Lecture 4 Dr. Dalia M. Mohsen Prof. of Microbiology

Medical Microbiology

Dr. Dalia M. Mohsen Prof. In Microbiology CLS 311

Lecture 4

Dr. Dalia M. Mohsen Prof. of Microbiology

Reproduction in Prokaryotes

Types of reproduction are -Binary fission (Bacteria)Budding (yeasts)Conidiospores (moulds and

yeasts)Fragmentation of filaments

(molds)



Binary Fission


Budding• Seen in yeast cells, and few bacterial species


Conidiospores

Produce chains of Conidiospores carried externally at the tip of the filaments


Generation time

• Time required for the cell to divide (and its population to double.

• Depends on– Type of bacteria– Environmental conditions , like temperature

• Average generation time is 1 to 3 hrs.– Can vary from 20 min to 24 hrs


Phases of Growth

• 4 Phases

1. Lag Phase

2. Log Phase

3. Stationary Phase

4. Death Phase


Lag Phase

Period of little or no cell division• Bacteria are first introduced into an environment or

media• Bacteria are “checking out” their surroundings• Undergo intense metabolic activity, synthesis of

enzymes and various molecules• There is increase in cells sizes but no increase in the

cells number• Last for few minutes to hours or several days


Log PhaseRapid cell growth rate (exponential growth)Cellular reproduction is most activepopulation doubles every generation timeGeneration time is constant – on graph is seen as a straight

linemicrobes are sensitive to adverse conditions

– antibiotics– anti-microbial agents– Elevated temperature– Starvation– Change in pH


Stationary Phase

• Growth rate slows, the number of microbial death balances the number of new cells produced.

• Death rate = growth rate• Cells begin to encounter environmental stress

– lack of nutrients– lack of water– not enough space– metabolic wastes– oxygen– pH

Endospores would form now at the end of this stage


Death Phase

Logarithmic decline phase

• The number of death eventually exceed the number of new cells formed

• Death rate > growth rate• Due to limiting factors in the environment• Tailing could occurs as a result of endospores formation• Phase continues until all the population is diminished



Limiting Factors in the environment for growth

of bacteria

a. Lack of water, nutrients and foodsb. Lack of Space and Oxygenc. Accumulation of metabolic wastesd. Changes in pHe. Temperature


Enumeration of Bacteria

• Direct measurement of microbial growth

– Plate counts

– Serial dilution

– Pour plate and spread plate techniques

– Filtration

– The most probable number method

– Direct microscopic count


• Estimation of microbial counts by indirect method

– Turbidity– Metabolic activity– Dry weight


Perform serial dilutions of a sample

Direct Measurements of Microbial Growth


Inoculate Petri plates from serial dilutions

Plate Count

Figure 6.16


• After incubation, count colonies on plates that have 25-250 colonies (CFUs)

Plate Count


Pour and spread plate method


Filtration


• Multiple tube MPN test

• Count positive tubes and compare to statistical MPN table.

Figure 6.18b

Most probable number method


Direct Microscopic Count

Figure 6.19


Turbidity

Estimating Bacterial Numbers by Indirect Methods


• Metabolic activity – amount of certain metabolic product is in direct proportion with the number of bacteria.Ex. – an acid or CO2

• Dry weight – good method for measuring filamentous organisms.Ex. - Fungus is removed from medium –

extraneous material removed – dried in a desiccator - weighed

Thank you


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Lecture 4 Dr. Dalia M. Mohsen Prof. of Microbiology