BIOCHEM (Supplemental Lecture)

Which enzyme facilitates entry of lactose across the bacterial plasma membrane?

β-galactoside permease

Which enzyme hydrolyzes lactose into glucose and galactose?

β-galactosidase

Late/delayed lactose fermenters (dLFs) typically possess which enzyme(s)?

β-galactoside permease but delayed β-galactosidase

Glucose, Lactose and Sucrose

TSI agar is primarily used to assess fermentation of which sugars?

Yellow

In TSI, what color indicates an acidic reaction?

K/A

Which TSI reaction indicates glucose fermentation only (no lactose/sucrose fermentation)?

A/A

Which TSI result shows fermentation of two or more sugars?

H₂S (hydrogen sulfide)

Blackening of the TSI butt indicates production of:

Phenol red

Which indicator is used in TSI and KIA?

Glucose fermented only

A deep red slant and yellow butt (K/A) after 24 hours suggests:

Cracks or lifting of medium / bubbles

Gas production in TSI is evidenced by:

A/A with H₂S

In TSI, which combination indicates both lactose fermentation and H₂S?

Similar purpose — both test glucose and lactose; KIA also detects H₂S

KIA differs from TSI mainly by:

0.1% glucose

Which sugar concentration in TSI prompts initial glucose fermentation followed by peptone usage?

Glucose fermentation

The MR-VP tests detect end products of:

Stable mixed-acid fermentation producing low pH (red)

Methyl Red (MR) positive indicates:

Acetoin (acetylmethylcarbinol) / proceeds to 2,3-butanediol

Voges-Proskauer (VP) positive indicates production of:

IMViC (Indole, MR, VP, Citrate)

MR and VP tests are part of which common identification group?

Yellow

Which tube color indicates MR negative after addition of methyl red?

α-naphthol in ethanol (Barritt’s reagent A)

VP reagent A is:

β-galactosidase (in organisms lacking permease or slow fermenters)

Which enzyme is detected by the ONPG test?

Late/delayed lactose fermenters (dLF) or organisms with β-galactosidase but no permease

ONPG is used primarily to detect:

Yellow (ortho-nitrophenol release)

ONPG gives a positive result when which color appears?

Ehrlich’s and Kovac’s reagents

Indole production from tryptophan is detected by which reagents?

Red ring

In Kovac’s reagent test for indole, a positive result yields:

Tryptone broth

Which medium is rich in tryptophan used for indole testing?

O₂ and H₂O

Catalase breaks down H₂O₂ into:

Catalase positive

A rapid evolution of bubbles upon addition of H₂O₂ indicates:

Staphylococci and Micrococci (and Bacillus)

Which organisms are typically catalase positive?

Blood agar (contains RBCs with catalase)

When performing a catalase test, which agar should you avoid using because it can give false positives?

Cytochrome oxidase / Indophenol oxidase

The oxidase test detects presence of:

Positive oxidase

A dark purple or black color after oxidase reagent indicates:

Pseudomonas, Neisseria, Vibrio, Campylobacter

Which genera are typically oxidase positive?

Metal loops with iron can give false positives; use nonreactive tools

Why should one use a wooden applicator or platinum loop with oxidase test?

Phenol red

Christensen’s urea agar uses which pH indicator?

Pink / Cerise (alkaline)

Urease positive reaction turns Christensen’s urea agar to which color?

Proteus spp.

Which organism is well-known for rapid urease production (2–4 hours)?

Ammonia, CO₂ and water

The urease reaction hydrolyzes urea into:

Remove carboxyl group from amino acids producing amines

Decarboxylase tests detect bacterial ability to:

Bromcresol purple and cresol red

Moeller decarboxylase medium uses which indicator(s)?

Cadaverine (plus CO₂)

Lysine decarboxylase converts lysine into:

Alkaline (purple)

A positive decarboxylation test typically shifts medium pH to:

Lysine decarboxylation or deamination and H₂S

The Lysine Iron Agar (LIA) primary purpose is to detect:

Dark purple (or black if H₂S present)

In LIA, lysine decarboxylation yields which color?

Reddish-purple (red slant) / plum

Lysine deamination in LIA produces which change?

Salmonella

Which organism is lysine positive as an example in your notes?

• A. Indole production

• B. Motility

• D. Ornithine decarboxylase

Motility-Indole-Ornithine (MIO) agar tests for all:

Clouding or hazy growth radiating from the stab line

Motility in semisolid media is observed as:

Purple throughout the medium

Ornithine decarboxylation positive in MIO yields which color?

Pink to red at top (after reagent)

Indole in MIO is detected by adding Kovac’s reagent — a positive result is: