The genus Yersinia in the family Enterobacteriaceae consists of 12 species, of which three are human pathogens. Yersinia enterocolitica and Yersinia pseudotuberculosis cause primarily gastrointes­tinal infections, whereas Yersinia pestis is the causative agent of plague.

Bacterio­phages (phages) are viruses that infect bacteria. They are the most abundant and versatile group of organisms on Earth and have a significant impact on microbial ecosystems. Several phages that can infect Yersinia are known, but only a few have been characterized in detail. The aim of this thesis was to obtain more information about yersiniophages, with a special interest in understanding the factors that determine their host specificity.

The phages studied in this thesis were phiYeO3-12 and phiR1-37, which infect Y. enterocolitica serotype O:3 (YeO3), and Y. pestis phage phiA1122. phiYeO3-12 and phiA1122 are T7 – related members of Podoviridae, whereas phiR1-37 belongs to the viral family Myoviridae.

These phages utilize different parts of Yersinia lipopolysaccharides as their receptor. For phiYeO3-12 the receptor is the YeO3 O-antigen. In this work, the YeO3 outer core was identified as the receptor for phiR1-37 and the core of Y. pestis and Y. pseudotuberculosis as the receptor for phiA1122.

The basic biological and genetic features of phages phiYeO3-12 and phiR1-37 were elucidated in this work. phiR1-37 was found to be exceptional for having its genome composed of DNA where thymidine is replaced by deoxyuridine. According to the N-terminal amino acid sequences of its structural proteins and a partial genomic sequence, no close relatives of phiR1-37 have been described. For phiYeO3-12, the non-essential regions in the genome were identified and the genes coding for DNA ligase and lysozyme were shown to be evolutionary factors important in the adaptation of phiYeO3-12 to grow on Yersinia.