DNA as a Pollutant: the Clinical Class 1 Integron

Abstract

When humans first tried to control bacterial growth with antimicrobials, DNA elements called integrons began to accumulate genes for resistance. The clinical class 1 integron accumulated diverse antibiotic resistance genes, spread into multiple bacterial species, colonized a wide range of animal and plant hosts, and spread to every continent. This review examines how this DNA element has become a significant environmental pollutant. DNA analysis suggests the clinical class 1 integron had a single origin in the early twentieth century. Human use of selective agents then drove a rapid increase in its abundance. Diverse environmental samples now commonly contain over 1 × 106 copies per gram of the class 1 integron. In areas exposed to human and animal wastes, its abundance can reach one copy per bacterial cell. Clearly, the class 1 integron has become spectacularly successful over a very short period of time. It has done so because it has a unique set of properties. It prospers under anthropogenic disturbance and is disseminated in vast numbers by virtue of colonizing humans and agricultural animals. And, unlike conventional pollutants, it continues to replicate after release into the environment. This replication is driven by the selective advantage it confers to its host cell in the presence of the antibiotics, metals, and disinfectants also released in waste streams. The clinical class 1 integron is just one example of a new type of DNA pollutant. Other resistance genes and mobile elements are also increasingly being reported as significant environmental contaminants.