From the Institutes of Medical Microbiology and Pathology and Department of Pediatrics, University of GSteborg, GSteborg, Sweden The Bactericidal Effect of Normal Human Serum on E. coli Strains from Normals and from Patients with Urinary Tract Infections S. Olling, L. A. Hanson, J. Holmgren, U. Jodal, K. Lincoln and U. Lindberg Summary: E. col~ strains originating from faeces of normal children or from urine of children with urinary tract infections were analysed with regard to sensi- tivity to the bactericidal activity of normal human serum. It was found that urine isolates from patients with symptomatic pyelonephritis or cystitis did not differ in sensitivity from then faecal strains, suggesting that resistance to the bactericidal activity of serum is not a virulence factor of urinary pathogens. A more detail- ed analysis revealed, however, that the most resistant strains of common O groups were twice as prevalent in urine from patients with symptomatic infections as in the faeces specimens. In contrast, children with asymptomatic infections had more sensitive strains than the other groups. These patients had a higher frequency of rough strains and strains of less common O groups which were shown to be more sensitive than those of the most prevalent O groups. Furthermore, the strains of the most common O groups isolated from children with asymptomatic bacteriuria were more sensitive than the strains of the same O groups causing symptomatic infections. These characteristics of the strains causing asymptoma- tic UTI may illustrate a special host-parasite relation- ship present in these patients. Zusammenfassung: Escherichia coli-St~mme, die aus dem Stuhl gesunder Kinder oder aus dem Urin yon Kindern mit Harnweginfektionen (HWI) isoliert werden konnten, wurden in ihrem Verhalten gegentiber der bakteriziden Wirkung des normalen Humanserums untersucht. Hierbei ergab sich kein Unterschied in der Empfindlich- keit gegeniiber der Serumbakterizidie bet St~mmen ver- schiedener Herkunft. Deswegen war zu vermuten, dad die Serumbakterizidie keinen Virulenzfaktor ffir die Erreger der HWI darstellt. Eine genauere Untersuchung zeigte jedoch, dab die am wenigsten beeinflufibaren St~imme der h~ufigen O-Grup- pen doppelt so oft ira Urin yon Patienten mit HWI zu finden waren als in Stuhlproben. Im Gegensatz hierzu zeigten Kinder mit asymptomatischen Bakterurien mehr Serumbakterizidie-empfindliche St~nme als die anderen Gruppen. Und zwar hatten diese Patienten einen h5heren Anteil an Rauh-St~mmen nnd Erregern, die weniger h~ufigen O-Gruppen zuzuordnen sind und die sich als Bakterizidie-empfindlicher als die vorherrschen- den O-Gruppen erwiesen. Ferner waren die bet Kin- dern mit einer asymptomatischen Bakteriurie isolierten St~mme Serumbakterizidie-empfindlicher als die Stem- me der gleichen O-Gruppe, die klinisch manifeste Infek- tionen hervorriefen. Die Charakteristika der bet einer asymptomatischen Bakteriurie isolierten St~mme weisen auf die spezielle Erreger-Wirtbeziehung hin, die bet diesen Patienten vorliegt. Introduction Urinary tract infections (UTI) are common in man, even in the absence of any detectable malfunction in the urinary tract (i). Recurrences often occur and renal parenchymal lesions appear in some of the patients (2, 3). It is not known if these infec- tions are due to defects in the host resistance, or if specific virulence factors exist in certain bacte- rial strains rendering them invasive to the urinary tract in spite of normal defense mechanisms of the host. A search for potential virulence factors of significance in the appearance and course of UTI might add to our understanding of the complex host-parasite relationship involved in this group of infections. Resistance to the bactericidal activity of serum has been reported to be a virulence factor of E. co~i strains tested in intraperitoneal infections in mice (4) and experimental pyelonephritis in rats (5). The aim of the present study was to compare the degree of resistance to bactericidal activity of normal human serum among strains isolated from faeces of healthy children with that among strains from urine of children with various forms of UTI, diag- nosed according to strict clinical and laboratory criteria. A further aim was to analyse the relation of this resistance to the type of O antigen of the bacteria. Kimball et al. (6) and Vosti and Randall (7) have undertaken somewhat similar analyses, not including, however, the comparison of infecting strains with normal faecal flora, which is pertinent since bacteria causing UTI usually seem to originate from the stool (8). Material and Methods Bacteria~ strains. Altogether 866 strains were ana- lysed. They consisted of 102 faecal strains from 24 Infection I (1973) Nr. I