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Chapter 8 : The Adaptive Genetic Arsenal of Pathogenic Vibrio Species: the Role of Integrons
Category: Environmental Microbiology
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This chapter discusses the specific roles of integrons in the adaptive capacity of the Vibrionaceae, with emphasis on pathogenic Vibrio species and antibiotic resistance. Even though many reports have demonstrated that the presence of antibiotic resistance genes in plasmids or integrons in V. cholerae was the cause of resistance to antimicrobial agents, the mechanism of resistance in other cases was unknown. Integrons likely correspond to one of the most refined tools selected by bacteria, as suggested by the data collected during the last 15 years. The authors recommend using the single term integron to describe all types of integron structures, supporting this suggestion with the fact that the different integrons use the same recombination processes and machinery. The integron gene cassettes for which an activity has been experimentally demonstrated, be they from superintegrons (SI) arrays or from soil DNA, encode proteins related to simple enzymatic functions; their recruitment is seen as providing the bacterial host with an adaptive advantage. Both experimental and phylogenetic data suggest that SIs are the source of the mobile integrons (MI) and resistance gene cassettes observed within clinical isolates.
Structural comparison of a “classical” mobile integron and the V. cholerae N16961 superintegron. (Top) Schematic representation of In40; the various resistance genes are associated with different attC sites (see text). Antibiotic resistance cassettes confer resistance to the following compounds: aacA4, aminoglycosides; qac, quaternary ammonium compounds; cmlA2, chloramphenicol; oxa9, beta-lactams. The sul gene, which provides resistance to sulfonamides, is not a gene cassette. (Bottom) The ORFs are separated by highly homologous sequences, the VCRs. See text for details.
Phylogenetic relationship of the integron intI genes among the proteobacteria. The dendrogram is based on known intI gene sequences. The tree was rooted using XerC and XerD from E. coli (Eco) and Thiobacillus denitrificans ATCC 25259 (Thd). The integrases from the five classes of MI are boxed. Abbreviations for the organism in which the integron integrases are found are as follows: Azoarcus sp. EbN1 (Azo), Dechloromonas aromatica (Dar), E. coli (Eco), Geobacter metallireducens (Gme), Listonella pelagia (Lpe), Vibrio anguillarum (Lan), Methylobacillus flagellatus (Mef), Microbulbifer degradans (Mid), Nitrosomonas europaea (Neu), P. alcaligenes (Pal), P. stutzeri BAM (PstBAM), P. stutzeri Q (PstQ), Rubrivivax gelatinosus (Rug), S. oneidensis (Son), Shewanella putrefaciens (Spu), T. denitrificans (Thd), Treponema denticola (Tde), V. cholerae (Vch), V. metschnikovii (Vme), V. mimicus (Vmi), V. parahaemolyticus (Vpa), Vibrio splendidus (Vsp), V. fischeri (Vfi), Xanthomonas campestris (Xca), X. oryzae (Xor), and Xanthomonas species (Xsp). The sources of IntI6, IntI7, and IntI8 are unknown. Branch lengths were drawn proportional to the amount of evolution based on genetic distances. Accession numbers (when available) can be found in Table 1 .
Bacterial species harboring chromosomal integrons and superintegrons