Two important problems of current medical microbiology are discussed in this paper: 1) a change of etiological structure of infectious morbidity-elevation of the incidence of infections caused by conditioned pathogenic causative agents, and 2) postvaccinal complications. In the light of these problems the tasks of microbiological investigations for the immediate future are discussed. Tasks and prospects of study of the molecular structure of the pathogenicity factors of importance for the creation of chemical vaccines and genetic investigations which would provide construction of strains and preparations useful in practice are formulated.

Stable L-forms were for the first time obtained under the effect of tetracycline on the Mec thnikov vibrio and NAG vibrio. The process of conversion of the L-forms under the effect of tetracycline was the same as under the action of penicilline. Three types of the L-forms were obtained: stable tetracycline-resistant L-forms, stable tetracyclin-superresistant and tetracycline-dependent L-forms. Under the reversion of the unstable L-forms strains with a typical complex of biological properties and atypical revertants were obtained.

Stable L-forms were obtained by the action of tetracycline on Mechnikov's vibrio and the NAG vibrio, for the first time. Conversion of the L-forms of the vibrios by tetracycline is similar to that obtained by the use of penicillins. By passage of the L-forms three types of cultures were obtained: stable tetracycline-resistant L-forms, stable L-forms highly resistant to tetracycline, and tetracycline-dependent L-forms. Strains of vibrios having the typical complex of biological properties as well as atypical strains of vibrios were formed by reversion of the unstable L-forms.

It is shown that the genetic determinants of type-specific antigens 1 to V of Shigella flexneri controlling similar functions are localized near the lactose-fermentation region on the chromosome; the determinant of antigen VI lies beyond it. The authors support the proposal that Shigella flexneri serotype 6 be excluded from Shigella flexneri and be considered as a subspecies in correspondence with the Soviet scheme of classification or even as a distinct species as Slopek and Mulczyk suggested. Genetic arguments based on the different behavior of S. flexneri strains in recombination with Escherichia coli K–12 Hfr are presented which support the division of S. flexneri serotype 5 into two subtypes, a and b, in correspondence with a similar division of other serotypes. The advisability of this division is confirmed by receptor analysis demonstrating differences in group-specific factors in strains of serotype 5. The antigenic formula of subtype 5b is V:7 … and of 5a, V:4 ….