MOLECULAR ANALYSIS OF SALMONELLA-ENTERITIDIS BY PULSED-FIELD GEL-ELECTROPHORESIS AND RIBOTYPING

A total of 61 isolates of Salmonella enteritidis were analyzed by the techniques of pulsed-field gel electrophoresis (PFGE) and ribotyping, Twenty-three of the isolates were from Zurich, Switzerland, and 38 iso lates were from the University Hospital, Kuala Lumpur, Malaysia. Five of the Malaysian isolates were hospital-related outbreak strains and w ere shown to be indistinguishable by PFGE analysis following digestion with three different restriction endonucleases, XbaI (5'-TCTAGA-3'), SpeI (5'-ACTAGT-3'), and AvrII (5'CCTAGG-3'). The PFGE pattern of an i solate from a suspected carrier staff nurse was found to be identical to those of the hospital outbreak isolates. These isolates were also i ndistinguishable by ribotyping with SmaI and SphI. The same single PFG E pattern was also detected in 29 of 32 sporadic isolates of S. enteri tidis, Four closely related ribotypes were detected among these 29 iso lates. Similarly, outbreak-related strains G om Switzerland showed clo se genetic identity by PFGE and ribotyping, Strains obtained from poul try showed more variations in their PFGE patterns and ribotypes, altho ugh the patterns were still closely related. In addition, SphI ribotyp es A and D among the Swiss strains correlated with phage types 4 and 8 , respectively, No correlation of phage types with PFGE pattern was no ted. Both PFGE and ribotyping indicate that the S. enteritidis strains circulating in Malaysia and Switzerland are very similar and may be c lonally related. Comparison of the PFGE patterns with the ribotypes fo r 23 S,wiss and 16 Malaysian isolates showed that there was a 69% conc ordance in the grouping of isolates. Overall analysis showed that one or two PFGE patterns and five ribotypes (using SphI) were detected amo ng 38 Malaysian isolates and two to four PFGE patterns and four riboty pes (using SphI),were present among 23 Swiss isolates, thus suggesting that PFGE is slightly less sensitive than ribotyping with regard to t he ability to discriminate between isolates. We conclude that the clos e genetic similarity observed between epidemiologically unrelated and outbreak-related isolates of S. enteritidis suggests that both PFGE an d ribotyping are of limited value in the epidemiological analysis of t hese particular isolates, possibly because of the highly clonal nature of pathogenic strains of S. enteritidis.