Serum sensitivity of Escherichia coli strains present in the clinical environment

It is well known that the growth of some Gram-negative microorganisms is inhibited by normal human serum; but there are no reports about the percentage of serum-sensitive Escherichia coli strains present in clinics. The aim of the present paper is to obtain data useful for elucidating this point. The study was carried out using 38 strains of E. coli isolated in clinics. Different concentrations of serum (from 2.5% to 20% in BHI broth) were used for evaluating its bactericidal activity. The bactericidal activity was measured both as colony forming units (CFU) and as a delay in the growth curve recorded by an Avantage-Abbott instrument. The concentration of serum inducing a killing effect greater than 99.9% was considered bactericidal. The results indicate that in the presence of 2.5%, 5%, 10% and 20% of serum in BHI broth, the percentage of serum-sensitive strains was 2.6%, 10.5%, 18.4% and 31.6%, respectively. These data are discussed in view of the possible application of the synergistic effect of serum with sub-MICs of different antibiotics on serum-sensitive E. coli strains present in the clinical environment.