Exercise 12: Hydrogen Sulfide Production + Motility

What does SIM stand for, and what can SIM medium detect?

Sulfide, indole, and motility. It detects hydrogen sulfide production, indole production from tryptophan, and bacterial movement through a semisolid medium.

Is SIM medium selective, differential, or both?

It is differential because it distinguishes bacteria by hydrogen sulfide production, indole production, and motility. It is not selective.

What are the significant components of SIM medium described in your lecture?

• Cysteine provides a sulfur-containing substrate for hydrogen sulfide production

• Ferrous ammonium sulfate (Fe(NH₄)SO₄) detects hydrogen sulfide

• Tryptophan supports indole testing

• Low agar concentration allows motility to be observed.

What enzyme breaks down cysteine to release hydrogen sulfide?

Cysteine desulfhydrase.

What are the products of cysteine breakdown (or sulfide production) by cysteine desulfhydrase?

• Hydrogen sulfide (H2S)

• Pyruvate

• Ammonium (NH4+)

What functional group does a desulfhydrase remove?

A sulfur-containing thiol group, or –SH group.

How can cysteine breakdown benefit a bacterium?

It produces pyruvate, which can enter metabolic pathways for energy production. Hydrogen sulfide and ammonium are also released.

Besides cysteine degradation, what other process can produce hydrogen sulfide?

Anaerobic respiration using inorganic sulfur-containing compounds as electron acceptors.

What inorganic sulfur-containing compounds discussed in your manual can be reduced to produce hydrogen sulfide?

Thiosulfate, sulfate, and sulfite.

What component of SIM medium detects hydrogen sulfide, and how does it work?

Ferrous ammonium sulfate supplies ferrous iron that reacts with sulfide to form insoluble black ferrous sulfide, FeS.

What is the black material in a positive hydrogen sulfide test?

Ferrous sulfide, FeS

• It is a precipitate formed when hydrogen sulfide produced by bacteria reacts with ferrous iron in the medium.

How are positive and negative hydrogen sulfide results interpreted?

Any black precipitate indicates a positive result. No blackening indicates a negative result, provided bacterial growth occurred.

Must a reagent be added after incubation to detect hydrogen sulfide in SIM medium?

No. The iron-containing detection component is already present, so black precipitate develops during incubation.

Which organisms are the positive and negative controls for hydrogen sulfide production in your course?

• Positive: Proteus vulgaris

• Negative: Enterococcus faecalis and Enterobacter aerogenes

Which medically relevant bacterial genera can produce hydrogen sulfide?

Salmonella and Proteus.

Why can SIM medium be used to detect motility?

It contains a low concentration of agar, making it semisolid. Motile bacteria can move away from the original inoculation line and grow throughout the surrounding medium.

What agar concentration is used in SIM medium?

0.35% agar, compared to approximately 1.2% in a standard solid medium

How should SIM medium be inoculated in your course?

Use an inoculation needle to make one straight stab approximately halfway down the medium. Withdraw the needle along the same path, then incubate at 35°C until the next lab period. Stabilization of the inoculation needle is necessary to keep the inoculum confined to one narrow path/visualize motility properly

What indicates a positive motility result in SIM medium? Negative?

• Positive: diffuse or cloudy growth spreading laterally away from the stab line into the surrounding medium

• Negative: growth remains confined to the stab line

Which organisms are the motility controls in your course?

• Positive: Proteus vulgaris and Enterobacter aerogenes

• Negative: Enterococcus faecalis

Can a bacterium be motile without producing hydrogen sulfide?

Yes. Motility and hydrogen sulfide production are separate characteristics. Enterobacter aerogenes is the course example: it spreads away from the stab line (motile) without blackening the medium.

What can happen when a motile organism also produces hydrogen sulfide in SIM medium?

Blackening may extend widely through the medium as the organism spreads. Extensive blackening can obscure the growth pattern, so motility should be judged cautiously if the stab line cannot be evaluated.

What component of SIM medium supports indole testing, and what enzyme is detected?

• Tryptophan is the substrate

• Indole production indicates activity of tryptophanase, which breaks tryptophan into indole, pyruvate, and ammonia.

What reagent is used to detect indole in SIM medium, and how are results interpreted?

Kovac’s reagent is added to the top of the agar without mixing

• Production of a pink to cherry-red upper reagent layer is positive; a layer that remains yellow is negative.

Why should hydrogen sulfide production and the motility growth pattern be recorded before adding Kovac’s reagent?

These observations depend on the appearance and distribution of growth in the medium. Recording them first avoids interference from the added reagent or disturbance of the tube.