Integrons and Superintegrons

Numerous studies have appeared that describe the importance of resistance integrons (RIs) and superintegrons (SIs) in antibiotic resistance, microbial physiology, and environmental adaptation in phylogenetically diverse gram-negative bacteria. This chapter describes the genetic organization of an integron; summarizes the different classes of RIs and highlights their importance in antibiotic resistance; describes the organization of an SI; and highlights the structure of the key component of the integron, the integrase, and its binding to an attC site. There are two types of recombinases: tyrosine recombinases (integrases) and serine recombinases (resolvases or invertases). The attC site is an imperfect inverted repeat located at the 3' end of the gene. Cassettes are always integrated in the same orientation and are cotranscribed from one or two common promoters (P1 or P2) located in the 5' conserved segment (CS). To date, five distinct integron classes have been found associated with cassettes that contain antibiotic resistance genes. Three main classes of integrons (classes 1, 2, and 3) have been described in gram-negative bacteria. The similarity between the three integrases (40 to 58% genetic identity) suggests that their evolutionary divergence extended beyond the introduction of antibiotics into clinical medicine.