Ways To Count Bacteria

Q:What is a Hemocytomer?
A: Counts white blood cells.

Q: How does a hemocytometer work?
A: It is a thick glass microscope slide with engraved rectangular identiation that creates a chamber. The depth and area is known therefore you can cound the number of cells by the specific volume of the cluid but on to the slide.

Opitical Density
Q: How does a spectrophotometer work and count bacteria?
A: A spectrophotometer is photometer that measures intensity a wavelength. A spectrophotometer counts bacteria by a light source shining through the sample. The sample transmits or reflects light. The detector detects how much light was relfected from or transmitted through the sample. The detector then converts how much light the sample transmitted or reflected into a number.

Viable count methods
*spread plate is used when the mixed populations of prokaryotes exist in a natural sample (i.e. soil, pond water, fecal material, etc.) and the researcher would like to obtain isolated colonies and enumerate the number of culturable prokaryotic cells in the sample.
Q: What is the protocol for making a spread plate of a sample containing bacteria?
A: .1ml, or less, of a diluted sample which contains bacteria is pipetted onto the surface of the agar and spread evenly using a sterile glass spreader.

  • pour plate- The sample is pipetted onto a sterile petri plate, in which the melted agar, or sterile medium, is poured over top of the pipetted sample and then mixed well. This allows a larger volume of the diluted sample to be used, where the Spread plate only .1ml or less is able to be used otherwise the excess solution would not be absorbed.

Q: How is the protocol for a pour plate different from that of a streak plate? Why would you use a pour plate rather than a streak plate?
A:With the pour plate the agar is placed after the diluted sample, and the streak plate the bacteria are placed over top of the agar. A benefit of the pour plate is that the bacteria will form colonies throughout the agar not just on the surface like the streak plate. However, take caution that if the bacteria cannot stand the high temperature of the melted agar poured onto it, they will not survive.

  • serial dilution

Q: How can you count viable bacteria in a liquid sample when there is a high concentration of bacteria in your sample?

Q:What are the advantages and disadvantages of these different methods of counting bacteria?

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