[Combined use of microbiological methods for the intraspecific differentiation of Shigella sonnei. II. The role of S. sonnei strains belonging to unclassifiable phagotypes in the etiology of dysentery in the People's Republic of Bulgaria].

The phage typing of S. sonnei strains in Bulgaria during 1973-1983 revealed that 10.07% of these strains could not be classified with the known phage types in accordance with the international scheme of Hammarstrom-Kallings. They were found to belong to so-called nonclassifiable (n.c.) phage types. Among 68 detected n.c. phage types only 24 (35.29%) were found to have a stable lytic picture and to be of epidemiological importance for the country. Experiments were carried out with the aim of finding out the mechanisms of the appearance of n.c. phage types by the study of the lysogenicity of the strains and the properties of R-plasmids isolated from them. The results of these experiments are discussed. The inclusion of some of the n.c. phage types of S. sonnei, occurring most frequently and isolated also in other countries, into the international scheme of Hammarström-Kallings is proposed.