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The spread of particular clones of S. sonnei throughout the territory of the German Democratic Republic within one year was evaluated by means of plasmid pattern analysis of the bacterial isolates. Such epidemic clones are described by their "clone formulae" comprising the phage type, biotype, colicinogenotype, antibiogram and the plasmid pattern type. They were not observed before 1982, but potential progenitor strains could be identified as having occurred sporadically in the years before. The additional application of plasmid pattern analysis revealed a better discrimination of the epidemic distribution of distinct bacterial clones than the use of conventional methods such as phage-typing, biotyping and colicinogenotyping only.

Salmonella typhimurium strains isolated in the German Democratic Republic between 1972 and 1980 mainly belong to phage type n.c.1/72/n.c. and biochemotype b. Nearly all of these strains are characterized by an identical basic plasmid pattern. Thus, they may be considered to be independent isolates of the same epidemic strain. Moreover, some multiple drug resistant offsprings of that epidemic strain have been characterized by their carriage of an additional IncH-1 R plasmid. However, a few strains of phage type n.c.1/72/n.c. were found to differ fundamentally in their plasmid patterns from that of the epidemic strain. The phage pattern of these strains is determined by a conjugative R plasmid belonging to the incompatibility group IncI4. Despite of the same phage- and biochemotype these strains must be considered non-related with the epidemic strain, therefore. The analysing of the plasmid pattern of bacterial strains turns out to be an important contribution to the investigation of epidemic processes.