| Ships' ballast has been implicated as a vector in the dispersal of invasive species into new environments. Numerous cases have been documented for the introduction of macroorganisms via ballasting operations, but what is less known is the role of ballast water and residuals in the spread of potentially harmful microbes, especially with reference to novel genotypes. Of particular interest are the genes encoding for various forms of antibiotic resistance, many of which are carried on mobile genetic elements. A four-year sampling effort has yielded over 300 putative isolates of Vibrio cholerae from ships' ballast tanks and various environmental sources, of which 208 have been profiled for antibiotic susceptibility using twelve diverse antibiotics. The results demonstrate widespread resistance to beta-lactam antibiotics (67%), especially in nearshore isolates as compared to those isolates derived from ships' ballast tanks. Plasmid extractions and restriction enzyme analyses have shown evidence of plasmids of approximately 38 and 23 kbp in many of these isolates, suggesting the potential for horizontal gene transfer (HGT). Additional work using restriction fragment length polymorphisms and PCR for specific antibiotic resistance genes has yet to reveal the genetic source of the observed antibiotic resistance. Knowledge from this study and future microcosm experiments will help to ascertain the potential for HGT in a ballast tank setting. |
