Phagocytosis and serum susceptibility of Escherichia coil cultured in iron-deplete and iron-replete media

Siderophore-mediated iron acquisition and modulation of host-bacterial interactions

Iron is an essential micronutrient for most life forms including the majority of resident bacteria of the microbiota and their mammalian hosts. Bacteria have evolved numerous mechanisms to competitively acquire iron within host environments, such as the secretion of small molecules known as siderophores that can solubilize iron for bacterial use. However, siderophore biosynthesis and acquisition is not a capability equally harbored by all resident bacteria. Moreover, the structural diversity of siderophores creates variability in the susceptibility to host mechanisms that serve to counteract siderophore-mediated iron acquisition and limit bacterial growth. As a result, the differential capabilities to acquire iron among members of a complex microbial community carry important implications for the growth and function of resident bacteria. Siderophores can also directly influence host function by modulating cellular iron homeostasis, further providing a mechanism by which resident bacteria may influence their local environment at the host-microbial interface. This review will explore the putative mechanisms by which siderophore production by resident bacteria in the intestines may influence microbial community dynamics and host-bacterial interactions with important implications for pathogen- and microbiota-driven diseases including infection, inflammatory bowel diseases and colorectal cancer.

Adherent-Invasive Escherichia coli Production of Cellulose Influences Iron-Induced Bacterial Aggregation, Phagocytosis, and Induction of Colitis

Adherent-invasive Escherichia coli (AIEC), a functionally distinct subset of resident intestinal E. coli associated with Crohn's disease, is characterized by enhanced epithelial adhesion and invasion, survival within macrophages, and biofilm formation. Environmental factors, such as iron, modulate E. coli production of extracellular structures, which in turn influence the formation of multicellular communities, such as biofilms, and bacterial interactions with host cells. However, the physiological and functional responses of AIEC to variable iron availability have not been thoroughly investigated. We therefore characterized the impact of iron on the physiology of AIEC strain NC101 and subsequent interactions with macrophages. Iron promoted the cellulose-dependent aggregation of NC101. Bacterial cells recovered from the aggregates were more susceptible to phagocytosis than planktonic cells, which corresponded with the decreased macrophage production of the proinflammatory cytokine interleukin-12 (IL-12) p40. Prevention of aggregate formation through the disruption of cellulose production reduced the phagocytosis of iron-exposed NC101. In contrast, under iron-limiting conditions, where NC101 aggregation is not induced, the disruption of cellulose production enhanced NC101 phagocytosis and decreased macrophage secretion of IL-12 p40. Finally, abrogation of cellulose production reduced NC101 induction of colitis when NC101 was monoassociated in inflammation-prone Il10(-/-) mice. Taken together, our results introduce cellulose as a novel physiological factor that impacts host-microbe-environment interactions and alters the proinflammatory potential of AIEC.

The effects of iron limitation and cell density on prokaryotic metabolism and gene expression: Excerpts from Fusobacterium necrophorum strain 774 (sheep isolate)

Fusobacterium necrophorum is a Gram-negative obligate anaerobe associated with several diseases in humans and animals. Despite its increasing clinical significance, there is little or no data on the relationship between its metabolism and virulence. Previous studies have shown that bacteria grown under iron-limitation express immunogenic antigens similar to those generated in vivo. Thus, this paper describes the relationship between F. necrophorum subsp. necrophorum (Fnn) metabolism and the expression of the encoded putative virulence factors under iron-restricted conditions. At the midlog phase, iron limitation reduced Fnn growth but the cell density was dependent on the size of the inoculum. Preferential utilization of glucose-1-phosphate, d-mannitol and l-phenylalanine; production of 2-hydroxycaproic acid and termination of dimethyl sulphide production were major Fnn response-factors to iron limitation. Ultimately, iron restriction resulted in an increased ability of Fnn to metabolize diverse carbon sources and in the expression of stress-specific virulence factors. Iron starvation in low Fnn cell density was associated with the up-regulation of haemagglutinin (HA) and leukotoxin (lktA) genes (2.49 and 3.72 fold change respectively). However, Fnn encoded Haemolysin (Hly), yebN homologue (febN) and tonB homologue, were down-regulated (0.15, 0.79 and 0.33, fold changes respectively). Interestingly, cell density appeared to play a regulatory role in the final bacteria cell biomass, induction of a metabolic gene expression and the expression pattern virulence factors in Fnn suggesting the role of a cell density-associated regulatory factor. This report suggest that future studies on differential expression of bacterial genes under altered environmental condition(s) should consider testing the effect of cell concentrations as this is often neglected in such studies. In conclusion, iron restriction induces preferential utilization of carbon sources and altered metabolism in Fnn with associated changes in the expression pattern of the virulence factors.

Influence of animal origin and lineage on survival of Escherichia coli O157:H7 strains in strong and weak acid challenges

Twenty-five strains of Escherichia coli O157:H7 isolated from humans, cattle, and pigs were maintained in HCl (pH 2.5) and in a volatile fatty acid (VFA) mixture (pH 4.0) for up to 6 h at 37 degrees C to assess their ability to survive in acidic conditions that simulate those of the stomach and ileum, respectively. In HCl, the average group survival of bovine strains was significantly higher than that of porcine and human strains, whereas in VFAs, porcine strains were significantly more resistant than bovine and human strains. Bovine strains exhibited significantly higher average survival in HCl than in VFAs. The average survival of strains classified as octamer-based genome scanning (OBGS) lineage II was significantly superior to that of strains classified as OBGS lineage I in HCl. The group of lineage I strains was more resistant in VFAs compared with lineage II, but only after 6 h of challenge. The possible involvement of urease in acid resistance of E. coli O157:H7 was also examined. Although the strains possessed the ureC gene, as shown by PCR, this gene did not appear to contribute to acid resistance under the conditions tested. The data indicate that there is a relationship between acid resistance and source or lineage of O157:H7 strains.