HIDRODINÁMICA
2.5. Movimiento de un sólido en un fluido viscoso
resistant to amplcillin, tetracycline and kananycln
is Indicated (0—
0) as is the theoretical washout
rate for a non-growing organism at this dilution
rate (--- ).
Figure <?. Competition between strains with and without plasmid
The kinetics of the takeover event were exponential and the rate of
reduction of the R+ population closely resembles that of the theoretical
washout rate (T.W.R.) of a non-growing organism at the dilution rate
employed in this experiment (0.3 hr *). This result implies that it is
the R+ strain that is at a disadvantage in mixed populations competing
for the same limiting substrate (in this case phosphate).
The degree of competition between W3110 and W3110 (RP4) can be
calculated from the kinetics of changeover in the chemostat population.
The growth rate of the strain coming to predominance (R- W3110) must be
at least equal to the dilution rate (0.3 hr- ^ ) and the growth rate of
the uncompetitive strain (W3110 (RP4)) can be calculated from the
washout kinetics by the formula of Jannasch (1967):
Pl - [ ( U xt - lit x c)/(T2 - Tj)] ♦ D
where pj • specific growth rate of the uncompetitive strain
xQ ■ uncompetitive population at time Tj
xt • uncompetitive population at time T 2
and D “ dilution rate
Using values of xQ and xt derived from figure 2, the calculated value of
Pj is 0.1 hr* a reduction compared to the competing strain of 66Z.
A similar result was obtained by Godwin and Slater (1979) and the
calculated result for RP1 (Helling et al., 1977) is very similar. These
values show that any R- cell arising within a population of R* cells
would be at a competitive advantage and would be sure to come to
predominate the culture. Thus, conclusions about the remarkable
and the utility of long term cultivation of RP4 containing populations
in the chemostat as a measure of pla6mid stability has been
demonstrated.
iv) Persistence of the R* strain at low levels in the chemostat
After the enforced competition between W3110 (RP4) and W3110 in the
chemostat had resulted in the percentage of R+ cells falling to about
0.1Z of the total population, competition appeared to stop and the R*
strain was maintained at 0.01Z-0.1Z of the total population until the
end of the experiment. A similar observation of persistence in
chemostat culture has been reported in other competition situations
(Dykhuizen, 1978; Melling et a l .. 1977; Zamenhof and Eichhorn, 196>;
Godwin and Slater, 1979; Adams e£ £ l ., 1979), all despite the
prediction of chemostat theory that uncompetitive strains should be
completely washed out of the chemostat (Powell, 1958).
A possibility that would explain such persistence, especially in the
case of plasmid bearing cells is that the presence of a plasmid alters
bacterial cell wall layers and enables selective adhesion of R* cells to
the walls or baffles of the chemostat vessel. Some adhesion properties
are known to reside on plasmids (e.g. Smith and Higgins, 1976) and so it
was of interest to examine the wall population of a chemostat after a
competition experiment to determine if indeed RP4 containing cells made
up a high proportion of the adhering population. Wall growth was
examined by rinsing a striped chemostat after a competition experiment
several times in saline (0.9Z w/v) and then swabbing the walls and
three types of antibiotic plate allowed an estimation of the R+ and R-
populations present. In all cases tested (several experiments) the
relative proportions of R+ and R- cells present in the wall populations
was the same as it was in free culture indicating a lack of specific
adhesion due to the presence of RP4.
v) Repeated competition with 'cycled' strains of W3110 (RP4)
and W3110
An alternative possibility for the persistance of R+ strains in mixed
culture as a low level population could be mutation of those survivors
to a higher growth rate such that peristance of these new 'fitter'
strains was allowed. This possibility was tested in the following way:
a culture of W3110 (RP4) maintained under phosphate limitation in the
chemostat was subject to an enforced competition event in the normal way
and allowed to washout to a residual (stable) level of 0.1Z. At this
point the R+ strain present in the chemostat was isolated on L-agar
plates containing tetracycline and. after restreaking, used as inoculum
for a fresh chemostat run under the aame conditions as the first. After
about 10 generations of limited growth this 'cycled' strain was
challenged with an R- (W3110) inoculum isolated in a similar way from
the original culture. If mutation had occurred in the persistant R+
population of the first chemostat and providing that the mutation was
inherited during subculture, then competition in the second chemostat
should be severely reduced or completely absent. Figure 3 shows the
kinetics of washout for the strain in each chemostat. Both rates of
washout are similar and, in addition, the final R+ level in each case