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Many of the 4.5 million Americans bitten by animals each year require either therapeutic or prophylactic antimicrobial therapy (12). Approximately 20% of patients are allergic to penicillin, which necessitates a selection of an alternative antimicrobial agent. Macrolides, such as erythromycin, have been used to treat wounds with infections following human and animal bites, but cases of therapeutic failure have been reported (5, 6). ABT-773 is a new ketolide agent with a broad spectrum of activity, including activity against macrolide-resistant pathogens. (A. L. Barry, P. C. Fuchs, and S. D. Brown, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2144, p. 348, 1999). To evaluate the expanded activity of ABT-773, we determined its comparative activities against 268 aerobic and 148 anaerobic strains recently isolated from infected skin and soft-tissue bite wounds in humans.

All isolates were identified by standard criteria (4, 7, 8, 11). The specific sources were dog bites (n = 184), cat bites (n = 191), human bites (n = 18), and other animal bites (n = 23). The numbers and species of isolates tested are given in Table 1. Standard laboratory powders were supplied as follows: ABT-773 and clarithromycin, Abbott Laboratories, Abbott Park, Ill.; azithromycin and ampicillin sulbactam, Pfizer Inc., New York, N.Y.; roxithromycin and cefotaxime, Hoechst Marion Roussel, Romanville, France; erythromycin, Eli Lilly & Co., Indianapolis, Ind.; cefuroxime, Glaxo Welcome, Research Triangle Park, N.C.; levofloxacin, Ortho McNeil Pharmaceuticals, Raritan, N.J.; and penicillin G and tetracycline, Sigma Chemical Co., St. Louis, Mo. The antimicrobial agents were reconstituted according to the manufacturers' instructions. Serial twofold dilutions of antimicrobial agents were prepared on the day of the test and were added to the media at various concentrations.

Frozen cultures were transferred twice on Trypticase soy agar supplemented with 5% sheep blood or chocolate agar (Hardy Diagnostics, Santa Maria, Calif.) for the aerobes and brucella agar supplemented with hemin, vitamin K₁, and 5% sheep blood (Anaerobe Systems, Morgan Hill, Calif.) for the anaerobes to ensure purity and good growth. Susceptibility testing was performed according to the standards of the National Committee for Clinical Laboratory Standards (9, 10). Brucella agar supplemented with hemin, vitamin K₁, and 5% laked sheep blood was the basal medium used for anaerobic species and for Eikenella corrodens, Bergeyella zoohelcum, and Capnocytophaga species. Mueller-Hinton agar was used for staphylococci, and Mueller-Hinton agar supplemented with 5% sheep blood was used for the remainder of the organisms.

The agar plates were inoculated with a Steers replicator (Craft Machine Inc., Chester, Pa.). The inoculum used for the aerobic bacteria was 10⁴ CFU per spot, and the inoculum used for E. corrodens and the anaerobic bacteria was 10⁵ CFU per spot. Control plates without antimicrobial agents were inoculated before and after inoculation of each set of drug-containing plates. Plates with aerobic isolates were incubated at 35°C in an aerobic environment for 18 to 20 h and were then examined. E. corrodens strains were incubated in 5% CO₂ for 42 to 44 h and were then examined. Plates with anaerobes were incubated in an anaerobic chamber (Anaerobe Systems) at 35°C for 48 h and then examined.

The control strains tested included Staphylococcus aureus ATCC 29213, Enterococcus faecalis ATCC 29212, Escherichia coli ATCC 25922, Bacteroides fragilis ATCC 25285, and Bacteroides thetaiotaomicron ATCC 29741. These strains were tested simultaneously with the appropriate plates and environments. The MIC was defined as the lowest concentration of an agent that yielded no growth or a marked change in the appearance of growth compared to the growth on the control plate. The full results of the study are noted in Table 1. ABT-773 was active against Pasteurella multocida, Pasteurella septica, and other Pasteurella species at 2.0 µg/ml. The activity of ABT-773 was within 1 dilution of that of azithromycin, and ABT-773 was four- to eightfold more active than clarithromycin against Pasteurella spp. ABT-773 was very active against gram-positive aerobes including Corynebacterium spp., S. aureus, streptococci, and most coagulase-negative staphylococci. ABT-773 was also active against unusual isolates such as E. corrodens and B. zoohelcum. ABT-773 was active (MICs at which 90% of isolates are inhibited [MIC₉₀s], 0.125 µg/ml) against the anaerobes Prevotella heparinolytica, other Prevotella spp., Porphyromonas spp., Bacteroides tectum, and Peptostreptococcus spp. The MIC of ABT-773 for 11 strains of Fusobacterium nucleatum ranged from 0.5 to 8 µg/ml, with an MIC₅₀ and an MIC₉₀ of 4 and 8 µg/ml, respectively, while the MIC of ABT-773 for 12 strains of other Fusobacterium spp. ranged from 0.015 to 8 µg/ml, with an MIC₅₀ and an MIC₉₀ of 0.5 and 0.5 µg/ml, respectively. ABT-773 was generally more active than macrolides against S. aureus, E. corrodens, and anaerobes.

ABT-773 exhibited good activity against all Pasteurella species and was generally more active than the macrolides tested against S. aureus, E. corrodens, and anaerobes. This improved activity against a broad range of potential pathogens from bite wounds compared to those of macrolides suggests that ABT-773 merits further evaluation. However, isolates of F. nucleatum were relatively resistant to ABT-773 and all the macrolides, as well as the ketolides HMR 3004 and telithromycin (HMR 3647) (1).

Goldstein et al. (1) previously reported on the activities of these other ketolides, HMR 3004 and telithromycin (HMR 3647), against human and animal bite wound isolates. The activity (MIC₉₀s) of telithromycin against P. multocida subsp. multocida (1 µg/ml), P. multocida subsp. septica (1 µg/ml), E. corrodens, Neisseria weaveri, and many other isolates was similar to that of ABT-773 on a weight basis. HMR 3004 was generally slightly more active than telithromycin (HMR 3647) in that study (1), but the compound is not being developed. The activity of ABT-773 was 1 to 2 dilutions greater than that of telithromycin against such aerobes as EF-4b and Corynebacterium aquaticum and against anaerobes including B. tectum, P. heparinolytica, and Porphyromonas gingivalis. Wexler et al. (13) studied the activity of the ketolide HMR 3004 against a variety of more usual anaerobes and found that it inhibited 93% of Bacteroides spp., 98% of Prevotella spp., and all Porphyromonas spp. tested at 4 µg/ml. Tentative breakpoints for ABT-773 have been recommended: 1 µg/ml for susceptibility and 4 µg/ml for resistance for nonfastidious species (Barry et al., 39th ICAAC). A breakpoint of 4 µg/ml for susceptibility was chosen for Haemophilus influenzae, and a breakpoint of 0.5 µg/ml for susceptibility was chosen for streptococci. Which breakpoint will apply to our isolates remains to be determined.

The addition of CO₂ to the atmosphere of incubation has been shown to affect erythromycin and other macrolide susceptibility results (2, 3), primarily because of a decrease in the pH on the agar surface. Ramer et al. (N. C. Ramer, D. F. McDaniel, P. M. Johnson, V. D. Shortbridge, Y. S. Or, Z. Ma, and R. K. Flamm, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2143, p. 348, 1999) have noted a decrease in the MIC of ABT-773 of 2 to 4 dilutions with an increase in the pH from 6.8 to 8.0. Barry et al. (39th ICAAC) studied the effect of CO₂ on the in vitro activity of ABT-773 and concluded that "incubation in increased CO₂ elevated MICs of ABT [ABT-773] by approximately 30%; azithromycin and erythromycin were more profoundly effected by increased CO₂." These higher MICs lead to estimation of a lower in vitro activity of the compound and more conservative data; whether these changes are clinically relevant or whether these changes will have an effect on the breakpoints remains to be determined.

ABT-773 has an improved and broader spectrum of activity compared to those of the currently available macrolides and, consequently, merits further evaluation as a therapeutic alternative for bite wound infections.

(This study was presented in part at the International Conference on Macrolides, Azolides, Streptogramins, Ketolides and Oxazolidinones 5, Seville, Spain, 26 January 2000.)