- Comparisons were made of the abilities of Streptococcus lactis,
Streptococcus cremoris and Streptococcus diacetilactis bacteriophages
to endure various laboratory isolation and storage treatments.
Neutralization of Cottage cheese whey containing phages offered no
survival advantage over unneutralized samples, refrigeration of
neutral and acid whey samples increased the survival of only the
S. diacetilactis phage. About 50 percent of the phages could be
recovered from milk coagulated by addition of lactic acid to provide
casein-free, clear, phage-containing whey. Sterilization of phage
lysates by membrane filtration allowed collection of 92 to 99 percent
of the phages; Seitz filtration allowed collection of a maximum of
24 percent. Whey samples containing phages could be concentrated
by removing water using polyethylene glycol. Lyophilization
decreased phage titers at least 50 percent.
Cross reaction patterns of phages for strains of lactic
streptococci revealed that similar strains were present in cultures
supplied by several companies. For example, phages which were
lytic for strains isolated from one brand of commercial mixed strain
starter cultures would lyse strains isolated from as many as three
different brands of starter cultures.
The lytic patterns of 60 bacteriophage races toward 100 single
lactic streptococcal strains allowed the establishment of eight phage
groups, A through H. The phages within a particular group were
generally species specific. However several exceptions were noted.
Group A phages attacked primarily strains of S. diacetilactis.
Groups B through H phages mainly attacked strains of S. lactis and
S. cremoris. Groups G and H consisted of one phage that attacked
only the homologous host.
Lytic reactions determined by multiple printing of phages on
lawns seeded with possible hosts were employed for selecting strains
to be used in mixed-strain starter cultures. A computer was used
in selecting strains for blending into mixed-strain cultures with
resulting maximum protection against phage. The strains selected
by the computer were arranged in a sequence or rotation system
suitable for use in dairy plant fermentations. Protection from phage
infection as determined by a resistance index remained high for six
successive cultures, but the addition of the seventh culture to the
rotation system increased the likelihood of phage attack significantly. Neutralization of representatives of each host range phage group by
standardized rabbit antisera prepared against four different phages
revealed similarity among several groups. However the neutralization
of phages was not constant within each group.
Electron photomicrographs of a phage for S. diacetilactis
revealed a tadpole-shaped particle. The head width and length were
each about 60 mμ , the tail width was about 10mμ and the tail length
was about 170 mμ . There also were suggestions for a polyhedral
head structure in enlarged (84,000X) pictures of the viruses.
Chemical compositional studies of the phage studied by electron
microscopy revealed that phosphorus represented 4.45 percent of
the phage dry weight. Nitrogen was found to constitute 12.8 percent
of the phage protein. A ratio of phage phosphorus to phage protein
nitrogen of 2.7 was determined.
Bacteriophages prepared in broth medium were recovered by
differential centrifugation and purified in cesium chloride density
gradients. An amino acid analysis revealed the following mole
percentages for the amino acids quantitated: lysine, 8.1; histidine,
1.7; arginine, 3.8; aspartic acid and asparagine, 10.0; threonine,
7.5; serine, 6.8; glutamic acid and glutamine, 12.1; proline, 3.4;
glycine, 9.0; alanine, 9.5; valine, 8.7; methionine, 1.1; isoleucine,
7.9; tyrosine, 2.2 and phenylalanine, 2.8.
The nucleic acid of the phage was determined to be
double-stranded DNA with a thermal melting point of 84.3°C,
indicating an average base composition of 35.7 percent guanine plus
Calculated values for percentage DNA and protein composition
of the phage were 44 and 56 percent respectively. The density of
the phage was calculated to be 1.47g/cc.