Streptococcus thermophilus autolytic strains are characterized by a typical
bell-shaped growth curve when grown under appropriate conditions. The cell
ular mechanisms involved in the triggering of lysis and the bacteriolytic a
ctivities of these strains were investigated in this study. Lactose depleti
on and organic solvents (ethanol, methanol, and chloroform) were shown to t
rigger a premature and immediate lysis of M17 exponentially growing cells.
These factors and compounds are suspected to act by altering the cell envel
ope properties, causing either the permeabilization (organic solvents) or t
he depolarization (lactose depletion) of the cytoplasmic membrane. The auto
lytic character was shown to be associated with lysogeny. Phage particles,
most of which were defective, were observed in the culture supernatants aft
er both mitomycin C-induced and spontaneous lysis. By renaturing sodium dod
ecyl sulfate-polyacrylamide gel electrophoresis, a bacteriolytic activity w
as detected at 31 kDa exclusively in the autolytic strains. This enzyme was
detected during both growth and spontaneous lysis with the same intensity.
We have shown that it was prophage encoded and homologous to the endolysin
Lyt51 of the streptococcal temperate bacteriophage phi 01205 (M. Sheehan,
E, Stanley, G. F. Fitzgerald, and D. van Sinderen, Appl. Environ. Microbiol
. 65:569-577, 1999). It appears from our results that the autolytic propert
ies are conferred to the S. thermophilus strains by a leaky prophage but do
not result from massive prophage induction. More specifically, we propose
that phagic genes are constitutively expressed in almost all the cells at a
low and nonlethal level and that lysis is controlled and achieved by the p
rophage-encoded lysis proteins.