|Abstract or Summary
- The most frequent cause of insufficient acid production during
fermentation of dairy products is bacteriophage infection of lactic
starter cultures. Previously, efforts to minimize this problem have
dealt primarily with the host range relationships of the viruses and
susceptibility patterns of the lactic streptococci. This study was
undertaken to characterize the host range, morphology, and nucleic
acid types of 35 lactic streptococcus bacteriophages; and to investigate
the interrelationship between these characters.
Preliminary experiments revealed a simple procedure for
obtaining high titer phage preparations. In addition, a simple procedure
for storage of stock lysates was found to be effective in maintaining
phage viability for more than two years.
Host range determinations showed that considerably more hosts
were lysed at high than at low titer. Groupings of phage were more
apparent with low titer host range patterns. Several hosts that
plated heterologous phages at low efficiency of pla.ting (E. O. P. ) were
further investigated. A single cycle of growth through these hosts
produced progeny phage that plated with low E. 0. P. on the original
hosts, due either to host controlled modification or host range
Electron microscopy studies revealed phages of the same
general shape, differing only in size of the component structures. All
of the phages examined had non-contractile tails with a tail end plate
structure, typical of the double stranded DNA phages of Bradleys
Group B. Head size ranged from 320-850 A in diameter. Tail
lengths ranged from 850-5200 X. Six types of ta.11 end plate,
structures were observed. The presence of a collar was observed
for nine of the lactic phages. Ten separate morphological groups
were characterized on the basis of similarity in head size and shape,
tail length, tail plate structure, and the presence or absence of a
Two phage species, w401x1 and w407x1, were unique from the
others with respect to head size and tail length. Head diameters
ranged from 700-850 X and tail lengths from 2000-5200 X . The
most striking feature of these two phages was the long winding tail of
variable length. In addition, these two phages did not form plaques on
lawns of the strains which gave rise to the lysate. An extensive
search for an indicator strain was not made. The morphology groups showed no correlation with groups based
on host range pattern.
The buoyant density of these phages covers a wide range, from
1. 428 to 1. 531 g /cc. On the basis of the DNA melting profile, all of
the phages in this study proved to be double stranded. The GC base
composition ranged from 32. 7 to 40.0 percent.
Genome sizes of the lactic phages were determined on the
basis of renaturation kinetics. Genome sizes ranged from 6 to
24 x 10⁶ daltons.
Renaturation kinetics were also used to assess polynucleotide
similarity among the lactic phages. DNA homology ranged from zero
to 100 percent.
An orderly relationship between percent homology, GC base
composition, morphology, and host range pattern was not revealed.