1. Microbiology Lab # 11 Medgar Evers College Dr. Santos
Isolation of Strep and Staph part II and gram – pathogen isolation part I
Microbiology Lab # 11
Medgar Evers College
Dr. Santos

2. Staph isolation /identification Part II
You should have from last week, three tubes of Staphylococcus broth with S.aureus from three sources; a control, a fomite, and your nose.
This week, you will attempt to isolate the colonies by plating them on solid media. You will use two media; MSA (MANNITOL SALT AGAR) and SM110 (STAPHYLOCOCCUS MEDIUM 110)

3. MSA This medium contains Mannitol 7.5 % sodium chloride
Phenol red indicator
S.aureus ferments mannitol and will turn the medium red to yellow due to the formation of acid. The other strains of Staph do not ferment mannitol and will appear red.

5. SM110
This medium contains mannitol, 7.5 % NaCl but lacks the phenol red so the medium doesn’t change to yellow. The advantage of using this medium is that it allows you to see the different colony colors produced by the different Staph strains.

6. SM110

7. Strep isolation/identification Part II
This week you will examine your blood agar plate from last week, and differentiate between alpha and beta hemolysis.
Alpha hemolysis is partial breakdown of red blood cells and beta hemolysis is total breakdown of red blood cells.
Greenish colonies are indicative of alpha hemolysis and clear colonies are indicative of beta hemolysis.
This week you will pick the colonies and make sub-cultures for next week. Use trypticase soy broth for this experiment.

8. Alpha hemolysis

9. Beta hemolysis

10. Gram – pathogen isolation Part I
Isolating pathogenic gram – organisms.
You will be given a sample of either Salmonella or Shigella to isolate (non lactose fermenters).
Streak a plate of MacConkey and a plate of Hektoen Enteric agar with the mixture provided.
These 2 media inhibit the growth of gram + bacteria.
Other media used are XLD (lysine, xylose deoxycholate) and EMB (eosin methylene blue)

11. Shigella/Salmonella
Other coliforms
MacConkey
Colorless
Red/dark centers
Hektoen Enteric
Greenish/blue
Salmon/orange
XLD
Shigella-red
Salmonella- red/ black center
yellow
EMB
Metallic green/black centers