1. Dr Rita Oladele Dept of Med Micro &Para CMUL/LUTH
Bacterial Growth
Dr Rita Oladele
Dept of Med Micro &Para
CMUL/LUTH

2. AIMS Understand the concept of microbial growth
Describe the steps involved in bacterial replication (binary fission)
Differentiate between bacterial cell growth, bacterial replication and bacterial population growth
Understand the calculation of population growth parameters
Describe the bacterial population growth curve (batch culture)
Describe what is happening to bacterial cells at each stage of the
batch cycle growth cycle
Describe commonly used methods for counting bacteria – direct
microscopic count, viable count, turbidometric methods
Differentiate batch culture from continuous culture

3. Bacterial growth
Increase in number of bacterial cells rather
than increase in size of individual bacteria
Bacterial species only maintained if population continues to grow
Growth depends on temperature, pH, osmotic pressure, oxygen, and nutrients

4. Microbial Growth contd
• Growth= an increase in the number of cells, not an increase in size • Generation=growth by binary fission • Generation time=time it takes for a cell to divide and the population to double; most are 1-3 hours (E.coli: every 20 min.)

5. Bacterial cell replication
• Bacterial cell is able to duplicate itself – asexualprocess – binary fission • Involves many chemical reactions – Energy transformation – Biosynthesis of small molecules (enzymes, cofactors) – Polymerisation of macromolecules from monomers • Assembly of macromolecules, formation of cellular structures (cell wall, cytoplasmic membrane ……….)

6. Bacterial division

7. Binary Division

8. Phases of Growth Lag phase Log or exponential Active growth Stationary
Death = Growth rate
Death
Nutrients consumed
pH too low (why?)
Optimize curves in production

9. Bacterial growth curve

10. 4/27/2017
Growth Curve –
Chapter 6

11. Lag phase
• When microbes inoculated into fresh medium they do not start to grow immediately (lag phase) • Length of lag phase variable – depends on history of the culture and growth conditions – exponentially growing culture inoculated into same media, -same growth conditions – no lag phase – old culture, same media & conditions – lag phase because cells need to replenish essential constituents to start growth & cell division cycle – Cells damaged (heat, radiation, toxic chemicals) lag phase as cells repair damage – Cells transferred from rich medium to poor culture medium, lag phase as cells have to synthesise more enzymes etc to enable synthesis of macromolecules not present in poor culture

12. Log or Exponential phase
• Each cell divides to form 2 cells; 2 cells divide to form 4 cells …… • Rate of exponential growth influenced by environmental conditions(temperature,composition of culture medium) & genetic characteristics of organism

13. Stationary phase
• In a batch culture exponential growth cannot occur indefinitely – Essential nutrients in medium used up and/or organism builds up to an inhibitory level. – Exponential growth ceases = stationary phase • In stationary phase – no net increase or decrease in cell numbers • Many cell functions continue – energy metabolism, biosynthesis • In some populations some slow growth may continue – some cells die and some grow – 2 processes balance out so no net change

14. Death phase
• If incubation continues after stationary phase, cells may remain alive and continue to metabolise OR they may die = death phase • In some cases cell death is accompanied by lysis • Rate of cell death generally slower than that of exponential growth

15. Requirements for bacterial growth
• Physical requirements
– Temperature, pH, osmotic pressure, oxygen
• Chemical requirements
– Carbon, nitrogen, sulfur, and phosphorous
– Trace elements

16. Optimum Growth Temperatures
• Psychrophiles-optimum 15oC • Psychrotrophs- range from 20-30oC • Mesophiles- range from 25-40oC • Thermophiles-range from 50-60oC

17. pH – Most bacteria grow between pH 6.5 and 7.5
– Molds and yeasts grow between pH 5 and 6
– Acidophiles grow in acidic environments

18. Osmotic Pressure
– Hypertonic environments, increase salt or sugar, cause plasmolysis; used to preserve foods – Extreme or obligate halophiles require high osmotic pressure – Facultative halophiles tolerate high osmotic

19. Requirements for bacterial growth
• Physical requirements – Temperature, pH, osmotic pressure, oxygen • Chemical requirements – Carbon, nitrogen, sulfur, and phosphorous – Trace elements

20. Requirements for bacterial growth
• Physical requirements – Temperature, pH, osmotic pressure, oxygen • Chemical requirements – Carbon, nitrogen, sulfur, and phosphorous – Trace elements

21. Requirements for bacterial growth
• Physical requirements – Temperature, pH, osmotic pressure, oxygen • Chemical requirements – Carbon, nitrogen, sulfur, and phosphorous – Trace elements

22. The Requirements for Growth: Chemical
• Nitrogen
– In amino acids, proteins
– Most bacteria decompose proteins
– Some bacteria use NH4 + or NO3−
– A few bacteria use N2 in nitrogen fixation
• Sulfur
– In amino acids, thiamine, biotin
– Some bacteria use SO4 2− or H2S Phosphorus
– In DNA, RNA, ATP, and membranes
– PO43− is a source of phosphorus
Carbon

23. Direct Measurement of microbial growth
• Population growth is measured by: Direct microscopic count – Following changes in the number of cells – Weight of some component of cell mass eg protein – Total dry weight of

24. Viable count
Only counts living cells • Viable cell = cell that is able to divide & form offspring • Usual method – determine the number of cells in the sample able to form colonies on suitable agar medium = platecount or colony count • Assumption: – Each viable cell can form one colony

25. Turbidity measurements of microbial growth

26. Continuous culture
• Continuous cultures – flow system of constant volume where fresh medium is added continuously and spent culture medium removed continuously – at a constant rate. • Once the system is in equilibrium, the cell number and nutrient status remain constant steady state

27. Questions?? Comments????
Well did I communicate?????