Abundance of Environmental Vibrio Species in Marine Environment and Characterization of Cryptic Plasmids

Water samples were collected from several previously determined sampling sites at Mai Po Nature Reserve (22°29′N to 22°31′N and 113°59′E to 114°03′E) and Cape d’Aguilar Marine Reserve (22°12′N and 114°15′E) of Hong Kong. Fifty environmental isolates of Vibrio species were isolated from water samples of Mai Po Nature Reserve and the Cape d’Aguilar Marine Reserve in Hong Kong and they were screened for the presence of plasmids. Mai Po is a wastewater-impacted area while the Cape d’Aguilar Marine Reserve is a pristine marine water. Plasmids were found in Vibrio isolates from both sites at similar frequencies and each site had distinctive plasmid profiles. These plasmid-bearing Vibrio isolates were identified to be different species of the Vibrio species by both biochemical test and subsequent full-length 16S rRNA sequences. Antibiotic resistance test showed that all these plasmid-bearing Vibrio isolates showed multiple resistance to the 21 antibiotics tested. In addition, selective isolates also showed tolerance to 10 μM Hg in culture medium and these isolates generally harbored large plasmid(s) (>30 kb). Among the plasmids detected, a novel cryptic plasmid, pVC, from an environmental isolate of Vibrio cholerae MP-1 from Mai Po Nature Reserve in Hong Kong has been isolated. The complete nucleotide sequence analysis (3,806 bp) revealed three major putative open reading frames (ORFs). Our results showed that high frequency of plasmid in Vibrio species of both polluted and pristine environments may be ecologically important to the survival of these bacteria in the environment. At the same time, public awareness of the associated problem with these bacteria should also be acknowledged. The specific functioning of the cryptic plasmids remains the focus of our current investigations.