Carsten Kröger

The metabolic island GEI4417/4436 of Salmonella enterica serovar Typhimurium ( S . Typhimurium) enables the degradation of myo -inositol (MI) as the sole carbon and energy source. The island encodes two regulatory factors, the autoregulated repressor IolR that inhibits the transcription of most iol genes in the absence of MI, and the activator ReiD that induces the expression of an iol gene operon that is not controlled by IolR. In this study, we investigated the putative role of global regulators in the control of MI utilization by S . Typhimurium. The histone-like nucleoid structuring protein H-NS is demonstrated here to interact with 16 regions of GEI4417/4436 and to silence the transcriptional activity of the promoters P reiD and P iolE , thus controlling the expression of genes initial for MI degradation. The cAMP-binding regulatory protein (CRP) is shown to bind numerous promoters in GEI4417/4436 and is required for the growth of S . Typhimurium strain 14028 in a minimal medium with MI. The binding kinetics of H-NS and CRP toward promoters of GEI4417/4436 were quantified by surface resonance spectroscopy, showing that H-NS weakly binds to promoters of initial genes and that CRP has a high affinity to promoters of all genes essential for MI utilization. Furthermore, three promoters of the metabolic island were identified here to belong to the virulon of the two-component system SsrA/SsrB of salmonellae as demonstrated by electrophoretic mobility shift assays and the use of chromosomal luciferase reporter fusions. IMPORTANCE The capacity to utilize myo-inositol (MI) as sole carbon and energy source is widespread among bacteria, among them the intestinal pathogen S . Typhimurium. This study elucidates the complex and hierarchical regulation that underlies the utilization of MI by S . Typhimurium under substrate limitation. A total of seven regulatory factors have been identified so far, allowing the pathogen an environment-dependent, efficient, and fine-tuned regulation of a metabolic property that provides growth advantages in different environments.

Horizontal gene transfer by plasmids can confer metabolic capabilities that expand a host cell’s niche. Yet, it is less understood whether the coalescence of specialized catabolic functions, antibiotic resistances and metal resistances on plasmids provides synergistic benefits. In this study, we report whole-genome assembly and phenotypic analysis of five Salmonella enterica strains isolated in the 1980s from milk powder in Munich, Germany. All strains exhibited the unusual phenotype of lactose-fermentation and encoded either of two variants of the lac operon. Surprisingly, all strains encoded the mobilized colistin resistance gene 9 ( mcr-9 ), long before the first report of this gene in the literature. In two cases, the mcr-9 gene and the lac locus were linked within a large gene island that formed an IncHI2A-type plasmid in one strain but was chromosomally integrated in the other strain. In two other strains, the mcr-9 gene was found on a large IncHI1B/IncP-type plasmid, whereas the lac locus was encoded on a separate chromosomally integrated plasmidic island. The mcr-9 sequences were identical and genomic contexts could not explain the wide range of colistin resistances exhibited by the Salmonella strains. Nucleotide variants did explain phenotypic differences in motility and exopolysaccharide production. The observed linkage of mcr-9 to lactose metabolism, an array of heavy-metal detoxification systems, and other antibiotic resistance genes may reflect a coalescence of specialized phenotypes that improve the spread of colistin resistance in dairy facilities, much earlier than previously suspected.