Dr Rita Oladele
Dept of Med Micro &Para
CMUL/LUTH
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
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
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.)
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 ……….)
Bacterial division
Binary Division
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
Bacterial growth curve
Growth Curve
–
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
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
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
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
Requirements for bacterial
growth
 • Physical requirements
 – Temperature, pH, osmotic pressure, oxygen
 • Chemical requirements
 – Carbon, nitrogen, sulfur, and phosphorous
 – Trace elements
Optimum Growth Temperatures
• Psychrophiles-optimum 15oC
• Psychrotrophs- range from 20-30oC
• Mesophiles- range from 25-40oC
• Thermophiles-range from 50-60oC
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
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
Requirements for bacterial
growth
• Physical requirements
– Temperature, pH, osmotic pressure, oxygen
• Chemical requirements
– Carbon, nitrogen, sulfur, and phosphorous
– Trace elements
Requirements for bacterial
growth
• Physical requirements
– Temperature, pH, osmotic pressure, oxygen
• Chemical requirements
– Carbon, nitrogen, sulfur, and phosphorous
– Trace elements
Requirements for bacterial
growth
• Physical requirements
– Temperature, pH, osmotic pressure, oxygen
• Chemical requirements
– Carbon, nitrogen, sulfur, and phosphorous
– Trace elements
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
– Most bacteria decompose proteins
– Some bacteria use SO4 2− or H2S Phosphorus
– In DNA, RNA, ATP, and membranes
– PO43− is a source of phosphorus
 Carbon
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
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
Turbidity measurements of
microbial growth
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
Well did I communicate?????