Integrons in Enterobacteriaceae from Surface Waters

The spread of antibiotic resistant bacteria is a growing concern. One mechanism of dissemination of antibiotic resistant genes is via integrons that are embedded in bacterial chromosomes, plasmids or transposons. Integrons are genetic units that are capable of capturing and expressing genes within a gene cassette including, but are not limited to, antibiotic resistant genes. Such ability, coupled with plasmid association, promotes the spread of antibiotic resistance through horizontal transfer between bacteria. Family Enterobacteriaceae contains several bacteria implicated in the most severe nosocomial infections, which are also ubiquitous in nature. However, little information is available on the incidence of antibiotic resistance in environmental samples in the United States. The objectives of this study are to determine frequency of integrons, characterize integrons according to gene composition and to determine if the same integron can be carried by different species. 143 Enterobacteriaceae isolates were obtained from several public beaches and creeks in Orange County, California. Species identification was conducted using MALDI-TOF mass spectrometry or 16S rRNA gene. Presence of integrons was determined through polymerase-chain reaction (PCR) amplification, followed by DNA sequencing of integrase and gene cassettes. Integrons were detected in 42 bacterial isolates (29.4%), of which the most common were Escherichia coli (n=33), followed by Citrobacter freundii (4), Klebsiella pneumoniae (3), Enterobacter asburiae (1) and Proteus mirabilis (1). Lengths of the integrons ranged from 928 to 2869 bp. Integrons were classified into eleven distinct groups, based on gene cassette composition and sequence similarity. The most prominent integron group, encompassing nearly 60% of all isolates, included dfrA17, aadA5, sul1, and qacE1 genes encoding resistance against trimethoprim, aminoglycosides, sulfonamide, and quaternary ammonium compounds, respectively. This integron was shared between Escherichia coli and Citrobacter freundii isolates. Overall, five dfrA gene variants and three aadA gene variants were detected. Rare genes included catB3 and cmlA1, which code for chloramphenicol resistance. Two integron groups were observed to be novel in species such as, Escherichia coli and Enterobacter asburiae. This study shows that a considerable proportion of environmental Enterobacteriaceae possess diverse integrons containing antibiotic resistance genes, suggesting that the environment may play a role in the spread of integrons among diverse bacteria.