Jorn-rta/of Genera/ Microbiology (19j r ) , 67, 345-348
Printed in Crent Britnin
345
Exponential Growth of the Germ Tubes of Fungal Spores
By A. P. J. TRINCI
Microbiology Department, Queen Elizabeth College, Campden Hill Road, London, W. 8
(Acceptedfor publication 14 June 1971)
SUMMARY
The germ tubes of eight fungi studied initially grew exponentially in length.
Subsequently there was a deceleration in their growth rate. With the exception of
Geotrichum lactis the germ-tube specific growth rate of each organisms was substantially greater than its specific growth rate in submerged culture. The possible
reason for this difference is discussed.
INTRODUCTION
Although under favourable cultural conditions unicellular organisms increase in number
at an exponential rate, certain individual cells have been shown to grow linearly (Schizosaccharomyces pombe, Mitchison (I963) ; Tetrahymena piriformis, Cameron & Prescott
(1961); Escherichia coli, Kubitschek (1968); Chlorella pyrenoidosa, Williams (1965)).
Kubitschek (1970) has recently suggested that linear growth is the fundamental form for all
kinds of cells. Smith (1924) demonstrated exponential growth of fungal hyphae on solid
medium but his observations were only begun when the hyphae were about 230pm. long.
Plomley (I 959) found that the germ tubes of Chaetomium spores only grew exponentially
in length after an initial linear phase of growth which lasted for several hours. The germ
tubes of Aspergillus nidulans, Penicilliurn chrysogenum and Mucor hiemalis, however, grew
exponentially in length from their inception (Trinci, I 969); the germ-tube specific growth
rates of these organisms were substantially faster than their respective specific growth rates
in submerged culture.
RESULTS AND DISCUSSION
In the present study growth in germ-tube length was followed by time lapse cinephotography (Trinci, 1969) and the organisms were grown at 25' on DM medium (Trinci, 1971).
During the first 4 h. after inoculation Rhizopus stolonifer sporangiospores increase linearly
in diameter from 7.6 to 9-8,urn. Other fungal spores have also been shown to swell at linear
rates (Ekundayo & Carlile, 1964; Fletcher, 1969; Gull & Trinci, 1971). However, the cube
root of the volume of these spores increases linearly with time. That is,
v, = v,+ k (tl - to),
where Vois the volume of the spore at time to, V, is the volume of the spore at time tl, and
k is a constant.
Fig. I shows that the germ tube of a Rhizopus stolonij2r sporangiospore initially increased
in length at an exponential rate until it was about 60 pm. long and that subsequently there
was a deceleration in growth rate. The germ tubes of all the other organisms studied (Table I)
followed similar growth kinetics.
Linear kinetics may only be a feature of the growth of individual microbial cells, whereas
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P. J. T R I N C I
A.
346
c
Germ t ubc
formed at
this stage
\
2
4
6
Time (h.) since inoculation
0
Fig.
I.
8
Growth of a germ tube of Rhizopus stolonifer at 25'. The medium was inoculated
with a freshly prepared suspension of sporangiospores.
Table I . ComDarison between aerm-tube pro wth and growth in submerged
shake-$ask culture
v
Y
Y
v
Cultures gIown at 25" on DM medium.
Increase in mould dry
weight in submerged culture?
Germ-tube growth*
h
I
I.
Mean specific
growth rate
.
v
Mucor racernosus
Actinomucor repens
Absidia glauca (+)
Geotrichumlactis
Aspergillus wentii
A . niger
Penicillium chrysogenum
*
VLU
\
Mean doubling
time
I r n
1
Mean specific
growth rate
,
v VU4)
0.809 (k 0.055)
0.655 (k0.170)
0.451 ( 2 0.075)
0.4 I 7 ( & 0'022)
0.303 (20.062)
0.282 (i0.049)
0.296 (+o.rw)
h
> [
0.81 ( f0.05)
1-16 (k0.31)
I 3 8 (i0.28)
1.68 (20.15)
2'50 ( L 0.47)
2-54 ( i0.42)
2-60 (k 0-66)
" '33
0'102
0.181
0.I 24
0.353
0.147
0'1rg
0.164
\
Mean
doubling
time
Irn
1
,
7-5
4.0
5'8
2'0
4'8
5'9
4'3
is the mean of 5 to 6 germ tubes from at least two separate experiments.
From Trinci, 1971.
$ Standard deviation.
ctK, each rate
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41
9-1
3'6
3'6
1.2
2'1
2.4
1.8
Growth of fungal-spore germ tubes
347
a branching coenocytic hypha should be regarded as being analogous to a population of
unicellular organisms and hence it is not surprising that it grows exponentially. However,
an exponential increase in the volume of the germ tube does not necessarily mean that it is
increasing in dry weight at the same rate (Mitchison, 1963).
The germ tube and submerged culture specific growth rates of each organism are compared in Table I. With the exception of Geotrichum lactis, the germ-tube specific growth
rates were substantially greater than the organisms’ specific growth rates in submerged
culture. The specific growth rate of G. lactis as measured by increase in turbididy (as opposed
to dry-weight measurements, Table I) of a submerged shake flask culture at 25’ was 0.403
(doubling time, 1.72 h.), a figure almost identical with the organism’s germ-tube specific
growth rate. Geotrichum lactis arthrospores are unusual spores in that they contain large
vacuoles and do not swell prior to germ-tube emergence. It is possible that, unlike other
fungal spores, the metabolism (and nutrition) of G. lactis arthrospores is similar to that of
its vegetative hyphase. This may explain why the organism’s germ tubes and vegetative
hyphae grow at the same rate.
In the present (Table I) and previous studies (Trinci, 1969; Trinci & Gull, 1970)it was
found that the ratio, germ-tube specific growth rate: specific growth rate in submerged
culture (a,/a,) was higher for non-septate than for septate fungi.
Salmonella typhimurium has a faster specific growth rate (I -87) when grown upon nutrient
broth (a complex medium) than when grown upon a defined minimal medium (specific
growth rate, 0,626) with only glucose as the energy source (Kjeldgaard, Maalrae & Schaechter,
1958); the ratio, specific growth rate on nutrient broth:specific growth rate on minimal
medium was thus 2-1. The germ-tube specific growth rate of an organism may be higher
than its specific growth rate in submerged culture (i.e. the normal growth rate of vegetative
hyphae) because the initial growth of the germ tube is supported in part by the complex
organic substances mobilized from the spore’s endogenous reserves, whereas the only
nutrients available for vegetative hyphal growth are those in the medium (in this case a
relatively simple defined medium). The germ-tube specific growth rate of a fungus may
thus represent its maximum growth rate upon a complex medium. Furthermore, the point
at which the deceleration in germ-tube growth commences (Fig. I) may be correlated with
exhaustion or depletion of the spores’ endogenous reserves and may thus be regarded as the
start of a transition state.
These results indicate the germ-tube specific growth rate should not be used to study the
influence of nutrients upon growth.
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348
A. P. J . T R I N C I
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