Biochemical Testing

What does the Oxidase Test check for?

It checks if bacteria produce the enzyme cytochrome c oxidase, part of the aerobic electron transport chain.

What indicates a positive Oxidase Test?

The colony turns dark blue or purple within 10–60 seconds.

What indicates a negative Oxidase Test?

No color change or color appears after 60 seconds.

What does the Starch Hydrolysis Test check for?

The enzyme α-amylase, which breaks down starch into glucose.

What indicates a positive Starch Hydrolysis Test?

A clear halo around the growth after adding iodine. the starch has been digested.

What indicates a negative Starch Hydrolysis Test?

The medium turns dark blue/purple/black after iodine. starch is still present.

What does the Citrate Utilization Test determine?

Whether bacteria can use citrate as their sole carbon source, via the enzyme citrate permease.

What indicates a positive Citrate Test?

the agar turns blue. the pH increased

What indicates a negative Citrate Test?

the agar remains green. no color change

What does the Gelatin Utilization Test check for?

It checks if bacteria produce the enzyme gelatinase, which breaks down gelatin into amino acids and peptides.

What is a positive Gelatin Utilization Test result?

The medium liquefies (melts) and stays liquid after cooling. the gelatin was digested.

What is a negative Gelatin Utilization Test result?

The medium remains solid after cooling. the gelatin was not broken down.

What does the Urea Test check for?

The enzyme urease, which breaks down urea into ammonia and carbon dioxide.

What is a positive Urea Test result?

The medium turns bright pink/magenta, showing an alkaline reaction from ammonia.

What is a negative Urea Test result?

The medium stays yellow or shows no color change.

what does it mean if something is more pH

more alkaline, base

what does it mean if something is less pH

more acidic, lower base

What does the SIM Test stand for?

Sulfide, Indole, and Motility. It’s a triple test for three bacterial characteristics.

In the SIM Test, what does Sulfide detect?

Whether bacteria produce hydrogen sulfide

In the sim test, what is a positive Sulfide result?

Black precipitate appears in the medium

in the simWhat is a negative Sulfide result?

No black color forms in the medium.

In the SIM Test, what does Indole detect?

Whether bacteria produce tryptophanase, which converts tryptophan into indole

in the SIM test What is a positive Indole result?

A red layer forms on top after adding Kovac’s reagent.

in the SIM test What is a negative Indole result

The layer is yellow or unchanged after adding Kovac’s reagent

In the SIM Test, what does Motility test for?

Whether bacteria are motile

in the sim test What is a positive Motility result?

The medium looks cloudy or turbid, showing growth spreading from the stab line

in a SIM test What is a negative Motility result?

Growth is restricted to the stab line. bacteria are non-motile.

What does the Carbohydrate Fermentation Test check for?

Whether bacteria can ferment sugars (like glucose, lactose, or sucrose) and produce acid and/or gas as byproducts.

What indicates acid production in carbohydrate fermentation?

The medium (like phenol red broth) turns yellow — pH drops below 6.8 (acidic).

What indicates no acid production in carbohydrate fermentation?

The medium stays red — pH stays above 7.0 (neutral/alkaline).

What indicates gas production in carbohydrate fermentation?

A bubble or air pocket forms in the Durham tube.

What indicates no gas production in carbohydrate fermentation?

No bubbles form in the Durham tube.

What is the TSI (Triple Sugar Iron) Test used for?

It checks if bacteria can ferment glucose, lactose, and/or sucrose, and whether they produce hydrogen sulfide gas

In the TSI Test, what does A/A (acid/acid) mean?

Yellow slant / Yellow butt = fermentation of glucose, lactose, and/or sucrose.

In the TSI Test, what does K/A (alkaline/acid) mean?

Red slant / Yellow butt = fermentation of glucose only.

In the TSI Test, what does K/K (alkaline/alkaline) mean?

Red slant / Red butt = no sugar fermentation.

In the TSI Test, what does Gas (G) indicate?

the agar will rise up from the bottom

In the TSI Test, what does H₂S (Hydrogen Sulfide) indicate?

A black precipitate (ferrous sulfide) forms in the butt.

What does no H₂S production look like in the TSI Test?

No blackening in the agar.