Long-Term Trends in Susceptibility of Moraxella Catarrhalis: A Population Analysis

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A retrospective, population analysis of antimicrobial susceptibility patterns 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 β-lactamase production expanded from 30 to 96% in the population. MICs of penicillin G, cefamandole, ceftriaxone, amoxycillin/clavulanate, imipenem, clarithromycin, tetracycline, ciprofloxacin and trimethoprim/sulphamethoxazole for a representative sample of 375 isolates were determined. Analyses were conducted to test for variation in susceptibility among isolates, correlations of susceptibility levels among different antimicrobial agents, and temporal patterns in susceptibility. All antimicrobials except clarithromycin displayed significant differences among isolates within years, and mean MICs of all antimicrobial agents except tetracycline and clarithromycin varied significantly between years. Temporal trends to a reduction in susceptibility were detected to four of five β-lactam antimicrobials (all except cefamandole). Significant correlations in MICs were uncovered among all pairs of four β-lactam antimicrobials in both producers and non-producers of β-lactamase. In contrast, cefamandole MICs were correlated only with ceftriaxone and penicillin, and these were limited to β-lactam producing isolates; cefamandole and amoxycillin/clavulanate showed a correlation limited to non-producing isolates. For some antimicrobials, trends toward decreasing susceptibility may have been caused by an increased proportion of β-lactamase producing isolates in the population, but the observation of significant decreases in susceptibility limited to β-lactamase-producing isolates suggests that the underlying factors were different forms of β-lactamase, β-lactamase-dependent modifiers and/or additional factors.