THE STABLE ASSOCIATION BETWEEN STREPTOCOCCUS THERMOPHILUS DSM 20617 AND THE TEMPERATE BACTERIOPHAGE 20617 IMPROVES THE BACTERIAL GROWTH FITNESS

S. thermophilus DSM20167 (ATCC19258, CNRZ1358) showed an autolytic behaviour when cultured in lactose and sucrose limited concentration and was characterized by a typical bell-shaped growth curve. The entity of the phage-induced cell lysis was strikingly inversely related (r 0.86) with the sugar concentration of the medium. Experiments carried out using a glycolytic inhibitor highlighted that the induction of lytic cycle was dependent on the energetic status of the cells. Genome sequence analysis of the strain DSM20617 revealed the presence of a pac-type temperate bacteriophage of 41007 bp designed ? 20617. The genomic organization and structure of ? 20617 resemble those of temperate and virulent streptococcal bacteriophages. The prophage was integrated at the 3' end of the gene coding from the glycolytic enzyme enolase (eno) without affecting the integrity and the transcriptional levels of the eno gene. The site of recombination between ? 20617 and the host chromosome was identified in a 43 bp AT reach region. Comparative experiments carried out between the wild-type and a phage-cured derivative strain revealed that cell-wall integrity of the infected strain was compromised also in cells grown in high lactose concentration in absence of detectable cell lysis. Transcription of the phage holin and lysin was indeed detected at the early exponential phase of growth at high lactose concentration. Comparative and competitive experiments, carried out between wild-type and the phage-cured derivative strain revealed that the "association" between S. thermophilus and the temperate bacteriophage ? 20617 improves the bacterial growth fitness.