Resistance of Enterococcus faecium to neutrophil-mediated phagocytosis

Enterococcus faecalis bearing aggregation substance is resistant to killing by human neutrophils despite phagocytosis and neutrophil activation

Enterococcus faecalis aggregation substance (AS) mediates efficient bacterium-bacterium contact to facilitate plasmid exchange as part of a bacterial sex pheromone system. We have previously determined that AS promotes direct, opsonin-independent binding of E. faecalis to human neutrophils (PMNs) via complement receptor type 3 and other receptors on the PMN surface. We have now examined the functional consequences of this bacterium-host cell interaction. AS-bearing E. faecalis was phagocytosed and internalized by PMNs, as determined by deconvolution fluorescence microscopy. However, these bacteria were not killed by PMNs, and internalized bacteria excluded propidium iodide, indicating intact bacterial membranes. Resistance to killing occurred despite activation of PMNs, as indicated by an increase in both functional and total surface Mac-1 expression, shedding of L-selectin, and an increase in PMN extracellular superoxide and phagosomal oxidant production. Deconvolution fluorescence microscopy also revealed that phagosomes containing AS-bearing bacteria were markedly larger than phagosomes containing opsonized E. faecalis, suggesting that some modification of phagosomal maturation may be involved in AS-induced resistance to killing. PMN phagosomal pH was significantly higher after ingestion of nonopsonized AS-bearing E. faecalis than after that of opsonized bacteria. The novel ability of AS to promote intracellular survival of E. faecalis inside PMNs suggests that AS may be a virulence factor used by strains of E. faecalis.

Enterococcus faecalis aggregation substance promotes opsonin-independent binding to human neutrophils via a complement receptor type 3-mediated mechanism

Enterococcus faecalis aggregation substance (AS) mediates efficient adhesion between bacteria, thereby facilitating plasmid exchange as an integral part of a bacterial sex pheromone system. We examined the interaction of AS-bearing E. faecalis with human neutrophils (PMNs), an important component of the host defense system. AS promoted a markedly increased opsonin-independent bacterial binding to PMNs. Adhesion was dependent on the expression of the enterococcal Asc10 protein, which contains two Arg-Gly-Asp (RGD) sequences, and addition of exogenous RGD-containing peptides inhibited AS-mediated binding by 66%. AS-mediated adhesion was inhibited by 85% by anti-human complement receptor type 3 (CR3) monoclonal antibodies or by use of PMNs from a patient with leukocyte adhesion deficiency. However, AS-bearing E. faecalis cells were unable to bind to CHO-Mac-1 cells, expressing functionally active CR3, suggesting the potential need for additional PMN surface receptors for bacterial adhesion. Monoclonal antibodies against integrin-associated protein (CD47) and L-selectin, both of which may interact with CR3 and bind to ligands on E. faecalis, also inhibited AS-dependent binding. The non-opsonic binding of E. faecalis to PMNs may play an important role in this organism's pathogenesis.

Loss of resistance to ingestion and phagocytic killing by O(-) and K(-) mutants of a uropathogenic Escherichia coli O75:K5 strain

To determine the importance of the O75 O antigen and the K5 capsular antigen in resistance to phagocytosis and phagocytic killing, we used previously described O75(-) and K5(-) mutants from an O75(+) K5(+) wild-type uropathogenic Escherichia coli strain in phagocytosis assays with polymorphonuclear leukocytes (PMNs) and monocytes. At a 10-to-1 ratio of bacteria to phagocytes and in the presence of 10% serum, the parental strain GR-12 was resistant to both PMNs and monocytes over a 2-h incubation period. The O75(-) and K5(-) mutants were similar in sensitivity to killing by both PMNs and monocytes, decreasing in viability by 80% in the first hour. Yet, a significant difference in killing between the O75(-) and K5(-) mutants was observed in the first 15 min of incubation. The K5(-) mutant decreased in numbers by almost 60%, while the O75(-) mutant increased in numbers similarly to GR-12 in the first 15 min. The difference in killing was found not to be due to the rate of opsonization. To further determine the mechanism of resistance, a fluorescence assay was used to differentiate attached and internalized bacteria. The K5 capsule hindered the association of both the wild-type strain and the O75(-) mutant in the initial incubation time with PMNs. In conclusion, both the K5 capsule and O75 O antigen play crucial roles in resistance to phagocytosis over time.

Enterococci: new aspects of an old organism

Enterococci are a long-known cause of bacterial endocarditis and a more recently recognized cause of nosocomial infection and superinfection. While much is known about the many antibiotic resistances of enterococci, less is known about the organism itself and how it causes disease. This article presents a brief overview of enterococci and its possible virulence factors and summarizes the authors' efforts to understand the features of this organism that may contribute to its disease potential.

Isolation and chemical characterization of a capsular polysaccharide antigen shared by clinical isolates of Enterococcus faecalis and vancomycin-resistant Enterococcus faecium

Enterococci are a common cause of serious infections, especially in newborns, severely immunocompromised patients, and patients requiring intensive care. To characterize enterococcal surface antigens that are targets of opsonic antibodies, rabbits were immunized with various gentamicin-killed Enterococcus faecalis strains, and immune sera were tested in an opsonophagocytic assay against a selection of clinical isolates. Serum raised against one strain killed the homologous strain (12030) at a dilution of 1:5,120 and mediated opsonic killing of 33% of all strains tested. In addition, this serum killed two (28%) of seven vancomycin-resistant Enterococcus faecium strains. Adsorption of sera with the homologous strain eliminated killing activity. The adsorbing antigens were resistant to treatment with proteinase K and to boiling for 1 h, but were susceptible to treatment with sodium periodate, indicating that the antigen inducing opsonic activity is a polysaccharide. Antibodies in immune rabbit sera reacted with a capsule-like structure visualized by electron microscopy both on the homologous E. faecalis strain and on a vancomycin-resistant E. faecium strain. The capsular polysaccharides from E. faecalis 12030 and E. faecium 838970 were purified, and chemical and structural analyses indicated they were identical glycerol teichoic acid-like molecules with a carbohydrate backbone structure of 6-alpha-D-glucose-1-2 glycerol-3-PO4 with substitution on carbon 2 of the glucose with an alpha-2-1-D-glucose residue. The purified antigen adsorbed opsonic killing activity from immune rabbit sera and elicited high titers of antibodies (when used to immunize rabbits) that both mediated opsonic killing of bacteria and bound to a capsule-like structure visualized by electron microscopy. These results indicate that approximately one-third of a sample of 15 E. faecalis strains and 7 vancomycin-resistant E. faecium strains possess shared capsular polysaccharides that are targets of opsonophagocytic antibodies and therefore are potential vaccine candidates.

Encapsulated Enterococcus faecalis: role of encapsulation in persistence in mouse peritoneum in absence of mouse lethality

Two nonhemolytic, mucoid, encapsulated strains of Enterococcus faecalis lacked lethality for 23 white mice when inoculated (10(9) cells/mL) intraperitoneally. Bacteremia was short lived (2 to 3 days), but peritoneal cultures remained positive for 7 days postinoculation. Although encapsulation did not result in animal lethality, encapsulation may have delayed peritoneal clearance by interference with phagocytosis.

Multiple-drug resistant enterococci: the nature of the problem and an agenda for the future

Enterococci, leading causes of nosocomial bacteremia, surgical wound infection, and urinary tract infection, are becoming resistant to many and sometimes all standard therapies. New rapid surveillance methods are highlighting the importance of examining enterococcal isolates at the species level. Most enterococcal infections are caused by Enterococcus faecalis, which are more likely to express traits related to overt virulence but--for the moment--also more likely to retain sensitivity to at least one effective antibiotic. The remaining infections are mostly caused by E. faecium, a species virtually devoid of known overt pathogenic traits but more likely to be resistant to even antibiotics of last resort. Effective control of multiple-drug resistant enterococci will require 1) better understanding of the interaction between enterococci, the hospital environment, and humans, 2) prudent antibiotic use, 3) better contact isolation in hospitals and other patient care environments, and 4) improved surveillance. Equally important is renewed vigor in the search for additional drugs, accompanied by the evolution of new therapeutic paradigms less vulnerable to the cycle of drug introduction and drug resistance.

Intracellular activities of RP 59500 (quinupristin-dalfopristin) and sparfloxacin against Enterococcus faecium

RP 59500, a combination of the streptogramins quinupristin and dalfopristin, and sparfloxacin are new antibiotics with good in vitro activities against Enterococcus faecium, which is an increasingly important nosocomial pathogen with resistance to multiple antimicrobials. Since fluoroquinolones and related macrolides have displayed high intracellular concentrations inside host cells, we evaluated the intracellular activities of these agents inside neutrophils against three strains each of vancomycin-susceptible E. faecium (VSEF) and vancomycin-resistant E. faecium (VREF). At concentrations equal to four times the MIC, RP 59500 and sparfloxacin decreased the number of intracellular VSEF organisms, while both antibiotics were at best bacteriostatic against intracellular VREF strains. At concentrations equal to one-fourth of the MIC, both antibiotics were bacteriostatic against intracellular VSEF strains but were ineffective in inhibiting the growth of VREF strains. Despite their anticipated markedly higher intracellular human neutrophil (PMN) concentrations, RP 59500 and sparfloxacin activities in medium alone were equal to or greater than those inside PMNs against almost all strains. We conclude that the intracellular PMN concentrations of these antibiotics may not be directly related to their intracellular activities in our assay. The reason for the differences in their activities against VSEF versus VREF remains undefined.

Nonlethal adherence to human neutrophils mediated by Dr antigen-specific adhesins of Escherichia coli

Uropathogenic Escherichia coli strains express a variety of adhesins, including members of the Dr adhesin family such as the Dr hemagglutinin, AFAI, and AFAIII. Certain E. coli adhesins (e.g., type 1 and S fimbriae) are known to mediate adherence to human polymorphonuclear leukocytes (PMNs). The receptor on erythrocytes for Dr family adhesins, decay accelerating factor, is also present on PMNs. To determine whether Dr family adhesins mediate adherence to PMNs and to characterize the specificity and consequences of such adherence, we studied agglutination of PMNs and adherence to PMNs by recombinant E. coli strains expressing various mannose-resistant or mannose-sensitive adhesins, in the presence or absence of inhibitors of adherence. Dr family adhesins, like type 1 fimbriae, mediated concentration-dependent adherence to PMNs. Adherence to PMNs was mannose sensitive for type 1 fimbriae but mannose resistant for Dr family adhesins. Chloramphenicol inhibited PMN adherence for the Dr hemagglutinin with the same potency as that with which it inhibited hemagglutination, but it was inactive against PMN adherence and hemagglutination mediated by other members of the Dr adhesin family. In contrast to PMN adherence mediated by type 1 fimbriae, adherence mediated by the Dr hemagglutinin did not lead to significantly increased bacterial killing. These data suggest that Dr family adhesins mediate a novel pattern of adherence to PMNs, probably by recognizing decay accelerating factor, with minimal consequent bacterial killing.