Use of trans-complementation method to determine the effects of various <i>ftsl</i> mutations on β-lactamase-negative ampicillin-resistant (BLNAR) <i>Haemophilus influenzae</i> strains

Haemophilus influenzae is a causative agent of serious infections, especially among children. β-lactam antibiotics are commonly used for the treatment of these infections. Among H. influenzae isolates, β-lactam resistance is due to the presence of β-lactamase, or to mutations in the ftsI gene that generate altered PBP3 (penicillin-binding protein 3) with reduced affinity for β-lactams (BLNAR-β-lactamase-negative, ampicillin-resistant). Wild-type ftsI gene encoding for PBP3 was amplified in whole from β-lactam susceptible H. influenzae Rd and cloned in pLS88 plasmid to obtain pADUTAS17, which was then used to transform known BLNAR strains, susceptible strains, and a strain (CF55) with wild-type ftsI but unexplained reduced β-lactam susceptibility. Ampicillin and cefotaxime MICs (minimum inhibitory concentration) were determined after transformation with pLS88 and pADUTAS17 plasmids. The results showed that antibiotic susceptibilities were not affected by trans-complementation for isolates carrying wild-type ftsI gene. However, trans-complementation for all BLNAR strains showed decreases between - 0.957 and 0.5-fold for ampicillin and cefotaxime, confirming the role of the PBP3 substitutions in the BLNAR phenotype of these isolates. The first article showed that trans-complementation might be a useful tool in the investigation of decreased β-lactam susceptibility in H. influenzae.