β-Lactamase production and antibiotic susceptibility pattern of Moraxella catarrhalis isolates collected from two county hospitals in China

BACKGROUND: Moraxella catarrhalis (M. catarrhalis) is an important bacterial pathogen. However, its antibiotic susceptibility patterns in different areas are difficult to compare because of the use of different methods and judgement criteria. This study aimed to determine antimicrobial susceptibility and β-lactamase activity characteristics of M. catarrhalis isolates collected from two county hospitals in China, and to express the results with reference to three commonly used judgement criteria. RESULTS: Nasopharyngeal swabs were obtained from child inpatients with respiratory tract infections at the People’s Hospital of Zhongjiang County and Youyang County from January to December 2015. M. catarrhalis strains were isolated and identified from the swabs, and susceptibility against 11 antimicrobials was determined using the E-test method or disc diffusion. Test results were interpreted with reference to the standards of the European Committee on Antimicrobial Susceptibility Testing (EUCAST), the Clinical and Laboratory Standards Institute (CLSI), and the British Society for Antimicrobial Chemotherapy (BSAC). Detection of β-lactamase activity was determined by the chromogenic cephalosporin nitrocefin. M. catarrhalis yield rates were 7.12 and 9.58% (Zhongjiang County, 77/1082 cases; Youyang County, 101/1054 cases, respectively). All isolates were susceptible to amoxicillin–clavulanic acid. The susceptibility rate to meropenem was 100% according to EUCAST; no breakpoints were listed in CLSI or BSAC. The non-susceptibility rate to sulfamethoxazole-trimethoprim differed significantly between the two hospitals regardless of the judgemnet criteria used, with isolates from Zhongjiang showing higher susceptibility to those from Youyang (Fisher’s exact test, P < 0.05). According to CLSI, the total non-susceptibility rate to erythromycin was 70.8% (Zhongjiang County, 79.2%; Youyang County, 64.3%), and the rate reached 92.1% (Zhongjiang County, 90.9%; Youyang County, 93.1%) on the basis of EUCAST or BSAC. The total positive rate of β-lactamase was 99.4% (177/178 cases) (Zhongjiang County, 100%, 77/77 cases; Youyang County, 99.0%, 100/101 cases). CONCLUSIONS: Ninety nine percent of M. catarrhalis isolates produce β-lactamase. The isolates showed poor susceptibility to ampicillin and erythromycin, and high susceptibility to the third- and fourth-generation cephalosporins and amoxicillin–clavulanic. Significant discrepancies between different antimicrobial susceptibility judgemnet criteria were noted.