PHAGE TYPING COMBINED WITH PULSED-FIELD GEL-ELECTROPHORESIS AND RANDOM AMPLIFIED POLYMORPHIC DNA INCREASES DISCRIMINATION IN THE EPIDEMIOLOGIC ANALYSIS OF SALMONELLA-ENTERITIDIS STRAINS

Phage typing (PT) combined with pulsed-field gel electrophoresis (PFGE ) and a random amplified polymorphic DNA (RAPD) fingerprinting method was used to characterize Salmonella enteritidis strains. Twenty-four e pidemiologically unrelated isolates, sampled from diverse ecological n iches and fifteen isolates from four well-defined outbreaks of foodbor ne gastroenteritis, were studied. Seven phage types, with a predominan ce of PT 4 (63% of isolates), were observed when analysing the epidemi ologically unrelated group. PT 4 was detected in all of the ecological niches studied, including food and fecally polluted river and beach w ater. The discriminatory power for phage typing, the average probabili ty that the typing system will assign a different type to two unrelate d strains randomly sampled in the microbial population, was 0.62. Ten PFGE pattern types were obtained with Xba I restriction endonuclease e nzyme among the unrelated isolates; thirteen isolates belonged to PFGE pattern type 1 and the rest of the PFGE types were assigned to one or two isolates. The Dice coefficient clustered the similarities of the PFGE patterns between 80-100%. PFGE showed a discriminatory power of 0 .72. Five clearly distinct RAPD patterns were observed with the OPS-19 oligonucleotide, but the discrimination obtained was low (0.46). The combination of the three typing methods increased the number of types to seventeen, giving high discrimination (0.92). Seven of the isolates recovered from various ecological niches belonged to the combination PT 4/PFGE 1/RAPD A and other combinations were unique or included only two strains. The four epidemiologically well-defined foodborne outbre aks were associated with the PT 4 phage type. In two of the outbreaks, other phage types (PT 7a and RDNC) were also observed in two isolates . Most of the isolates belonging to the foodborne outbreaks had an ide ntical PFGE pattern (PFGE pattern type 1), but a difference in a restr iction band was observed in an isolate belonging to an outbreak. Two R APD patterns were observed in the outbreaks; RAPD pattern type A was d etected in three of the four outbreaks. When the combined typing metho d was applied to the study, high concordance was observed and most of the outbreak strains belonged to the combination PT 4/PFGE 1/RAPD A. I t is concluded that the combination of phage type with PFGE and RAPD p rovides a powerful discriminatory tool for the epidemiological analysi s of unrelated and related strains of S. enteritidis. (C) 1998 Elsevie r Science B.V.