Question

List four selective type of media, four differential media and two general purpose media. Site examples of bacteria that can grow on each type and the appearance of the media or bacterium after incubation.

Discuss four ways bacteria can be enumerated or counted.

Answer 1

selective media is composed of specific ingredients to inhibit the growth of certain species of microbes in a mixed culture while allowing others to grow.

Egample of selective media are:-

1. MacConky agar--- it is for gram negative bacteria

2. Eosin methylene blue-- support growth of gram negative bacteria.

3. Hektoen enteric agar--- selective for gram negative bacteria.

4. Mannitol salt agar-- selective for gram positive bacteria.

A differential media contains specific ingredients or chemicals that help us to visually distinguish which species do or don't carry out a specific biochemical process.

Egample of differential media are :-

1. Blood agar-- it becomes transparent in presence of hemolytic.

2. Streptococcuseosin methylene blue-- differential for lactose and sucrose fermentation.

3. X-gal plates--- differential for lac operon mutants.

4. XLD agar

Some media are considered general purpose media and support growth of large variety of bacteria.

Egample of general media are:-

1. Tryptic soy broth--- to grow pathogenic bacteria.

2. Brain heart infusion agar--- for various organism such as yeast.pneumococcus etc.

Bacteria can be counted by following ways:-

1. Direct counting:-Direct counting methods are used to determine bacterial concentration without the need for advanced equipment.

2. Viable cell counting:-Plate counting is used to estimate the number of viable cells that are present in a sample.

3. By absorbtiometer or nephalometer

4. By chemical assay with nitrogen.