Hydrogen Sulfide Negative Salmonella and their Implication for Standard Culture-based Identification

Salmonella typically produces hydrogen sulfide (H₂S) gas, which appears as a black precipitate on most selective culture media and serves as a key diagnostic feature in many laboratories. However, the emergence of H₂S-negative Salmonella serovars raises concerns about their potential to evade conventional isolation protocols. This review explores the phenotypic variability in H₂S production among Salmonella and summarizes recent global reports of H₂S-negative serovars. Additionally, it discusses the limitations of current H₂S-dependent isolation methods and proposes improved strategies for effectively detecting these atypical strains. We identified several factors, including media composition and incubation duration, that can influence H₂S production in Salmonella. By analyzing various methods employed in recent years to isolate H₂S-negative strains, we advocate for the early integration of non-H₂S-dependent detection techniques to guide isolation workflows and prevent the unintentional exclusion of viable H₂S-negative serovars. Specifically, we recommend parallel plating of suspect Salmonella cultures on both H₂S-indicating media and non-H₂S-based media, such as chromogenic agars, followed by the confirmation of both black and nonblack colonies. In conclusion, this review underscores the continued occurrence of H₂S-negative Salmonella serovars and offers practical recommendations to enhance their detection, thereby supporting more comprehensive and accurate pathogen identification.