Liquid media
Water-based solutions that do not solidify at temperatures above freezing
and that tend to flow freely when the container is tilted.
Broths, milks, infusions.
Nutrient 'broth' (NB) contains beef extract and peptone in water
Methylene blue, milk and litmus milk (milk and dyes) are classified as opaque liquid media
Fluid thioglycolate broth is used to determine oxygen utilization in
bacteria is a viscous liquid medium.
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Semi-solid media Provide a clot-like consistency.
Contain an amount (about 0.3 - 0.5%) of solidifying agent (agar or gelatin) that thickens but do not produce a firm substrate.
Uses include determination of bacterial motility and to localize a reaction at a specific site.
Motility Test Medium
Sulfur Indole Motility Medium (SIM)
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Solid media
Provides a firm surface on which cells can form visible colonies.
Generally used for the isolation and culturing of bacteria and fungi.
Two forms of solid media are:
Liquefiable solid media (also called reversible solid media)
A solidifying agent that changes its physical properties according to temperature.
Agar (polysaccharide isolated from a red algae (Gelidium).
Not digested by microbes.
Melts or liquifies at 100°C and can be poured into plates at 45-50°C.
Do not resolidify when it reaches 42°C.
A contribution from Frau Fanny Hesse to Robert Koch's lab.
==> Any medium containing 1 - 5% agar usually has the word agar in its name.
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Non-Liquefiable solid media
Do not melt
Rice grains (for fungi)
Cooked meat media (for anaerobic bacteria)
Potato slides
Other preparations containing egg or serum then coagulate and become solid by moist heat.
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Chemically-defined or synthetic media
Made using an exact formula.
Contain precise ingredients.
Standardized and reproducible media are most useful in research and cell culture
A medium for Leishmania (a protozoan) contains 75 different chemicals!!
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Non-synthetic or complex media
If even one ingredient of a given medium is not chemically definable then it can not be represented by an exact chemical formula.
Include substances such as extracts from animals, plants, or yeasts, ground-up cells, tissues or secretions.
Blood serum, meat extracts and infusions are also classified within non-synthetic or complex media.
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General purpose media
Designed to grow as broad spectrum of microbes as possible that do not have special growth requirements.
Mostly non-synthetic (complex) with nutrients suitable to support the growth of a variety of bacteria and fungi.
Nutrient agar (NA)
Nutrient broth (NB)
Brain-heart infusion
Trypticase soy agar (TSA)
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Enriched media
Contain complex organic substances (blood, serum, hemoglobin, or special growth factors such as specific vitamins, amino acids).
Bacteria that require enriched media are called "Fastidious".
Blood agar is used to grow fastidious streptococci, and other pathogens.
They also differentiate among colonies by the zones of hemolysis (destruction of red blood cells) they show.
Blood can be from sheep, horse or rabbit.
Neisseria gonorrhea, the causative agent of gonorrhea, is grown on Thayer-Martin medium (chocolate agar) which is made by heating blood agar.
Also used to grow Haemophilus influenzae. See Fig. 3.7.
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Media used for identification purposes
Selective media
Contain one or more agents that inhibit the growth of a certain microbe or microbes.
They suppressed the growth of unwanted microbes and allow the growth of the desired ones.
Mannitol salt agar (MSA) contains 7.5% NaCl is use to growth Staphylococcus aureus.
This concentration of salt is inhibitory to most human pathogens.
Methylene Blue (EMB) for the isolation of intestinal microorganisms ('coliforms') (G- bacteria)
Bile salts (a component of human feces) inhibits Gram (+) but allow Gram (-) bacteria to grow.
ncluded in MacConkey agar, Hektoen enteric agar and others.
Methylene blue and crystal violet inhibit certain G (+) bacteria.
Tellurite for oral Streptococcus from saliva.
Selenite and brilliant green dyes are use for the isolation of Salmonella from feces.
Sodium azide for the isolation of enterococci from water and food.
Examples of selective media are presented in Table 3.3.
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Differential media
For the growth of several types of microorganisms but are designed to bring out visible differences among those microorganisms.
Differences include colony size or color, in media color changes, or in the formation of gas bubbles and precipitates.
Some media are designed to show 3 or 4 different reactions (TSIA).
Examples of differential media are presented in Table 3.4.
Media that are both selective and differential
Mannitol salt agar
(7.5%) for Staphylococcus aureus.
Contains a dye (phenol red) that changes color under variations of pH, and mannitol, a sugar that can be converted into acid. S. epidermis do not use mannitol but S. aureus does use it.
MacConkey agar
Contains lactose and neutral red, a dye that is yellow when neutral and pink or red when acidic.
E. coli gives off acid when it metabolizes lactose (pink-reddish color).Salmonella does not give off acid.
Its colonies remain in their natural color (off-white).
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Miscellaneous media
Reducing media
This type of medium (thioglycollic acid or cysteine) absorb oxygen or slow its penetration into a medium; reduces its availability.
Carbohydrate fermentation media
Contain sugars that can be fermented (converted into acids) and a pH indicator.
Small Durham tubes inside test tube with a liquid culture.
Transport media
Maintain and preserve specimens to be held for a period of time.
Stuart's and Amies media contain salts, buffers, absorbents to prevent cell destruction by enzymes, pH changes, and toxic substances but will not support growth.
Assay media
To test the effectiveness of antimicrobial growth, assessment of disinfectants, antiseptics, cosmetics, and preservatives on the growth of microorganisms.
Used by industrial and environmental microbiologists.
Enumeration media
To count microbes (water, milk, food, soil, etc.).
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Animals as media
Animals are used when microorganisms can not grow on artificial
media.
Guinea pigs, mice, rabbits, rats, hamsters, dogs, monkeys, cats, chickens, turkeys, armadillos and others.
Animal viruses (chimpanzees, monkeys, and others)
The spirochete of syphilis (mice)
The leprosy bacillus (armadillos)
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Advantages of using animals in research
Source of antibodies, antisera, antitoxins
Useful to check pathogenicity of certain bacteriaUseful to check the effect of drugs.
Essential for vaccine production, e.g. the flu virus in chicken embryos
Disadvantages of using animals in research
Confined to limited space
FeedingCare
EnvironmentSacrifice
There is an active movement to avoid the use of animals in research.
Strong promotion of tissue culture and molecular techniques to replace
live animals.
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Remember to read your textbook, study tables, graphs and illustrations.
Develop a strategy to administer your time so that when exams come you do not have to cram.
Attend lectures and ask questions.
Lecture notes are posted BEFORE lecture is given thereafter they will be removed.