Characterization of extended-spectrum β-lactamase-producing isolates of Haemophilus parainfluenzae

Objectives: To characterize the β-lactam resistance mechanisms of two clinical isolates of cefotaxime-resistant Haemophilus parainfluenzae recovered from patients in South Africa. Methods: The relatedness of isolates and plasmids was assessed using PFGE and restriction enzyme analysis, respectively. Plasmid-mediated and chromosomally integrated bla_TEM genes and ftsI genes were sequenced, and the plasmid-mediated bla_TEM-15 was used to transform a range of control organisms. Results: The two isolates were found to be unique according to PFGE, but had an identical 3.7 kb plasmid encoding a TEM-15 β-lactamase. Both isolates also had substitutions in penicillin binding protein 3 (PBP3) consistent with substitutions known to exist in β-lactamase-negative ampicillin-resistant (BLNAR) strains of Haemophilus influenzae. The cefotaxime MICs for control strains of H. influenzae, H. parainfluenzae and BLNAR H. influenzae transformed with the plasmid-mediated bla_TEM-15 were 1.0, 1.0 and 4.0 mg/L, respectively, compared with 16.0 and 8.0 mg/L, respectively, for the two parent H. parainfluenzae. Conclusions: The high-level cefotaxime resistance in the H. parainfluenzae isolates was due to a combination of a plasmid-mediated TEM-15 extended-spectrum β-lactamase with altered PBP3 probably contributing. Other contributing resistance mechanisms could not be excluded.