Integrons are genetic elements found in bacteria, serving as platforms for acquiring and expressing new genes. While not self-mobile, they often reside on larger mobile genetic elements like plasmids and transposons, facilitating their spread. This allows bacteria to rapidly adapt to environmental pressures, including antibiotics. Understanding integrons is important for comprehending bacterial evolution and addressing public health challenges.
What are Integrons?
Integrons are genetic platforms within bacteria that function as “gene-capturing machines.” They are characterized by three core components that acquire and express foreign DNA.
The intI gene encodes an enzyme called integrase, which facilitates genetic recombination.
The attI site is a specific DNA sequence recognized by the integrase, acting as the primary insertion point for incoming genetic material.
A promoter sequence, located upstream of the attI site, drives the transcription and expression of any captured genes.
These three elements enable the capture and expression of gene cassettes. Gene cassettes are small, mobile DNA segments that contain a single gene and a recombination site called attC. While integrons are not mobile elements themselves, their association with plasmids and transposons allows them to move between different bacterial hosts.
How Integrons Work
Integrons operate through site-specific recombination to acquire and express gene cassettes. The integrase enzyme, encoded by the intI gene, recognizes both the attI site within the integron and the attC recombination site on circular gene cassettes. This allows the integrase to insert these cassettes into the attI site of the integron.
When a gene cassette is inserted, it positions itself within the integron’s gene array, near the intI gene and its associated promoter. This arrangement ensures that the newly acquired gene, which often lacks its own promoter, can be efficiently transcribed and expressed by the integron’s promoter. Multiple gene cassettes can be sequentially inserted, forming an array, with those closer to the promoter generally expressed at higher levels.
The process is reversible; integrase can also catalyze the excision of gene cassettes, allowing them to form free circular DNA elements. This dynamic process of insertion and excision provides bacteria with a flexible system for adapting to changing conditions and rapidly gaining new traits.
Integrons and Antibiotic Resistance
Integrons are major contributors to the rise and spread of antibiotic resistance in bacteria. Many gene cassettes captured by integrons carry genes that confer resistance to various antibiotics. This allows bacteria to acquire new defense mechanisms against antimicrobial drugs. The integron’s ability to efficiently capture and express these resistance genes contributes to the emergence of multidrug-resistant bacteria.
For instance, Class 1 integrons, common in clinical settings, frequently carry genes like aadA and aadB, conferring resistance to aminoglycoside antibiotics. Other resistance genes found within integrons include those that produce beta-lactamases, such as blaCARB-2, which break down beta-lactam antibiotics. Integrons can also harbor genes like dfrA, providing resistance to trimethoprim.
The prevalence of integrons, especially Class 1 integrons, in clinically relevant bacteria like Escherichia coli, Klebsiella spp., and Acinetobacter baumannii, highlights their role in disseminating resistance. These resistance traits can spread between different bacterial species through horizontal gene transfer, often facilitated by plasmids and transposons that carry the integrons. This rapid dissemination complicates the treatment of bacterial infections and poses a public health challenge.
The Broader Implications of Integrons
Beyond their role in antibiotic resistance, integrons are drivers of bacterial evolution and adaptation. They provide bacteria with a mechanism for acquiring new genetic information, enabling rapid adjustments to diverse environments. Integrons can capture a wide array of genes that confer other adaptive traits, such as increased virulence or altered metabolic capabilities.
Integrons are found in various environments, including water and soil, not just clinical settings. This widespread distribution underscores their importance in shaping microbial diversity and evolution. The ability of integrons to acquire novel genes and rearrange existing cassette arrays provides a rapid means for bacteria to generate new functions and adapt to changing ecological niches.
The challenges posed by integrons necessitate continuous surveillance of bacterial populations to track the emergence and spread of new resistance mechanisms. Their dynamic nature means bacteria can quickly evolve new strategies to overcome interventions. Understanding these elements is important for developing effective strategies to combat the adaptability of bacterial pathogens and manage public health risks.