Population growth Population growth patterns patterns Learning objectives: Learning objectives: • To be able to describe a typical To be able to describe a typical growth curve of a bacterial growth curve of a bacterial population and reasons for lag population and reasons for lag phase, exponential phase, phase, exponential phase, stationary phase and death phase. stationary phase and death phase. • To be able to describe the effect To be able to describe the effect of temperature, pH, nutrient of temperature, pH, nutrient availability and oxygen availability and oxygen availability on growth. availability on growth.
Transcript
Population growth patternsPopulation growth patterns
Learning objectives:Learning objectives:• To be able to describe a typical growth To be able to describe a typical growth
curve of a bacterial population and curve of a bacterial population and reasons for lag phase, exponential reasons for lag phase, exponential phase, stationary phase and death phase, stationary phase and death phase.phase.
• To be able to describe the effect of To be able to describe the effect of temperature, pH, nutrient availability temperature, pH, nutrient availability and oxygen availability on growth.and oxygen availability on growth.
4 phases of bacterial growth4 phases of bacterial growthlag phase when the cells are active but there is little lag phase when the cells are active but there is little
increase in number as the organisms acclimatise to their increase in number as the organisms acclimatise to their environmentenvironment
Individual bacteria may be increasing in size.Individual bacteria may be increasing in size.Enzymes may be being synthesised to utilise the Enzymes may be being synthesised to utilise the
nutrient mediumnutrient medium
Log phase (or exponential) nutrients in plentiful supply, Log phase (or exponential) nutrients in plentiful supply, ample space, little waste. State of balanced growthample space, little waste. State of balanced growth
Stationary or constant phase nutrients become depleted, Stationary or constant phase nutrients become depleted, fall in pH as CO2 and other metabolites build up. fall in pH as CO2 and other metabolites build up.
Death rate = birth rateDeath rate = birth rate
Death phase more cells die than are produced so the Death phase more cells die than are produced so the number of living cells declines. number of living cells declines.
Causes of death include starvation, lack of oxygen and Causes of death include starvation, lack of oxygen and toxicity due to waste productstoxicity due to waste products
– In general growth rate increases with temperature In general growth rate increases with temperature until the microbes' enzymes denature. until the microbes' enzymes denature.
– Bacteria are found in an astonishing variety of Bacteria are found in an astonishing variety of habitats, sometimes with extreme temperatures, habitats, sometimes with extreme temperatures, where no other organisms can survive. where no other organisms can survive.
– Different bacteria can grow in the range -7°C to Different bacteria can grow in the range -7°C to 118°C. 118°C.
– These cells are adapted so that their enzymes These cells are adapted so that their enzymes have unusual optimum temperatures. have unusual optimum temperatures.
– Bacteria can be classified according to their Bacteria can be classified according to their optimum temperature:optimum temperature:
Optimum Optimum
temperaturetemperature
Name of Name of group group
examplesexamples
20-45°C 20-45°C
>45°C >45°C
<20°C<20°C
Mesophiles Mesophiles
Thermophiles Thermophiles
PsychrophilesPsychrophiles
•Mammalian symbiotic bacteria. Mammalian symbiotic bacteria. These are the commonest These are the commonest bacteria.bacteria.
•Rotting vegetation, hot Rotting vegetation, hot springs, volcanic vents. These springs, volcanic vents. These are useful in biotechnology, are useful in biotechnology, since there enzymes do not since there enzymes do not denature easily (e.g. in PCR or denature easily (e.g. in PCR or biological detergents).biological detergents).
•Soil bacteria or polar ocean Soil bacteria or polar ocean dwellers. These bacteria can dwellers. These bacteria can cause food spoilage in cause food spoilage in refrigerators.refrigerators.
• In school labs bacteria must not be In school labs bacteria must not be grown above 30°C by law. grown above 30°C by law.
• Why?Why?
• This prevents the growth of human This prevents the growth of human pathogenic bacteria (which are mostly pathogenic bacteria (which are mostly mesophiles).mesophiles).
pHpH
• Most microbes grow best in neutral pH and die in Most microbes grow best in neutral pH and die in extreme acid or alkali conditionsas their enzymes extreme acid or alkali conditionsas their enzymes slow down. slow down.
• The optimum pH for different microbes variesfrom 5 The optimum pH for different microbes variesfrom 5 to 7.5. to 7.5.
• A few acid-tolerant species can grow at pH 2, but A few acid-tolerant species can grow at pH 2, but almost all microbes are killed below pH 4, which is almost all microbes are killed below pH 4, which is why human stomach acid is so effective.why human stomach acid is so effective.
• The pH of a growth medium can change over time The pH of a growth medium can change over time due to production of waste acids or alkalis, so it is due to production of waste acids or alkalis, so it is important to use a pH buffer when growing important to use a pH buffer when growing microbes.microbes.
NutrientsNutrients
• All microbes need to be provided with the All microbes need to be provided with the major elements CHONSP in an appropriate major elements CHONSP in an appropriate form. form.
• Organotrophs require simple carbohydrates Organotrophs require simple carbohydrates such as glucose or sucrose for respiration such as glucose or sucrose for respiration (and some may be able to digest starch), (and some may be able to digest starch), together with minerals, especially a source of together with minerals, especially a source of nitrogen such as ammonia or amino acids. nitrogen such as ammonia or amino acids. Why?Why?
• to make proteins. to make proteins. • Lithotrophs require mush simpler inorganic Lithotrophs require mush simpler inorganic
nutrients, such as methane, carbon dioxide, nutrients, such as methane, carbon dioxide, water and minerals.water and minerals.
Oxygen Oxygen • Many microbes need molecular oxygen Many microbes need molecular oxygen
for respiration, just as animals do, and for respiration, just as animals do, and are termed obligate aerobes. are termed obligate aerobes.
• In a solid agar medium these microbes In a solid agar medium these microbes will only grow on the surface, and a will only grow on the surface, and a liquid medium must be well aerated.liquid medium must be well aerated.
• Some microbes are facultative anaerobes, Some microbes are facultative anaerobes, which means they will use aerobic which means they will use aerobic respiration if oxygen is available, but can respiration if oxygen is available, but can switch to anaerobic respiration if oxygen is switch to anaerobic respiration if oxygen is absent. absent.
• These include yeasts and lactic acid bacteria, These include yeasts and lactic acid bacteria, and their metabolic products can be and their metabolic products can be controlled by varying the amount of oxygen. controlled by varying the amount of oxygen.
• For example in brewingFor example in brewing
• A few microbes are obligate anaerobes, A few microbes are obligate anaerobes, which means they die in the presence which means they die in the presence of oxygen.of oxygen.
• Anaerobes are useful industrially as Anaerobes are useful industrially as they will grow throughout a medium, they will grow throughout a medium, whereas aerobes will only grow on whereas aerobes will only grow on surfaces, or in well-stirred liquid media.surfaces, or in well-stirred liquid media.
• Any of these factors can be controlled Any of these factors can be controlled to select particular microbes, or to select particular microbes, or particular metabolic products, or to particular metabolic products, or to control the growth rate. control the growth rate.
• You don't always want the fastest You don't always want the fastest growth of cells, but the fastest growth of cells, but the fastest production of a metabolite, which may production of a metabolite, which may occur under different conditions.occur under different conditions.
Water potentialWater potential
• Very salty or sugary solutions can draw Very salty or sugary solutions can draw water out of organisms by osmosiswater out of organisms by osmosis
• Organisms adapted to very salty Organisms adapted to very salty conditions are called halophilesconditions are called halophiles
Industrial MicrobiologyIndustrial Microbiology
• Microbes produce many useful Microbes produce many useful products, and humans have made use products, and humans have made use of this for thousands of years.of this for thousands of years.
• Today there is a wide range of products Today there is a wide range of products made by microbial biotechnology, most made by microbial biotechnology, most of which are too complex to be of which are too complex to be synthesised by purely chemical synthesised by purely chemical techniques. techniques.
These include:These include:
• food (bread, cheese, yoghurt, single food (bread, cheese, yoghurt, single cell protein (SCP)); cell protein (SCP));
steroids); steroids); • plastics; etc.plastics; etc.
Microbes are particularly useful Microbes are particularly useful for industrial purposes because:for industrial purposes because:
• They have fast growth rates.They have fast growth rates.• They have simple nutritional requirements and can They have simple nutritional requirements and can
often be fed on cheap or even waste substrates such as often be fed on cheap or even waste substrates such as molasses, whey, wood pulp, etc.molasses, whey, wood pulp, etc.
• They can be grown indoors and their growth does not They can be grown indoors and their growth does not depend on seasons, climate, latitude, etc.depend on seasons, climate, latitude, etc.
• They are often tolerant of a wide range of temperatures They are often tolerant of a wide range of temperatures and pH.and pH.
• There are fewer ethical problems, when compared to There are fewer ethical problems, when compared to animals.animals.
• Prokaryotes can be more easily genetically modified Prokaryotes can be more easily genetically modified than eukaryotic cells, since they don't have a nucleus. than eukaryotic cells, since they don't have a nucleus. – Genes from other species can easily be inserted into Genes from other species can easily be inserted into
the bacterial DNA to produce a range of gene the bacterial DNA to produce a range of gene products by fermentation, or products by fermentation, or
– the microbe can be altered to produce far more of the the microbe can be altered to produce far more of the product than normal.product than normal.
QuestionsQuestions
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sampling time following innoculation
viab
le p
late
co
un
t p
er m
m3
med
ium
glucose only
lactose only
lactose and glucose
SimilaritiesSimilarities• Lag phase. Lag phase.
– Little increase in cell number;Little increase in cell number;– Individual bacteria may be increasing in size;Individual bacteria may be increasing in size;– Enzymes may be being synthesised to utilise the Enzymes may be being synthesised to utilise the
– The population shows exponential phase;The population shows exponential phase;– Nutrient supply is not a limiting factor;Nutrient supply is not a limiting factor;
• Stationary phase.Stationary phase.– The population growth slows down;The population growth slows down;– The number of new cells formed is balanced by The number of new cells formed is balanced by
the number of cells dying;the number of cells dying;– Limiting factors, such a nutrient supply and Limiting factors, such a nutrient supply and
metabolic wastes have started to influence further metabolic wastes have started to influence further increase in population size; max 2increase in population size; max 2
DifferencesDifferences• Lag phase for lactose alone longer; nutrient Lag phase for lactose alone longer; nutrient
less readily available;less readily available;• Growth of population less than with glucose; Growth of population less than with glucose;
fewer cells and longer to increase;max.2fewer cells and longer to increase;max.2• Growth with lactose and glucose gives a Growth with lactose and glucose gives a
greater population;greater population;• Has a second lag; around 100-120 minute Has a second lag; around 100-120 minute
and then increases again;and then increases again;• Uses glucose first as growth curve similar to Uses glucose first as growth curve similar to
glucose alone; and then uses lactose;max 3glucose alone; and then uses lactose;max 3• Death phase is occurring for glucose alone Death phase is occurring for glucose alone
as all the energysource/glucose has run as all the energysource/glucose has run out.1out.1
