Microbial Control

Lab 4

Selective and Differential Media

We have completed Isolation of bacteria using steak plate and spread plate

This is a good beginning, but now we must determine more about the bacteria using:

Selective Media and Differential Media

Selective and Differential Media

Selective Media – chemicals added that allow certain bacteria to grow and inhibit others.

Differential Media – Allows all bacteria to grow but has chemicals to cause a change in appearance of certain bacteria.

Selective and Differential Media

Media can be selective, differential or both

The 3 media we will use today in Exercises 1-3 are BOTH selective and differential

Selective and Differential Media

• Exercise 1 – Manitol Salt Agar (MSA)– Manitol – carbohydrate (differential)– Sodium Chloride – salt (selective)– Phenol Red – pH indicator

Only pathogenic forms of S. aureus can ferment manitol which produces acid and therefore causes the pH indicator to turn yellow

Only staphylococci bacteria can live in saltMSA is Selective for all staphylococci and

differential for S. aureus

Selective and Differential Media

Exercise 2 - Eosin Methylene Blue Agar (EMB)– Lactose – carbohydrate (differential)– Sucrose – carbohydrate (differential)– Eosin Y – Dye (selective)– Methylene Blue – Dye (selective)

Dye inhibits growth of Gram + BacteriaSelective for Gram – bacteria and is Differential

for fecal coliform bacteria by how actively it ferments the carbohydrate (fecal coliform will have a green metalic sheen)

Selective and Differential Media

Exercise 3 – Mac Conkey Agar– Lactose – carbohydrate (differential)– Bile salts – salt (selective)– Neutral Red – pH indicator– Crystal Violet – Dye (selective)

Bile salts and dye inhibit Gram + bacteriaSelective for Gram – bacteria and Differential

for gram – especially enterbacteriaceae because they can ferment lactose and create acid causeing pH indicator to turn red

Microbial Control

Exercise 4 – Kirby bauer Antibiotic Sensitivity

• Sensitive – if a bacteria is sensitive to an antibiotic, the antibiotic will inhibit or slow growth of that bacteria and will have a “zone of inhibition” (an area of no bacteria growth) around the antibiotic.

Microbial Control

Exercise 4 – Kirby bauer Antibiotic Sensitivity – (cont)

• Resistant – if a bacteria is resistant to an antibiotic, then the presence of the antibiotic will have no affect on the growth of the bacteria

Microbial Control

Exercise 5 - Effectiveness of Disinfectants

We will use paper disks soaked in various disinfectants to determine the effectiveness of those products

Make sure to use a disk soaked in sterile water placed in center of agar plate for our control group

Microbial Control

Exercise 6 – Effect of pH

• pH scale ranges fro 0 – 14• pH 7 is neutral• Below 7 is acidic• Above 7 is basic• Each number is 10 times more than the

one before it

Microbial Control

Exercise 6 – Effect of pH (cont)

We want to determine the optimum pH for each bacteria

Below pH 5.5 – Acidophiles

pH 5.5 – 8.5 – Neutrophiles

Above pH 8.5 - Alkaphiles

Microbial Control

Exercise 7 – Effect of Temperature

• We want to determine the optimum temperature for bacteria

• Cardinal Temperatures for a bacteria are their optimum temperature, minimum temperature and maximum temperature

Microbial Control

Exercise 7 – Effect of Temperature (cont)

• Below 20° C – psychorphiles• Freezing to 30 ° C - psychrotrophs• 15 ° - 45 ° C – mesophiles

– Usually bacteria in human body are mesophiles

• Above 40 ° C – thermophiles• 65 ° – 110 ° C – Extreme thermophiles