Microorganisms' Growth

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Chapter 6
Microbial Growth

1

Growth
• increase in cellular constituents that may result
in:
– increase in cell number
• e.g., when microorganisms reproduce by budding or binary fission

– increase in cell size
• e.g., coenocytic microorganisms have nuclear divisions that are not accompanied by cell divisions

• microbiologists usually study population growth
rather than growth of individual cells

2

The Growth Curve
• observed when microorganisms are
cultivated in batch culture
– culture incubated in a closed vessel with a
single batch of medium

• usually plotted as logarithm of cell number
versus time
• usually has four distinct phases

3

population growth ceases

maximal rate of division
and population growth

decline in
population
size

no increase

Figure 6.1
4

Lag Phase
• cell synthesizing new components
– e.g., to replenish spent materials
– e.g., to adapt to new medium or other conditions

• varies in length
– in some cases can be very short or even absent

5

Exponential Phase
• also called log phase
• rate of growth is constant
• population is most uniform in terms of
chemical and physical properties during this
phase

6

cells are dividing and doubling in number at regular intervals

7

each individual
cell divides at a
slightly different
time

curve rises
smoothly rather
than as discrete
steps

Figure 6.3
8

Balanced growth
• during log phase, cells exhibit balanced
growth
– cellular constituents manufactured at constant
rates relative to each other

9

Unbalanced growth
• rates of synthesis of cell components vary
relative to each other
• occurs under a variety of conditions
– change in nutrient levels
• shift-up (poor medium to rich medium)
• shift-down (rich medium to poor medium)

– change in environmental conditions

10

Effect of nutrient concentration on
growth

Figure 6.2
11

Stationary Phase
• total number of viable cells remains constant
– may occur because metabolically active cells stop
reproducing
– may occur because reproductive rate is balanced
by death rate

12

Possible reasons for entry into
stationary phase





nutrient limitation
limited oxygen availability
toxic waste accumulation
critical population density reached

13

Starvation responses
• morphological changes
– e.g., endospore formation

• decrease in size, protoplast shrinkage, and
nucleoid condensation
• production of starvation proteins
• long-term survival
• increased virulence

14

Death Phase
• cells dying, usually at exponential rate
• death
– irreversible loss of ability to reproduce

• in some cases, death rate slows due to
accumulation of resistant cells

15

16

The Mathematics of Growth
• generation (doubling) time
– time required for the population to double in size

• specific growth rate
– ln 2 / generation time

17

Figure 6.4

18

Calculating Generation Time
• Generation Time (g) = t/n
g = (0.301) t / logNt – logN0
• If a bacterial population increases
from 1X103 cells to 1X109 cells in 10
hrs, what is g?
g = (.301) 10 / log 109 – log 103
= 3.01 / 9 – 3
= 0.5 hrs
19

20

Measurement of
Microbial Growth
• can measure changes in number of cells in a
population
• can measure changes in mass of population

21

Measurement of Cell Numbers
• Direct cell counts
– counting chambers
– electronic counters
– on membrane filters

• Viable cell counts
– plating methods
– membrane filtration methods

22

Counting chambers
• easy, inexpensive,
and quick
• useful for counting
both eucaryotes and
procaryotes
• cannot distinguish
living from dead cells

Figure 6.5
23

Electronic counters
• microbial suspension forced through small
orifice
• movement of microbe through orifice impacts
electric current that flows through orifice
• instances of disruption of current are counted

24...
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