4.4 Microbiology
Classifying Bacteria - shape
According to shapeCoccus - plural Cocci
Bacillus - plural Bacilli
Spirillum – plural Spirilli
Classifying Bacteria – Gram Stain
Classifying Bacteria – Gram Positive or Gram Negative
Cell wall gives a bacterium shape and prevents them from bursting osmotically.
Gram Positive Bacteria• Retain the Gram Stain• Have a thick layer of peptidoglycan• Are affected by antibiotics such as Penicillin and
the enzyme lysozyme (in tears) which interfere with the cross linking of peptidoglycan
Gram Negative Bacteria• Do not retain the Gram Stain, only pick up the
counter stain• Have a thinner but more complex 2 layered cell
wall• Very little peptidoglycan but outer layer is another
membrane.
Classifying Bacteria – Gram Stain
Culturing Bacteria - Growth
Lag Phase – Cells adapting to new environment – synthesising new enzymes etc.
Log Phase / Exponential Growth – No limiting factors
Stationary Phase – Limiting factors affecting growth death rate equals birth rate
Death / Decline Phase – All nutrients used, toxin accumulating. Oxygen concentration decreasing.
Culturing Bacteria – Conditions Needed for Growth• Nutrients – Carbon / Nitrogen Sources as
well vitamins and minerals• Water - Solvent for biochemical
reactions, cell expansion• Temperature & pH - Particular to specific species• Oxygen- Obligate aerobes (need O2 all
the time)Obligate anaerobes find oxygen toxic
Facultative anaerobes – grow better in the presence of oxygen
Aseptic Technique• Compete with cultured microbe, may produce toxins and/or
contaminate the product / Protect from pathogenic species
Measuring Bacterial Growth• Turbidity – indirect method (total or viable?)
Measuring Bacterial Growth• Haemocytometer (total or viable?)
May need to dilute beforehand and then factor in the dilution factor.
Total count.
Measuring Bacterial Growth
Plating and Counting Colonies
Fermentation
Culturing of microorganisms both aerobically and anaerobically in fermentation tanks.Metabolism – reactions of living organismsMetabolites – products from the reactionsTwo types of metabolite:• Primary metabolite – produced by an
organism as part of its normal growth• Secondary metabolite – substances produced
by an organism that are not part of its normal growth (usually begins after the log phase)
Industrial Scale Fermenters
Batch Versus ContinuousNutrients added
Product removed
Growth rates
Product quality
Repercussions if contaminated
Penicillin Production• Secondary metabolite• Batch culture• Penicillium chrysogenum or Penicillium
notatum• Fungus – Aerobic / 24oC / pH6.5
• Inhibits peptidoglycan cross links – Cell wall weakened – osmotic bursting
