Biochemical and Immunochemical Analysis of Rickettsia rickettsii

Transcription

1 INFECTION AND IMMUNITY, June 1984, p /84/ $02.00/0 Copyright 1984, American Society for Microbiology Vol. 44, No. 3 Biochemical and Immunochemical Analysis of Rickettsia rickettsii Strains of Various Degrees of Virulence ROBERT L. ANACKER,l* ROBERT N. PHILIP,2 JIM C. WILLIAMS,'t ROBERT H. LIST,' AND RAYMOND E. MANN' Laboratory of Microbial Structure and Function' and Epidemiology Branch,2 Rocky Mountain Laboratories, Hamilton, Montana Received 27 December 1983/Accepted 27 February 1984 Six strains of Rickettsia rickettsii from Montana and North Carolina were examined in an effort to identify rickettsial constituents associated with virulence. Fever responses, scrotal reactions, and mortalities of male guinea pigs inoculated intraperitoneally with 1,000 PFU of rickettsial strains revealed that the two Montana patient strains (Sheila Smith and Norgaard) and one Montana strain (Sawtooth Y 2) from the wood tick, Dermacentor andersoni, could be placed in the group of highest virulence, the two North Carolina strains (Morgan and Simpson) in the group of lesser virulence, and the Montana strain (HLP) from the rabbit tick, Haemaphysalis leporispalustris, in the group of lowest virulence. The HLP strain was differentiated from the other strains by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by staining with Coomassie brilliant blue or with silver. The patient strains could not be differentiated from each other by these procedures. All of the strains apparently had three heat-modifiable proteins. Analysis of proteinase K-digested rickettsial lysates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis suggested that the strains had a complex mixture of polysaccharides. These putative polysaccharides probably were not related to the differences in virulence of the strains, since the patterns for all of the strains were identical. At least five antigens (molecular weights of 128,000, 105,000, 84,000, 30,500, and 20,500) were demonstrated by radioimmune precipitation tests employing extracts from radioiodine-labeled rickettsiae and antibodies from infected guinea pigs. With these same sera a minimum of 14 antigens was detected in these strains by an immunoblotting procedure. The apparent molecular weights of several of the HLP antigens differed from those of the presumed corresponding antigens of the other strains. The electrophoretic techniques utilized in this study were not sufficiently sensitive to demonstrate compositional differences in the patient strains which differed in their virulence for guinea pigs. A variety of evidence suggests that strains of the etiological agent of Rocky Mountain spotted fever, Rickettsia rickettsii, differ in virulence for humans and experimental animals. Before the advent of effective antibiotics, only about 5% of individuals afflicted with Rocky Mountain spotted fever in south-central Idaho succumbed, whereas the majority of unvaccinated patients in certain areas, such as the Bitter Root Valley of western Montana and Thermopolis, Wyo., died (11). Similarly, strains isolated from the American dog tick, Dermacentor variabilis, collected in the eastern United States were less virulent for guinea pigs than were some of the strains isolated from the western wood tick, Dermacentor andersoni (15). Also, strains isolated from the rabbit tick, Haemaphysalis leporispalustris, were found to be less virulent for guinea pigs than were the most virulent strains from D. andersoni (12). Recently, it was found that one strain (HLP), although serologically closely related to the D. andersoni strains of R. rickettsii and still considered to belong to the R. rickettsii species, could be distinguished from the wood tick strains by the microimmunofluorescence test (13). Thus, it seems clear that R. rickettsii can exist in forms which vary with respect to the severity of illness induced in their hosts. The factors responsible for the virulence of R. rickettsii are unknown. In an effort to identify those components associated with virulence, eastern and western strains of R. rickettsii from patients and ticks were compared by a variety * Corresponding author. t Present address: U.S. Army Medical Institute of Infectious Diseases, Fort Detrick, Frederick, MD of methods. The studies, outlined in this report, demonstrated that the strains selected differed in their virulence for guinea pigs and that differences in the composition of one of these strains could be revealed by the experimental methods employed. It has not yet been established whether these structural differences are indeed related to differences in virulence. MATERIALS AND METHODS Rickettsial strains. The six strains used in this study are described in Table 1. Four of the strains (Sheila Smith, Norgaard, Morgan, and Simpson) have been cloned. The Norgaard strain was isolated from a nymph obtained from a patient 3 days after the onset of Rocky Mountain spotted fever. Since the nymphal D. andersoni was the only tick on the patient, this isolate is presumed to be the one which caused the illness in this patient. In all cases, rickettsiae from the last one or two egg passages were used in these experiments. Cultivation of rickettsiae. The rickettsiae were grown in the yolk sacs of chicken embryos or in L cells as previously described (16). Guinea pigs. Male Hartley strain guinea pigs raised at Rocky Mountain Laboratories, Hamilton, Mont., were used in the virulence test. Purification of rickettsiae. Rickettsiae were purified by centrifugation in Renografin density gradients (17). The purified rickettsiae were suspended in 10 mm potassium phosphate buffer (ph 7.0) to a concentration of about 5 to 10 mg of protein per ml and held in 0.1- to 0.2-ml amounts at -75 C until used.

2 560 ANACKER ET AL. INFECT. IMMUN. TABLE 1. Strains of R. rickettsii studied Strain (reference) Host Geographical source Year No. of passagesa Sheila Smith (4) Human Western Montana EP/1 TC/1 GP/4 TC/2-4 EP Norgaard (13) D. andersoni Western Montana M/4 EPI4 TC/2-3 EP nymph Sawtooth Y 2 (5) D. andersoni adult Western Montana EP Morgan (13) Human North Carolina M/3 EP/S TC/2-3 EP Simpson (13) Human North Carolina M/3 EP/5 TC/2-3 EP HLP (12) H. leporispalustris Western Montana EP/9 TC/3-4 EP adult a EP, Egg passage; M, Microtus; TC, tissue culture; GP, guinea pig. Radioiodination of rickettsiae. Rickettsiae were iodinated in the presence of 1,3,4,6-tetrachloro-3a,6a-diphenylglycoluril (Iodogen; Pierce Chemical Co., Rockford, Ill.) (9). Purified rickettsiae were thawed and diluted with K-36 buffer (18) to contain 2 pug of protein per pl. A sample (0.1 ml) of the appropriate suspension was then incubated with 10 pi of carrier-free Na125I (50 puci/pi) for 5 min with occasional gentle stirring in a small glass vial coated with 10 pg of lodogen. Next, the labeled rickettsiae were washed twice with 1.0 ml of K-36 buffer (Microfuge B centrifuge; 5 min). The final pellet was resuspended in 0.5 ml of 10 mm potassium phosphate buffer (ph 7.0). SDS-PAGE. Samples were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) essentially by the procedure of Laemmli (8). The rickettsial suspensions were thawed, mixed with solubilizing buffer containing 4% SDS and 8% 2-mercaptoethanol, and heated at the desired temperature. Samples (25 pli) containing 25 pig of rickettsial protein were added to each well and electrophoresed on gels consisting of a 5% stacking gel and a 12.5% separating gel at a constant current of 40 ma. The gels were fixed in 25% isopropanol-7% acetic acid and stained either with Coomassie brilliant blue R-250 or with silver for polysaccharide or polysaccharide and protein (6). For some experiments, the rickettsial samples were solubilized at 100 C for 5 min, and then the samples (60 pi) were treated twice with proteinase K (20 pu1 containing 2.5 mg of proteinase K per ml of solubilizing solution) (Boehringer Mannheim Biochemicals, Indianapolis, Ind.) at 60 C for 1 h each time to digest proteins (6). Unlabeled high- and low-molecular-weight protein standards (Bio-Rad Laboratories, Richmond, Calif.) or a 14C_ methylated protein mixture (Amersham Corp., Arlington Heights, Ill.) were generally run along with the rickettsial preparations on the polyacrylamide gels. Virulence of rickettsiae for guinea pigs. Virulence was assessed basically as described earlier (1). A total of 1,000 PFU of each strain grown in chicken embryos and diluted in Snyder I solution (7) was inoculated intraperitoneally into guinea pigs, which weighed ca. 450 g, in groups of six. Rectal temperatures were determined daily for 12 days with a telethermometer (Yellow Springs Instrument Co., Yellow Springs, Ohio). Areas under the fever curves were determined with a model 9100A Hewlett-Packard calculator coupled to a 9125B Hewlett-Packard calculator plotter and compared by the student t test (2). Scrotal reactions of infected guinea pigs were assigned a score of 0 to 4+ on the basis of grossly observable responses, from undetectable (0) to severe with necrotic changes (4+). Radioimmune precipitation test. Radioiodinated rickettsiae (200 p.g of protein) were extracted for 30 min at 37 C with lysing buffer (0.1% SDS, 0.5% sodium deoxycholate, 0.5% Triton X-100, 0.85% NaCl, 0.02% NaN3 in 10 mm Tris [ph 7.4]) and then centrifuged at 39,000 x g and 20 C for 30 min. A sample (50 RI) of the supernatant was incubated with 50,u1 of pooled, inactivated guinea pig sera for 30 min at 37 C in microfuge tubes (1.5 ml). Then 100 1LI of 5% protein A- Sepharose CL-4B (Pharmacia Fine Chemicals, Inc., Piscataway, N.J.) in 67 mm phosphate buffer plus 0.85% NaCI (ph 7.4) was added, and incubation was continued for another 30 min at 37 C. Afterwards, the preparations were centrifuged at 9,600 x g for 3 min, and the pellets were washed three times with 1 ml of lysing buffer, with centrifugation as described above. The pellet obtained after the last wash was heated in 100 R1 of sample solubilizing solution at 100 C for 5 min. Samples (25 pi) were loaded onto polyacrylamide gels and electrophoresed in the standard way. The gels were fixed in 25% isopropanol-7% acetic acid, dried, and incubated with Kodak X-Omat AR film. Immunoblotting. Rickettsial constituents separated by SDS-PAGE were transferred to nitrocellulose paper (HAHY; Millipore Corp., Bedford, Mass.) in a Trans-Blot cell (Bio-Rad) by the procedure of Batteiger et al. (3). Unoccupied protein-binding sites on the nitrocellulose paper were blocked with 0.05% Tween 20 in 50 mm phosphate buffer (ph 7.4) containing 150 mm NaCl and 0.02% sodium azide (PBS-T), and the paper was then incubated for 2 h with 0 w cc D I- w DAYS POST CHALLENGE FIG. 1. Fever curves for male Hartley strain guinea pigs inoculated intraperitoneally with 1,000 PFU of the appropriate strain of R. rickettsii. Each point represents the mean temperature of six animals. V, Sawtooth 9 2; 0, Norgaard; *, Sheila Smith; A, Simpson; O, Morgan; Q, HLP; 0, diluent control. Points where two or more curves interact are noted by asterisks.

3 VOL. 44, : 100 dilutions of inactivated guinea pig immune sera or antiyolk sac serum in PBS-T. After the removal of unbound serum proteins by washing in phosphate-buffered saline, the nitrocellulose paper was incubated with 125I-labeled protein A in PBS-T, washed again in phosphate-buffered saline, dried, and incubated with Kodak X-Omat AR film. RESULTS Virulence of strains for guinea pigs. Average temperatures of guinea pigs inoculated intraperitoneally with 1,000 PFU of the apropriate strain are presented in Fig. 1. Analyses of the areas under the fever curves showed that the two strains from Montana patients and the Sawtooth 9 2 strain from D. andersoni caused significantly more fever than did the strains from the North Carolina patients (P < 0.01). The North Carolina strains induced more fever than did the strain isolated from the rabbit tick (P < 0.001). Differences in the areas under the fever curves for guinea pigs within the above groups were not significant (P > 0.2). The identical assignment of these strains into groups was made on the basis of the severity of the scrotal reactions. There was no grossly observable scrotal reaction in animals infected with the HLP strain. Mean maximum scores for the other Montana strains ranged from 2.1 (Sheila Smith) to 3.0 (Norgaard), whereas scores for the North Carolina strains were 0.5 (Morgan) and 1.3 (Simpson). During the 21 days the animals were held after infection, two animals inoculated with the Norgaard strain and one animal inoculated with the Sheila Smith strain died. Based on these criteria, Montana strains Sheila Smith, Norgaard, and Sawtooth 9 2 were most virulent, North Carolina strains Morgan and Simpson were less virulent, and the HLP strain was least virulent. All but the Sawtooth 9 2 strain were selected for further study r an ts* f* i}x_w 4 N _. ~~~~~~~7 FIG. 2. Coomassie brilliant blue-stained SDS-PAGE profiles of rickettsial strains solubilized at either 37C for 30 min (lanes 1 to 5) or at 1000C for 5 mmn (lanes 6 to 10). All of the strains had three heatmodifiable proteins (noted by asterisks). Major differences in patterns between HLP and other strains are marked with arrows. Lanes 1 and 6, Sheila Smith; lanes 2 and 7, Morgan; lanes 3 and 8, Norgaard; lanes 4 and 9, Simpson; lanes 5 and 10, HLP. MW, Molecular weight... ANALYSIS OF RICKETTSIA RICKETTSII STRAINS ^* UA on4it U...s <'if.,-.4 FIG. 3. Silver-stained SDS-PAGE profiles of rickettsial strains solubilized at either 37 C for 30 min (lanes 1 to 5) or at 100 C for 5 min (lanes 6 to 10). Major differences in patterns between HLP and other strains are noted with arrows. Lanes 1 and 6, Sheila Smith; lanes 2 and 7, Morgan; lanes 3 and 8, Norgaard; lanes 4 and 9, Simpson; lanes 5 and 10, HLP. MW, Molecular weight. Comparisons of strains by SDS-PAGE. Whole-cell preparations 'of strains solubilized at either 37 or 100 C were examined by SDS-PAGE, followed by staining with Coomassie brilliant blue (Fig. 2). All of the strains had three major heat-modifiable proteins (noted by asterisks in Fig. 2). Two of the heat-modifiable proteins appeared to be identical in all strains, but the intermediate heat-modifiable protein from the HLP strain migrated slightly faster than did the comparable proteins from the other strains. The SDS-PAGE profile of the HLP strain differed from that of the other strains. Some differences between HLP and the other strains are noted by arrows in Fig. 2. Next, whole-cell lysates of the test strains were subjected to SDS-PAGE and then stained with silver to determine whether this stain might reveal differences (Fig. 3). The heatmodifiable proteins and the proteins differentiating the HLP strain from the patient strains were observed as in the Coomassie brillant blue-stained gel, but silver staining did not indicate compositional differences in the Montana and North Carolina patient strains. When rickettsial lysates were treated with proteinase K, electrophoresed, and then stained with the silver stain for polysaccharides, the patterns presented in Fig. 4 were obtained. A nearly continuous series of seemingly identical bands was observed in the lysates of all of the strains of R. rickettsii. Particularly obvious were the relatively intense bands occurring in the middle and upper portions of the lanes. When this gel was reoxidized to clear the silver stain and then restained with Coomassie brilliant blue, only the two lowest bands stained (very slightly), suggesting that these substances which stained with silver but not with Coomassie brilliant blue were polysaccharide in nature. Radioimmune precipitation of R. rickettsii antigens. Pooled 21-day sera from guinea pigs inoculated intraperitoneally

4 562 ANACKER ET AL. INFECT. IMMUN. 30-_m mf._ L. La..;Vw * o.: Amw * FIG. 4. Silver-stained SDS-PAGE profiles of rickettsial strains solubilized at 100 C for 5 min (100 p.g of rickettsial protein in 60,ul) and then incubated consecutively at 60 C for 1 h with two additions of 20,ul of solubilizing solution containing 50,ug of proteinase K. Lane 1, Sheila Smith; lane 2, Morgan; lane 3, Norgaard; lane 4, Simpson; and lane 5, HLP. MW, Molecular weight. with 1,000 PFU of the patient strains (Fig. 1) were tested for their ability to precipitate rickettsial antigens in the radioimmune precipitation test (Fig. 5). Qualitatively, antibodies to at least five antigens (molecular weights of 128,000, 105,000, 84,000, 30,500, and 20,500) not observed in significant amounts in the control lanes were detected in all sera, although there appeared to be some quantitative differences in the antibodies in some sera as judged by the amount of labeled antigen bound. Immunoblotting of R. rickettsii antigens. Immunoblotting results obtained with the five strains tested against pooled sera from guinea pigs inoculated intraperitoneally with 1,000 PFU of the Sheila Smith strain or anti-yolk sac serum are presented in Fig. 6. (Results obtained with antisera to the other strains were comparable.) Antigens detected in the lysates of the rickettsial strains had apparent molecular weights ranging from ca. 15,000 to 165,000. The HLP strain had several antigens, noted by asterisks in Fig. 6, which differed slightly in mobility from the presumed corresponding antigens present in the patient strains. DISCUSSION Our results from limited virulence assays of various strains of R. rickettsii in guinea pigs corroborate the conclusion of Price that some western strains are more virulent than some eastern strains (14, 15). In our tests the two patient strains and the D. andersoni strain from western Montana produced higher and longer-lasting fever and more severe scrotal reactions than did the two patient strains from North Carolina. In addition, only guinea pigs from those groups inoculated with the western strains died during the period of observation. On the other hand, the HLP strain of R. rickettsii obtained from a rabbit tick in western Montana was the least virulent of the strains tested; it produced in guinea pigs on the average only 1 day of fever and no visible scrotal reaction. This result also is consistent with an earlier report (12). Our attempts to differentiate our selected strains of R. rickettsii were only partially successful. The HLP strain could be distinguished from the Montana and North Carolina patient strains by several of the procedures we tried. With the SDS-PAGE technique and Coomassie brilliant blue or silver staining, bands with apparent molecular weights of ca. 14,000, 18,500, and 27,000 were demonstrated in lysates of the strain of least virulence, HLP; lysates of the more virulent strains from patients did not exhibit bands with these molecular weights. In addition, several antigenic differences between the HLP strain and the patient strains were revealed by immunoblotting experiments. Possibly some of the differences in composition of these strains shown by these techniques are responsible for the slight but demonstrable differences of these strains in microimmunofluorescence tests (13). Although our methods showed differences between the HLP strain and the patient strains, none of the electrophoretic techniques was satisfactory for differentiating the western patient strains of highest virulence from the eastern strains of lesser virulence. The reason for our inability to differentiate by electrophoretic methods patient strains which differ in their virulence for experimental animals is not clear. Perhaps the differences in the virulence factors for the two groups are relatively minor, so minor that they are not reflected in differences in electrophoretic properties. Alternatively, quantitative, rather than qualitative, differences may be responsible for the virulence difference, or the virulence factors may not be demonstrable by the techniques utilized for this study. At present the significance of the differences observed by SDS-PAGE and immunoblotting between HLP and the other strains of R. rickettsii is unknown. Perhaps the "modified" constituents present in the patient strains are responsible for their greater virulence, but we cannot exclude the possibility that some or all of these differences may be completely unrelated to the kinds of interaction between parasite and host usually associated with virulence. Instead, the observed differences may represent evolutionary changes in the rickettsiae, permitting enhanced survival in their particular tick and mammalian hosts. It is unclear why disparate results were obtained in radioimmune precipitation and immunoblotting experiments, but current experiments with monoclonal antibodies to rickettsial antigens may provide at least a partial explanation (R. L. Anacker, R. H. List, R. E. Mann, S. F. Hayes, and H. C. Caldwell, manuscript in preparation). Some of our monoclonal antibodies are able to reveal carbohydrate antigens by immunoblotting but not by radioimmune precipitation tests, in which only proteins are labeled with radioiodine. Conversely, other monoclonal antibodies function in the radioimmune precipitation test but not in immunoblotting tests. Possibly the later antibodies are specific for "conformational" antigens which are destroyed by the SDS and 2-mercaptoethanol solubilization treatment required before electrophoresis. The presence of multiple silver-staining components in

5 VOL. 44, W 46- _ J ANALYSIS OF RICKETTSIA RICKETTSII STRAINS M- VẈ WW'W 46-* X _, 0 ii 13-4, 4 i0 4, A : a& * A 30 amow4 L 4,464w,WlaIt"" ;FI- * 4 FIG. 5. Precipitation of antigens labeled extrinsically with 1251 and extracted from rickettsiae with a mixture of SDS, sodium deoxycholate, and Triton X-100 by pooled sera from guinea pigs infected with strains of R. rickettsii. Antigens precipitated by sera from infected guinea pigs but not by the control guinea pig anti-yolk sac serum are noted by asterisks. Source of extracts: panel A, Sheila Smith, lanes 1 to 4 and 9; Morgan, lanes 5 to 8 and 10; panel B, Norgaard, lanes 1 to 4 and 9; Simpson, lanes 5 to 8 and 10. Sera (both panels): anti-sheila Smith, lanes 1 and 5; anti-morgan, lanes 2 and 6; anti-norgaard, lanes 3 and 7; anti-simpson, lanes 4 and 8; anti-yolk sac, lanes 9 and 10. The soluble extracts were run in panel A, lane 11 (Sheila Smith), lane 12 (Morgan); panel B, lane 11 (Norgaard); lane 12 (Simpson). MW, Molecular weight. proteinase K-digested lysates of rickettsiae suggest that rickettsiae have a complex mixture of polysaccharides. In addition, the patterns of these silver-staining rickettsial constituents are reminiscent of the ladder-like patterns characteristic of lipopolysaccharides extracted from wild-type enteric bacteria (6). Although polysaccharides have been Xm w S * FIG. 6. immunoblotting with pooled guinea pig sera from animals infected with the Sheila Smith strain of R. rickettsii. Immune serum, lanes 1 to 5; control anti-yolk sac serum, lanes 6 to 10. Lysate in lanes 1 and 6, Sheila Smith; lanes 2 and 7, Morgan; lanes 3 and 8, Norgaard; lanes 4 and 9, Simpson; lanes 5 and 10, HLP. The bands in lane 5 Detection of rickettsial antigens in whole-cell lysates by identified by asterisks are the antigens which differentiate the HLP strain from the patient strains. MW, Molecular weight. B implicated in virulence of gram-negative bacteria (10), the fact that the rickettsial strains examined in this study have presumed identical polysaccharide compositions argues against the concept of a major role for polysaccharides as a differential determiner of virulence of R. rickettsii. ACKNOWLEDGMENTS The authors are grateful to Alan Barbour, Harlan Caldwell, John Swanson, and Penny Hitchcock for their helpful advice and to Susan Smaus for her excellent secretarial assistance. LITERATURE CITED 1. Anacker, R. L., T. F. McCaul, W. Burgdorfer, and R. K. Gerloff Properties of selected rickettsiae of the spotted fever group. Infect. Immun. 27: Anacker, R. L., R. F. Smith, R. E. Mann, and M. A. Hamilton New assay of protective activity of Rocky Mountain spotted fever vaccines. J. Clin. Microbiol. 4: Batteiger, B., W. J. Newhall V, and R. B. Jones The use of Tween 20 as a blocking agent in the immunological detection of proteins transferred to nitrocellulose membranes. J. Immunol. Methods 55: Bell, E. J., and E. G. Pickens A toxic substance associated with the rickettsias of the spotted fever group. J. Immunol. 70: Burgdorfer, W Investigation of "transovarial transmission" of Rickettsia rickettsii in the wood tick, Dermacentor andersoni. Exp. Parasitol. 14: Hitchcock, P. J., and T. M. Brown Morphological heterogeneity among Salmonella lipopolysaccharide chemotypes in silver-stained polyacrylamide gels. J. Bacteriol. 154: Jackson, E. B., and J. E. Smadel Immunization against scrub typhus. II. Preparation of lyophilized living vaccine. Am. J. Hyg. 53: Laemmli, U. K Cleavage of structural proteins during the assembly of the head of bacteriophage T4. Nature (London) 227: Markwell, M. A. K., and C. F. Fox Surface-specific ^

6 564 ANACKER ET AL. INFECT. IMMUN. iodination of membrane proteins of viruses and eucaryotic cells using 1,3,4,6-tetrachloro-3a,6ot-diphenylglycoluril. Biochemistry 17: rskov, F Virulence factors of the bacterial cell surface. J. Infect. Dis. 137: Parker, R. R Rocky Mountain spotted fever: results of ten years' prophylactic vaccination. J. Infect. Dis. 57: Parker, R. R., E. G. Pickens, D. B. Lackman, E. J. Bell, and F. B. Thrailkill Isolation and characterization of Rocky Mountain spotted fever rickettsiae from the rabbit tick Haemaphysalis leporis-palustris Packard. Public Health Rep. 66: Philip, R. N., E. A. Casper, W. Burgdorfer, R. K. Gerloff, L. E. Hughes, and E. J. Bell Serologic typing of rickettsiae of the spotted fever group by microimmunofluorescence. J. Immunol. 121: Price, W. H The epidemiology of Rocky Mountain spotted fever. l. The characterization of strain virulence of Rickettsia rickettsii. Am. J. Hyg. 58: Price, W. H Variation in virulence of Rickettsia rickettsii under natural and experimental conditions, p In F. W. Hartman, F. T. Horsfall, Jr., and J. G. Kidd (ed.), Dynamics of virus and rickettsial infections. The Blakiston Co., Inc., New York. 16. Stoenner, H. G., D. B. Lackman, and E. J. Bell Factors affecting the growth of rickettsias of the spotted fever group in fertile hens' eggs. J. Infect. Dis. 110: Weiss, E., J. C. Coolbaugh, and J. C. Williams Separation of viable Rickettsia typhi from yolk sac and L cell host components by Renografin density gradient centrifugation. App. Microbiol. 30: Weiss, E., H. B. Rees, Jr., and J. R. Hayes Metabolic activity of purified suspensions of Rickettsia rickettsi. Nature (London) 213: Downloaded from on November 8, 2018 by guest