The role of Bacteroides encapsulation in the lethal synergy between Escherichia coli and Bacteroides species studied in a rat fibrin clot peritonitis model

Effect of growth of Bacteroides fragilis at different redox levels on potential pathogenicity in a HeLa cell system: demonstration by confocal laser scanning microscopy

During trauma, the intestinal anaerobe, Bacteroides fragilis, may enter into a pathogenic state. The process coincides with changing environmental conditions particularly the redox level in situ. To gain insight into this phenomenon B. fragilis was grown at different redox levels, and the invasive potential was examined using an in vitro model consisting of HeLa cell monolayers. The clinical strain AIP 5-86 was taken from a small collection of B. fragilis strains able to penetrate into tissue cell monolayers when selected by an acridine orange-crystal violet fluorescent staining technique. Following preliminary investigation by confocal laser scanning microscopy (CLSM), this particular strain was regarded as representative for examining the invasive potential. After growth in a defined medium under oxidizing, reducing or intermediate Eh7 conditions, the washed mid-log phase bacteria were allowed to interact with HeLa cell monolayers for 45 min at 37 degrees C. The results were extensively monitored by CLSM to follow the reactions in a stereoscopic dimension. In addition, the bacteria were examined by transmission and scanning electron microscopy before interaction to distinguish characteristics in surface configuration. The growth of the bacteria at particular redox levels seemed to influence their potential for pathogenicity. After growth at relatively high Eh, the bacteria easily penetrated into the HeLa cells, but not at low Eh, as determined by the laser scanning studies. Examination of the bacteria alone by transmission and scanning electron microscopy revealed small vesicles and a tendency to aggregate after growth at the low redox level while there were rather few vesicles and an implied dispersion at the high redox level. This leaves it open whether the invasiveness was based on the alterations found during growth of the bacteria. Different redox levels as well as the respective changes of the bacterial surface may help to discern the commensal from the pathogenic state of B. fragilis.

Transferrins and heme-compounds as iron sources for pathogenic bacteria

The low concentration of free iron in body fluids creates bacteriostatic conditions for many microorganisms and is therefore an important defense factor of the body against invading bacteria. Pathogenic bacteria have developed several mechanisms for acquiring iron from the host. Siderophore-mediated iron uptake involves the synthesis of low molecular weight iron chelators called siderophores which compete with the host iron-binding glycoproteins lactoferrin (LF) and transferrin (TF) for iron. Other ways to induce iron uptake, without the mediation of siderophores, are the possession of outer membrane protein receptors that actually recognize the complex of TF or LF with iron, resulting in the internalization of this metal, and the use of heme-compounds released into the circulation after lysis of erythrocytes. In this review, the nonsiderophore-mediated iron-uptake systems used by certain pathogenic bacteria are emphasized. The possible contribution of these iron-uptake systems to the virulence of pathogens is also discussed.

Human immune response to an iron-repressible outer membrane protein of Bacteroides fragilis

Under conditions of iron starvation, Bacteroides fragilis expresses various iron-repressible outer membrane proteins (IROMPs). A 44-kDa protein appears to be one of the major outer membrane proteins (OMPs) in B. fragilis under iron stress and plays a role in heme uptake by this bacterium. To determine whether the 44-kDa IROMP of B. fragilis is expressed in vivo and whether this protein is immunogenic, we used Western immunoblotting to examine serum samples from patients with an infection caused by Bacteroides species. All the serum samples from patients and from normal controls showed reactivity with several proteins of B. fragilis. Only serum samples from patients infected with B. fragilis showed immunoreactivity with the 44-kDa protein. We also used a rat infection model to study the immune response against this protein during the process of an intra-abdominal infection in these animals. During the first 8 days of infection a gradual increase of antibodies to the 44-kDa protein in the rat was detected. These results suggest that the 44-kDa IROMP is expressed in vivo, since it induces an antibody response in patients and animals. We also analyzed 85 strains of the B. fragilis group for the presence of proteins antigenically related to the B. fragilis 44-kDa protein. The data indicate that this protein was conserved in B. fragilis strains and was absent in the other bacterial strains tested.