Surveillance of the mutation of gene encoding Vi antigen of Salmonella typhi isolated in Vietnam during 1995 and 2005

Background: Salmonella typhi (S.typhi) is the major cause of human typhoid fever outbreaks. In fact, there were various typhoid fever outbreaks that occurred in China, and India that was caused by S.typhi strain without Vi antigen. Objective: To determine whether the S.typhi strains with mutation of gene encoding Vi antigen exists in Vietnam and the rate of mutation (if they exists). Subject and methods: 450 S.typhi isolates were collected in the Northern, Central and Southern Region of Vietnam during 1995 and 2005. The isolates were analyzed by the PCR method in order to detect mutants by using 2 primer pairs of tviB and DE. Results and Conclusion: There was no clear evidence on the relationship between the widely used Typhi Vi vaccine in Vietnam and the existence and spread of the mutation of gene encoding Vi antigen of S.typhi. 30 out of 450 isolates mutated losing the gene encoding of Vi antigen, making it 6.67%. These isolates were spread out between 1995 and 2005 throughout the Northern, Central and Southern Regions of Vietnam, with a peak in 1999. A noteworthy point was the rate of mutation of S.typhi losing the gene encoding of Vi antigen in Vietnam during the period of study. However, the mutation rate of S.typhi in Vietnam was still higher than the ratio of similar mutations being published in the other countries worldwide and higher than the recommended level of the World Health Organization.

Preliminary study on the capacity to transfer the antibiotic resistance of S.typhi isolates collected from Northern, Central and Southern regions of Vietnam

126 multiply resistant S.typhi isolates were screened among 363 isolates collected from Northern, Central and Southern regions of Vietnam during the period 1995-1997. Transfer experiments of the resistance to E.coli K12-J53 from these S.typhi isolates were performed to determine their transferable capacity of resistance factors and its frequency. The results indicated that 74.78% of multiply resistant S.typhi isolates (AM, CL, TE, TR, SU) have R-factors, most of them contained all resistance genes characterizing the resistance of wild strains. The frequency of transfer of the resistance varied from 10-5 to 10-8.

Plasmid encoded multidrug resistance in Salmonella typhi in Vietnam

129 isolates of S.typhi multi-drug resistant to Cm, Ap, Te, Tr, Su from Northern, Southern and Central regions of Vietnam during 1995-1997 were examined to detect the ability to transfer gene encoding the resistance to antibiotics and to study molecular characters of it’s R-plasmids. Results indicated the presence of self-transferable plasmid in 90% S.typhi strains from Southern, and 100% strains from Northern and Central regions. These strains carried from 1 to 2 plasmids of molecular weighed 110 MD and 80 MD. Gene encoding the resistance to Cm, Ap, Te, Tr, and Su were found to be located on a large plasmid of 100 MD. All S.typhi strains from 3 regions showed to have large plasmids with identical molecular structure, this indicated the same origin of these S.typhi strains.

Transfer of antibiotic resistance genes of shigella isolates from three regions (Northern, Southern and Central) of Viet Nam

119 isolates of Shigella with multi-drug resistant (100 Shigella flexneri) to chloramphenicol (Cm), ampicillin (Ap), tetracyclin (Te), trimethoprim (Tr), sulfamethoxazole (Su) from Northern, Central and Southern regions of Viet Nam during a period of 1995-1997 were investigated for the ability to transfer gene trains by method of conjugation. 59.2% strains were transferable in vitro to E.coli K12 with frequence from 10-7 to 10-5 (65.8% strains from Northern, 54.8% strains from Central and 57.5% strains from Southern regions). Almost all the antibiotic resistance genes were transfered with 10 different patterns of resistance; Ap, Tr, Su being the most frequent patters: 45.7% strains fro Northern, 30.8% strains from Southern and 25.6% strains from Central regions.