Microbial Growth

When microbes are provided with nutrients & required environmental factors, they become metabolically active and grow

1Lecturer:: Mohamed Ali. Bsc, M.Eng

College of Agriculture, University Of Hargeisa.

Chapter contents:1- Bacterial Division/reproduction2- Stages of Bacterial growth curve3- Measurement of Microbial

growth4- Environmental factors effect

microbial growth

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What is Microbial growth? involves an increase in the number of

cells rather than in the size of individual cells.

2 levels of growth:

1. A cell synthesizes new components, increase its size

2. Increase number of cells in the population

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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., some microorganisms have nuclear

divisions that are not accompanied by cell divisions

microbiologists usually study population growth rather than growth of individual cells

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1- The Bacterial division/reproduction

Bacteria normally reproduce by a method called binary fission

What is Binary Fission.?Binary Fission is the process of bacteria

reproduction where one cell become two

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Stages of Binary Fission6

Generation Time (Doubling time) What is Generation

Time? The time required

for a cell to divide (and its population to double) is called the generation time

As you seen in the picture, cell’s division produces two cells, two cells’ divisions produce four cells, and so on.

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Bacterial Generation (Doubling) Time

Examples’;

Escherichia coli 20 minutes Mycobacterium tuberculosis 18 hours Mycobacterium leprae 14 days

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Bacteria Undergo Exponential Growth

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cells are dividing and doubling in number at regular intervals

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2-Stages in Bacterial Growth curve When a few bacteria are inoculated into a liquid growth

medium, it is possible to plot a bacterial growth curve that shows the growth of cells over time (See picture). There are four basic phases of growth: 1- the lag, 2- log (Exponential) , 3- stationary, and 4-death phases.

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Stages of Bacterial growth curve

Bacterial populations follow a sequential series of growth phases: the lag, log, stationary, and death phases.

Knowledge of the bacterial growth curve is critical to understanding population dynamics and population control in the course of infectious diseases, and in food preservation .

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no increase

maximal rate of divisionand population growth

population growth ceases

decline inpopulationsize

4 Stages of Bacterial growth Curve

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a) Lag Phase Bacteria are first introduced into an environment

or media culture. Bacteria are trying to adapt to nutrients

Lag phase-preparing to grow in size and synthesize enzymes etc.

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

Stages of Bacteria growth Curve

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b) Log (Exponential) Phase also called log phase rate of growth is constant Population number of cells undergoing

binary fission doubles at a constant interval called generation time

Continue as long as cells have adequate nutrients & good environment

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c) Stationary Phase If exponential growth continued

unchecked, startlingly large numbers of cells could arise.

In this stage microbial death is equal to microbial growth e-g Death=growth.

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Possible reasons for entry into stationary phase

nutrient limitation limited oxygen availability toxic waste accumulationcritical population density reached

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d) Death Phase Decline in the number of viable cells cells dying, usually at exponential rate Death; loss of ability to reproduce in some cases, death rate slows due to

accumulation of resistant cells Slower than log phase

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3- Measurement of Microbial Growth

can measure changes in number of cells in a population

can measure changes in mass of population

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Measurement of Cell Numbers What are the lab equipment's used to

measure bacteria cell numbers? There are two types :- 1- Direct cell counts:-

a) counting chambers

b) electronic counters

c) on membrane filters

2- Viable cell counts plating methods (spread, pour plate) membrane filtration methods 20

a) Counting chambers easy, inexpensive, and quick useful for counting both eucaryotes and

procaryotes cannot distinguish living from dead cells

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b) Electronic counters Such as Coulter counter microbial suspension forced

through small hole or orifice movement of microbe through

orifice impacts electric current that flows through orifice

cannot distinguish living from dead cells

quick and easy to use useful for large

microorganisms and blood cells, but not prokaryotes

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c) membrane filters

cells filtered through special membrane that provides dark background for observing cells

cells are stained with fluorescent dyes useful for counting bacteria with certain dyes, can distinguish living from dead

cells

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4- Important environmental factors affect microbial growth

1.pH

2.Temperature

3.Gas requirement

4.Pressure

5.And other environmental factors e-g radiation, water activities etc.

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1. pH negative

logarithm of the hydrogen ion concentration

Affect the activity & integrity of enzymes & structural components of a cell

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pH Optimum pH for most microbes ranges

approximately from 6 to 8 acidophiles

growth optimum between pH 0 and pH 5.5 neutrophiles

growth optimum between pH 5.5 and pH 7 alkalophiles

growth optimum between pH8.5 and pH 11.5

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2. Temperature

organisms exhibit distinct cardinal growth temperatures minimum maximum optimum

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Pychrophiles grow well at OoC optimum growth at 15oC or lower (cold loving microbes ) Cannot grow above 20oC

Psychrotroph can grow at 0-7oC optimum between 20-30oC, max around 35oC

Mesophiles growth optimum around 20-40oC (moderate temp. loving microbes)

Thermophiles Growth range is 45oC and 80oC optimum between 55-65oC (heat loving microbes)

Hyperthermophiles optimum growth between 80oC and 100oC May grow to temperatures of 120oC

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Depends the kinds of microorganism, some microorganism need oxygen while others not need oxygen to grow

3. Gas Requirements

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Thioglycollate broth:1. Aerobic (pseudomonas aeruginosa)2. Facultative (Staphylococcus aureus)3. Facultative (Escherichia coli)4. Obligate Anaerobe (Clostridium butyricum)

needoxygen

preferoxygen

ignoreoxygen

oxygen istoxic

Require< 2 – 10%oxygen

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4. Pressure

Microbes obtain almost all their nutrients in solution from surrounding water

barophile organismsAdapted to life under high pressure require or grow more rapidly in the

presence of increased pressureExample : bottom dwellers in the ocean

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Review Questions1) What are the four stages of microbial growth? Explain

what happens each phase?

2) Name 3 direct methods use to measure microbial cell numbers?

3) Name 2 methods used measure viable cell counts

4) What are the four environmental factors effect microbial growth?

5) What is binary fission?

6) What is generation (doubling time)?

7) State 4 possible reasons why bacterial growth curve entry into stationary phase?

8) How does pH effect microbial growth?

9) What is microbial growth? 34