This document discusses various biochemical tests and diagnostic media used to identify bacteria. It describes selective and differential media, including Mannitol salt agar, Eosin methylene blue (EMB) agar, and Brilliant green agar. It explains how these media selectively inhibit or allow growth of certain bacteria and can differentiate bacteria based on properties like lactose fermentation or catalase production.

1. Biochemical Tests and
Diagnostic Media
• Labs #6 (white) and #7 (green)
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• Overall scheme: Lab #6 (white) , last page
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2. SIM medium: Motility test
POSITIVE: NEGATIVE:
Organism moves Organism does NOT
away from stab move away from
stab

3. SIM medium: Sulfur test
H2S-NEGATIVE: H2S-POSITIVE:
no black black precipitate
precipitate formed
formed

4. Simmon’s citrate agar slant
negative positive

5. Diagnostic Media
• Mannitol salt agar
• Eosin methylene blue (EMB) agar
• Brilliant green agar

6. Selective medium
Selects for the GROWTH of some
organisms and inhibits the growth of
other organisms.
i.e. Some organisms grow on a selective medium
whereas others do not grow (or do not grow well) on that
same medium.

7. Differential medium
Differentiates between one type of
organism and another type of organism.
Different types of organism look different from each
other when grown on differential media.

8. Mannitol salt agar
Staphylococcus Serratia
aureus marcescens
grows does not grow
Selective medium
High salt (NaCl) concentration in medium favors organisms
that tolerate high salt concentration, e.g. Staphylococcus.

9. Mannitol salt agar
Differential medium
Differentiates among staphylococci

10. Mannitol salt agar
Differential medium
Differentiates among staphylococci

11. Mannitol salt agar
Staphylococcus
ferments
mannitol
(yellow)
Differential medium
Differentiates among staphylococci:
• pathogenic Staphylococcus species ferment mannitol to
produce acid (yellow medium)

12. Mannitol salt agar
Staphylococcus
Micrococcus does not ferments
ferment mannitol mannitol
(pink) (yellow)
Differential medium
Differentiates among staphylococci:
• pathogenic Staphylococcus species ferment mannitol to
produce acid (yellow medium)
• less pathogenic Micrococcus (a staphylococcus)

13. Levine Eosin-Methylene Blue (EMB)
Agar
Gram negatives
(growth)
Gram positive
(growth inhibited)
SELECTIVE MEDIUM

14. Levine Eosin-Methylene Blue (EMB)
Agar
Lactose Nonlactose
fermenter fermenters
DIFFERENTIAL MEDIUM

15. High acid producers on EMB
e.g. E. coli
blue-black colonies or metallic sheen

16. Levine Eosin-Methylene Blue (EMB)
Agar
Lactose fermenter Non-lactose fermenter
(high acid producer:
black colonies)
Gram positive
(growth inhibited)
Lactose fermenter
(low acid producer:
pink colonies)

17. Brilliant Green Agar
SELECTIVE
• Growth of all gram positive and
many gram negative bacteria
inhibited
• Growth of many Salmonella species
occurs
DIFFERENTIAL

18. Brilliant Green Agar
DIFFERENTIAL
Differentiates lactose fermenters from
nonlactose fermenters

19. Brilliant Green Agar
Lactose fermenter Non-lactose fermenter
E. coli Salmonella

20. Catalase Test
Many bacteria produce the enzyme catalase.
catalase
H2O2 H2O + O2
hydrogen oxygen
peroxide (gas)

21. Catalase Test
bubbles (gas)
Catalase positive organism