You are working with a Gram positive bacterium that can be grown in Tryptic Soy

Question

You are working with a Gram positive bacterium that can be grown in Tryptic Soy Broth, a complex medium, or in a defined medium containing glucose, ammonium sulfate, potassium phosphate, ferric chloride, sodium phosphate and magnesium sulfate.

Describe two direct methods of measuring growth in the tryptic soy broth.

Explanation / Answer

Growth is an orderly increase in the quantity of cellular constituents. It depends upon the ability of the cell to form new protoplasm from nutrients available in the environment. Measuring techniques involve direct counts, visually or instrumentally, and indirect viable cell counts. 1. Direct microscopic counts are possible using special slides known as counting chambers. Dead cells cannot be distinguished from living ones. Only dense suspensions can be counted (>107 cells per ml), but samples can be concentrated by centrifugation or filtration to increase sensitivity. 2. Indirect viable cell counts, also called plate counts, involve plating out (spreading) a sample of a culture on a nutrient agar surface. The sample or cell suspension can be diluted in a nontoxic diluent (e.g. water or saline) before plating. If plated on a suitable medium, each viable unit grows and forms a colony. Each colony that can be counted is called a colony forming unit (cfu) and the number of cfu's is related to the viable number of bacteria in the sample.

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