Microbial Control Lab 4 Selective and Differential Media

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 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 – p. H indicator Only pathogenic forms of S. aureus can ferment manitol which produces acid and therefore causes the p. H 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) – – 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 – – Lactose – carbohydrate (differential) Bile salts – salt (selective) Neutral Red – p. H 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 p. H 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 p. H • • • p. H scale ranges fro 0 – 14 p. H 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 p. H (cont) We want to determine the optimum p. H for each bacteria Below p. H 5. 5 – Acidophiles p. H 5. 5 – 8. 5 – Neutrophiles Above p. H 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