Integrons: the hidden architects of bacterial adaptation, evolution, and the challenges of antimicrobial resistance.

Integrons, a diverse group of genetic elements, have emerged as key players in bacterial adaptation and evolution. These elements, commonly found in both environmental as well as clinical settings, facilitate the acquisition, exchange, and expression of integron cassettes, allowing bacteria to rapidly adapt to changing environments and acquire antibiotic resistance. This review provides an in-depth exploration of the various classes of clinical integrons, including class 1, 2, and 3, highlighting their origins, distribution, and associated mobile elements. We delve into the astonishing success of "class 1 integrons", emphasizing their ability to recognize diverse attachment sites known as "attC sites" and getting integrated within many different integron cassettes from diverse sources. Class 1 integrons are able to propagate widely among bacterial hosts due to their lack of host specificity, interaction with transposons, and broad host range plasmids. Moreover, we discuss the substantial impact of class 1 integrons in antimicrobial resistance, as they accumulate an array of resistance genes through strong positive selection. Additionally, we address the challenging issue regarding the evolution and function of integrons and integron cassettes, including the role of promoters, origins of integron cassettes, and the abundance of unknown proteins encoded within them. The future prospects of integron research are also explored, highlighting the need to understand cassette expression patterns, assess the contribution of chromosomal/superintegron arrays to host fitness, unravel the mechanisms of cassette generation, and investigate the connection between the SOS induction and horizontal gene transfer. Overall, this review underlines the significance of integrons as hidden architects driving bacterial adaptation and evolution, providing valuable insights into their ecological and evolutionary dynamics, and shaping the future direction of research in this field.