Meropenem-Clavulanic Acid Has High In Vitro Activity against Multidrug-Resistant Mycobacterium tuberculosis

We investigated the activity of meropenem-clavulanic acid (MEM-CLA) against 68 Mycobacterium tuberculosis isolates. We included predominantly multi- and extensively drug-resistant tuberculosis (MDR/XDR-TB) isolates, since the activity of MEM-CLA for resistant isolates has previously not been studied extensively. Using Middlebrook 7H10 medium, all but four isolates showed an MIC distribution of 0.125 to 2 mg/liter for MEM-CLA, below the non-species-related breakpoint for MEM of 2 mg/liter defined by EUCAST. MEM-CLA is a potential treatment option for MDR/XDR-TB.     

Comparative In Vitro Activity Profile of Oritavancin against Recent Gram-Positive Clinical Isolates

Oritavancin activity was tested against 15,764 gram-positive isolates collected from 246 hospital centers in 25 countries between 2005 and 2008. Organisms were Staphylococcus aureus (n = 9,075), coagulase-negative staphylococci (n = 1,664), Enterococcus faecalis (n = 1,738), Enterococcus faecium (n = 819), Streptococcus pyogenes (n = 959), Streptococcus agalactiae (n = 415), group C, G, and F streptococci (n = 84), and Streptococcus pneumoniae (n = 1,010). Among the evaluated staphylococci, 56.7% were resistant to oxacillin. The vancomycin resistance rate among enterococci was 21.2%. Penicillin-resistant and -intermediate rates were 14.7% and 21.4%, respectively, among S. pneumoniae isolates. Among nonpneumococcal streptococci, 18.5% were nonsusceptible to erythromycin. Oritavancin showed substantial in vitro activity against all organisms tested, regardless of resistance profile. The maximum oritavancin MIC against all staphylococci tested (n = 10,739) was 4 μg/ml; the MIC₉₀ against S. aureus was 0.12 μg/ml. Against E. faecalis and E. faecium, oritavancin MIC₉₀s were 0.06 and 0.12, respectively. Oritavancin was active against glycopeptide-resistant enterococci, including VanA strains (n = 486), with MIC₉₀s of 0.25 and 1 μg/ml against VanA E. faecium and E. faecalis, respectively. Oritavancin showed potent activity against streptococci (n = 2,468); MIC₉₀s for the different streptococcal species were between 0.008 and 1 μg/ml. These data are consistent with previous studies with respect to resistance rates of gram-positive isolates and demonstrate the spectrum and in vitro activity of oritavancin against a wide variety of contemporary gram-positive pathogens, regardless of resistance to currently used drugs. The data provide a foundation for interpreting oritavancin activity and potential changes in susceptibility over time once oritavancin enters into clinical use.Gram-positive infections remain a clinical challenge due to increasing rates of resistance to currently available antimicrobial agents (37). Among Staphylococcus aureus strains, the prevalence of methicillin (meticillin) resistance in both hospital and community settings is increasing (19). Reports of increased numbers of S. aureus isolates with decreased susceptibility to glycopeptides have also emerged (2). Similar increases in vancomycin-resistant enterococci, penicillin-nonsusceptible pneumococci, and erythromycin-nonsusceptible streptococci have been reported (13, 27, 28, 31). Against this backdrop, the need to develop new agents is clear, with special attention to agents that can overcome existing mechanisms of resistance.Oritavancin is a semisynthetic bactericidal lipoglycopeptide under clinical development for the treatment of serious infections caused by a variety of gram-positive species, including drug-resistant enterococci, staphylococci, and streptococci (30). Like the glycopeptides vancomycin and teicoplanin, oritavancin inhibits cell wall synthesis (1, 4, 25). Additionally, oritavancin differs from vancomycin and teicoplanin by partially inhibiting RNA synthesis (4) and collapsing transmembrane electrochemical potential and increasing membrane permeability (6). These additional activities help to explain the rapid concentration-dependent bactericidal activity of oritavancin in vitro, even against isolates with reduced susceptibility to vancomycin and teicoplanin (12, 26, 30). Oritavancin''s multiple mechanisms of action are hypothesized to forestall the development of high-level resistance to this agent.The recent development of methods for oritavancin susceptibility testing indicates that oritavancin MICs reported prior to 2006 underestimate the potency of the drug because of the physicochemical property of the drug to bind to laboratory plasticware (3). A revised broth microdilution method for oritavancin, one that includes polysorbate 80 to minimize binding to labware, has been approved by the Clinical Laboratory Standards Institute (10). This method was used in the present surveillance study, and our study represents the first report of an oritavancin surveillance program using polysorbate 80 methodology.Current and ongoing surveillance initiatives seek to establish an in vitro activity profile of oritavancin against contemporary gram-positive bacterial populations, including those resistant to currently available agents that may be used to treat gram-positive infections. The goals of the present study were to research the potential utility of oritavancin against clinical pathogens and to establish baseline MIC susceptibility data prior to the availability of oritavancin in clinical settings, against which further susceptibility studies could be compared. To these ends, we collected 15,764 recent gram-positive clinical isolates between 2005 and 2008 from 246 geographically dispersed hospital centers and tested their susceptibility to oritavancin as well as to antimicrobial agents currently used in the clinical setting.(Parts of this study have been presented previously in abstract form [15, 16, 18, 34].)     

Efficacy of Nitazoxanide against Clinical Isolates of Mycobacterium tuberculosis

Nitazoxanide (NTZ) has bactericidal activity against the H37Rv laboratory strain of Mycobacterium tuberculosis with a MIC of 16 μg/ml. However, its efficacy against clinical isolates of M. tuberculosis has not been determined. We found that NTZ''s MIC against 50 clinical isolates ranged from 12 to 28 μg/ml with a median of 16 μg/ml and was unaffected by resistance to first- or second-line antituberculosis drugs or a diversity of spoligotypes.     

In Vitro Activity of Cefotaxime Against Cephalothin-Resistant Clinical Isolates

Cefotaxime is more active than six other cephalosporins against 150 cephalothin-resistant Enterobacteriaceae strains and is the only drug which is more active than ampicillin against Haemophilus. It shows a potentially useful activity against Pseudomonas.     

In Vitro Antibacterial Activity of AZD0914 against Human Mycoplasmas and Ureaplasmas

In this study, susceptibilities were determined for AZD0914, a spiropyrimidinetrione DNA gyrase inhibitor, azithromycin, doxycycline, and levofloxacin against Mycoplasma and Ureaplasma species. The activity of AZD0914 was comparable to that of levofloxacin and doxycycline against Mycoplasma genitalium and Mycoplasma pneumoniae. The AZD0914 MIC₉₀ against Mycoplasma hominis was 8-fold greater than that for levofloxacin. The AZD0914 MIC₉₀ against Ureaplasma species was 4-fold less than that for azithromycin and 8-fold less than that for levofloxacin and doxycycline.     

In Vitro Activity of Isavuconazole and Comparators against Clinical Isolates of the Mucorales Order

The in vitro activity of isavuconazole against Mucorales isolates measured by EUCAST E.Def 9.2 and CLSI M38-A2 methodologies was investigated in comparison with those of amphotericin B, posaconazole, and voriconazole. Seventy-two isolates were included: 12 of Lichtheimia corymbifera, 5 of Lichtheimia ramosa, 5 of group I and 9 of group II of Mucor circinelloides, 9 of Rhizomucor pusillus, 26 of Rhizopus microsporus, and 6 of Rhizopus oryzae. Species identification was confirmed by internal transcribed spacer (ITS) sequencing. EUCAST MICs were read on day 1 (EUCAST-d1) and day 2 (EUCAST-d2), and CLSI MICs were read on day 2 (CLSI-d2). Isavuconazole MIC₅₀s (range) (mg/liter) by EUCAST-d1, CLSI-d2, and EUCAST-d2 were 1 (0.125 to 16), 1 (0.125 to 2), and 4 (0.5 to >16), respectively, across all isolates. The similar values for comparator drugs were as follows: posaconazole, 0.25 (≤0.03 to >16), 0.25 (0.06 to >16), and 1 (0.06 to >16); amphotericin, 0.06 (≤0.03 to 0.5), 0.06 (≤0.03 to 0.25), and 0.125 (≤0.03 to 1); voriconazole, 16 (2 to >16), 8 (1 to >16), and >16 (8 to >16), respectively. Isavuconazole activity varied by species: Lichtheimia corymbifera, 1 (0.5 to 2), 1 (1 to 2), and 2 (1 to 4); Lichtheimia ramosa, 0.25 (0.125 to 0.5), 1 (0.5 to 2), and 2 (0.5 to 4); Rhizomucor pusillus, 0.5 (0.5 to 1), 1 (0.125 to 1), and 2 (1 to 2); Rhizopus microsporus, 1 (0.5 to 4), 0.5 (0.125 to 1), and 4 (1 to 8); and Rhizopus oryzae, 1 (0.5 to 4), 1 (0.125 to 2), and 4 (0.5 to 8), respectively, were more susceptible than Mucor circinelloides: group I, 8 (4 to 8), 4 (2 to 4), and 16 (2 to 16), respectively, and group II, 8 (1 to 16), 8 (1 to 8), and 16 (4 to >16), respectively. This was also observed for posaconazole. The essential agreement was best between EUCAST-d1 and CLSI-d2 (75% to 83%). Isavuconazole displayed in vitro activity against Mucorales isolates with the exception of Mucor circinelloides. The MICs were in general 1 to 3 steps higher than those for posaconazole. However, in the clinical setting this may be compensated for by the higher exposure at standard dosing.     

Tigecycline Potentiates Clarithromycin Activity against Mycobacterium avium In Vitro

The in vitro activities of clarithromycin and tigecycline alone and in combination against Mycobacterium avium were assessed. The activity of clarithromycin was time dependent, highly variable, and often resulted in clarithromycin resistance. Tigecycline showed concentration-dependent activity, and mycobacterial killing could only be achieved at high concentrations. Tigecycline enhanced clarithromycin activity against M. avium and prevented clarithromycin resistance. Whether there is clinical usefulness of tigecycline in the treatment of M. avium infections needs further study.     

Comparative In Vitro Activity Profiles of Novel Bis-Indole Antibacterials against Gram-Positive and Gram-Negative Clinical Isolates

Antimicrobial susceptibilities of 233 Gram-positive and 180 Gram-negative strains to two novel bis-indoles were evaluated. Both compounds were potent inhibitors of Gram-positive bacteria, with MIC₉₀ values of 0.004 to 0.5 μg/ml. One bis-indole, MBX 1162, exhibited potent activity against all Gram-negative strains, with MIC₉₀ values of 0.12 to 4 μg/ml, even against high-level-resistant pathogens, and compared favorably to all comparator antibiotics. The bis-indole compounds show promise for the treatment of multidrug-resistant clinical pathogens.Antibiotic resistance is reaching a crisis level because few options remain to treat certain pathogenic bacteria—mainly those causing hospital-acquired infection, but with the potential to occur in the community (8) and on the battlefield (2). Of special note are the following particularly problematic pathogens: multidrug-resistant (MDR) Acinetobacter baumannii, extended-spectrum β-lactamase (ESBL)-producing Escherichia coli and Klebsiella species, Pseudomonas aeruginosa, vancomycin-resistant enterococci (VRE [Enterococcus faecium]), methicillin-resistant Staphylococcus aureus (MRSA), and coagulase-negative staphylococci such as methicillin-resistant Staphylococcus epidermidis (MRSE).The continuing erosion of the efficacy of current antibiotics requires the discovery and development of new antibacterials that are not subject to existing mechanisms of target-based resistance. This can be accomplished by building derivatives of existing antibiotics which escape resistance mechanisms or by the development of entirely new chemical classes of antibiotics. The latter approach is preferred because preexisting target-based resistance mechanisms are unlikely to be present in the bacterial population. Here, we report MIC₉₀ values versus several problematic bacterial pathogens for a recently described series of bis-indole compounds (6, 7). MBX 1066 (Fig. ​(Fig.1),1), along with MBX 1090, 1113 and 1128 (7) were identified in a screen of the NCI repository for compounds active against Bacillus anthracis. The indole groups of MBX 1066 and MBX 1090 face each other in a symmetrical fashion (“head-to-head”), while they are positioned in a tandem arrangement (“head-to-tail”) in MBX 1113 and 1128. The head-to-head compounds were found to be more potent than the head-to-tail compounds against Gram-negative species while being nearly equipotent against Gram-positive species. MBX 1066 exhibited low cytotoxicity against HeLa cells (50% cytotoxic concentration [CC₅₀], 33 μg/ml) upon 3-day exposure, while the other compounds were slightly more cytotoxic (7). MBX 1066 displayed rapid bactericidal activity against both Gram-positive (Bacillus anthracis and B. subtilis) and Gram-negative (Yersinia pestis) bacteria (7) and demonstrated efficacy in murine models of Gram-positive (B. anthracis and S. aureus) and Gram-negative (Y. pestis) infections (7). MBX 1066 and related compounds bound serum proteins less than 25% (M. Butler, unpublished observation). Finally, experiments to isolate MBX 1066-resistant mutants by serial passage and spontaneous mutation selection were unsuccessful against both S. aureus and E. coli, although mutants resistant to a closely related compound, MBX 1090, were isolated (6, 7).Open in a separate windowFIG. 1.Structures of bis-indole compounds MBX 1066 and MBX 1162.We have conducted a structure activity relationship (SAR) program around the head-to-head compounds, typified by MBX 1066. While the molecular target(s) of these compounds is not known, the fact that they share some structural features with compounds that bind in the minor groove of duplex DNA (1) suggests that these compounds may inhibit DNA synthesis by binding to DNA. In fact, we have shown that they are potent inhibitors of DNA synthesis (7). However, their efficacy in murine models of infection, together with favorable in vitro selectivity indices, indicates that these compounds discriminate to some degree between bacterial and mammalian targets (7).Analogs of MBX 1066, particularly MBX 1162 (Fig. ​(Fig.1),1), exhibit improved Gram-negative activity while maintaining the Gram-positive potencies displayed by the parent compound. MBX 1162 is remarkably potent against antibiotic-resistant bacterial strains such as MDR A. baumannii, ESBL-producing Klebsiella pneumoniae, VRE, and MRSA, making it a promising new antibacterial agent. While MBX 1162 appeared somewhat more cytotoxic than MBX 1066 against HeLa cells (CC₅₀, 4 μg/ml) upon 3-day exposure, it retained the favorable features of MBX 1066, including bactericidal activity against both Gram-positive and Gram-negative pathogens, low serum binding (M. Butler, unpublished), and absence of susceptibility to resistance development (T. Opperman, unpublished observation), and its enhanced antibacterial activity provided selectivity index values (CC₅₀/MIC) comparable to those of MBX 1066 for Gram-negative species. In addition, MBX 1162 exhibited potent inhibition of DNA synthesis (T. Opperman and M. Butler, unpublished), suggesting its mechanism of action is similar to that of MBX 1066. Although we have observed an exceptionally broad antimicrobial profile for MBX 1066 and 1162 against single isolates of a variety of species, it is important to determine efficacy against larger groups of single species isolates, obtained from several clinical sources, looking specifically at populations of antibiotic-resistant clinical pathogens. To this end, we analyzed potencies against multiple strains of eight Gram-positive and eight Gram-negative species.Two hundred thirty-three Gram-positive strains and 180 Gram-negative aerobic strains were tested by the broth microdilution method (4) against MBX 1066 and 1162 as well as four comparator antibiotics. The comparator antibiotics were selected to be the most appropriate for each family as well as for verifying particular resistances and were thus different for Gram-positive versus Gram-negative isolates. These included linezolid (ChemPacifica), daptomycin (Cubist), vancomycin (Sigma-Aldrich), and imipenem (United States Pharmacopeia) for the Gram-positive aerobic bacteria and imipenem, tigecycline (Wyeth), gentamicin (Sigma-Aldrich), and ciprofloxacin (United States Pharmacopeia) for the Gram-negative aerobic bacteria. In addition, 18 isolates of the Gram-positive anaerobe Clostridium difficile were analyzed (3) using clindamycin (Sigma-Aldrich), imipenem, and metronidazole (Sigma-Aldrich) as comparators. The growth medium used in these studies was the CLSI-recommended Mueller-Hinton broth II (MHB II), with the exception of the streptococci (MHB II plus 2% lysed horse blood), Haemophilus influenza (HTM medium), and C. difficile (supplemented brucella broth). The quality control reference strains, S. aureus ATCC 29213, E. faecalis ATCC 29212, Streptococcus pneumoniae ATCC 49619, E. coli ATCC 25922, P. aeruginosa ATCC 27853, and Bacteroides fragilis ATCC 25285, were tested in accordance with CLSI methodology, and the results were within published ranges (5). The locations of the sources of the clinical isolates are listed in Table ​Table11.

TABLE 1.

Activities of MBX 1066 and MBX 1162 and selected comparators against Gram-positive and Gram-negative isolates

In Vitro Activity of the New Glycopeptide LY333328 against Multiply Resistant Gram-Positive Clinical Isolates

The in vitro activity of LY333328 was compared with those of vancomycin and teicoplanin against 425 gram-positive clinical isolates, including a variety of multiply resistant strains. LY333328 at ≤4 μg/ml inhibited all microorganisms tested, including methicillin- and teicoplanin-resistant staphylococci, glycopeptide-resistant enterococci, penicillin- and multiply resistant pneumococci, and viridans and beta-hemolytic streptococci.     

In Vitro Activity of Nemonoxacin,a Novel Nonfluorinated Quinolone,against 2,440 Clinical Isolates

The in vitro activity of nemonoxacin (TG-873870), a novel nonfluorinated quinolone, was tested against 2,440 clinical isolates. Nemonoxacin was at least fourfold more active than levofloxacin and moxifloxacin against most gram-positive cocci tested (shown by the following MIC₉₀/range [μg/ml] values; community-associated methicillin [meticillin]-resistant Staphylococcus aureus, 0.5/0.015 to 2; Staphylococcus epidermidis, 0.5/0.015 to 4 for methicillin-susceptible staphylococci and 2/0.12 to 2 for methicillin-resistant staphylococci; Streptococcus pneumoniae, 0.015/≤0.008 to 0.25; Enterococcus faecalis, 1/0.03 to 128). Nemonoxacin activity against gram-negative bacilli was similar to levofloxacin and moxifloxacin (MIC₉₀/range [μg/ml]; Escherichia coli, 32/≤0.015 to ≥512; Klebsiella pneumoniae, 2/≤0.015 to 128; K. oxytoca, 0.5/0.06 to 1; Proteus mirabilis, 16/0.25 to ≥512; Pseudomonas aeruginosa, 32/≤0.015 to ≥512; Acinetobacter baumannii, 1/0.12 to 16).Nemonoxacin (TG-873870) (TaiGen Biotechnology Co. Ltd.) is a novel C-8-methoxy nonfluorinated quinolone that is currently being investigated for clinical use (Fig. ​(Fig.1).1). On the basis of other fluoroquinolones with similar chemical structures, nemonoxacin is expected to have a broad spectrum of activity and reduced toxicity. C-8-methoxy substituents have been associated with an improved spectrum of activity, including increased activity against gram-positive cocci, and reduced mutant selection (1, 13). The removal of the fluorine residue may reduce the incidence of toxic side effects (2).Open in a separate windowFIG. 1.Nemonoxacin chemical structure. Me, methyl group.The activity of nemonoxacin against Mycobacterium tuberculosis and Nocardia spp. has been described previously (9, 15). Current studies with nemonoxacin indicate that it is active against a variety of gram-negative and gram-positive organisms, including antibiotic-resistant organisms like methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) (8, 12, 16). Good safety and efficacy data have been reported for animal studies (6-8). Nemonoxacin was noted to have a safety profile similar to that of levofloxacin in the treatment of community-acquired pneumonia (16).The purpose of this study was to assess the activity of nemonoxacin and other fluoroquinolones against gram-positive and gram-negative organisms obtained from Canadian hospitals as part of the CANWARD 2007 study. The most prevalent gram-positive and gram-negative pathogens collected as part of the CANWARD study (www.can-r.ca) were included in this analysis.(Abstracts of this data were presented at a joint meeting of the 48th Interscience Conference on Antimicrobial Agents and Chemotherapy and the 46th Infectious Diseases Society of America, Washington, DC, 2008, abstr. C1-1957 and F1-2057.)Clinical isolates were collected as part of CANWARD, an ongoing national surveillance system designed to assess pathogen prevalence and antibiotic resistance from respiratory, skin and soft tissue, urinary, and bacteremic infections in Canadian hospitals (18). Twelve sentinel hospitals from across Canada submitted clinical isolates from blood, respiratory, urine, and wound/intravenous site specimens from patients affiliated with hospital clinics, emergency rooms, medical/surgical wards, and intensive care units. All organisms were deemed clinically significant and identified at the originating center using local site criteria.The organisms evaluated in this study included 374 methicillin-susceptible S. aureus (MSSA) isolates, 127 MRSA (25 community-associated MRSA [CA-MRSA] isolates and 99 hospital-associated MRSA [HA-MRSA] isolates), 43 methicillin-susceptible Staphylococcus epidermidis (MSSE) isolates, 9 methicillin-resistant S. epidermidis (MRSE) isolates, 655 Streptococcus pneumoniae isolates (including 32 penicillin-resistant isolates), 81 Enterococcus faecalis isolates, 38 Enterococcus faecium isolates, 599 Escherichia coli isolates,199 Klebsiella pneumoniae isolates, 32 Klebsiella oxytoca isolates, 72 Enterobacter cloacae isolates, 33 Proteus mirabilis isolates, 137 Pseudomonas aeruginosa isolates, 26 Stenotrophomonas maltophilia isolates, and 15 Acinetobacter baumannii isolates.In vitro susceptibilities were determined by the broth microdilution method in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines (3). The fluoroquinolones tested in this study included ciprofloxacin, levofloxacin, moxifloxacin, and nemonoxacin. Custom-designed 96-well microdilution panels containing doubling dilutions of the antimicrobial agents in cation-adjusted Mueller-Hinton broth with 5% lysed horse blood were produced to determine the MICs. Quality control of the broth microdilution panels was conducted using appropriate CLSI organisms and MIC ranges (3). Quality control for nemonoxacin was performed using the following ATCC quality control organisms with moxifloxacin ranges: S. pneumoniae 49619, S. aureus 29213, E. faecalis 29212, E. coli 25922, and P. aeruginosa 27853. MICs were interpreted on the basis of CLSI breakpoints (4).MRSA were assigned to the Canadian epidemic strain types (CMRSA-1 to CMRSA-10) (14) by pulsed-field gel electrophoresis (PFGE) or staphylococcal protein A (spa) typing (5, 10) as previously described (11). CA-MRSA and HA-MRSA were differentiated genotypically (by PFGE pattern), as epidemiologic data were unavailable (11). CMRSA-7 (USA400) and CMRSA-10 (USA300) isolates were identified as CA-MRSA, while organisms with all other CMRSA patterns were considered HA-MRSA. Isolates that were not assigned to one of the epidemic strains by PFGE or spa typing were labeled “unique” and were not considered HA-MRSA or CA-MRSA (11).In 2007, 7,881 clinical isolates were collected as part of CANWARD (18). The in vitro activity of nemonoxacin was tested against 2,440 gram-positive cocci and gram-negative bacilli.Table ​Table11 presents the MIC distributions and MIC₉₀s for nemonoxacin and other fluoroquinolones against gram-positive cocci. Nemonoxacin displayed greater activity than the other fluoroquinolones tested against the MSSA (MIC₉₀, 0.12 μg/ml). In addition, nemonoxacin displayed slightly greater activity than the other fluoroquinolones tested against the MRSA (nemonoxacin, 4 μg/ml; ciprofloxacin, ≥16 μg/ml; levofloxacin, ≥32 μg/ml; moxifloxacin, 8 μg/ml [MIC₅₀s shown]). The activity of all of the fluoroquinolones was reduced against MRSA, but nemonoxacin was the least affected (Table ​(Table1).1). The higher nemonoxacin MICs of ≥4 μg/ml were noted only among the HA-MRSA that displayed high levels of resistance to levofloxacin and moxifloxacin. By PFGE, the majority of these isolates were genetically unrelated to other strains in the study (40%) or were in small clusters of two or three isolates (28%) (11). Interestingly, nemonoxacin remained highly active against CA-MRSA (MIC₅₀, 0.25 μg/ml; MIC₉₀, 0.5 μg/ml). The activity of nemonoxacin was significantly greater against S. aureus with levofloxacin MICs of <2 μg/ml (MIC₉₀, 0.06 μg/ml) than isolates with levofloxacin MICs of ≥2 μg/ml (MIC₉₀, 16 μg/ml). Nemonoxacin was at least eightfold more active than the other fluoroquinolones against S. epidermidis (MSSE and MRSE). The activity of nemonoxacin against S. pneumoniae (MIC₉₀, 0.015 μg/ml), including penicillin-resistant strains (MIC₉₀, 0.03 μg/ml), was the greatest of the fluoroquinolones tested. Similarly, nemonoxacin was the most active fluoroquinolone against E. faecalis. Nemonoxacin was more active against E. faecalis (MIC₉₀, 1 μg/ml) than E. faecium (MIC₉₀, 128 μg/ml).

TABLE 1.

In vitro activity of nemonoxacin and other fluoroquinolones against gram-positive cocci

In Vitro Activity of Ceftaroline against 623 Diverse Strains of Anaerobic Bacteria

The in vitro activities of ceftaroline, a novel, parenteral, broad-spectrum cephalosporin, and four comparator antimicrobials were determined against anaerobic bacteria. Against Gram-positive strains, the activity of ceftaroline was similar to that of amoxicillin-clavulanate and four to eight times greater than that of ceftriaxone. Against Gram-negative organisms, ceftaroline showed good activity against β-lactamase-negative strains but not against the members of the Bacteroides fragilis group. Ceftaroline showed potent activity against a broad spectrum of anaerobes encountered in respiratory, skin, and soft tissue infections.With the continuing emergence of novel patterns of resistance to commonly used antimicrobial agents, alternative therapies are needed to treat serious infections. Ceftaroline is a novel, parenteral, broad-spectrum cephalosporin that exhibits bactericidal activity against Gram-positive organisms, such as methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-intermediate S. aureus, and multidrug-resistant Streptococcus pneumoniae (MDRSP) strains, as well as common Gram-negative pathogens (8, 12, 14, 16, 18-22). Ceftaroline is currently in development for the treatment of complicated skin and skin structure infections and community-acquired pneumonia.Anaerobic bacteria are common pathogens in a variety of pleuropulmonary infections, including aspiration pneumonia, lung abscesses, and empyema (1, 3, 6, 15). However, many laboratories do not culture for anaerobes (9), diminishing awareness of the role of anaerobes in these infections. The main anaerobic pathogens isolated from these infections include Prevotella melaninogenica (∼25%), Prevotella intermedia (∼30%), Fusobacterium species (∼39%), Gram-positive cocci (∼30%), and Veillonella species (∼35%) (7). Cephalosporins such as cefoxitin have been used for the therapy of aspiration pneumonias. Although cefoxitin is active against most respiratory anaerobes, it has poor activity against the newer resistant strains of members of the family Enterobacteriaceae and MRSA. The activity of ceftaroline against Gram-positive anaerobes is similar to that of amoxicillin-clavulanate, and non-β-lactamase-producing Gram-negative strains generally have low ceftaroline MICs (present study), suggesting that ceftaroline might have an adequate spectrum of activity for therapy for some cases of aspiration pneumonia.To investigate the broader potential of ceftaroline, we compared its in vitro activity against 623 unique clinical isolates of anaerobic bacteria representing 5 Gram-negative bacterial genera and 17 Gram-positive bacterial genera to the activities of ceftriaxone, metronidazole, clindamycin, and amoxicillin-clavulanate.The reference agar dilution procedure described in CLSI document M11-A7 was used (5). The organisms were recovered from a variety of clinical specimens and were stored at −70°C in 20% skim milk. Identification was accomplished by standard phenotypic methods or by partial 16S rRNA gene sequencing for strains that could not be identified phenotypically (13, 17). Quality control strains Bacteroides fragilis ATCC 25285, Clostridium difficile ATCC 700057, and Staphylococcus aureus ATCC 29213 were included on each day of testing.The antimicrobial agents were obtained as follows: ceftaroline was from Forest Laboratories, Inc. (New York, NY); ceftriaxone, vancomycin, and metronidazole were from Sigma-Aldrich, Inc. (St. Louis, MO); and amoxicillin and clavulanate were from GlaxoSmithKline (Research Triangle Park, NC). The agar dilution plates were prepared on the day of testing.The strains were taken from the freezer and transferred twice to ensure purity and good growth. Cell paste from 48-h cultures was suspended in brucella broth to achieve the turbidity of a 0.5 McFarland standard, and the mixture was applied to plates with a Steers replicator to deliver approximately 10⁵ CFU/spot. The plates were incubated for 44 h at 37°C in an anaerobic chamber. The MIC was the lowest concentration that completely inhibited growth or that resulted in a marked reduction in growth compared with that for the drug-free growth control (5).A summary showing the MIC range, MIC₅₀, MIC₉₀, and percent susceptibility is presented in Table ​Table1.1. The cumulative ceftaroline MIC distributions for all groups of strains are displayed in Table ​Table22.

TABLE 1.

Summary of ceftaroline and comparator agent MICs, by species or group

In Vitro Activity of Delafloxacin against Clinical Neisseria gonorrhoeae Isolates and Selection of Gonococcal Delafloxacin Resistance

We evaluated the in vitro activity of delafloxacin against a panel of 117 Neisseria gonorrhoeae strains, including 110 clinical isolates collected from 2012 to 2015 and seven reference strains, compared with the activities of seven antimicrobials currently or previously recommended for treatment of gonorrhea. We examined the potential for delafloxacin to select for resistant mutants in ciprofloxacin-susceptible and ciprofloxacin-resistant N. gonorrhoeae. We characterized mutations in the gyrA, gyrB, parC, and parE genes and the multidrug-resistant efflux pumps (MtrC-MtrD-MtrE and NorM) by PCR and sequencing and by whole-genome sequencing. The MIC₅₀, MIC₉₀, and MIC ranges of delafloxacin were 0.06 μg/ml, 0.125 μg/ml, and ≤0.001 to 0.25 μg/ml, respectively. The frequency of spontaneous mutation ranged from 10⁻⁷ to <10⁻⁹. The multistep delafloxacin resistance selection of 30 daily passages resulted in stable resistant mutants. There was no obvious cross-resistance to nonfluoroquinolone comparator antimicrobials. A mutant with reduced susceptibility to ciprofloxacin (MIC, 0.25 μg/ml) obtained from the ciprofloxacin-susceptible parental strain had a novel Ser91Tyr alteration in the gyrA gene. We also identified new mutations in the gyrA and/or parC and parE genes and the multidrug-resistant efflux pumps (MtrC-MtrD-MtrE and NorM) of two mutant strains with elevated delafloxacin MICs of 1 μg/ml. Although delafloxacin exhibited potent in vitro activity against N. gonorrhoeae isolates and reference strains with diverse antimicrobial resistance profiles and demonstrated a low tendency to select for spontaneous mutants, it is important to establish the correlation between these excellent in vitro data and treatment outcomes through appropriate randomized controlled clinical trials.     

In Vitro Synergistic Activity of Antimicrobial Agents in Combination against Clinical Isolates of Colistin-Resistant Acinetobacter baumannii

Emerging resistance to colistin in clinical Acinetobacter baumannii isolates is of growing concern. Since current treatment options for these strains are extremely limited, we investigated the in vitro activities of various antimicrobial combinations against colistin-resistant A. baumannii. Nine clinical isolates (8 from bacteremia cases and 1 from a pneumonia case) of colistin-resistant A. baumannii were collected in Asan Medical Center, Seoul, South Korea, between January 2010 and December 2012. To screen for potential synergistic effects, multiple combinations of two antimicrobials among 12 commercially available agents were tested using the multiple-combination bactericidal test (MCBT). Checkerboard tests were performed to validate these results. Among the 9 colistin-resistant strains, 6 were pandrug resistant and 3 were extensively drug resistant. With MCBT, the most effective combinations were colistin-rifampin and colistin-teicoplanin; both combinations showed synergistic effect against 8 of 9 strains. Colistin-aztreonam, colistin-meropenem, and colistin-vancomycin combinations showed synergy against seven strains. Colistin was the most common constituent of antimicrobial combinations that were active against colistin-resistant A. baumannii. Checkerboard tests were then conducted in colistin-based combinations. Notably, colistin-rifampin showed synergism against all nine strains (100%). Both colistin-vancomycin and colistin-teicoplanin showed either synergy or partial synergy. Colistin combined with another β-lactam agent (aztreonam, ceftazidime, or meropenem) showed a relatively moderate effect. Colistin combined with ampicillin-sulbactam, tigecycline, amikacin, azithromycin, or trimethoprim-sulfamethoxazole demonstrated limited synergism. Using MCBT and checkerboard tests, we found that only colistin-based combinations, particularly those with rifampin, glycopeptides, or β-lactams, may confer therapeutic benefits against colistin-resistant A. baumannii.     

In Vitro and In Vivo Activities of Moxifloxacin and Clinafloxacin against Mycobacterium tuberculosis

On 10% oleic acid–albumin–dextrose–catalase-enriched 7H11 agar medium, the MIC at which 90% of the isolates are inhibited for 20 strains of Mycobacterium tuberculosis was 0.5 μg of sparfloxacin (SPFX) or moxifloxacin (MXFX) per ml and 1.0 μg of clinafloxacin (CNFX) per ml, indicating that the in vitro activities of SPFX and MXFX were virtually identical and were slightly greater than that of CNFX. However, the in vivo activities of these drugs in a murine tuberculosis model differed considerably. Female Swiss mice were infected intravenously with 6.2 × 10⁶ CFU of the H37Rv strain and treated for 4 weeks, beginning the next day after infection, with isoniazid (INH) serving as the positive control. By the criteria of 30-day survival rate, spleen weight, gross lung lesion, and mean number of CFU in the spleen, treatment with CNFX at up to 100 mg/kg of body weight six times weekly displayed no measurable effect against M. tuberculosis, whereas both SPFX and MXFX were effective; administration six times weekly of either of the latter two drugs demonstrated dosage-dependent bactericidal effects, as measured by enumeration of CFU in the spleens, and MXFX appeared more bactericidal than the same dosage of SPFX. Of the three fluoroquinolones, only MXFX at 100 mg/kg six times weekly appeared as bactericidal as INH at 25 mg/kg six times weekly. Thus, MXFX may be an important component of the newer combined regimens for treatment of tuberculosis.     

In Vitro Antimicrobial Activity of Cinoxacin Against 2,968 Clinical Bacterial Isolates

Cinoxacin demonstrated effective in vitro antimicrobial activity against the Enterobacteriaceae, but negligible activity against Pseudomonas aeruginosa and gram-positive cocci. The activity of cinoxacin was slightly greater than that of nalidixic acid.