Chapter 6
Microbial growth
• Microbial growth – increase in the number
of cells
• Depends on environmental factor such as
temperature.
• Divided into groups based on their
temperature requirement for growth
• Microbial growth is also influenced by pH.
• Optimum growth pH 6.5 to 7.5
• Acidophiles – like acidic pH
• Sulfolobus – temperature 70oC
pH 2 (thermophile)
• Alkalophiles – basic pH
• Nitrosomonas – pH 8 to 8.8
• Nitrosomonas – soil
• Fungi – pH 5 to 6
• Osmotic environment
• Most bacteria cannot survive in hypertonic
environment.
• Plamolysis
• High concentration of salt is used to
preserve food products.
• Halophiles – need high concentration of
salt for growth ( at least 30% salt)
• Salt lakes, dead sea
• Halobacterium
• Facultative halophiles –can grow in an
environment that has 2 to 15% salt.
• Do not need high concentration of salt for
growth.
• Grow in the presence or absence of salt
• Staphylococcus aureus
Chemical requirement for growth
• Carbon – backbone of the structures
• Chemoheterotrophs – carbon and energy
from organic molecules. E. coli
• Chemoautotrophs – carbon from CO2
energy – inorganic molecules – sulfur
Thiobacillus
• Photoautotrophs = carbon from CO2
• Energy – sunlight, Cyanobacteria
• Nitrogen – proteins, DNA, RNA
• Amino acids, nitrate ions
• Phosphorus – DNA, RNA, ATP,
phospholipids
• Phosphate ions
• Sulfur proteins – sulfate ions
Oxygen requirement
• Obligate aerobes – need O2 for growth
• Micrococcus
• Facultative anaerobe – can grow in the
presence or absence of O2. it grows better
in the presence of O2. E.coli
• Obligate anaerobe – only grow in the
absence of O2. Clostridium
• Cells use O2 during cellular respiration.
• Small amount of toxic O2 – super oxide free
radical is made – damage to the structures
• Obligate aerobes and facultative anaerobes –
enzymes to neutralize the toxic O2
• Super oxide dismutase (SOD)
• Free radical – hydrogen peroxide (H2O2)
• Catalase – H2O2 – water and oxygen
• Aerotolerant anaerobes – tolerate O2 – do not
use O2. Lactobacillus
Microaerophiles
• Need small amount of O2 for growth
• Cannot grow in an O2 rich environment
• Do not have enough enzymes to
neutralize all the toxic O2 produced in an
O2 rich environment.
• Neisseria gonorrhoeae
Culture Media
• Chemically defined medium – know what
is in the medium – how much of it is in the
medium
• Complex medium – know what is in the
medium – do not know how much
• Selective medium – prevents the growth of
some bacteria – allows the growth of
certain other bacteria
• Used to isolate identify bacteria
• Sabouraud dextrose agar – selective for
fungi – pH 5.6
• Differential medium- one group from
another
• Blood agar – hemolytic bacteria – clearing
of RBCs around the colony
• Non hemolytic – no clearing of RBCs
• Generation time – time it takes for one cell
to divide into 2
• Depend on the environmental conditions
and the species.
• E.coli – 20 minutes
• Mycobacterium leprae – 14 days
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Lag phase – no increase in the # cells.
Cells are active – DNA, proteins, RNA
Log phase – cells are actively dividing.
Sharp increase in the number of cells
Most sensitive to antibiotics – penicillin
Stationary phase - #cells dividing equals the
#cells dying.
• Death phase - #cells dying exceeds the # new
cells formed.
Measurement of microbes
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Plate count procedure
Determine the #cells in a sample.
Public health officials - #cells in milk
Pasteurized – less than 30,000 cells/ml
Unpasteurized – less than 50,000 cells/ml
Plates – colonies between 25 to 250
Serial dilution
• # of cells in a sample = # colonies X
reciprocal of the dilution
• 50 X10000 = 500,000 cells/ ml of milk