Role of Horizontal Gene Transfer in the Development of Multidrug Resistance in Haemophilus influenzae

Abstract

<i>Haemophilus influenzae</i> colonizes the respiratory tract in humans and causes both invasive and noninvasive infections. Resistance to extended-spectrum cephalosporins in <i>H. influenzae</i> is rare in Europe. In this study, we defined acquired resistance gene loci and <i>ftsI</i> mutations in multidrug-resistant (MDR) and/or PBP3-mediated beta-lactam-resistant (rPBP3) <i>H. influenzae</i> strains, intending to understand the mode of spread of antibiotic resistance determinants in this species. Horizontal transfer of mobile genetic elements and transformation with resistance-conferring <i>ftsI</i> alleles were contributory. We found one small plasmid and three novel integrative conjugative elements (ICEs) which carry different combinations of resistance genes. Demonstration of transfer and/or ICE circular forms showed that the ICEs are functional. Two extensively MDR genetically unrelated <i>H. influenzae</i> strains (F and G) from the same geographical region shared an identical novel MDR ICE (Tn<i>6686</i>) harboring <i>bla</i> _TEM-1, <i>catA2</i>-like, and <i>tet</i>(B). The first Nordic case of MDR <i>H. influenzae</i> septicemia, strain 0, originating from the same geographical area as these strains, had a similar resistance pattern but contained another ICE [Tn<i>6687</i> with <i>bla</i> _TEM-1, <i>catP</i> and <i>tet</i>(B)] with an overall structure quite similar to that of Tn<i>6686.</i> Comparison of the complete <i>ftsI</i> genes among rPBP3 strains revealed that the entire gene or certain regions of it are identical in genetically unrelated strains, indicating horizontal gene transfer. Our findings illustrate that <i>H. influenzae</i> is capable of acquiring resistance against a wide range of commonly used antibiotics through horizontal gene transfer, in terms of conjugative transfer of ICEs and transformation of chromosomal genes.<b>IMPORTANCE</b> <i>Haemophilus influenzae</i> colonizes the respiratory tract in humans and causes both invasive and noninvasive infections. As a threat to treatment, resistance against critically important antibiotics is on the rise in <i>H. influenzae</i> Identifying mechanisms for horizontal acquisition of resistance genes is important to understand how multidrug resistance develops. The present study explores the antimicrobial resistance genes and their context in beta-lactam-resistant <i>H. influenzae</i> with coresistance to up to four non-beta-lactam groups. The results reveal that this organism is capable of acquiring resistance to a wide range of commonly used antibiotics through conjugative transfer of mobile genetic elements and transformation of chromosomal genes, resulting in mosaic genes with a broader resistance spectrum. Strains with chromosomally mediated resistance to extended-spectrum cephalosporins, co-trimoxazole, and quinolones combined with mobile genetic elements carrying genes mediating resistance to ampicillin, tetracyclines, and chloramphenicol have been reported, and further dissemination of such strains represents a particular concern.