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 Staphylococcus aureus, Streptococcus spp., Klebsiella spp., Escherichia coli and Pseudomonas aeruginosa. Four strains of Bacteroides 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.