1
Biotechnology
RA
Techniques in Microbial Culture
Bacteria and fungi may be cultured in liquid or solid media. These comprise a base of agar to which is added the nutrients required for microbial growth. Agar is a gelatinous colloidal extract of red algae, and can be used in solid or liquid form. It is used because of its two unique physical properties. Firstly, it melts at 100˚C and remains liquid until cooled to 40˚C, at which point it gels. Secondly, few microbes are capable of digesting agar so the
medium is not used up during culture. The addition of microbes to an agar plate, or to liquid agar, is called inoculation and must be carried out under aseptic conditions. Aseptic techniques involve the sterilisation of equipment and culture media to prevent cross contamination by unwanted microbes. Sterilisation is a process by which all organisms and spores are destroyed, either by heat or by chemicals.
Conditions for the Culture of Bacteria and Fungi Fungi
Bacteria
Temperature: Most fungi have an optimum temperature for growth
Temperature: Most bacteria cultured in the school laboratory are
of 25˚C, but most are adapted to survive between 5 and 35˚C.
classified as mesophiles. Mesophiles prefer temperatures between 20 and 40˚C.
pH: Fungi prefer a neutral (pH 7) growing g
pH p : Most bacteria grow optimally in media with w a pH between 6 and 8. Very few bacteria can grow in acidic conditions. b
environment, although most species can n tolerate slightly acidic conditions.
Nutrients: Fungi require a source
Nutrients: Bacteria need a source
of carbon and nitrogen to produce protein. They also require trace elements such as potassium, phosphorus and magnesium. Growth factors can be added to increase the rate of fungal growth.
of carbon, nitrogen and mineral salts as raw ingredients for cellular growth.
Water potential: Fungi are 85-90% water by mass. Water is constantly lost from the hyphae via evaporation and must be replaced through absorption from the media. To aid water uptake, media have a water potential that is less negative than that of the fungal tissue.
Water potential: All bacteria require water for growth. To prevent cell lysis or dehydration, the water potential of the medium must be such that net water fluxes into and out of the bacterial cell are minimised.
Gaseous environment: The majority of fungi are aerobic and very few species can tolerate anaerobic conditions. This is why fungi always grow on the surface of a culture medium, not inside it.
Gaseous environment: Aerobic bacteria will grow only in oxygenated environments, whereas obligate anaerobes (e.g. Clostridium) do not tolerate oxygen. Facultative anaerobes grow under aerobic conditions, but are able to metabolise anaerobically when oxygen is unavailable. All bacterial cultures benefit from a low concentration of carbon dioxide.
Inoculating Solid Media
1
Hold the inoculating loop in the flame until it glows red hot. Remove the lid from the culture broth and pass the neck of the bottle through the flame.
2
Dip the cool inoculating loop into the broth. Flame the neck of the bottle again and replace the lid.
3
Raise the lid of the plate just enough to allow the loop to streak the plate. Streak the surface of the media. Seal the plate with tape and incubate upside down.
1. Explain why inoculated plates must be stored upside down in an incubator:
2. Outline the correct procedure for the disposal of microbial plates and cultures:
3. Suggest a general method by which you could separate microorganisms through culturing:
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2
Biotechnology
Serial Dilution
RDA
The growth of microorganisms in culture can be measured in a number of ways. Some indirect methods measure culture dry weight or turbidity, both of which are often directly proportional to cell density. More commonly used are methods that directly or indirectly count the number of cells in a culture. Because
microbial populations are often very large, most counting methods rely on counting a very small sample of the culture. A commonly used indirect method is serial dilution followed by plate counts (illustrated below).
Measuring Microbial Growth Using Serial Dilution Serial dilution can be performed at different stages during the culture growth. By making a series of dilutions and then counting the colonies that arise after plating, the density of the original inoculum (starting culture) can be calculated. Colonies should be well separated and the number of colonies counted should ideally be neither too small nor too large (about 15-30 is good). CALCULATION:
No. of colonies on plate X reciprocal of sample dilution = no. of bacteria per cm3.
EXAMPLE:
28 colonies on a plate of 1/1000 dilution, then the original culture contained: 28 x 1000 = 28 x 103 cm–3 bacterial cells
Plate counts are widely used in microbiology. It is a useful technique because only the viable colonies are counted, but it requires some incubation time before colonies form. For quality control purposes in some food industries where the food product is perishable (e.g. milk processing) this time delay is unacceptable and direct methods (e.g., cell counts using oil immersion microscopy) are used.
1 cm3 Original inoculum from culture
Dilutions
1 cm3
1 cm3
1 cm3
1 cm3
9 cm3 of nutrient broth in each tube
1:100
1:10
1 cm3
1 cm3
1:1000
1 cm3
Thick growth
1:10 000
1 cm3
1:100 000
1 cm3
Isolated colonies
1. In the example of serial dilution above, use the equation provided to calculate the cell concentration in the original culture:
2. (a) Define the term viable count:
(b) Explain why dilution plating is a useful technique for obtaining a viable count:
(c) Investigate an alternative technique, such as turbidimetry, and identify how the technique differs from dilution plating:
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3
Biotechnology
Strain Isolation
RA
In nature, bacteria exist as mixed populations. However, in order to study them in the laboratory they must exist as pure cultures (i.e. cultures in which all organisms are descendants of the same organism or clones). The most common way of separating bacterial cells on the agar surface is the streak plate method. This provides a simple and rapid method of diluting the sample by mechanical means. As the loop is streaked across the agar surface, more and more bacteria are rubbed off until individual
separated organisms are deposited on the agar. After incubation, the area at the beginning of the streak pattern will show confluent growth (growth as a continuous sheet), while the area near the end of the pattern should show discrete colonies. Isolated colonies can then be removed from the streak plate using aseptic techniques, and transferred to new sterile medium. After incubation, all organisms in the new culture will be descendants of the same organism (i.e. a pure culture).
In each streak, the loop picks up bacteria from the previous series, diluting the number of cells each time.
The streaking starts here. Streaks are made in the order indicated by the numbers on the plate. The first streak is made from the initial bacterial mixture.
Individual colonies (arising from one cell) should be obtained here. These can be removed and then cultured separately.
Latex gloves ensure no contamination from either bacteria or fungi on the hands.
After incubation
BBT Inc.
.cnI TB B
The inoculating loop is sterilised with flame and alcohol after each streak.
CDC
GT
Rough colonies on blood agar
When approximately 10 to 100 million bacterial cells are present, colonies become visible. Note the wellisolated colonies in the photo above. A single colony may be removed for further investigation.
A swab containing a single strain of bacteria is used to inoculate additional nutrient plates to produce pure cultures of bacteria (clones).
Smooth colonies on bicarbonate agar
To test purity, a sample of a culture can be grown on a selective medium that promotes the growth of a single species. The photo above shows a positive encapsulation test for Bacillus anthracis.
1. Explain the basis by which bacteria are isolated using streak plating:
2
Discuss the basic principles of aseptic technique, outlining why each procedure is necessary:
3. Comment on the importance of aseptic (sterile) technique in streak plating:
4. State how many bacterial cells must be present on the plate before the colony becomes visible to the naked eye:
5. Outline when it might be necessary to use selective media to culture bacteria:
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© Biozone International 1995-2003