Discovering the unusual plasmid-localized class 1 integron that enables bacteria to trade resistance genes
Imagine cooking dinner with fresh chicken from the local market, only to discover that this simple meal could leave you with a bacterial infection that antibiotics cannot treat. This scenario is becoming increasingly common worldwide due to a phenomenon called antimicrobial resistance (AMR), where bacteria evolve to survive the very medicines designed to kill them.
In Colombia, scientists made a crucial discovery that helps explain how this happens: an unusual genetic element in Salmonella bacteria that acts like a "genetic swap meet," allowing them to trade antibiotic resistance genes with ease 2 .
This article explores the fascinating science behind this discovery, its implications for food safety in Colombia and beyond, and what it means for our ongoing battle against antibiotic-resistant superbugs.
To understand the significance of the Colombian discovery, we first need to understand integrons—the genetic elements at the heart of this story.
| Type | Location | Size | Primary Role | Example |
|---|---|---|---|---|
| Chromosomal Integrons | Bacterial chromosome | Large (hundreds of cassettes) | Genomic evolution, environmental adaptation | Vibrio cholerae superintegron 4 |
| Mobile Integrons | Plasmids or transposons | Smaller (1-10 cassettes) | Antibiotic resistance dissemination | Class 1 integron in Salmonella 7 |
| Class 1 Integrons | Typically on mobile elements | Variable | Clinical antibiotic resistance | Found in Salmonella Typhimurium and Anatum 2 |
Class 1 integrons, the type discovered in Colombian Salmonella, are particularly concerning because they're usually located on plasmids—small, circular DNA molecules that can easily transfer between different bacteria. This mobility makes them exceptionally effective at spreading antibiotic resistance through bacterial populations 7 8 .
In the mid-2000s, Colombian researchers embarked on a mission to understand why antibiotics were increasingly failing to treat Salmonella infections. Their investigation led to a groundbreaking study published in 2006 titled "Antimicrobial resistance in nontyphoidal Salmonella from food sources in Colombia: evidence for an unusual plasmid-localized class 1 integron in serotypes Typhimurium and Anatum" 2 .
The research team collected 72 Salmonella isolates representing 18 different serotypes from various food samples throughout Colombia.
The researchers first exposed the bacterial collection to different antibiotics to determine resistance patterns. They discovered alarmingly high resistance rates to important veterinary and human medicines 2 .
They used polymerase chain reaction (PCR) to screen for class 1 integrons. The results were striking—approximately 61% of the multidrug-resistant (MDR) isolates contained one or more class 1 integrons 2 .
By sequencing the gene cassettes within these integrons, they identified a "mosaic" of resistance genes, including incomplete open reading frames and a complete bla-(oxa2) cassette that provides resistance to ampicillin and related antibiotics 2 .
Through plasmid profiling and incompatibility typing, the team confirmed these unusual integrons were located on conjugative plasmids belonging to Inc groups A/C, P, and W—essentially making them mobile resistance packages that could spread between different bacterial strains 2 .
| Antibiotic Class | Specific Antibiotic | Resistance Rate | Clinical Significance |
|---|---|---|---|
| Cephalosporins | Ceftiofur | 15% | Critical for human medicine; resistance limits treatment options |
| Aminoglycosides | Neomycin | 11% | Commonly used in veterinary and human medicine |
| Tetracyclines | Oxytetracycline | 10% | Broad-spectrum antibiotic used in humans and animals |
| Multiple Classes | 3+ antibiotic classes | Significant portion | Classified as multidrug-resistant (MDR) |
Understanding how researchers identify and study these genetic elements helps demystify the process and highlights the sophistication of modern microbiological methods.
Amplifies specific DNA sequences to detect integron integrase genes (intI1) in bacterial isolates 8 .
Determines exact genetic code to identify specific resistance genes in cassettes 2 .
Identifies and characterizes plasmids to locate integrons on mobile genetic elements 2 .
Measures effectiveness of antibiotics to establish resistance patterns across bacterial isolates 9 .
Provides complete genetic blueprint for comprehensive identification of resistance genes and their contexts 1 .
Studies gene transfer between bacteria to demonstrate mobility of resistance genes 8 .
The 2006 discovery was not an isolated finding. Recent studies confirm that antimicrobial resistance in Colombian Salmonella remains a serious and evolving problem.
Another 2025 study examining pig samples from 2022-2023 found that 59% of Salmonella isolates displayed multidrug resistance 9 .
These findings highlight how the genetic machinery discovered in 2006 continues to enable the evolution and spread of resistant Salmonella strains through Colombian food production systems.
The most common resistance profile included antibiotics critical for human medicine: ampicillin, cephalosporins, fluoroquinolones, tetracyclines, and trimethoprim-sulfamethoxazole 5 .
Some serotypes showed 100% resistance to ampicillin, penicillin, and several other antibiotics 9 .
The plasmid-localized nature of these integrons continues to facilitate the spread of resistance genes between bacterial populations.
The discovery of plasmid-localized class 1 integrons in Colombian Salmonella provides more than just an explanation for how resistance spreads—it offers insights for combating this threat.
Reducing unnecessary antibiotic use in agriculture and human medicine decreases the selective pressure that favors resistant bacteria 3 .
Implementing better hygiene, biosecurity, and animal health management reduces the need for antibiotics in food production 1 .
Research into phage therapy, probiotics, antimicrobial peptides, and plant-derived compounds offers potential alternatives to traditional antibiotics 3 .
Proper food handling, cooking, and hygiene practices can reduce the risk of infection from resistant bacteria in food 3 .
Sharing surveillance data and resistance patterns internationally helps track the global spread of resistant strains.
The discovery of an unusual plasmid-localized class 1 integron in Colombian Salmonella represents more than just a local phenomenon—it provides a window into the remarkable adaptability of microorganisms and the challenges we face in maintaining effective antibiotics. These genetic elements function as nature's own genetic engineering toolkit, allowing bacteria to rapidly evolve and share defense mechanisms against our most important medicines.
As research continues, scientists are increasingly recognizing that combating antimicrobial resistance requires understanding these genetic mechanisms and implementing comprehensive strategies that address antibiotic use in both human medicine and agriculture. The "unusual" integron discovered in Colombian food sources serves as both a warning and a guide—revealing the sophistication of bacterial evolution while pointing toward more effective approaches to preserve our antibiotic resources for future generations.
The battle against antibiotic-resistant bacteria is indeed a race against microbial evolution, but with continued research and strategic action, it's a race we can still win.