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80 CHAPTER 4

This type of curve is typical of a batch culture,

i.e. a closed culture system such as a flask in which all

the nutrients are present initially. The rate of growth

during the exponential phase is termed the specific

growth rate (μ)of the organism, and if all conditions

are optimal then the maximum specific growth rate,

μμmax, is obtained. This is a characteristic of a particu-

lar organism or strain.

The value of μμis calculated by measuring log 10 of the

number of cells (N 0 ) or the biomass at any one time

(t 0 ) and log 10 of cell number (Nt) or biomass at some

later time (t), according to the equation:

log 10 Nt−log 10 N 0 =

where 2.303 is the base of natural logarithms.

Rewritten, this equation becomes:

μ=[ (log 10 Nt−log 10 N 0 )/t−t 0 ] × 2.303

If, for example, N 0 = 103 cells ml−^1 and Nt= 105 cells

ml−^1 , 4 hours later, then:

From this we can compute the mean doubling

time, or generation time(g), of the organism, as the

time needed for a doubling of the natural logarithm,

according to the equation:

So, in our example, g=0.60 h. For S. cerevisiaeat

30°C, near-maximum values of μμand gare 0.45 h−^1

and 1.54 h respectively. For the yeast Candida utilisat

30°C, μμ=0.40 h−^1 and g=1.73 h.

Mycelial fungi also grow exponentially because they

have a duplication cycle. Averaged for a colony as a

whole, they grow as hypothetical “units,” one producing

two in a given time interval, two producing four,

and so on. Representative values of μμmaxand gfor

mycelial fungi are: 0.35 h−^1 and 1.98 h for Neurospora

crassa at 30°C; 0.28 h−^1 and 2.48 h for Fusarium

graminearum at 30°C, and 0.80 h−^1 and 0.87 h for

Achlya bisexualis(Oomycota) at 24°C.

These values compare quite favorably with those of

yeasts. However, it is difficult to maintain exponential

growth of mycelial fungi, because the hyphae do not

disperse freely. Instead, they form spherical pellets in

shaken liquid culture, and this leads to problems of

nutrient and oxygen diffusion. This problem can be

overcome to some degree by using compounds (para-

morphogens) that alter the hyphal branching pattern,

g

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mycorrhizal fungus and grown in peat against the face

of a transparent perspex box. The root system itself

is quite limited: it consists of the region marked by

double arrowheads (<<) where the roots are enveloped

by a mycorrhizal sheath (Chapter 13). Most of the

branching network that we see is a system of aggregated

fungal hyphae, termed mycelial cords (Chapter 5)

whch explore the soil for nutrients. When they find

a localized pocket of organic nutrients (see the large

arrowhead in Fig. 4.14) they produce a mass of hyphae

to exploit the nutrient-rich zone.

Kinetics of fungal growth

Growth can be defined as an orderly, balanced increase

in cell numbers or biomass with time. All components

of an organism increase in a coordinated way during

growth – the cell number, dry weight, protein content,

nucleic acid content, and so on.

Figure 4.15 shows a typical growth curve of a yeast

in shaken liquid culture, when the logarithm of cell

number or dry weight is plotted against time. An ini-

tial lag phaseis followed by a phase of exponential

or logarithmic growth, then a decelerationphase, a

stationaryphase, and a phase of autolysis or cell

death. During exponential growth one cell produces

two in a given unit of time, two produce four, four

produce eight, and so on. Provided that the culture

is vigorously shaken and aerated, exponential growth

will continue until an essential nutrient or oxygen

becomes limiting, or until metabolic byproducts accu-

mulate to inhibitory levels.

Fig. 4.15Typical growth curve of a batch culture: a, lag
phase; b, exponential or logarithmic growth phase; c, decel-
eration phase; d, stationary phase; e, phase of autolysis.