isolates that require arginine—i.e., either citrulline (C), or ornithine (O)—uracil (U) and hypoxanthine (H) have generally been considered to be similar when characterised by auxotype, serovar and plasmid content. The MICs of penicillin, tetracycline, erythromycin, spectinomycin, cefoxitin and ceftriaxone for 552 isolates belonging to serovar IA-2 with these phenotypes were found to be similar. Therefore, restriction fragment length polymorphism analysis of rRNA genes (ribotyping), restriction enzyme (RE) analysis of chromosomal DNA, and pulsed-field gel electrophoresis (PFGE) were evaluated to determine whether these isolates could be distinguished by molecular methods. A subset of 27 isolates of that were OUH-requiring, CUH-requiring or OH-requiring, belonged to serovar IA-2 and carried a 2·6-MDa plasmid, were selected for further study. Based on the RE analysis of I-digested genomic DNA, the 27 isolates fell into a single RE pattern, five ribotypes and 17 PFGE profiles which did not correlate with the specific arginine-requiring subtypes of these isolates. Each ribotype varied by the presence of only a single fragment, which was of a different size in each pattern, and 17 (63%) of the 27 isolates belonged to ribotype I. PFGE yielded the highest level of discrimination with 17 different profiles.


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