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 salt
MSA is Selective for all staphylococci and
differential for S. aureus
Selective and Differential Media
Exercise 2 - Eosin Methylene Blue Agar (EMB)
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–
–
–
Lactose – carbohydrate (differential)
Sucrose – carbohydrate (differential)
Eosin Y – Dye (selective)
Methylene Blue – Dye (selective)
Dye inhibits growth of Gram + Bacteria
Selective 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
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–
–
–
Lactose – carbohydrate (differential)
Bile salts – salt (selective)
Neutral Red – pH indicator
Crystal Violet – Dye (selective)
Bile salts and dye inhibit Gram + bacteria
Selective 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