (40/50) of the strains were resistant to amoxycillin, 4% (2/50) to cefadroxil and 2% (1/50) to cefuroxime and cefprozil. No resistance to cephalexin, ceftibuten, tetracycline, erythromycine, azithromycine and chloramphenicol was observed. The activity of amoxycillin was strongly enhanced in the presence of clavulanic acid in all strains. Older cephalosporins were equally active but third generation cephalosporin ceftibuten displayed significantly lower MICs compared to older compounds. Among non β -lactam antibiotics tetracycline and erythromycine showed similar activity. Azithromycine had markedly stronger inhibitory activity on comparison with erythromycine and tetracycline. Crude b-lactamases extracts from M. catarrhalis strains antagonized the activities of disks containing amoxicillin (30 mg). Enzymes did not affect the inhibition zones around cephalosporin disks. Incidence of β -lactamase producing M. catarrhalis strains is higher in Croatia than in other countries of the world except of Japan.The results of the susceptibility tests were in concordance with the results reported by other investigators. Β -lactamase positive strains of M. catarrhalis are generally susceptible to all antibiotics except of unprotected penicillins. In order to prevent the emergence of antibiotic resistant isolates, the rational use of antimicrobials and monitoring of the antibiotic resistance of respiratory pathogens will be required. Wide range of MICs for amoxycillin alone and combined with clavulante could be attributed to various level of enzyme production by M. catarrhalis strains According to our results amoxycillin combined with clavulanic acid should be the antibiotic of choice for the treatment of infections caused with β -lactamase positive isolates of M. catarrhalis. Oral cephalosporins, tetracycline, macrolides or azithromycine could be an option too.

Moraxella catarrhalis; antibiotic susceptibilities; beta-lactamases

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