J. gen. Virol. (t974), 22, 431-435
43I
Printed in Great Britain
Lysis of RNA
Phage-infected Cells depends upon Culture Conditions
(Accepted I4 November I973)
SUMMARY
As the generation time ofEscherichia coli is increased, lysis caused by RNA phage
infection occurs later after infection and decreases in amount. RNA phage is unable
to cause lysis of E. coli approaching or in stationary phase. A mechanism of lysis
is suggested.
At present the mechanism of cell lysis caused by RNA bacteriophage infection is unknown.
RNA bacteriophage codes for only three proteins - polymerase, maturation protein and coat
protein (Stavis & August, ~97o). It has been suggested that coat protein might cause lysis
since cells infected with t"2 coat-protein mutants do not lyse (Zinder & Lyons, I968). As
shown below, however, failure of lysis may also occur in cells producing infective RNA
bacteriophage. There is then no evidence that any of the virus-coded proteins has a direct
lytic action. Therefore the changes in the cell surface that result in lysis are probably
secondary to other changes (Stavis & August, I97O) in the host after MS2 infection. These
changes could result in failure of one of the steps involved in growth and division of the cell
envelope. Cell division is coupled to DNA replication (Clark, I968; Schwarz, Asmus &
Frank, I969; Hoffmann, Messer & Schwarz, I972; Jones & Donachie, i973) which in turn
is dependent upon a round of RNA and protein synthesis (Lark, 1966; Helmstetter & Cooper,
I968). If MS2 induced lysis is a result of aberrant cell division secondary to changes in host
RNA and protein synthesis, then the time of lysis and perhaps the occurrence of lysis should
be dependent upon the host's doubling time, and lysis should not occur in cells that are not
dividing.
Therefore, the O.D.450 ofEscherichia coli growing at different rates in different media was
followed after MS2 infection. The O.D.~50 of MS2-infected E. coli growing in C medium
(Roberts et al. ~957) containing both casamino acids and glucose with a doubling time of
27 rain and of MS2-infected E. coli growing in C medium and casamino acids without
glucose with a doubling time of 72 min was followed (Fig. I). In both cases the O.D.
follows that of the uninfected control for a little more than one doubling time and then
stops increasing. In both cases the O.D. starts to drop before three generation times,
but drops farther in the more rapidly growing culture.
To check that the differences in lysis are due to a difference in doubling time rather than
to some other aspect of the nutritional differences between the cultures, Escherichia coli in
C medium and glucose which had a doubling time of 62 rain were infected with MS2
(Fig. 2a). After infection the changes in the O.D. of the culture growing in C medium
and glucose were similar to those of the culture growing with a nearly similar doubling time
in C medium and casamino acids. E. coli growing in C medium and glycerol with a doubling
time of IOO rain were also infected. The O.D. of this culture, like that of the others,
levelled off after a little more than one doubling time (Fig. 2b). The O.D. of the culture
growing in C medium and glycerol, however, did not drop, and only 13 ~o of the phage made
were extracellular even after five generation times as compared with 72 % released from the
culture grown in C medium and glucose. In all media the phage yields were about the same,
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Fig. I. MS2 infection o f Escherichia eoli growing in basic salts m e d i u m a n d c a s a m i n o acids with a n d
w i t h o u t glucose. E. coli 3coo (Hfr, thi-, reD) was grown to 2 to 3 x io 7 cells/ml in C m e d i u m
(Roberts et aL I955) a n d 0-15 ~ c a s a m i n o acids with a n d without o'5 ~ glucose. T h e cultures were
halved a n d MS2 at a multiplicity of 10 was a d d e d to one half. T h e O.D.45o of the infected a n d uninfected
cultures was followed o n a B a u s c h a n d L o m b Spectronic 2o. Samples were t a k e n at different times,
artifically lysed a n d titred for MS2. (A) Culture in C m e d i u m , c a s a m i n o acids a n d glucose. (B)
Culture in C m e d i u m a n d c a s a m i n o acids. O---O, O.D.450 o f uninfected culture; [] - - [], O.D.4~o o f
infected culture; x . . . . x , MS2 p.f.u./ml.
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Fig. 2. MS2 infection o f Escherichia coil growing in C m e d i u m a n d glucose a n d in C m e d i u m a n d
glycerol. Escherichia coli 3ooo were g r o w n to 2 to 3 x io 7 cells/ml in C m e d i u m a n d o'5 ~ glucose
or in C m e d i u m a n d o'5 ~ glycerol. T h e cultures were halved, a n d MS2 at a multiplicity o f Io was
a d d e d to one half. T h e O.D.4~0 was followed. Samples were t a k e n at different times, artificially lysed
a n d titred for MS2. (A) Culture in C m e d i u m a n d glucose. (B) Culture in C m e d i u m a n d glycerol.
0 - - 0 , O.D.45o o f uninfected culture; [] - - D , O.D.4o, o o f infected culture; x . . . . x , MS2 p.f.u./ml.
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Fig. 3. Result of MSz infection of samples removed from a culture of Escherichia coii at different
growth states. Escherichia coli 3ooo was grown overnight from a slant and diluted I : zoo into C
mediumsupplementedwitho'5glucoseToando'I5%casaminoacids. O.D.~50wasdetermined. Before
reading O.D.45ogreater than o'5, the samplewas diluted 1:5- At different times (marked by arrows)
samples were infectedwith MSz at a multiplicity of 5, and the O.D.45o,followed. 0--0, Uninfected
culture; [~- - -D, sample infected at O.D.45o = o.o2; O .... O, sample infected at O.D. 45o= o'o4;
x- - -x, sample infected at O.D.~50 = oqo; ± .... A, sample infected at O.D.450 = 0"30.
and while there were some differences in the rates of phage synthesis, the changes in extinction pattern did not correspond to the amount of phage made. The correlation of the levelling
and drop of extinction of MS2 infected cultures with growth rate suggests that these changes
in extinction are correlated with events in cell cycle.
Other experiments with cells infected at higher cell densities in these media showed that
Escherichia coli infected at O . D . o . ~ in C medium and casamino acids without glucose
produced a normal phage yield, but the O.D.~s0 of the culture increased exactly as did that of
the corresponding uninfected culture. In this medium at O.D.45o = o.I, the cells are in late
logarithmic phase because the culture reaches stationary phase much earlier (E450 = o'45)
than it does in media containing glucose or glycerol. If cell lysis is related to cell division,
then resting cells should not lyse. In addition, the balance between the macromolecular
syntheses and division must be altered as cells approach stationary phase, so cells in late
logarithmic phase might also be expected not to lyse. Therefore the result of infection of
a culture at different ages in the medium in which lysis was most pronounced (C medium
containing both casamino acids and glucose) was determined. Since in all but nitrogenlimited medium the capacity of E. coli to make infective virus drops Ioo-fold shortly after
the bacteria are fully in stationary phase (Ricciuti & Haywood, I973), the experiment was
completed in early stationary phase. While it takes just over one doubling time for the
growth of infected E. coli to cease in early and middle logarithmic phase, as the cells approach
stationary phase the O.D. of the infected culture does not level off, but follows that of
29
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434
Short communications
the uninfected control into stationary phase (Fig. 3). The cells infected in late logarithmic
phase (O.D.45o = o'3) make a normal phage yield, so the absence of lysis is not due to a defect
in MS2 synthesis. Thus not only do infected cells not lyse when they are not dividing, but
also no change in cell growth is observed as the infected cell enters stationary phase.
The experiment shown in Fig. 3 also demonstrates that experiments on lysis of cells
which are in late logarithmic phase (Engelberg & Soudry, I970 should be regarded with
caution, although cells in different media approach stationary phase differently. The experiment in Fig. 3 was repeated with a culture in sulphur-limited medium taken fully into
stationary phase (Ricciuti, I972) which in this medium occurs at a cell density near 7 x lo 8
cells/ml. The yield of p.f.u, shown in that work is about Ioo-fold less than the yield of cells
in logarithmic phase, so the significance of the absence of lysis is not clear.
Consistent with the hypothesis that MS2-induced cell lysis is the result of aberrant cell
division secondary to alterations in host syntheses are the facts that the changes in the
O.D. of MS2 infected cells depend upon the cells' doubling times and that cells infected
as they approach and enter stationary phase do not lyse. It is possible that under conditions
where the growth of infected cells is nearly normal the cell wall may be only slightly damaged,
so that the cells[may be able to leak virus (Hoffmann-Berling & Mazr, 1964; Engelberg &
Soudry~971 ) but be able to repair themselves so they do not lyse. Unbalanced cell growth
could cause defects in division of either the murein layer or the membrane layer of the cell
envelope. From what is presently known, the murein layer seems the most likely layer to be
implicated in lysis. Weidel & Pelzer (I964) have pointed out that the murein layer is one
continuous polymer, so for cell division to occur murein hydrolases must break bonds
between subunits, and an accurately co-ordinated supply of precursor subunits must be
available to make new bonds. When this system is thrown out of balance, there is autolytic
destruction of the murein layer and resulting lysis (Weidel & Pelzer, ~964; Schwarz et al.
I969). An increase in penicillin sensitivity which implies an increase in murein hydrolase
activity occurs shortly after completion of DNA replication, and fails to occur not only
when DNA synthesis is inhibited but also when protein synthesis is inhibited (Hoffmann et al.
I972). Protein synthesis is necessary for cell division (Jones & Donachie, i973). Consistent
with this is the observation that Rw-infected cells do not lyse when protein synthesis
inhibited (Engelberg & Soudry, 1971). When murein synthesis is inhibited in actively growing
cells, electron micrographs show the development of a bulge between the two undistorted
ends of the cell (Schwarz et al. 1969). A similar morphology has been described for Pseudomonas aeruginosa just prior to lysis due to infection with the RNA phage PP7 (Bradley,
I966).
This work was supported by U.S.P.H.S. grant AI o7788.
Microbiology Department
Yale University School of Medicine
New Haven, Connecticut o65 I o
A . M . HAYWOOD*
* Present address: Biophysics Unit, Agricultural Research Councir, Babraham, Cambridge CB2 4AT,
England.
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435
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(Received 5 October I 9 7 3 )
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