In Vitro Activities of Telithromycin (HMR 3647) against Rickettsia rickettsii, Rickettsia conorii, Rickettsia africae, Rickettsia typhi, Rickettsia prowazekii, Coxiella burnetii, Bartonella henselae, Bartonella quintana, Bartonella bacilliformis, and Ehrlichia chaffeensis

Abstract

In vitro activities of telithromycin compared to those of erythromycin against Rickettsia spp., Bartonella spp., Coxiella burnetii, and Ehrlichia chaffeensis were determined. Telithromycin was more active than erythromycin against Rickettsia, Bartonella, and Coxiella burnetii, with MICs of 0.5 μg/ml, 0.003 to 0.015 μg/ml, and 1 μg/ml, respectively, but was inactive against Ehrlichia chaffeensis.

Telithromycin belongs to a new class of erythromycin derivatives, the ketolides, that have activity against a wide variety of bacterial species (1, 20), including intracellular bacteria such as Chlamydia pneumoniae and Legionella spp. (15, 18). In this study we have evaluated the antimicrobial activities of telithromycin against various rickettsial pathogens, including Rickettsia spp., Ehrlichia spp., and Bartonella spp., which belong to the α group of Proteobacteria, and Coxiella burnetii, which belongs to the γ group of Proteobacteria (17). Tetracyclines are the mainstay antibiotics to treat patients with rickettsiosis, acute Q fever, and ehrlichiosis, with fluoroquinolones being alternatives for the two former diseases. However, both agents are contraindicated in children and pregnant women. Erythromycin is the first-line antibiotic therapy for bacillary angiomatosis due to Bartonella quintana or Bartonella henselae, with tetracyclines as a possible alternative. Ketolides may represent a safe alternative for treatment of all of these diseases, especially during childhood and pregnancy.

The antibiotics used in this study were doxycycline, erythromycin, and telithromycin (Hoescht Marion Roussel, Romainville, France). The strains tested are specified in Table 1 and Fig. 1. The bacteriostatic activities of telithromycin and erythromycin against Rickettsia spp. were evaluated by a dye uptake assay and a plaque assay in Vero cells, as previously described (13). For Bartonella spp. a modified version of the antibiotic agar dilution method of the National Committee for Clinical Laboratory Standards was used for determination of MICs (9, 19). The bactericidal activities of antibiotics against Bartonella spp. were determined in a broth assay, using liquid Schaedler medium (Bio Merieux, Lyon, France) supplemented with 10% horse blood. One milliliter of this medium with or without antibiotics was placed in a glass tube filled with 10⁶ CFU of bacteria per ml for 24 h. Tenfold serial dilutions of each tube culture were plated onto blood-agar and reincubated for 5 to 6 days before enumeration of colonies. The minimal bactericidal concentration was defined as the minimal concentration of the antibiotic inducing a 99.9% decrease in bacterial inocula.

TABLE 1.

MICs of telithromycin and erythromycin against Rickettsia and Bartonella species and against C. burnetii

Organism or strain(s)a	MIC (μg/ml)
	Telithromycin	Erythromycin
Rickettsia conorii Moroccan (ATCC VR-141)	0.5	8
Rickettsia conorii serotype Israeli	0.5	8
Rickettsia rickettsii Sheila Smith (ATCC VR-149)	0.5–1b	8
Rickettsia africae	0.5	8
Rickettsia typhi Wilmington (ATCC VR-144)	0.5	0.5–1b
Rickettsia prowazekii Breinl (ATCC VR-142)	0.5	0.125–0.25b
Bartonella henselae Houston 1 (ATCC 49882) and UR.LI.E.9	0.003	0.06–0.25
Bartonella quintana Oklahoma (ATCC 49793) and UR.P.IE.H2	0.006	0.006–0.12
Bartonella bacilliformis KC583 (ATCC 35685), KC584 (ATCC 35686), Acochaca 812, and Monzon 269	0.015	0.06
Coxiella burnetii
Nine Mile	1	8
Q212	1	8
Priscilla	1	8

^aAlternative strain designations appear in parentheses. 

^bFor Rickettsia spp., MICs obtained with the dye uptake assay and the plaque assay are specified only when different. 

FIG. 1.

Susceptibilities of E. chaffeensis to doxycycline and telithromycin, as determined by a microplate colorimetric assay (4). Only results obtained with 8 μg of telithromycin per ml or 1 μg of doxycycline per ml are presented.

The bacteriostatic activity of telithromycin against C. burnetii was determined by the shell-vial assay with HEL cells (14), whereas the bactericidal activity was evaluated by a quantitative model previously described (8). Antibiotic susceptibility for Ehrlichia was determined by a microplate assay using DH82 cells, as previously described (4). Antibiotics were added to the culture medium, and the MIC was defined as the lowest antibiotic concentration capable of preventing growth during the 72 h of antibiotic exposure. Bactericidal effect was defined as lack of bacterial regrowth within the 6 days following antibiotic removal. Telithromycin was tested at twofold serial concentrations ranging from 0.25 to 8 μg/ml, and doxycycline was tested at concentrations ranging from 0.25 to 1 μg/ml.

The results of the testing of the in vitro activities of telithromycin against different bacterial strains are summarized in Table 1. Growth of all strains of Rickettsia spp., C. burnetii, and Bartonella spp. was inhibited by telithromycin at concentrations of ≤0.5 μg/ml. Telithromycin was more active than erythromycin against all these bacteria, except for Rickettsia typhi and Rickettsia prowazekii, which were inhibited by similar concentrations of erythromycin or telithromycin. Telithromycin did not display any bactericidal activity at concentrations of ≤4 μg/ml against Rickettsia spp. or Bartonella spp. The bactericidal activity of telithromycin was evaluated on the C. burnetii Nine Mile and Q212 strains. The primary inoculum for the Nine Mile strain was 3 × 10⁵ IU/ml, and the primary inoculum for the Q212 strain was 6 × 10⁶ IU/ml. Residual concentrations of viable bacteria, after 24 h of incubation with telithromycin at 4 μg/ml, were 10⁵ and 2 × 10⁶ IU/ml for the Nine Mile and Q212 strains, respectively. No significant differences in bacterial titers, as determined by Student's t test at the 95% confidence limit, were noted between untreated controls and cultures receiving 4 μg per of telithromycin per ml. Telithromycin at concentrations of 0.25 to 8 μg/ml was ineffective against Ehrlichia chaffeensis, as the infection increased in telithromycin-containing cultures in the same manner as in untreated controls (Fig. 1).

We found that telithromycin was more active than erythromycin against Rickettsia sp., C. burnetii, and Bartonella sp. strains, whereas it was not effective against E. chaffeensis. These results are in accordance with previous reports on the in vitro activities of various macrolide compounds against these pathogens (6, 7, 10, 16). As for rickettsiosis, tetracyclines remain the antibiotics of choice, with fluoroquinolones being an alternative (11). Nevertheless, adverse effects of these compounds either limit or contraindicate their use, especially during pregnancy and childhood. A wide variability in the susceptibilities of rickettsiae to different macrolide compounds has been described (6, 10, 16). We have previously shown that josamycin is the most effective macrolide compound in vitro against rickettsiae (16), and successful treatment with josamycin in children and pregnant women suffering from Mediterranean spotted fever (2, 12) has been reported; thus, telithromycin may represent a safe alternative in these situations.

Erythromycin is not currently considered a safe alternative in acute Q fever treatment owing to the fact that therapeutic failures with this antibiotic, probably due to heterogeneous susceptibility to erythromycin among different C. burnetii strains, have been reported (10). The lower MICs of telithromycin that have been found may correspond to a better in vivo activity.

Infections due to E. chaffeensis are treated with tetracyclines. In vitro, rifampin is also effective (4), but clinical trials using this drug are lacking. Like erythromycin, telithromycin was not effective against E. chaffeensis and should not be evaluated in human ehrlichiosis.

Erythromycin is the antibiotic therapy currently recommended for immunocompromised patients with bacillary angiomatosis (8), and azithromycin has been used successfully to treat patients with local suppuration or systemic manifestations of cat scratch disease (3, 5).

In conclusion, the new ketolide compound telithromycin is highly effective against Rickettsia and Bartonella species and C. burnetii, and clinical trials using this drug, at least for rickettsiosis, acute Q fever, and bacillary angiomatosis, are warranted.