Probiotic Bacteria Reduce Salmonella Typhimurium Intestinal Colonization by Competing for Iron Original paper

What was studied?

This study evaluated how probiotic Escherichia coli Nissle 1917 iron competition limits intestinal colonization by Salmonella enterica serovar Typhimurium in mouse models of acute and chronic colitis. The authors asked a simple question. Can EcN utilize the same iron sources that Salmonella requires during inflammation, thereby starving the pathogen? They measured gut iron levels during Salmonella infection and showed that inflammation created a low-iron niche and administered wild-type EcN or EcN mutants lacking key iron uptake systems. The authors tracked pathogen burden, EcN colonization, gut inflammation, and host lipocalin-2 responses. They also tested mice that were unable to produce lipocalin-2 to determine if host iron sequestration was necessary for the probiotic effect.

Who was studied?

Female C57BL/6 mice with streptomycin-facilitated Salmonella colitis were used for the acute model. Female 129X1/SvJ mice with functional Nramp1 formed the chronic persistent model. All mice received oral Salmonella Typhimurium. They then received either wild-type EcN, EcN tonB mutants that cannot power high-affinity iron transport, or EcN Δ4 mutants lacking salmochelin, aerobactin, yersiniabactin, and heme uptake receptors. A separate group of lipocalin-2 knockout mice was infected to test host dependence. Bacterial counts were taken from feces and colon contents. Cecal tissue was scored blindly for inflammation. Host transcripts for lipocalin-2, Tnf-α, Ifn-γ, Cxcl-1, and Il-17a were quantified.

Most important findings

Probiotic Escherichia coli Nissle 1917 iron competition worked. A single oral dose of wild-type EcN sharply reduced Salmonella fecal shedding. In the chronic model, the reduction was more than 100-fold and lasted for the three-week follow-up. EcN also established stable colonization. When EcN could not import iron because tonB was disrupted, it still colonized, but it no longer suppressed Salmonella. The same loss of effect was seen with the Δ4 strain in acute colitis. This proved that iron competition was the probiotic mechanism.

The host factor lipocalin-2 was also essential. EcN lowered Salmonella only in lipocalin-2–sufficient mice. In lipocalin-2–deficient mice, Salmonella and EcN coexisted because the host no longer blocked enterochelin. EcN itself resisted lipocalin-2 because it used several lipocalin-2–insensitive siderophores. This gave EcN an edge over Salmonella even inside an inflamed, iron-poor gut. EcN also reduced cecal inflammation and the expression of inflammatory transcripts. That anti-inflammatory effect was present even when EcN mutants failed to displace Salmonella. So EcN had two partly separate actions. One depended on iron competition. One relied on mucosal immune modulation. Together they generate a clear microbiome signature. It is characterized by an Enterobacteriaceae-rich, inflamed gut, high lipocalin-2 levels, low free iron, the presence of a strain carrying redundant siderophore systems, and a resulting decline in Salmonella counts.

Key implications

Clinicians can interpret this as mechanistic evidence that some probiotic effects in infection are nutritional, rather than solely immunological. EcN can augment the host’s nutritional immunity arm and make lipocalin-2 work better. This matters because antibiotics are often avoided in uncomplicated nontyphoidal Salmonella. A live EcN product could therefore become a supportive option during or after acute colitis to lower pathogen burden. The work also warns that not every EcN-like strain will work. The iron uptake arsenal must be intact. The host must still make lipocalin-2. These two conditions identify patients and settings where an EcN-style probiotic is more likely to succeed.