V. BACTERIOPHAGE TYPING OF SHIGELLA
277
VII. REPRODUCIBILITY AND STABILITY OF TYPE
PATTERNS
A. Molecular basis of instability
Due to a high frequency of lysogenisation and frequent phage type
changes upon loss of lysogenising phage, or uptake of extrachromosomal
DNA (phage, R-factors, colicin factors, etc.), spontaneous lysotype modifications are to be expected. The question is to what extent such changes
occur and whether practical implications are recognisable. One problem is
associated with the gradual change in phage typing pattern upon storage.
Strains should be typed shortly after isolation.
Even if phase I1 colonies are subcultured a mixture of both phase I and
phase I1 variants may develop. Ald6va et al. (1975) noted that type instability and changes on storage particularly affect certain phage types.
B. Type stability in vitro
Hammarstrom (1949) investigated type stability by a variety of methods:
(1) periodic retyping of the same strains,
(2) retyping after serial passage,
(3) testing several R-form colony variants of the same strains.
Reduced sensitivity to the phages was observed upon retyping. Phages
that were originally reactive often did not later attack the same bacteria.
Such changes could follow mutation or selection after five to eight months
in vitro. Hammarstrom (1949) found a change in 3% of the cultures and in
14% after storage for 27 months.
Junghans (1961) only noted changes in 6% among 1732 strains after
two to three years. I n 2.5% increased resistance against phages had
developed. Reportedly, less change is observed if strains are subcultured
every six months-after passage on endo-agar on which atypical colonies
are quite easily avoided morphologically. Storage as freeze dried cultures
may conserve typing patterns. Even lyophilisation, though, is not without
its problems, since it may induce loss of phages (Bergan and Midtvedt,
1975).
Often, survival of Shigella outside the body does not change the phage
type as evidenced by storage in water and soil samples for two to six weeks,
and in various media, at different temperatures for up to six months
(Grunow, 1965b).
C. Type stability in vivo
The question of type stability in vivo can only be evaluated in patients
who are isolated from epidemic reservoirs and thus do not run the risk of
277
VII. REPRODUCIBILITY AND STABILITY OF TYPE
PATTERNS
A. Molecular basis of instability
Due to a high frequency of lysogenisation and frequent phage type
changes upon loss of lysogenising phage, or uptake of extrachromosomal
DNA (phage, R-factors, colicin factors, etc.), spontaneous lysotype modifications are to be expected. The question is to what extent such changes
occur and whether practical implications are recognisable. One problem is
associated with the gradual change in phage typing pattern upon storage.
Strains should be typed shortly after isolation.
Even if phase I1 colonies are subcultured a mixture of both phase I and
phase I1 variants may develop. Ald6va et al. (1975) noted that type instability and changes on storage particularly affect certain phage types.
B. Type stability in vitro
Hammarstrom (1949) investigated type stability by a variety of methods:
(1) periodic retyping of the same strains,
(2) retyping after serial passage,
(3) testing several R-form colony variants of the same strains.
Reduced sensitivity to the phages was observed upon retyping. Phages
that were originally reactive often did not later attack the same bacteria.
Such changes could follow mutation or selection after five to eight months
in vitro. Hammarstrom (1949) found a change in 3% of the cultures and in
14% after storage for 27 months.
Junghans (1961) only noted changes in 6% among 1732 strains after
two to three years. I n 2.5% increased resistance against phages had
developed. Reportedly, less change is observed if strains are subcultured
every six months-after passage on endo-agar on which atypical colonies
are quite easily avoided morphologically. Storage as freeze dried cultures
may conserve typing patterns. Even lyophilisation, though, is not without
its problems, since it may induce loss of phages (Bergan and Midtvedt,
1975).
Often, survival of Shigella outside the body does not change the phage
type as evidenced by storage in water and soil samples for two to six weeks,
and in various media, at different temperatures for up to six months
(Grunow, 1965b).
C. Type stability in vivo
The question of type stability in vivo can only be evaluated in patients
who are isolated from epidemic reservoirs and thus do not run the risk of