| Salmonella typhi and Salmonella paratyphi A are the main pathogens of typhoid fever and paratyphoid respectively which are always worldwide public health problems, especially in Southeast Asia, Africa and other developing countries. According to WHO in 2004, the cases of typhoid and paratyphoid fevers were estimated to be about 21.65 million and 5.41 million respectively, and the dealths were totally 220,000 worldwide. In China, typhoid and paratyphoid fevers are also the key prevention and treatment infectious diseases and classified as Group B diseases. The incidence rates of typhoid and paratyphoid fevers fluctuated between 10 per 100 thousand and 50 per 100 thousand before 1990, and decreased to the range of 4.08 per 100 thousand and 10.45 per 100 thousand after 1990. But in recent years, there was a new epidemic trend that had been observed. S. paratyphi A emerged and prevailed soon in several provinces. In some regions, the proportion of cases due to S. paratyphi A was more than S. typhi and S. paratyphi A has become predominant pathogenic bacteria. Moreover, the epidemic areas expanded gradually.In infectious diseases surveilliance, outbreak identification and tracking back areas, molecular typing technologies play important roles for etiological analysis. Moreover, PFGE is an important technology of molecular typing. Because of its good reproducibility, results stability and easy to standardization, PFGE has been applied widely in molecular epidemiology analysis.By using PFGE protocol for Salmonella typhimurium, which is recommended by PulseNet Asia Pacific, we set up the primary PFGE typing database for S. typhi strains in China. The database includes 930 S. typhi strains, which were isolated from eight provinces from 1959 to 2007 and preserved in ICDC. We used Xbaâ… as the endonuclease. PFGE electrophoresis images were analyzed by BioNumerics (Rersion 4.0) software packages and then the patterns were clustered by Unweighed Pair Group Method (UPGMA) using Arithmetic average. The clustering similarity coefficient was between 41.4% and 100%. We finally got 456 different Xbaâ… patterns. Each pattern included 1 to 44 strains and 2 strains on average. 38 kinds of patterns were discovered in different provinces and the other 418 patterns belonged to the unique pattern of each province. There were 63 kinds of patterns discovered in different years and the other 393 patterns belonged to the unique pattern of every year. Numbers of clones with significant difference were existed in different areas and appeared at different time. These results suggested that S. typhi strains isolated in China had a large variability. However, the same pattern discovered in various years indicated that S. typhi strains were relatively stable in continual variation. This database has potential not only to connect and share data with international laboratory surveillance net, but also to analyze China typhoid fever surveillance and to serve for international typhoid fever prewarning and emergency reaction.The incidence rate of S. paratyphi A was low, between 0.4 per 100 thousand and 3.7 per 100 thousand, and most cases were sporadic in the developed countries. Salmonella PFGE standardization protocol released by PulseNet is targeted mainly at Serotype S. typhimurium. In recent years, S. paratyphi A caused epidemic in China and Southeast Asia counties. During the surveillance and annalysis of S. paratyphi A isolates, we found that the discrimination of S. typhimurium PFGE typing mehod was limited when it applied to the analysis of S. paratyphi A strains. In order to explore the suitable PFGE analysis protocol for S. paratyphi A strains, we screened and optimized the DNA endonuclease and PFGE electrophoresis parameters, and chose three kinds of restriction enzyme Xbaâ… , Speâ… and Xhoâ… as preferred endonuclease. Among several electrophoresis parameters of S. paratyphi A PFGE molecular typing, we selected the PFGE analysis protocol of a preferred enzyme by comparing the electrophoresis parameters of each enzyme, which had the best discriminatory ability. We chose different electrophoresis parameters for each enzyme on the analysis of S. paratyphi A strains. By comparing the discrimination of different parameters, we selected the best discriminatory ability of different electrophoresis parameters with respect to each enzyme, and then chose the best enzyme. Through the classification and comparative analysis of similarity and discrimination of 33 strains, we found that Xbaâ… digestion with PFGE electrophoresis parameter 1.5-29s, 20h; Speâ… digestion with electrophoresis parameter 1-20 s, 20h; and Xhoâ… digestion with electrophoresis parameter 2.2-29 s, 20h, had the best discrimination, which could divided into 17, 23 and 15 patterns each at the most. In addition, we found that Speâ… had the best discrimination in comparison with all three enzymes, so we proposed that Speâ… should be used as the preferred enzyme to replace Xbaâ… . The discrimination of S. paratyphi A PFGE typing isolates was improved by using these optimum conditions, and they would be of great benefit to the laboratory surveillance and analysis on outbreak.
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