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Legionella pneumophila is a facultative intracellular bacterial pathogen that parasitizes human monocytes and alveolar macrophages. Previous studies from this laboratory have shown that monocyte complement receptors CR1 and CR3 and complement component C3 in serum mediate L. pneumophila phagocytosis. In this study, we have explored C3 fixation to L. pneumophila. We developed a whole-cell enzyme-linked immunosorbent assay (ELISA) to measure C3 fixation to the bacterial surface. By this assay, C3 fixes to L. pneumophila that are opsonized in fresh nonimmune serum, and C3 fixation takes place via the alternative pathway of complement activation. Immunoblot analysis of opsonized L. pneumophila indicated that C3 fixes selectively to specific acceptor molecules of L. pneumophila. Consistent with this, when nitrocellulose blots of whole L. pneumophila or bacterial components are incubated in fresh nonimmune serum, C3 fixes exclusively to the major outer membrane protein (MOMP) of L. pneumophila, a porin; C3 does not fix to L. pneumophila LPS on these blots. To further explore the role of MOMP in C3 fixation and phagocytosis, we reconstituted purified MOMP into liposomes. By the ELISA, MOMP-liposomes, but not plain liposomes lacking MOMP, avidly fix C3. Consistent with a dominant role for MOMP in C3 fixation, MOMP-liposomes form a C3 complex of the same apparent molecular weight as whole L. pneumophila in nonimmune serum. Opsonized radioiodinated MOMP-liposomes avidly adhere to monocytes, and adherence is dose dependent upon serum. By electron microscopy, opsonized MOMP-liposomes are efficiently phagocytized by human monocytes, and phagocytosis takes place by a conventional appearing form of phagocytosis. This study demonstrates that C3 fixes selectively to the MOMP of L. pneumophila, and that, in the presence of nonimmune serum, MOMP can mediate phagocytosis of liposomes and, potentially, phagocytosis of intact L. pneumophila by human monocytes.

Animal studies with Streptococcus pneumoniae have provided valuable models for drug development. In order to monitor long-term pneumococcal infections noninvasively in living mice, a novel gram-positive lux transposon cassette, Tn4001 luxABCDE Kmr, that allows random integration of lux genes onto the bacterial chromosome was constructed. The cassette was designed so that the luxABCDE and kanamycin resistance genes were linked to form a single promoterless operon. Bioluminescence and kanamycin resistance only occur in a bacterial cell if this operon has transposed downstream of a promoter on the bacterium's chromosome. S. pneumoniae D39 was transformed with plasmid pAUL-A Tn4001 luxABCDE Kmr, and a number of highly bioluminescent colonies were recovered. Genomic DNA from the brightest D39 strain was used to transform a number of clinical S. pneumoniae isolates, and several of these strains were tested in animal models, including a pneumococcal lung infection model. Strong bioluminescent signals were seen in the lungs of the animals containing these pneumococci, allowing the course and antibiotic treatment of the infections to be readily monitored in real time in the living animals. Recovery of the bacteria from the animals showed that the bioluminescent signal corresponded to the number of CFU and that the lux construct was highly stable even after several days in vivo. We believe that this lux transposon will greatly expand the ability to evaluate drug efficacy against gram-positive bacteria in living animals using bioluminescence.

We have developed a rapid, continuous method for real-time monitoring of biofilms, both in vitro and in a mouse infection model, through noninvasive imaging of bioluminescent bacteria colonized on Teflon catheters. Two important biofilm-forming bacterial pathogens, Staphylococcus aureus and Pseudomonas aeruginosa, were made bioluminescent by insertion of a complete lux operon. These bacteria produced significant bioluminescent signals for both in vitro studies and the development of an in vivo model, allowing effective real-time assessment of the physiological state of the biofilms. In vitro viable counts and light output were parallel and highly correlated (S. aureus r = 0.98; P. aeruginosa r = 0.99) and could be maintained for 10 days or longer, provided that growth medium was replenished every 12 h. In the murine model, subcutaneous implantation of the catheters (precolonized or postimplant infected) was well tolerated. An infecting dose of 10 (3) to 10 (5) CFU/catheter for S. aureus and P. aeruginosa resulted in a reproducible, localized infection surrounding the catheter that persisted until the termination of the experiment on day 20. Recovery of the bacteria from the catheters of infected animals showed that the bioluminescent signal corresponded to the CFU and that the lux constructs were highly stable even after many days in vivo. Since the metabolic activity of viable cells could be detected directly on the support matrix, nondestructively, and noninvasively, this method is especially appealing for the study of chronic biofilm infections and drug efficacy studies in vivo.

Scrapie is a degenerative neurologic disease in sheep and goats which can be experimentally transmitted to laboratory rodents. Considerable evidence suggests that the scrapie agent is composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc). Inactivation of scrapie prions by ionizing radiation exhibited single-hit kinetics and gave a target size of 55,000 +/- 9000 mol wt. The inactivation profile was independent of the form of the prion. Scrapie agent infectivity in brain homogenates, microsomal fractions, detergent-extracted microsomes, purified amyloid rods, and liposomes exhibited the same inactivation profile. Our data are consistent with the hypothesis that the infectious particle causing scrapie contains approximately 2 PrPSc molecules.

Prions were purified from scrapie-infected hamster brains and incubated for 24 hr at 65 degrees with 2 mM Zn2+ or 5 mM Mg2+; no loss of infectivity was observed. Bacteriophage M13, tobacco mosaic virus (TMV), potato virus X, and potato spindle tuber viroid were all inactivated by divalent metal ions under these conditions. Prions also resisted inactivation by prolonged digestions with DNase I, RNases A and T1, and micrococcal nuclease. Prions were resistant to psoralen photoadduct formation using high concentrations of psoralens; in contrast, M13 bacteriophage was inactivated by low concentrations of all these psoralens. Hydroxylamine failed to inactivate prions even after lengthy exposures to concentrations as high as 1 M, while TMV and M13 were both inactivated. Sonication of prions failed to decrease infectivity even though rod-shaped aggregates were disrupted while both M13 and TMV lost infectivity.