INTRODUCTION

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Gemifloxacin mesylate (SB 265805, LB20304), (R,S)-7-(3-aminomethyl-4-syn-methoxyimino-1-pyrrolidinyl)-1-cyclopro pyl-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid methanesulfonate, is a new fluoroquinolone with a broad spectrum of antimicrobial activity and enhanced activity against both aerobic and anaerobic gram-positive bacteria (3, 4, 6, 7, 9).

While premarket in vitro testing of new antimicrobial compounds is often extensive, these studies tend to focus on typical anaerobic bacterial pathogens such as the Bacteroides fragilis group, Clostridium perfringens, and Clostridium difficile (3, 6, 7). In a prior study (4), we reported the activity of gemifloxacin against typical anaerobic bacteria; it showed activity against B. fragilis and some Prevotella and Porphyromonas strains, but only limited activity against Bacteroides thetaiotaomicron, Bacteroides distasonis, and Bacteroides ovatus. Little or no data are available about the activities of these new compounds against many of the less-frequently encountered anaerobic pathogens, such as Actinomyces spp., Anaerobiospirillum spp., Porphyromonas spp., and Bilophila wadsworthia.

We determined the activity of gemifloxacin against the large variety of less-usual anaerobic species that are encountered in human clinical infections and compared its activity with that of other commonly used oral agents.

	MATERIALS AND METHODS

Top Abstract Introduction Materials and Methods Results and Discussion References

The strains used in this study were previously isolated from human clinical specimens from a variety of sources and were identified by standard criteria (1, 2, 5, 10). Almost all of these isolates were different from those strains used in our prior study (4) in those cases when the same species were used. B. fragilis ATCC 25285 and B. thetaiotaomicron ATCC 29741 were tested simultaneously as control strains. The numbers and species of isolates tested are given in Table 1.

Standard laboratory powders were supplied as follows: gemifloxacin and amoxicillin-clavulanate, SmithKline Beecham Pharmaceuticals, Philadelphia, Pa; trovafloxacin and azithromycin, Pfizer Inc., New York, N.Y.; clarithromycin, Abbott Laboratories, Abbott Park, Ill.; clindamycin, Pharmacia Upjohn Co., Kalamazoo, Mich.; metronidazole, Searle Research & Development, Skokie, Ill.; erythromycin, Eli Lilly & Co., Indianapolis, Ind.; and penicillin G, Sigma Chemical Co., St. Louis, Mo.

Frozen cultures were transferred at least twice on brucella agar supplemented with hemin, vitamin K₁, and 5% sheep blood to ensure purity and good growth. Susceptibility testing was performed according to National Committee for Clinical Laboratory Standards standards (8). Brucella agar supplemented with hemin, vitamin K₁, and 5% laked sheep blood was the basal medium used. For B. wadsworthia, the agar was also supplemented with pyruvate. 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 in varying concentrations.

The agar plates were inoculated with a Steers replicator (Craft Machine, Inc., Chester, Pa.). The inoculum used was 10⁵ CFU per spot. Control plates without antimicrobial agents were inoculated before and after each set of drug-containing plates. The MIC was defined as the lowest concentration of an agent that yielded no growth or a marked change in the appearance of growth as compared to the growth control plate.

	RESULTS AND DISCUSSION

Top Abstract Introduction Materials and Methods Results and Discussion References

Since anaerobic susceptibility testing is not routinely performed in most clinical laboratories, if it is performed at all, the clinician must rely on published studies to help guide both empirical therapy and specific therapy in situations that involve less-commonly isolated or identified anaerobes or mixed infections at other sites. Most in vitro studies of gemifloxacin against anaerobic bacteria focus their attention on common intra-abdominal pathogens such as B. fragilis and C. perfringens or lump the anaerobes into large groups without describing specific species (3, 7). Consequently, there is need for data regarding certain species often recovered from respiratory and gynecological infections as well as less-frequently identified components of mixed abdominal infections.

Our study showed (Table 1) that gemifloxacin compared favorably with trovafloxacin against the gram-positive anaerobes tested as well as against the other unusual isolates studied. Cormicon and Jones (3) studied 10 strains of peptostreptococci and found a MIC at which 90% of the isolates are inhibited (MIC₉₀) of 2 µg of gemifloxacin per ml, which is in contrast to our study which included 45 strains of peptostreptococci from four species, all of which were susceptible to 0.25 µg of gemifloxacin per ml. The reason for this discrepancy cannot be accounted for by methodological variations since both studies used brucella agar and an agar dilution method. Marco et al. (7) studied 18 strains of peptostreptococci and also found a MIC₉₀ of 2 µg of gemifloxacin per ml (range, 0.25 to 8 µg/ml).

Differences in the susceptibilities of different Clostridium species to gemifloxacin were apparent in our study, with Clostridium clostridioforme and Clostridium innocuum being relatively susceptible, while C. difficile was often resistant to gemifloxacin. In the present study, gemifloxacin had a MIC₉₀ of 1 µg/ml for the 10 Clostridium ramosum isolates studied, while in our prior study (4) the MIC₉₀ of gemifloxacin for the 14 isolates studied was 8 µg/ml. With the exception of two strains, all isolates in the two studies were different and most came from blood cultures. The apparent disparity comes from the higher MICs of gemifloxacin for 3 of 14 strains in the prior study and highlights the problem of testing small numbers of isolates of a single species. Cormicon and Jones (3) studied 10 clostridial isolates and found a maximum gemifloxacin MIC of 2 µg/ml. Marco et al. (7) reported all 19 unspeciated clostridial isolates they studied to be susceptible to 2 µg of gemifloxacin per ml.

Fusobacterium naviforme, usually isolated from oral sources, and Fusobacterium necrophorum, usually isolated from oral and gastrointestinal sources, were susceptible to 2 µg of gemifloxacin per ml, while Fusobacterium varium strains, a gastrointestinal, skin, and soft-tissue pathogen, exhibited a bimodal pattern; the other Fusobacterium species, such as Fusobacterium ulcerans, isolated from infected skin ulcers in Africa, and Fusobacterium russii, which is associated with animal bite wound infections, were relatively resistant to gemifloxacin (MIC₉₀s, 4 µg/ml).

Of note, Prevotella bivia, an important gynecological pathogen, and Prevotella melaninogenica, an oral and respiratory pathogen, were relatively resistant to gemifloxacin (MIC₉₀s, 8 µg/ml) and several of the other agents tested. In contrast, other Prevotella species such as Prevotella intermedia (MIC₉₀, 0.5 µg/ml), isolated from skin, soft-tissue, and oral infections, Prevotella oris and Prevotella buccae (MIC₉₀, 2 µg/ml), both oral and gastrointestinal isolates, and Prevotella heparinolytica (MIC₉₀, 0.5 µg/ml), a frequent animal bite wound pathogen, were relatively susceptible to gemifloxacin.

The data presented shows that there is marked variation in the susceptibility patterns of different anaerobic genera and species to trovafloxacin and gemifloxacin and that important clinical anaerobic isolates should have individual strain susceptibilities determined. It is difficult to predict susceptibility based on a grouping of several species in a less-commonly encountered or identified genus.

Overall, gemifloxacin exhibited good activity against gram-positive anaerobes, especially the four Peptostreptococcus species tested, as well as the Porphyromonas species tested.