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What is a Super Integron?
Introduction
Super-integron term was first applied in 1998 (but without definition) to the integron with a long cassette array on the small chromosome of Vibrio cholerae. The term has since been used for integrons of various cassette array lengths or for integrons on bacterial chromosomes (plasmids).
The use of "super-integron" is now discouraged since its meaning is unclear. In more modern usage, an integron located on a bacterial chromosome is termed a sedentary chromosomal integron, and one associated with transposons or plasmids is called a mobile integron. Two groups of integrons are known: resistance integrons and super-integrons. Gene cassettes in super-integrons encode a variety of different functions.
Super-integrons are located on the bacterial chromosome. The gene cassettes in resistance integrons probably originated from super-integrons. The recent finding of super-integron (SI) structures in the genomes of several bacterial species have expanded their role in genome evolution.
The Vibrio cholerae super integron is gathered in a single chromosomal super-structure harboring hundreds of gene cassettes. A comparison of the cassette contents of super-integrons from remote Vibrio species suggests that most of their cassettes are species-specific.
Many bacterial species belonging to several distinct genera of the γ- and β-proteobacteria undoubtedly carry or show strong evidence for the presence of chromosomal SIs. If each bacterial species harboring a SI has its own cassette pool, the resource in terms of gene cassette availability may be immense.
Super Integron
Our five-decade-long battle against bacteria is a testament to the genetic flexibility of these organisms. Not long after their introduction, we were witnessing the emergence of bacterial resistance to new antimicrobial agents. It is now clearly established that the prevailing strategy adopted by bacteria to evade antimicrobial activity is via acquisition of a gene from an exogenous source that confers resistance by any means. Integrons can now be divided into two major groups: the resistance integrons (RI) and the super-integrons (SI).
RI carry mostly gene cassettes that encode resistance against antibiotics and disinfectants and can be located either on the chromosome or on plasmids. The large chromosomally located integrons, which contain gene cassettes with a variety of functions, belong to the SI group.
SI are not given a specific name. The integron originally designated as class 4 is now named Vibrio cholerae SI. SI have been described for Geobacter sulfurreducens, Listonella pelagia, Nitrosomonas europaea, Pseudomonas alcaligenes, Pseudomonas mendocina, Pseudomonas spp., Pseudomonas stutzeri, Shewanella oneidensis, Shewanella putrefaciens, Treponema denticola, Vibrio anguillarum, Vibrio cholerae, Vibrio fischerii, Vibrio metschnikovii, Vibrio mimicus, Vibrio parahaemolyticus and Xanthomonas campestris.
Three classes of multi-resistant (MR) integrons have been defined based on the homology of the integrase genes and each class appears to be able to acquire the same gene cassettes. The integron platforms are defective for self-transposition but they are often found associated with insertion sequences (ISs), transposons, and/or conjugative plasmids which can serve as vehicles for the intra- and interspecies transmission of genetic material.
The potency of a highly efficient gene capture and expression system in combination with broad host range mobility is self-evident. The proficiency of this partnership is confirmed by the marked differences in codon usage among cassettes within the same integron, indicating that the antibiotic resistance determinants are of diverse origins
Such a system permits bacteria to stockpile exogenous genetic loci and MR integrons harboring up to five different cassettes have been characterized (In30).
Several observations suggest that integron structures impact genome evolution to a much greater extent than initially believed. First, the degree of homology between the three integrase classes (45-58%) suggests that their evolutionary divergence has extended over a longer period than the 50 years of the antibiotic era.
Second, the bias towards the propagation of resistance gene cassettes is likely due to the selective pressure of antibiotic therapy regimes driving the specific capture of resistance cassettes, implying that cassette genesis is not restricted to resistance determinants. It is conceivable that any ORF can be structured as a gene cassette.
Recently a new type of integron, a super-integron (SI) harboring hundreds of cassettes and differing in several ways from the MR integrons, has been identified in the Vibrio cholerae genome. This review focuses on this type of integron and gives the current state of knowledge on their characteristics and distribution.
Conclusion
The gene cassettes found in SI encode a wide variety of different functions, in contrast to the functions of gene cassettes found in RI. The number of resistance genes carried by the same plasmid, and even in the same integron, appears to rise. The integration of virulence factors and resistance determinants on the same plasmid may have even greater implications for public health. These bearers of multi-resistance are likely to remain because the physical association of integrons with other resistance determinants will lead to their continuous selection.
The role of SI in the evolution of bacterial species has been barely touched upon, but their apparent ubiquity suggests that they play an important role in bacterial evolution. The variety of structures found among class 1 integrons and their genetic surroundings after slightly more than half a century of antibiotic usage bears testament to the genetic flexibility and adaptability of the bacterial genome under environmental stress, making these microorganisms ultimate survivors.
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