Question

If you wanted to determine how many people were in your class, you would look around the room and count each individual. Even the smallest amounts of broth contain too many bacteria to count individually. To determine how many bacteria are in a given sample, microbiologists use a number of different methodologies including standard plate counts following serial dilutions, direct microscopic counts, determining the most probable number, using filtration followed by standard plate counts, measuring turbidity with a spectrophotometer, measuring the metabolic products of a population, and determining the dry weight of the cells in the culture. (1) Which of these methods are best to use to determine the number of viable cells? What do these methods have in common? (2) Which of these methods would you most likely use if you expected your density of cells to be very low? (3) Explain for which methodologies you might have to make a serial dilution and why dilutions may be necessary. (4) If you plated 1 ml of a 1:10,000 dilution and recovered an average of 27 colonies, how many cells per ml were there in the undiluted (stock) culture?

Answer 1

Ans1) there are a number of methods that can be used for counting the viable number of cells. The plate counting method is one of the most common methods that is used for counting viable number of cells. It helps in identifying the number of actively growing cells in a sample. Apart from plate counting, the spread plate method is also used for counting the actively dividing cells. The pour plate method is also used for analysis of the viable number of cells. Apart from that most probable number is another method that is used for counting the viable number of cells. In all of these methods the plate is incubated under certain conditions and the colonies are counted.

Ans 2) The microscope counting chamber can be used for counting low density of cells in a population. The cell density can be determined with help of spectrophotometrically. This helps in clearly identifying the viable cells when the population of the cells is low.

Ans 3) The spread plate method required serial dilution so that we are able to count the growth of the viable cells. it helps in reducing the dense culture of cell so that it forms individual colonies easy to be counted.