ma ~ CLM
NOV 241955 Guadlin - 916

Phenotypic variation affecting motility In Salmonella

Motility is a bacterial character which can be stucied both
in populations and in individual cells. Motility behaviour of
populations can be inferred from the colony patterns produced
during growth on semi-solid medium, itn which motile cells can
swim. It seems as if many non-motile Salmonella strains
characteristically contain a small proportion of motile cells,
which do not differ genetically from the non-motile members of
the strain. When pour-plates of non-motile Salmonella strains
are incubated each viable cell produces a colony; in 20 of 47
strains tested these colonies are surrounded by smaller "Satellite"
micro-colonies. On subculture, satellites yield non-motile growth
indistinghishable from the parent strain. Satellites are inferred
to arise from motile cells which arise during the growth of the
arent non-motile strain. Satellite formation also occurs in some
non-motile strains of E.coli, Vibrio cholerae and Proteus. It
seems that phenotypic variation of this kind is widespread. The
rate at which motile variants é@éccur in a non-motile strain has been
estimated, by making use of the Poisson PDistributim. Pour-plates
in gelatine agar medium are incubated at 37°C, chilled and then
incubated at room temperature. The medium is solid at room
temperature and no further bacterial movement can occur3 but

Comenate
growth continues till centinues and satellites are visible. In
-Q-
SW 545 { Stocker, Zinder & Lederberg ,f1953, J.gen.Microbiol .9,410),
about one-third of the colonies have no satellites, after five hours

yr othe eat 37°C. Assuming that “events” leading to the production of
satellites occur at random amongst the population, then the mean
number of such events can be calculated from the Poisson
Distribution. From the number of viable cells per colony, at
the time of chilling, the event frequency per cell per bacterial
generation can be found. In SW 545 it is about 107. The
distribution of numbers of satellites per colony indicates, that
in this strain, an event results in an average of 2-5 satellites.
The frequency of events appears to be a strain characteristic -
4{t seems as if each cell has a small, genetically determined,
probability of undergoing an event during it/s life-time. Motile
cells have been isolated from SW 545 by micro-manipulation.
The megority of the progeny of such cells were non-motile.
However, in most populations one, two, three or four motile cells
persist amongst the non-motile progeny. The motile cells behave
as if their motility resulted from the presence of non-
multiplying motility-conferring particles.

To sum up, plate experiments indicate that, in some non-
motile strains "events" occur at low frequency, each event giving
rise to a small number of motile cells. Micromanipulation of
single cells provides evidence for the presence of motility -
conferring particles, which suggests that an “event” its the synthesis
of a small number of motility-conferring particles. We now have
Se

evidence sugsesting that a motility- conferring particle
corresponds with flagelietten. Presumably some sort df basal
granule structuré, secreting the flagellum, is involved: AN ts
event is probably the synthesis of a small number of flagella, by
@ cell; these flagella are distributed out to the progeny and
are maintained for an indefinite period.

We have here a situation in which spontansous phenotypic
variation is cocurring amongst ocelis of uniform genotype, in an
environment which is as near uniform as we can male it. As
described here “events” have a genetically determined, low
probability of cocurrence. Possibly strains showing a high
proportion of motile celis have a correspondingly high event
frequency. The situation in SW 545, and strains of similar
behaviour, may be an extreme case of normal variability. For
variation of this type we propose the term residual variation;
that is spontaneous variation occurring amongst cells of uniform

>

genotype in a "uniform" environment.

C Quadling and BAD Stocker,
The Lister Institute,