The local enhancement of infection by exogenous ferric iron, as ferric ammonium citrate, and by ferrous iron as guinea-pig haemoglobin, was assessed in studies with 55 strains of bacteria injected into the skin of guinea-pigs. The test organisms included spp., spp., and . Four strains of spp. were tested with haemoglobin only. As previously reported with other strains, enhancement of infection by members of a given species by ferric iron was variable; in this study infection with only 11 of 59 strains was enhanced. Haemoglobin either of equal or lesser iron content was a more potent enhancer, affecting 27 of the 59 strains. The enhancement ranged from two-fold to 80-fold, the higher figures on the whole being characteristic of haemoglobin enhancement. Some few instances of depression by both haemoglobin and ferric ammonium citrate were noted. A few tests were made with systemic haemoglobin but the concentrations attainable 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 ferric or ferrous iron, represent human subjects at risk of infection because of clinical circumstances characterised by excess of available iron—either exogenous or as a result of haemolysis—our 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 iron excess.


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