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

• 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

• 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