Further Studies on the Susceptibility of <i>Serratia marcescens </i>to the Bactericidal Activity of Human Serum

Abstract

13 isolates of Serratia marcescens of various human serum-susceptibility categories and Escherichia coli strain ATCC 25922 were not inhibited by 80 or 50 vol% of conventionally heat-inactivated (56 °C, 30 min) human serum. Conversely, the growth of control strain E. coli C was retarded by 56 °C 30 min heat-inactivated serum. This strain was subject to Fe deprivation by unsaturated human serum transferrin (TF). Addition of either 100 or 50 mm of ferrous or 50 and 25 µM of ferric iron (Fe) ions to 80vol% of heat-inactivated human serum, or heat inactivation of 50 vol% of human serum at 65 °C for 30 min prevented the bacteriostatic effect against E. coli C. Addition of 10 and 5 mg of human TF/ml of 50 vol% of 65 °C 30 min heat-inactivated human serum resulted in a bacteriostatic effect. None of the S. marcescens and E. coli strains examined degraded TF either in conventionally heat-inactivated human serum or in a TF-supplemented, protein-free defined medium, as determined immunoelectrophoretically. All 13 isolates of S. marcescens were resistant against 0.01 m of disodium ethylenediamine tetra-acetate (EDTA). However, E. coli strains C and ATCC 25922 were inhibited by 0.01 m of EDTA in fresh and heat-inactivated human serum and in tryptic soy broth. This effect was due to chelation of divalent cations by EDTA. All strains of S. marcescens and both E. coli strains produced Fe chelators in an Fe-free defined broth medium. These were assayed in an Fe-free solid defined medium (SCA) that contained from 10 to 20 Mg/ml of the Fe chelator ethylenediamine diorthohydroxy-phenyl acetic acid (EDDA). E. coli strain C was regularly inhibited by 10 µg EDDA/ ml of SCA, an effect that was overcome by 0.2 µg of actually delivered ferrous Fe, whereas all S. marcescens isolates and E. coli strain ATCC 25922 required > 50 µg EDDA/ml of SCA for complete inhibition of growth. Similarly, E. coli strain C was inhibited by 10 and 5 mg of TF/ml of defined medium, whereas selected S. marcescens isolates and E. coli strain ATCC 25922 were not affected by 10 mg/ml of TF. Therefore, control strain E. coli C was more susceptible to serum transferring-ediated Fe deprivation and chelation of Fe ions by EDDA than the other bacterial strains.

Interactions between bacteria and iron binding proteins

“…Milk and serum inhibit the growth of various gram positive and gram negative bacteria (Schade, 1963;Masson & Heremans, 1966;Bullen et al, 1972;Rogers, 1973;Traub, 1977). The effects, attributed to the presence of Tr and Lf could be abolished by iron supply (Bull~n et al, 1967;Griffiths & Humphreys, 1977).…”