Stress and the periodontal diseases: growth responses of periodontal bacteria to Escherichia coli stress-associated autoinducer and exogenous Fe.

Psychological stress is known to increase the circulating levels of the catecholamine hormones noradrenaline and adrenaline, which have been shown to influence the growth of a large number of bacterial species by acting in a siderophore-like manner or by inducing the production of novel autoinducers of growth. As we have previously demonstrated that periodontal organisms display differing growth responses to noradrenaline and adrenaline, the aim of this study was to determine whether these growth effects were based upon either siderophore-like or autoinducer mechanisms. Initial inocula of 43 microbial organisms normally found within the subgingival biofilm were established under anaerobic conditions (35 degrees C). Each strain was re-inoculated into a serum-based minimal medium and growth was assessed by optical density (OD(600 nm)) with test and control cultures performed in triplicate. Test cultures were supplemented with either 50 mum ferric nitrate or a previously described Escherichia coli autoinducer of growth. Significant growth effects for supplementation with ferric nitrate (13 species responding positively) and E. coli autoinducer (24 species responding positively) were observed, with differences in growth response within bacterial species and within microbial complexes. When data for all organisms were compared with published responses to catecholamines there were only weak correlations with Fe (r = 0.28) and E. coli autoinducer (r = 0.34) responses. However, large positive responses (> 25% increase) to free Fe and/or E. coli autoinducer were significantly more prevalent in the group of organisms (n = 12) known to exhibit similar responses to catecholamine hormones (P < 0.01; chi2 = 4.56). The results support the view that catecholamines may exert their effects on subgingival organisms by initiating autoinducer production, or simply by acting in a siderophore-like manner, scavenging bound iron from the local environment. It is possible that autoinducer mechanisms may play an important role in the response of oral microorganisms to stress hormones, thereby contributing to the clinical course of stress-associated periodontal diseases.

Ferrioxamine-mediated Iron(III) utilization by Salmonella enterica.

Utilization of ferrioxamines as sole sources of iron distinguishes Salmonella enterica serotypes Typhimurium and Enteritidis from a number of related species, including Escherichia coli. Ferrioxamine supplements have therefore been used in preenrichment and selection media to increase the bacterial growth rate while selectivity is maintained. We characterized the determinants involved in utilization of ferrioxamines B, E, and G by S. enterica serotype Typhimurium by performing siderophore cross-feeding bioassays. Transport of all three ferric siderophores across the outer membrane was dependent on the FoxA receptor encoded by the Fur-repressible foxA gene. However, only the transport of ferrioxamine G was dependent on the energy-transducing protein TonB, since growth stimulation of a tonB strain by ferrioxamines B and E was observed, albeit at lower efficiencies than in the parental strain. Transport across the inner membrane was dependent on the periplasmic binding protein-dependent ABC transporter complex comprising FhuBCD, as has been reported for other hydroxamate siderophores of enteric bacteria. The distribution of the foxA gene in the genus Salmonella, as indicated by DNA hybridization studies and correlated with the ability to utilize ferrioxamine E, was restricted to subspecies I, II, and IIIb, and this gene was absent from subspecies IIIa, IV, VI, and VII (formerly subspecies IV) and Salmonella bongori (formerly subspecies V). S. enterica serotype Typhimurium mutants with either a transposon insertion or a defined nonpolar frameshift (+2) mutation in the foxA gene were not able to utilize any of the three ferrioxamines tested. A strain carrying the nonpolar foxA mutation exhibited a significantly reduced ability to colonize rabbit ileal loops compared to the foxA+ parent. In addition, a foxA mutant was markedly attenuated in mice inoculated by either the intragastric or intravenous route. Mice inoculated with the foxA mutant were protected against subsequent challenge by the foxA+ parent strain.

Incidence of the aerobactin iron uptake system among Escherichia coli isolates from infections of farm animals.

To assess the importance of aerobactin-mediated iron uptake as a bacterial virulence determinant in animal infections, a total of 576 strains of Escherichia coli isolated from cattle, chickens, sheep and pigs were screened by colony hybridization to determine the presence of the aerobactin genetic determinants, and by a bioassay to detect aerobactin secretion in iron-limited conditions. Results obtained by the two complementary methods correlated well. The incidence of the aerobactin system was very high among septicaemia isolates, particularly those from cattle and chickens, an observation that strongly suggests an important role for this mechanism of iron assimilation in pathogenesis. On the other hand, the incidence of the aerobactin system among mastitis strains was not significantly higher than among faecal isolates from healthy animals. No classical enterotoxigenic E. coli strains tested carried the aerobactin genetic determinants. Although most strains that produced aerobactin were also able to make colicin V, the fact that the two characteristics existed separately in a significant minority of isolates suggested that colicin testing alone could not be reliably used to determine the presence of the aerobactin system.

Novel iron uptake system specified by ColV plasmids: an important component in the virulence of invasive strains of Escherichia coli.

The enhanced virulence of invasive strains of Escherichia coli carrying ColV plasmids was shown to be due to a novel plasmid-mediated iron uptake system. Possession of a ColV plasmid conferred strong selective advantage on the host bacterial strain in experimental infections unless excess iron was administered in the inoculum. Moreover, supplementation of defined minimal medium with transferrin to complex available iron caused marked limitation of the growth of plasmid-free strains but had no effect on strains carrying a ColV plasmid. The activity of an efficient iron uptake process was clearly shown by experiments with a mutant of E. coli deficient in enterochelin biosynthesis. Although the mutant was dependent on the presence of citrate in the growth medium to facilitate iron transport, colicinogenic derivatives did not require added citrate for growth. Radioactive iron was shown to be taken up rapidly by nongrowing cells of the plasmid-carrying strain. Furthermore, it was observed that repression of the synthesis of specific outer membrane proteins normally induced by conditions of iron deficit was maintained after a shift of the colicinogenic strains from a rich medium to a medium low in iron. The ColV plasmid-mediated iron uptake system was independent of the active iron transport mechanisms known in E. coli, but like them it required tonB activity as a source of energy.