• Labs #6 (whitewhite) and #7 (green)• Overall scheme: Lab #6 (whitewhite) , last page

Biochemical Tests and Diagnostic Media

SIM medium: Motility test

POSITIVE:Organism moves away from stab

NEGATIVE:Organism does NOT move away from stab

SIM medium: Sulfur test

H2S-POSITIVE:black precipitate

formed

H2S-NEGATIVE:no black

precipitate formed

Simmon’s citrate agar slant

positivenegative

Diagnostic Media

• Mannitol salt agar• Eosin methylene blue (EMB) agar• Brilliant green agar

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.

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.

Mannitol salt agar

Selective mediumHigh salt (NaCl) concentration in medium favors organisms that tolerate high salt concentration, e.g. Staphylococcus.

Staphylococcus aureusgrows

Serratia marcescens

does not grow

Mannitol salt agar

Differential medium

Differentiates among staphylococci

Mannitol salt agar

Differential medium

Differentiates among staphylococci

Mannitol salt agar

Differential medium Differentiates among staphylococci:• pathogenic Staphylococcus species ferment mannitol to produce acid (yellow medium)(yellow medium)

Staphylococcus ferments mannitol(yellow)(yellow)

Mannitol salt agar

Differential medium Differentiates among staphylococci:• pathogenic Staphylococcus species ferment mannitol to produce acid (yellow medium)(yellow medium)• less pathogenic Micrococcus (a staphylococcus) do not ferment mannitol (red/pink medium)

Staphylococcus ferments mannitol(yellow)(yellow)

Micrococcus does not ferment mannitol

(pink)

Levine Eosin-Methylene Blue (EMB) Agar

Gram negatives (growth)

Gram positive(growth inhibited)

SELECTIVE MEDIUM

Levine Eosin-Methylene Blue (EMB) Agar

Nonlactose fermenters

Lactose fermenter

DIFFERENTIAL MEDIUM

High acid producers on EMBe.g. E. coli

blue-black colonies or metallic sheen

Levine Eosin-Methylene Blue (EMB) Agar

Lactose fermenter (low acid producer:

pink colonies)

Non-lactose fermenter Lactose fermenter (high acid producer:

black colonies)

Gram positive(growth inhibited)

Brilliant Green Agar

SELECTIVE

DIFFERENTIAL

• Growth of all gram positive and many gram negative bacteria inhibited

• Growth of many Salmonella species occurs

Brilliant Green AgarDIFFERENTIAL

Differentiates lactose fermenters lactose fermenters from nonlactose fermenters

Brilliant Green AgarNon-lactose fermenter Lactose fermenter

SalmonellaE. coliE. coli

Catalase Test

Many bacteria produce the enzyme catalase.

H2O2 H2O + O2catalase

hydrogenperoxide

oxygen (gas)

Catalase Test

bubbles (gas)

Catalase positive organism