Molecular epidemiology of Shigella flexneri in a long-stay psychiatric nursing center during 2001 to 2003

Abstract

With six separate wards accommodating more than 1,600 patients, V Nursing Center (VNC) is a long-stay psychiatric nursing center in eastern Taiwan. During 2001 to 2003, 39 shigellosis cases occurred in VNC. Different from the notion that most cases of shigellosis are caused by Shigella sonnei, all except one of these cases were caused by S. flexneri, with the remaining one caused by an S. sonnei isolate. O-antigen serotyping showed that the 38 S. flexneri strains were of either type 1a (n = 20) or 4a (n = 18), two less prevalent serotypes in Taiwan. NotI-based pulsed-field gel electrophoresis analyses performed with 8 type 1a non-VNC strains and 9 type 4a non-VNC strains isolated from 1996 to 2003 for comparison divided the 28 type 1a strains and the 27 type 4a strains into 7 and 10 subtypes, designated subtypes P1A to P1G and subtypes P4A to P4J, respectively. Subtypes P1A and P4A, which appeared in three consecutive years in VNC as well as outside of VNC, are the most prevalent subtypes. Analyses of the relatedness of the VNC strains on the basis of the banding patterns grouped the type 1a and 4a strains into four and five clusters, respectively. All except one of the type 1a strains had 95% similarity, indicating that they had a common parent, whereas the type 4a strains had similarities that ranged from 77 to 93%, suggesting that they were of diverse origins. In two of the outbreaks, less related subtypes of the type 4a strains were found in the same VNC wards in consecutive years, suggesting the possible existence of different subtypes in VNC all the time. Antibiotic susceptibility testing showed that all except one of the S. flexneri strains were sensitive to at least seven antibiotics; the remaining isolate was sensitive to three antibiotics. The data from the latter tests should be helpful for selection of proper treatments for S. flexneri infections in Taiwan.

FIG. 1.

Map of Taiwan. The three counties in the east are labeled. The distributions of the strains are indicated by P1A to P1G and P4A to P4J, which represent the subtypes of S. flexneri types 1a and 4a, respectively, obtained by NotI-based PFGE. The numbers in parentheses indicate the numbers of isolate obtained.

FIG. 2.

Dendrogram of the S. flexneri type 1a strains determined on the basis of NotI-based PFGE patterns. Twenty VNC isolates and 8 non-VNC strains are included.

FIG. 3.

Dendrogram of the S. flexneri type 4a strains determined on the basis of the NotI-based PFGE patterns. Eighteen VNC isolates and 9 non-VNC strains are included.

FIG. 4.

Profiles of plasmids extracted from representative type 1a and 4a S. flexneri strains. Plasmids were prepared as described in Materials and Methods and subjected to agarose gel (0.8%) electrophoresis. Lanes: M, markers containing 11 supercoiled plasmids (numbers to the left of the gel are in kilobases); 1aI, S. flexneri 90e4802; 1aII, S. flexneri 91e110; 1aIII, S. flexneri 86e48398; 1aIV, S. flexneri N02.115; 4aI, S. flexneri 92e0537; 4aII, S. flexneri 90e6312; 4aIII, S. flexneri 91e1351; 4aIV, S. flexneri 90e2459.