The Variable Response of Bacteria to Free Haemoglobin in the Tissues

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Abstract
The local enhancement of infection by exogenous Fe3+, as ferric ammonium citrate, and by Fe2+, as guinea pig Hb, was assessed in studies with 55 strains of bacteria injected into the skin of guinea-pigs. The test organisms included Staphylococcus aureus, Streptococcus spp., Klebsiella spp., Escherichia coli and Pseudomonas aeruginosa. Four strains of Bacteroides spp. were tested with Hb only. As previously reported with other strains, enhancement of infection by members of a given species by Fe3+ was variable; in this study infection with only 11 of 59 strains was enhanced. Hb either of equal or lesser Fe content was a more potent enhancer, effecting 27 of the 59 strains. The enhancement ranged from 2- to 80-fold, the higher figures on the whole being characteristic of Hb enhancement. Some few instances of depression by both Hb and ferric ammonium citrate were noted. A few tests were made with systemic Hb but the concentrations attaainable were largely ineffective. Enhancement of infection did not appear to be related to the capacity of a strain to lyse or digest host red blood cells. In so far as guinea-pigs whose antibacterial defences are lowered by Fe3+ or Fe2+ represent human subjects at risk of infection because of clinical circumstances characterized by excess of available Fe, (either exogenous or as a result of hemolysis) these results with organisms of a kind commonly associated with infection in hospitals suggest that only a small proportion of environmental bacteria can take advantage of any decreased resistance associated with Fe excess.