Nitrate-dependent Iron oxidation in Salmonella enterica Serovar Typhimurium strain SL1344

Background: Nitrate-dependent Iron (Fe2+) oxidation (NDFO) is an important biogeochemical process, but whether NDFO provides a direct metabolic benefit to facultative anaerobes with diverse lifestyles, such as Salmonella enterica, and what role the respiratory nitrate reductases play in this process is unknown.

This study investigated NDFO in Salmonella enterica serovar Typhimurium strain SL1344. We compared the wild-type (WT) strain with a ∆narGHIJ ∆narZYW mutant (∆nar), which lacks the primary respiratory nitrate reductase (Nar). Cultures were grown anaerobically in Lysogeny Broth (LB) medium amended with 4 mM nitrate and 10 mM FeSO₄. Growth, nitrate depletion, nitrite accumulation, and Fe2+ oxidation were monitored over 14 days. Abiotic controls amended with either nitrate or nitrite were included to control for abiotic iron oxidation.

There was no significant difference in the growth rate and biomass production between the WT and ∆nar strains. However, there was significant variation in nitrite reduction and iron oxidation. 56.9 % of the nitrate was depleted from the growth medium (initially 4mM) with the WT cultures compared to 19.7% with the ∆nar strain. Fe2+ oxidation in the WT cultures reached a Fe²⁺/Fe total ratio of ~0.52-0.61 during exponential growth and was consistent during the stationary phase, whereas for the mutant, the maximum Fe²⁺/Fe total ratio was ~0.78, which returned to a more reduced state in stationary phase (Fe²⁺/Fe total ~0.95). Abiotic controls amended with nitrite showed rapid Fe2+ oxidation, highlighting the role of nitrite as an oxidant.

The respiratory nitrate reductases are the primary drivers of NDFO in S. enterica SL1344 and are required for the generation of nitrite, which then abiotically oxidises Fe2+to Fe3+. This did not confer a growth advantage, suggesting NDFO is an indirect consequence of nitrate respiration rather than a direct energy-conserving pathway in this organism. - Received: - Version Posted: Funding - Science and Technology Facilities Council - Principal Award Recipient: Alex Price - The Open University - Principal Award Recipient: Michael Christopher Macey - Research England (Award Expanding Excellence in England 124.18) - Principal Award Recipient: Karen Olsson-Francis