Genes Conferring Resistance to Aminoglycosides in Aeromonas from Natural Habitats

Rise of antibiotic resistant bacteria is a serious threat to public health. Aminoglycosides are used to treat many gram-negative bacterial urinary tract infections. Aminoglycoside antibiotics are concentration-dependent bactericidal drugs which inhibit protein synthesis by binding to the 16S ribosomal RNA of the 30S ribosomal subunit at the A site of the bacterial ribosome. The mechanisms of resistance to aminoglycosides include enzymatic modification of the drug and modification of the ribosome to prevent antibiotic binding. Enzymatic genes conferring resistance to aminoglycosides are classified as nucleotidyltranferases, phophotransferases, or acetyltranferases. Aeromonas species are fresh water, gram-negative anaerobic rod bacteria that are clinically important as they causes serious wound infections, gastroenteritis, inflammation of the gastrointestinal tract, and bacteremia contributing to many illnesses and deaths every year. Many studies investigated the presence of aminoglycoside resistance genes in Aeromonas within the clinical setting, but very little is known about the presence of aminoglycoside resistance genes within free-living Aeromonas species in natural habitats. The objective of this study is to find out which genes that confer resistance to aminoglycosides are present in Aeromonas from natural environments, to estimate their frequency and to evaluate their nucleotide and amino acid polymorphism. 20 strains of Aeromonas species were isolated from water samples obtained from Southern California shores. Taxonomical identification was performed using mass spectrometry (MALDI-TOF). Six aminoglycoside resistant genes (aadA, strA-strB, aa3-IV, aphA1, aphA2, and aadB) were screened using PCR and gel electrophoresis. All genes detected by PCR were sequenced and compared to those deposited in GenBank. Altogether, four genes were detected. 11 (55%) Aeromonas strains contained aadA gene, 3 (15%) strains had aadB, another 3 strains had a variant of strA-strB and 2 strains had a variant of aphA1. Comparison with GenBank database showed that aphA1 has not been previously reported in Aeromonas, and there were only few sequences of strA-strB and aadB from Aeromonas. Our study shows that Aeromonas from natural habitats are rich in genes conferring resistance to aminoglycosides. The fact that a relatively small sample of Aeromonascarried such a big load of aminoglycoside resistant genes, some of them previously not observed in this genus, suggests that natural environments may promote exchange of antibiotic resistant genes between diverse bacteria.