Long-term trends in susceptibility of Moraxella catarrhalis: a population analysis

A retrospective, population analysis of antimicrobial susceptibility patter ns was performed on Moraxella catarrhalis isolates recovered from a single medical centre to detect temporal trends and infer potential mechanisms of reduced susceptibility. The duration of this study, June 1984 to July 1994, encompassed the period during which the frequency of beta-lactamase produc tion expanded from 30 to 96% in the population. MICs of penicillin G, cefam andole, ceftriaxone, amoxycillin/clavulanate, imipenem, clarithromycin, tet racycline, ciprofloxacin and trimethoprim/sulphamethoxazole for a represent ative sample of 375 isolates were determined. Analyses were conducted to te st for variation in susceptibility among isolates, correlations of suscepti bility levels among different antimicrobial agents, and temporal patterns i n susceptibility. All antimicrobials except clarithromycin displayed signif icant differences among isolates within years, and mean MICs of all antimic robial agents except tetracycline and clarithromycin varied significantly b etween years. Temporal trends to a reduction in susceptibility were detecte d to four of five beta-lactam antimicrobials tall except cefamandole). Sign ificant correlations in MICs were uncovered among all pairs of four beta-la ctam antimicrobials in both producers and non-producers of beta-lactamase. In contrast, cefamandole MICs were correlated only with ceftriaxone and pen icillin, and these were limited to beta-lactam producing isolates; cefamand ole and amoxycillin/clavulanate showed a correlation limited to non-produci ng isolates. For some antimicrobials, trends toward decreasing susceptibili ty may have been caused by an increased proportion of beta-lactamase produc ing isolates in the population, but the observation of significant decrease s in susceptibility limited to beta-lactamase-producing isolates suggests t hat the underlying factors were different forms of beta-lactamase, beta-lac tamase-dependent modifiers and/or additional factors.