Does Salmonella Produce H2S? A Key Diagnostic Marker
Salmonella bacteria are widely recognized for their ability to produce hydrogen sulfide (H2S), a gas known for its distinctive “rotten egg” smell. This characteristic is a distinguishing feature used in laboratory settings to help identify these microorganisms.
The Mechanism of H2S Production
Salmonella produces hydrogen sulfide through specific biochemical pathways involving the breakdown of sulfur-containing compounds. A primary enzyme involved in this process is thiosulfate reductase, which facilitates the reduction of thiosulfate, an inorganic sulfur compound. This enzyme’s activity, particularly in Salmonella enterica, relies on a proton motive force under anaerobic conditions to drive electron flow. The genes responsible for this enzymatic activity belong to the phs operon.
Another pathway involves the enzymatic breakdown of sulfur-containing amino acids like cysteine. The enzyme cysteine desulfhydrase degrades cysteine to produce pyruvate, ammonia, and sulfide. Salmonella enterica possesses a highly efficient cysteine desulfhydrase, which is a primary enzyme for cysteine degradation and sulfide production under aerobic conditions. The resulting sulfide then combines with hydrogen ions to form hydrogen sulfide gas.
H2S as a Diagnostic Tool
The production of hydrogen sulfide by Salmonella is a valuable characteristic in microbiology laboratories for identification. This feature is utilized in specialized culture media that contain iron salts, such as Triple Sugar Iron (TSI) agar, Hektoen Enteric (HE) agar, and Xylose Lysine Deoxycholate (XLD) agar. These media include components like sodium thiosulfate as a sulfur source and iron compounds, such as ferric ammonium citrate or ferrous sulfate, as indicators. When Salmonella produces H2S, the gas reacts with these iron salts.
This reaction forms a black precipitate, specifically ferrous sulfide (FeS), which is visibly noticeable within the agar medium. On TSI agar, this blackening typically appears in the butt (bottom) of the tube. For HE and XLD agar, Salmonella colonies often present with black centers. This distinct black coloration serves as a key differential characteristic, assisting microbiologists in presumptively identifying Salmonella species from other enteric bacteria.
Other H2S-Producing Bacteria
While Salmonella is well-known for H2S production, it is not the only bacterium capable of this metabolic activity. Several other bacterial genera can also produce hydrogen sulfide, including Proteus, Citrobacter, Edwardsiella, Clostridium, Escherichia, Enterobacter, and Klebsiella species. Because of this shared capability, H2S production is considered a useful differential test but not a definitive confirmatory test for Salmonella on its own.
Therefore, additional biochemical, immunological, or molecular tests are necessary for a conclusive identification of Salmonella. It is important to note that some Salmonella serovars may be H2S-negative, making it crucial for laboratories to employ a range of detection techniques to avoid missing these atypical strains.