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