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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)
Dr. Dalia M. Mohsen Prof. of Microbiology
Dr. Dalia M. Mohsen Prof. of Microbiology
Binary Fission
Dr. Dalia M. Mohsen Prof. of Microbiology
Budding• Seen in yeast cells, and few bacterial species
Dr. Dalia M. Mohsen Prof. of Microbiology
Conidiospores
Produce chains of Conidiospores carried externally at the tip of the filaments
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
Phases of Growth
• 4 Phases
1. Lag Phase
2. Log Phase
3. Stationary Phase
4. Death Phase
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
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
Dr. Dalia M. Mohsen Prof. of Microbiology
• Estimation of microbial counts by indirect method
– Turbidity– Metabolic activity– Dry weight
Dr. Dalia M. Mohsen Prof. of Microbiology
Perform serial dilutions of a sample
Direct Measurements of Microbial Growth
Dr. Dalia M. Mohsen Prof. of Microbiology
Inoculate Petri plates from serial dilutions
Plate Count
Figure 6.16
Dr. Dalia M. Mohsen Prof. of Microbiology
• After incubation, count colonies on plates that have 25-250 colonies (CFUs)
Plate Count
Dr. Dalia M. Mohsen Prof. of Microbiology
Pour and spread plate method
Dr. Dalia M. Mohsen Prof. of Microbiology
Filtration
Dr. Dalia M. Mohsen Prof. of Microbiology
• Multiple tube MPN test
• Count positive tubes and compare to statistical MPN table.
Figure 6.18b
Most probable number method
Dr. Dalia M. Mohsen Prof. of Microbiology
Direct Microscopic Count
Figure 6.19
Dr. Dalia M. Mohsen Prof. of Microbiology
Turbidity
Estimating Bacterial Numbers by Indirect Methods
Dr. Dalia M. Mohsen Prof. of Microbiology
• 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
Dr. Dalia M. Mohsen Prof. of Microbiology