Culture methods in microbiology

Culture methods in microbiology

Culture methods in microbiology

Culture methods in microbiology are very crucial in a microbiology laboratory.

Various culture methods are carried out to:

  • Isolate bacteria in pure culture and identification by performing various tests.
  • Demonstrate biochemical, antigenic, and other phenotypic and genomic properties of the isolated colonies.
  • Demonstrate susceptibility of the isolated bacteria to antibiotics, bacteriophages, bacteriocins, etc.
  • Prepare antigens for various uses.
  • Maintain stock culture.

Methods of Culture:

  • Various methods are used for culturing of bacteria.

These include:

  1. streak culture
  2. lawn culture
  3. pour-plate culture
  4. stroke culture
  5. stab culture
  6. liquid culture

Streak Culture:

  • Streak culture is the most useful method for obtaining discrete colonies of the bacteria.
  • It is carried out by streaking on the surface of a solid media plate using a platinum or nichrome loop of 2–4 mm diameter.
  • In this method, a loopful of the inoculum is placed near the peripheral area of the plate.
  • The inoculum is then spread with the loop to about one-fourth of the plate.
  • From the primary inoculum, it is spread thinly over the plate by streaking with the loop in parallel lines.
  • The loop is flamed and cooled in between the streaks to obtain isolated colonies.
  • The inoculated culture plate is incubated at 37°C overnight for the growth  of colonies.
  • Single isolated colonies obtained by this method are very useful to study various properties of bacteria.
  • Streak culture is the most useful method for obtaining discrete colonies of the bacteria.

streak culture

Lawn Culture:

  • The lawn culture provides a uniform layer of bacterial growth on a solid medium.
  • It is carried out by flooding the surface of the solid media plate with a liquid culture or suspension of bacteria, pipetting off the excess inoculum, and finally incubating the plate overnight at 37°C.
  • The culture plate may be inoculated by a sterile swab soaked in liquid bacterial culture or suspension and incubating overnight for the growth of the bacterial colonies.

Lawn culture method is useful:

  1. To carry out antibiotic sensitivity testing by disc diffusion method.
  2. To carry out bacteriophage typing.
  3. To produce a large amount of bacterial growth required for preparation of bacterial antigens and vaccines.

Pour-Plate Culture:

  • The pour-plate culture is used to determine approximate number of viable organisms in liquids, such as water or urine.
  • It is used to quantitate bacteria in urine cultures and also to estimate the viable bacterial count in a suspension.
  • This method is carried out in tubes, each containing 15 mL of molten agar.
  • The molten agar in tubes is left to cool in a water bath at 45°C.
  • The inoculum to be tested is diluted in serial dilution.
  • Then 1 mL each of diluted inoculum is added to each tube of molten agar and mixed well.
  • The contents of tubes are poured into sterile Petri dishes and allowed to set.
  • After overnight incubation of these Petri dishes at 37°C, colonies are found to be distributed throughout the depth of the medium, which can be counted using a colony counter.

Stroke Culture:

  • Stroke culture provides a pure growth of bacteria for carrying out slide agglutination and other diagnostic tests.
  • It is carried out in tubes usually containing nutrient agar slopes.

Stab Culture:

  • Stab culture is prepared by stabbing the medium in tubes with a long, straight wire and incubating at 37°C.

Stab culture is frequently used for:

  1. Maintaining stock cultures.
  2. Demonstration of oxygen requirement of bacteria.
  3. Demonstration of gelatin liquefaction of bacteria.

Liquid Culture:

  • Liquid culture is prepared in a liquid media enclosed in tubes, flasks, or bottles.
  • The medium is inoculated by touching with a charged loop or by adding the inoculum with pipettes or
    syringes and incubating at 37°C, followed by subculture on to solid media for final identification.

Liquid culture is specifically used:

  1. For blood culture and for sterility tests, where the concentration of bacteria is expected to be small.
  2. For culture of specimens containing antibiotics and other antibacterial substances, as these are rendered ineffective by dilutions in the medium.
  3. When large yields of bacteria are required.

Culture methods in microbiology

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