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Growth, Detection and Characterization
Gram negative bacteria do not retain crystal violet dye in the gram staining protocol. Gram positive
bacteria stain dark blue or violet by gram staining. Classification is based on the structural differences
of their cell walls: Gram positive have thick peptidoglycan layer.
Binary fission is asexual reproduction performed by bacteria which produces genetically identical
The requirements for bacterial multiplication are :
Ideal temperature psychrophiles reproduce in temperatures below 20 degrees, mesophiles
between 20-45 degrees and thermopiles over 45 degrees.
Water water activity is the intensity at which water associates with various non-aqueous molecules.
Higher water active substances tend to support more microorganisms.
Time, nutrients, Oxygen, anaerobic conditions, absence of competition, absence of antibacterial
conditions and PH are also all contributing factors that can affect microbial growth.
The 5 I's of culturing microbes:
Innoculation and Isolation :
If an individual bacterial cell is separated from other cells and has space on a nutrient plate
after it has been inoculated it will grow into a mould of cells an isolated colony. A colony
consists of one species.
Bacteria have to be grown for them to be inspected and identified.
Agar is used for preparing solid medium. It is obtained from seaweeds a complex
polysaccharide isolated from red algae. No nutritive value but provides moisture. It is not
affected by the growth of the bacteria and not digestible for most microbes. It melts at 98
degrees and solidifies at 42 degrees. 2 % agar is used in a solid medium.
Incubation involves humidity, oxygen, carbon dioxide, temperature and the microbe
multiplies and becomes macroscopic.
Inspection is micro and macroscopic observation. A pure culture grows only single species of
microorganisms. Mixed cultures hold 2 or more identified species. Contaminated culture was
once pure that has unwanted microbes growing.
Types of Culture Media:
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Special Media Based on oxygen requirement, aerobic/anaerobic media. A simple way is a
stab culture. Anaerobic jar is used to culture anaerobic bacteria, it contains sodium
bicarbonate and sodium borohydride.
Liquid Media No agar. Used for inoculum preparation for blood samples, or isolation of
pathogens. E.g Nutrient broth
Semi-solid medium 0.5% agar for observation of motile bacteria e.g pseudomonas
Simple/Basal Media E.g Nutrient broth consists of peptone, meat extract or sodium