Sixty-two selected strains of serotype Enteritidis of 33 phage types (PTs), and one strain classified as RDNC, were characterised by four different chromosomally based typing methods to elucidate genetic relationships among strains of different phage types. Based on IS200-hybridisation patterns, two major groups, containing strains of the most commonly encountered phage types, and six minor groups (seven with the RDNC strain included) were observed. IS200 pattern was a stable epidemiological marker in strains of all phage types except PT 6a and 14b. Ribotyping separated strains of the phage types into one major and five minor groups; the pattern of the RDNC strain was not seen with other strains. More than one ribotype was observed among strains of Enteritidis PTs 6, 7, 14b and 21. By pulsed-field gel electrophoresis, strains of 21 of the 33 phage types formed one large cluster when bands > 125 kb were used as the criterion for separation. Among strains belonging to PTs 1, 6, 7 and 14b, more than one pattern was observed by this method. By probing with five random cloned fragments of the Enteritidis chromosome, strains from 27 of 31 phage types examined showed the same hybridisation pattern. With the combined use of four genotypic methods, two groups of strains, representing eight and seven of 33 Enteritidis phage types, were formed; these two groups may be considered as the main evolutionary lines of Enteritidis. Strains of the remaining phage types, and the RDNC strain, belonged to separate groups.


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