Miles and Misra method

The Miles and Misra Method (or surface viable count) is a technique used in Microbiology to determine the number of colony forming units in a bacterial suspension or homogenate. The technique was first described in 1938 by Miles, Misra and Irwin who at the time were working at the LSHTM.[1] The Miles and Misra method has been shown to be precise.[2]

Materials

  • A calibrated dropping pipette, or automatic pipette, delivering drops of 20μl.
  • Petri dishes containing nutrient agar or other appropriate medium.
  • Phosphate Buffered Saline (PBS) or other appropriate diluent.
  • Bacterial suspension or homogenate.

Method

  • The inoculum / suspension is serially diluted by adding 1x of suspension to 9x of diluent. When the quantity of bacteria is unknown, dilutions should be made to at least 10−8.
  • Three plates are needed for each dilution series, for statistical reasons an average of at least 3 counts are needed.
  • The surface of the plates need to be sufficiently dry to allow a 20μl drop to be absorbed in 15–20 minutes.
  • Plates are divided into equal sectors (it is possible to use up to 8 per plate). The sectors are labelled with the dilutions.
  • In each sector, 1 x 20 μl of the appropriate dilution is dropped onto the surface of the agar and the drop allowed to spread naturally. In the original description of the method a drop from a height of 2.5 cm spread over an area of 1.5-2.0 cm. It is important to avoid touching the surface of the agar with the pipette.
  • The plates are left upright on the bench to dry before inversion and incubation at 37 °C for 18 – 24 hours (or appropriate incubation conditions considering the organism and agar used).
  • Each sector is observed for growth, high concentrations will give a confluent growth over the area of the drop, or a large number of small/merged colonies. Colonies are counted in the sector where the highest number of full-size discrete colonies can be seen (usually sectors containing between 2-20 colonies are counted).
  • The following equation is used to calculate the number of colony forming units (CFU) per ml from the original aliquot / sample:
CFU per ml = Average number of colonies for a dilution x 50 x dilution factor.

Advantages

  • Faster than other methods.
  • Produce less bacterial contamination of the working surface.

See also

References
  1. Miles, AA; Misra, SS; Irwin, JO (Nov 1938). "The estimation of the bactericidal power of the blood". The Journal of Hygiene. 38 (6): 732–49. doi:10.1017/s002217240001158x. PMC 2199673. PMID 20475467.
  2. Hedges, AJ (Jun 25, 2002). "Estimating the precision of serial dilutions and viable bacterial counts". International Journal of Food Microbiology. 76 (3): 207–14. doi:10.1016/s0168-1605(02)00022-3. PMID 12051477.
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