Lab Practical 2 LAB #13 Population Count

Standard plate count

(or viable count) Bacterial colonies are viewed through the magnifying glass against a colony-counting grid; Quebec colony counter.

How would you inoculate a plate to get 1:100 dilution?

-Mix 1 ml of a culture with 99 ml of water and plate 1 ml of the dilution.

How would you inoculate a plate to get a 1:10 dilution?

-Plate 0.1 ml of culture -Mix 1 ml of culture with 9ml of water and plate 1 ml of dilution.

Advantages of a counting chamber

-easier to standardize than manual counts -cells visible while counting -live/dead are counted -the cell diameter is measurable

What is serial dilution?

This is a process of reducing the number of microorganisms in a solution, it creates a sample of organisms that is capable of counting.

Disadvantages of a standard plate count

This method can only count bacteria which are able to develop under growing conditions provided.. Thermophils or dead colonies cannot be counted using this procedure.

Why is it necessary to perform a plate count in conjunction with the turbidimetry procedure?

Turbidimetry gives quick count of biomass (dead and alive cells) and plate count give you the number of colonies that arose from a single cell.

Viable count

a measurement of the concentration of live cells in a population

total count

refers to a method that considers all the cells in the sample whether they are alive or dead.

Naubauer counting chamber

A device used for determining the number of cells per unit volume of a suspension. (Grid-like, yeast suspension)

Quebec colony counter

A light box with a magnifying lens to closely observe and count the colonies

Colony forming units

A number of separable cells on the surface of a semisolid agar medium that create a visible colony. (CFU)

Turbidity

A spectrophotometer is used to measure the amount of cloudiness as a measure of growth.

Why do you want to count microbes only from a plate with 30-300 colonies?

Any thing higher than 300, it means most likely the colonies are too crowded. Anything lower than 30 means its not enough to get an accurate estimate.

Rules for counting microorganisms on a counting chamber:

Ignore cells touching the upper and left borders of the box, count the cells found on the lower left and right borders.

Advantages of a standard plate count

It has a very basic principle and technique that requires very minimal amount of equipment but still provides excellent results.

Disadvantages of a counting chamber

It is a very slow process.. counting and recording large numbers of cells is a laborious, time-consuming process through which relatively small volumes of cells are counted.

Total count or viable count?

MNP- viable count Microscopic count- total count Standard plate count- viable count Turbidity- total count

How are population counts conducted?

Microscopic counts, standard plate counts, and turbidity.

Microscopic counts

Special slides are used to count cells using a microscope.