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The complement phenotypes of Moraxella catarrhalis isolates obtained from adult patients with acute laryngitis were investigated using a microliter serum bactericidal assay and compared with those of other donor groups. Laryngitis isolates had a higher proportion (57%) of complement-resistant strains than did carrier strains from healthy 8- to 13-year-old schoolchildren (16%). The difference between these groups was statistically significant (chi2 [3 x 2 table] = 21.55; P < .001). The relatively frequent occurrence of the complement-resistant (virulence-associated) phenotype in adults with acute laryngitis supports the theory of an active role of M. catarrhalis in the pathogenesis of acute laryngitis.

The purpose of this study was to investigate complement resistance in Branhamella (Moraxella) catarrhalis isolated from healthy schoolchildren or sputum-producing adult patients. Two techniques were used: a serum bactericidal assay as the gold standard and an easier 'culture and spot' test. Children (age 4-13; n = 303) and patients (n = 1047) showed high colonization/infection rates with B. catarrhalis (31% and 19%, respectively). Complement resistance or intermediate sensitivity occurred frequently in patient isolates (62% and 27%, respectively) and less often in children (33% and 8.5%, respectively; P << 0.0001). In young children (age 4-5 years), the proportion of complement-resistant strains was around 50%. Complement resistance in B. catarrhalis is associated with illness and may hence be considered a virulence factor.

Recently, we showed that complement resistance is an important virulence factor of Moraxella (Branhamella) catarrhalis. Our study used a serum bactericidal assay to determine complement resistance in M. catarrhalis. Although the serum bactericidal assay is considered the "gold standard" for determining complement resistance, it is laborious and time-consuming and therefore not well suited for large-scale studies. Using a large number (n = 324) of M. catarrhalis isolates obtained from the sputa of patients with lower respiratory tract infections (n = 200) and young carriers (n = 124), we assessed the value of a simple "culture-and-spot" test as an alternative to the serum bactericidal assay. For both groups of isolates, the degree of concordance between the two tests used was very significant (P < 0.0001). The agreement between the two assays was estimated to be "excellent beyond chance" (as determined by Cohen's kappa test). The culture-and-spot assay is a valuable alternative to the serum bactericidal assay, not only for screening purposes as shown here but also for studying the mechanism of complement resistance in M. catarrhalis at the molecular level.

Resistance of microorganisms to antimicrobial agents is an increasing problem in the treatment of infectious diseases. In mixed infections, an interesting development can arise when one organism protects another from being killed by an antibiotic. Unfortunately, in the case of respiratory tract infections, experimental evidence of this development is poor. In this study, mice intranasally infected with a lethal number of pneumococci and treated with a curative dose of penicillin or amoxicillin died from pneumococcal pneumonia when they were coinoculated with beta-lactamase-producing Moraxella catarrhalis. beta-lactamase-negative M. catarrhalis did not show a similar indirect pathogenic effect. Treatment with a combination of amoxicillin and the beta-lactamase inhibitor clavulanic acid was not affected by beta-lactamase-producing M. catarrhalis. These findings help explain antibiotic failure in respiratory tract infections, even though the causative microorganism is sensitive to the antibiotic in vitro.