Precence of mobile genetic elements in Vibrios

Integrating conjugative elements (ICE’s) are mobile genetic elements that integrate and replicate along with the host chromosome. ICE’s of the SXT/R391 family have been studied quite extensively, though most of the research done to date has been on strains of Vibrio cholerae. ICE’s of the SXT/R391 family were first documented to confer resistance to antibiotics and heavy metals, however in since then they have also been found to help regulate motility and biofilm formation. The SXT/R391 ICE’s were also found to contain Restriction-Modification systems (RM). restriction (R) enzymes recognise specific DNA sequences cut the DNA, which the modification (M) enzymes then methylate. This functions as a defence mechanism against invading genetic material. Balado et al (2013) set out to examine the distribution and ecology of these particular genetic elements in the aquatic ecosystem. 

Balado et al (2013) collected 203 strains of 23 distinct species of Vibrio, which were screened for the presence of ICE’s. The strains were isolated from internal organs collected from 18 separate aquaculture facilities of the coast of Spain and Portugal. Cultures were grown at 25^oC on tryptic soy agar or broth, with added NaCl. Resistance to HgCl₂ and tetracycline was tested on strains found to contain ICE’s. Cultures were screened for ICE’s using PCR methods with primers targeting the integrase gene, resulting amplified ICE’s where then sequenced. ICE’s were also tested for motility, this was done by mixing both donor and recipient cultures and growing the mixture on LB agar plates. After initial incubation donor and recipient bacteria were separated growing on antibiotic containing agar using known variations resistance between the bacteria. Strains containing ICE’s were also subjected to a phage infection assay, to determine the effectiveness of RM systems found to be present. 

Initial PCR amplification uncovered the presence of two ICE’s of the SXT/R391 isolated from V. splendidus and V. alginolyticus, which were distinctly different from previously documented ICE’s from this family. It was found that these particular ICE’s are mobile, and may be transferred to a wide range of Gamaproteobacteria, and may confer resistance to both tetracycline and heavy metals. After testing for phage protection it was also concluded that both the of the new ICE’s conferred some degree of protection to bacteriophage infection. 

Overall Balado et al (2013) suggest that the presence of ICE’s is much more widespread in the marine environment than previously thought. The ability to spread to Gamaproteobacteria , which are often found in aquaculture, suggests that these genetic elements may have a much wider distribution than was previously expected. This link to aquaculture may have further effects as it may spread to pathogens and provide resistance to antibiotics. Finally the authors suggest that due to the presence of the RM systems spread and maintenance of these particular genes would be favourable, and these ICE’s may be maintained due to functionality past the antibiotic and heavy metal resistance already associated with the SXT/R391 family of ICE’s.

Balado, M., Lemos, M., L., Osorio, C., R.. (2013). Integrating conugative elements of the SXT/R391 family from fish-isolated Vibrios encode restriction-modification systems that confer resistance to bacteriophages. FEMS Microbial Ecology. 83 (1), 457-467.