Antibacterial Activity of Moxifloxacin (Bay 12-8039) against Aerobic Clinical Isolates, and Provisional Criteria for Disk Susceptibility Tests

 Moxifloxacin (Bay 12–8039), ciprofloxacin, and levofloxacin were compared in vitro against 1074 clinical isolates gathered from different medical centers throughout North America during the winter months of 1997. Moxifloxacin E tests and broth microdilution tests gave comparable results. Moxifloxacin was particularly potent against respiratory pathogens such as Haemophilus influenzae and Streptococcus pneumoniae. Ciprofloxacin was the most potent study drug against the family of Enterobacteriaceae and Pseudomonas spp. For tests of 5 μg moxifloxacin disks, zone size criteria of ≤17 mm for resistant (MIC ≥8 μg/ml) and ≥21 mm for susceptible (MIC ≤2 μg/ml) are provisionally proposed for use while clinical trials are under way.     

Comparative In Vitro Activity of Quinolones Against Stenotrophomonas maltophilia

 The susceptibility of 109 Stenotrophomonas maltophilia isolates, all characterized by pulsed-field gel electrophoresis, to nine quinolones was studied. Grepafloxacin, trovafloxacin, and moxifloxacin displayed similar intrinsic activities (MIC90, 0.5 μg/ml), which were lower than those of ofloxacin and ciprofloxacin (MIC90, 4 μg/ml), norfloxacin (MIC90, 64 μg/ml), and nalidixic acid (MIC90, 32 μg/ml). Nalidixic acid was generally one- to twofold dilutions more active than norfloxacin. According to the criteria of the National Committee for Clinical Laboratory Standards (NCCLS), the percentage of isolates susceptible to ciprofloxacin (breakpoint ≤1 μg/ml) was 76.1%. Using the NCCLS breakpoint for comparative purposes, the percentage of isolates susceptible to grepafloxacin, moxifloxacin, and trovafloxacin was 95.4, 96.4, and 96.4%, respectively. These results indicate that new quinolones may potentially be used for the management of Stenotrophomonas maltophilia infections.     

Ciprofloxacin- and Methicillin-Resistant Staphylococcus aureus Susceptible to Moxifloxacin, Levofloxacin, Teicoplanin, Vancomycin and Linezolid

 In order to determine the comparative efficacy of vancomycin, teicoplanin, levofloxacin, moxifloxacin, and linezolid against methicillin- and ciprofloxacin-resistant Staphylococcus aureus, each agent was tested against 65 genetically different strains using the microbroth dilution method. All of the isolates were typed using the enterobacterial repetitive intergenic consensus polymerase chain reaction to exclude multiple isolates of epidemic clones. Susceptibility testing revealed that all of the isolates were susceptible to vancomycin and teicoplanin. Linezolid exhibited minimum inhibitory concentration (MIC) levels ranging from 1 to 4 mg/l (MIC90, 4 mg/l). The MICs of moxifloxacin and levofloxacin ranged from 0.01 to 8 mg/l (MIC90, 8 mg/l) and 0.25 to 32 mg/l (MIC90, 16 mg/l), respectively. Thus, linezolid is active against methicillin- and ciprofloxacin-resistant Staphylococcus aureus, whereas moxifloxacin may need to be administered at a dose higher than recommended in order to successfully treat serious infections.     

New Colorimetric Microdilution Method for In Vitro Susceptibility Testing of Borrelia burgdorferi Against Antimicrobial Substances

 A newly developed colorimetric microdilution method was used to analyze the activity of 12 antimicrobial agents against nine Borrelia burgdorferi isolates, including all three genospecies pathogenic for humans. In addition, in vitro antimicrobial resistance patterns of Borrelia valaisiana and Borrelia bissettii tick isolates were investigated. The applied test system is based upon color changes that occur in the presence of phenol red and result from the accumulation of nonvolatile acid produced by actively metabolizing spirochetes. After 72 h of incubation, minimal inhibitory concentrations (MICs) were determined from the decrease of absorbance by software-assisted calculation of growth curves. MIC values were lowest for azlocillin (MIC, ≤0.125 μg/ml), ceftriaxone (MIC range, ≤0.015–0.06 μg/ml), and azithromycin (MIC range, ≤0.015–0.06 μg/ml). Whereas tobramycin (MIC range, 8–64 μg/ml) exhibited little activity, spectinomycin (MIC range, 0.25–2 μg/ml) showed in vitro antimicrobial activity against Borrelia burgdorferi. The MICs of penicillin G for Borrelia afzelii isolates were ten times higher than those for Borrelia burgdorferi, Borrelia valaisiana, and Borrelia bissettii isolates (P<0.05) and 100 times higher than those for isolates belonging to the genospecies Borrelia garinii (P<0.05). Further significant differences with respect to the MIC values of the other antimicrobial agents tested were not noted. The colorimetric microdilution method offered the advantages of reliability, reproducibility, and convenience and could handle large numbers of isolates and antibiotics.     

In vitro activity of fluoroquinolones against clinical isolates of Nocardia identified by partial 16S rRNA sequencing

Fluoroquinolones have several properties that make them potentially attractive candidates for the treatment of Nocardia infections, but information regarding their in vitro activity is limited. Minimum inhibitory concentrations (MIC) of five fluoroquinolones and other antimicrobials were determined by the reference broth dilution and E-test methods for 33 consecutive clinical isolates of Nocardia speciated by 16S rRNA gene sequences. The isolates included: Nocardia cyriacigeorgica (n = 6), N. nova (n = 8), N. farcinica (n = 8), N. brasiliensis (n = 3), N. asteroides (n = 4), and N. veterana (n = 4). MIC₅₀/MIC₉₀ results for ciprofloxacin, gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin by broth dilution were 32/32, 2/4, 1/4, 32/32, and 2/2 μg/ml, respectively. The MICs by broth dilution and E-test were within a two-fold doubling dilution for 94%, 97%, 97%, 100%, and 100% of isolates for ciprofloxacin, gatifloxacin, gemifloxacin, levofloxacin, and moxifloxacin, respectively. For ciprofloxacin, the E-test results showed either complete categorical agreement or minor error compared to the reference broth dilution method for 97% (32/33) of the isolates. For other fluoroquinolones, using Streptococcus pneumoniae breakpoints, 94% (124/132) of MIC results by E-test showed either complete agreement or minor error compared to the reference broth dilution method. Fluoroquinolones show variable in vitro activity against clinical isolates of Nocardia spp., and MICs determined by the E-test show reasonable agreement with those determined by the reference broth dilution method.     

Vancomycin MIC creep in MRSA blood culture isolates from Germany: a regional problem?

The aim of this study was to assess the vancomycin MIC distribution for MRSA blood culture isolates over a period of six years in Germany. The study examined 287 MRSA isolates from blood cultures collected at several hospitals in two German cities between 2004 and 2009. The vancomycin MIC was determined by Etest. Genotypic features of the MRSA strains with vancomycin MIC ≥ 1 mg/L were determined by semiautomated repetitive-sequence-based polymerase chain reaction. The range of vancomycin MIC as determined by Etest was 0.25 to 2.0 mg/L. The geometric mean MIC increased by 1.34-fold in city A over the study period (p < 0.05), but there was no meaningful change in city B (a 1.09-fold increase, p > 0.05). Furthermore, in city A a shift in vancomycin MICs occurred as an increase in the percentage of isolates with MIC ≥ 1 mg/L from period one (2004–2006) to period two (2007–2009) (p < 0.0001). Typing results showed that in city A a single clone was predominant (55% of the creep isolates). In this study, the creep phenomenon seems to be a regional problem. We suggest that all hospitals should monitor their local status of elevated vancomycin MICs in invasive MRSA isolates.     

Flucytosine Primary Resistance in Candida Species and Cryptococcus neoformans

 The in vitro activity of flucytosine (5FC) against 1,140 clinical isolates of Candida spp. and Cryptococcus neoformans was evaluated and compared with the activity of amphotericin B, fluconazole and itraconazole. Overall, 87.72% (1,000/1,140) of yeasts were susceptible to 5FC. This agent showed less potent in vitro activity against Candida glabrata, Candida krusei, Candida guilliermondii and Cryptococcus neoformans (MIC90s, 8–16 μg/ml) and intermediate activity or resistance to 6.5% of Candida albicans, 5.1% of Candida tropicalis and 0.8% of Candida parapsilosis strains. Amphotericin B showed potent activity against isolates with an MIC of 5FC≥8 μg/ml. A total of 112 of 140 strains that were 5FC-intermediate or -resistant showed decreased susceptibility to azoles (P<0.01).     

Comparative in Vitro Activity of Voriconazole and Itraconazole Against Fluconazole-Susceptible and Fluconazole-Resistant Clinical Isolates of Candida Species from Spain

 The in vitro activity of voriconazole was compared with that of itraconazole against 299 fluconazole-susceptible (MIC≤8 μg/ml) and 130 fluconazole-resistant (MIC≥16 μg/ml) clinical isolates of Candida spp. An adaptation of the National Committee for Clinical Laboratory Standards reference method was employed for determination of MICs. Voriconazole showed more potent activity than either fluconazole and itraconazole, even against some Candida albicans, Candida glabrata, and Candida krusei isolates resistant to fluconazole. However, for fluconazole-resistant isolates, the MICs of itraconazole and voriconazole were proportionally higher than for fluconazole-susceptible isolates. These data may indicate cross-resistance.     

In vitro evaluation of E-4695, a new fluoro-naphthyridine

Compound E-4695, a C-7 azetidinyl fluoro-naphthyridine, was compared to six structurally related quinolones and was generally two- to four-fold more active than ciprofloxacin. E-4695 was particularly active againstStaphylococcus aureus (MIC90 0.06 mg/l),Staphylococcus haemolyticus (MIC90 0.5 mg/l),Klebsiella spp. (MIC900.015 mg/l),Serratia marcescens (MIC90 0.06 mg/l),Acinetobacter spp. (MIC900.015 mg/l),Pseudomonas aeruginosa (MIC90 0.5 mg/l),Xanthomonas maltophilia (MIC90 0.5 mg/l), andNeisseria gonorrhoeae (MIC900.001 mg/l). This new quinolone-like drug requires further investigations to confirm these promising in vitro findings.     

Urinary Excretion and Bactericidal Activity of Intravenous Ciprofloxacin Compared with Oral Ciprofloxacin

 Twelve healthy volunteers participated in a randomized crossover study to compare urinary concentrations, serum parameters, and urinary bactericidal activity of ciprofloxacin after single intravenous (i.v.) doses of 200 mg and 400 mg and an oral (p.o.) dose of 500 mg. The median serum concentrations at 1 h after administration were 1 μg/ml, 4.3 μg/ml, and 2.2 μg/ml, respectively. Between the first collection period (0–2 h) and the last collection period (38–48 h), the median urinary concentrations decreased from 394 μg/ml, 675 μg/ml, and 585 μg/ml, respectively, to 0.3 μg/ml, 0.6 μg/ml, and 1 μg/ml, respectively. The urinary concentrations after the 400 mg i.v. and the 500 mg p.o. doses were not statistically different but were significantly higher than those after the 200 mg i.v. dose. The urinary bactericidal titers (UBTs), defined as the highest urinary dilution bactericidal for the organism tested, were determined against Escherichia coli (ATCC 25922) and eight uropathogens up to 48 h after administration of ciprofloxacin. The UBTs after the 400 mg i.v. and the 500 mg p.o. doses were similar and were significantly higher (P<0.05) than those following the 200 mg i.v. dose. After 400 mg i.v. and 500 mg p.o., median UBTs of ≥1 : 4 were present up to 48 h for all strains for which the MIC was ≤0.5 μg/ml, except for one nalidixic-acid resistant Escherichia coli strain for which the MIC was 0.25 μg/ml. Species for which the MIC is ≥1 μg/ml showed median UBTs of ≥1 : 4 for 8–16 h. Median UBTs of ≥1 : 4 were present up to 8 and 12 h for both Pseudomonas strains tested. A once-daily dosage of 400 mg i.v. or 500 mg p.o. might be sufficient for treatment of urinary tract infections caused by highly susceptible pathogens. A twice-daily dosing scheme seems to be preferable for complicated infections caused by pathogens with intermediate susceptibilty (MIC≥1 μg/ml) or for empiric therapy.     

Antibacterial Activity of Meropenem against Pseudomonas aeruginosa, Including Antibiotic-Induced Morphological Changes and Endotoxin-Liberating Effects

 The in vitro effects of meropenem on Pseudomonas aeruginosa were examined by studying (i) the inhibitory and bactericidal concentrations of meropenem versus those of imipenem for clinical isolates; (ii) changes in bacterial morphology during in vitro culture; and (iii) release of endotoxin induced by meropenem compared with that induced by other antipseudomonal compounds. Meropenem MIC90 and MBC90 values for 108 clinical isolates were 2 and 4.8 mg/l compared to 4.5 and 9.6 mg/l for imipenem. Morphological studies using phase-contrast and scanning electron microscopy showed that meropenem induced the formation of indeterminate bacterial cell forms at drug concentrations of 1–2.5 mg/l (0.5- to 1.25-fold the MIC), while spheroplasts predominated at drug levels exceeding 5 mg/l (2.5-fold the MIC). Determination of free and EDTA-releasable endotoxin activity by means of the Limulus lysate test showed that both meropenem and imipenem liberated significantly less endotoxin than did ceftazidime. Therefore, although meropenem binds to penicillin-binding proteins (PBPs) 2 and 3 (in contrast to imipenem, which binds to PBP2 only), endotoxin release should not be a cause of concern when treating systemic gram-negative infections with this drug.     

High-Level Mupirocin Resistance among Spanish Methicillin-Resistant Staphylococcus aureus

 Twelve strains of methicillin-resistant Staphylococcus aureus with high-level resistance to mupirocin were studied. The MIC of methicillin was 16 to 128 mg/l and the MIC of mupirocin ≥512 mg/l. All of the isolates carried a plasmid of about 30 MDa, and one strain lost resistance to mupirocin when the plasmid was cured. Three different resistance patterns were found: six strains were resistant to ciprofloxacin, kanamycin, and 10 mg/l of cadmium sulfate; two strains were resistant to these agents as well as to gentamicin, erythromycin, clindamycin, mercury chloride, and ethidium bromide; and four strains were resistant to the previously named agents as well as to rifampicin and tetracycline. The use of this valuable topical antibiotic, especially in long-term-care facilities, should be monitored to avoid dissemination of resistance.     

Comparative Activity of Eighteen Antimicrobial Agents against Anaerobic Bacteria Isolated in South Africa

 The in vitro activity of 18 antimicrobial agents was determined against 378 anaerobic bacteria isolated in Bloemfontein, South Africa, during 1996/97. Against the gram-positive isolates, MICs of penicillin and cefoxitin were >0.5 μg/ml and >16 μg/ml, respectively, for five and three strains of non-perfringens Clostridium spp. Seventeen Peptostreptococcus anaerobius strains were resistant to penicillin (MIC≥2 μg/ml). All gram-positive anaerobes tested except one Peptostreptococcus sp. and one Clostridium sp. were susceptible to dalfopristin-quinupristin (MICs≤1 μg/ml). The carbapenems exhibited excellent activity against the gram-positive isolates and were effective against most gram-negative anaerobes, with the exception of the fusobacteria. Only seven strains exhibited decreased susceptibility to trovafloxacin (MICs>2 μg/ml). In mixed anaerobic/aerobic infections, carbapenems and the fourth-generation quinolone trovafloxacin were the agents most suitable for us as broad-spectrum monotherapy.     

Activity of meropenem against imipenem-resistant bacteria and selection in vitro of carbapenem-resistantEnterobacteriaceae

The activity of meropenem against 106 imipenem-resistant (MIC 8 mg/l) clinical isolates, and the frequency of resistance to meropenem and imipenem among 24Enterobacteriaceae was determined. Both agents selected colonies on agar but 20–80 % were susceptible after one subculture and 72 % of the mutants reverted to susceptibility 1 to 6 months after selection. All isolates and stable mutants were inhibited by > 1 mg/l meropenem, although the MIC of imipenem was 4–16 mg/l. Three of sixXanthomonas maltophilia isolates were susceptible to meropenem (MICs 2–4 mg/l).Pseudomonas aeruginosa lacking outer membrane protein D2 were resistant to meropenem, although isolates with substantially reduced expression of this protein were susceptible. None of the imipenem-resistant gram-positive bacteria were susceptible to meropenem. There was no clear correlation between altered outer membrane protein expression and decreased susceptibility to carbapenems, and there was no apparent involvement of plasmid or chromosomal -lactamase.     

In Vitro Activity of the New Echinocandin Antifungal, MK-0991, against Common and Uncommon Clinical Isolates of Candida Species

 A broth macrodilution method, performed as recommended by the National Committee for Clinical Laboratory Standards, was used for comparative testing of the new echinocandin antifungal agent MK-0991 and fluconazole against 50 yeast isolates belonging to 12 species of Candida. MK-0991 was shown to be highly effective against both fluconazole-susceptible and -resistant Candida spp., yielding minimum inhibitory concentrations ranging from ≤0.19 to 0.78 μg/ml. Fungicidal activity was exerted at ≤1.5 μg/ml for 73% of the isolates tested. This study suggests that MK-0991 has significant potential for clinical development.     

Comparative Activity of Moxifloxacin in Vitro Against Obligately Anaerobic Bacteria

 The antimicrobial activity of moxifloxacin and seven other antibiotics (four of them quinolones) against 292 strains of obligately anaerobic bacteria was assessed employing a broth microdilution technique performed in Wilkens-Chalgren broth. MIC50/MIC90 values (mg/l) for moxifloxacin were as follows: Bacteroides fragilis (n=62) 0.25/2, Bacteroides ovatus (n=70) 1/4, Bacteroides vulgatus (n=29) 0.25/1, Bacteroides thetaiotaomicron (n=17) 2/2, Bacteroides caccae (n=11) 1/2, Prevotella spp. (n=11) 0.25/2, Fusobacterium spp. (n=17) 1/4, Bilophila wadsworthia (n=29) 0.5/1, and Clostridium spp. (n=29) 0.125/0.5, respectively. MIC50 values (mg/l) for Bacteroides distasonis (n=8) and Peptostreptococcus spp. (n=9) were 0.25. The results indicated that moxifloxacin was almost as active as trovafloxacin, as active as gatifloxacin, and more active than levofloxacin and ciprofloxacin against the anaerobes tested.     

Comparative In Vitro Potency of Gemifloxacin and Fluoroquinolones Against Recent European Clinical Isolates from a Global Surveillance Study

Gemifloxacin, a new fluoroquinolone with enhanced activity against gram-positive aerobes, was compared to ciprofloxacin, levofloxacin and ofloxacin against 21,464 recent isolates from 16 European countries. Gemifloxacin was the most potent fluoroquinolone against streptococci including penicillin-, macrolide- and ciprofloxacin-resistant Streptococcus pneumoniae, Staphylococcus aureus, coagulase-negative staphylococci, Acinetobacter spp., Haemophilus spp. and Moraxella catarrhalis. This drug was more potent than or comparable to ciprofloxacin against members of the family Enterobacteriaceae, Burkholderia cepacia, Pseudomonas aeruginosa and Stenotrophomonas maltophilia. Gemifloxacin is a promising fluoroquinolone with potent in vitro activity. Electronic Publication     

Characterization of Isoniazid-Resistant Strains of Mycobacterium tuberculosis on the Basis of Phenotypic Properties and Mutations in katG

 Forty isoniazid-resistant Mycobacterium tuberculosis isolates were characterized on the basis of phenotypic properties (i.e., catalase activity, MIC of isoniazid, and growth pattern in the presence of 7 different concentrations of isoniazid) and alterations in the katG gene (codons 315 and 463). Three different growth patterns could be distinguished: concentration-dependent inhibition of growth was observed in 29 strains, similar growth at all concentrations was seen in 7 strains, and enhanced growth at low concentrations of isoniazid was evident in 4 strains. The MIC of isoniazid was ≤4 μg/ml for 29 of 40 strains. Mutation at codon 315 of the katG was detected in 28 of 40 strains. However, only one of the seven strains for which the MIC of isoniazid was ≥16 μg/ml had mutation at this codon. Five of these seven strains for which the MIC was ≥16 μg/ml had no catalase activity. The results indicate that the MIC of isoniazid for a majority of strains is below the level achievable in serum. Therefore, isoniazid may be beneficial for the treatment of some cases of multidrug-resistant tuberculosis. Determination of catalase activity aids in the detection of isolates for which MICs are high and could, in conjunction with molecular methods, provide rapid detection of most isoniazid-resistant strains.     

In Vitro Susceptibility to Gemifloxacin and Trovafloxacin of Streptococcus pneumoniae Strains Exhibiting Decreased Susceptibility to Ciprofloxacin

 The in vitro susceptibility to trovafloxacin and gemifloxacin of Streptococcus pneumoniae strains exhibiting decreased susceptibility to ciprofloxacin (MIC ≥2 μg/ml; 30 strains with intermediate resistance [MIC 2 μg/ml] and 43 strains with complete resistance [MIC ≥4 μg/ml]) was determined. Seventy-three strains collected in a surveillance study carried out from May 1996 to April 1997 in Spain (prior to commercialisation of trovafloxacin and gemifloxacin) from patients with respiratory tract infections were tested. The antibacterial activity of gemifloxacin was affected to a lesser extent than that of trovafloxacin by the increase in the MIC of ciprofloxacin, with gemifloxacin showing significantly (P≤0.001) better antibacterial activity than trovafloxacin in all ciprofloxacin MIC categories (MIC50/MIC90 values of 0.015/0.03, 0.015/0.06, 0.03/0.06 and 0.12/0.25 μg/ml for gemifloxacin vs 0.12/0.12, 0.12/1, 0.25/0.5 and 2/4 μg/ml for trovafloxacin in the 2, 4, 8 and ≥16 μg/ml ciprofloxacin MIC categories, respectively). Nine (12.3%) of these 73 strains exhibited decreased susceptibility to trovafloxacin (≥2 μg/ml), whereas all strains were inhibited by 0.25 μg/ml of gemifloxacin.     

In vitro activity of the novel cephalosporin GR69153

The in vitro activity of GR69153 was compared to that of ceftazidime, ceftriaxone, imipenem and gentamicin against a total of 702 recent clinical isolates. MICs were determined by a standard agar dilution procedure and two inocula (10⁴ and 10⁸ cfu) were used throughout. GR69153 inhibited 90 % of isolates ofEscherichia coli, Klebsiella pneumonia andProteus mirabilis at 0.25 mg/l and 90 % of isolates ofPseudomonas aeruginosa at 1 mg/l.Citrobacter freundii (MIC90 16 mg/l),Morganella morganii (MIC90 128 mg/l) andEnterobacter spp. (MIC90>128 mg/l) were considerably more resistant to GR69153. GR69153 was four-fold more active than ceftazidime against methicillin-sensitiveStaphylococcus aureus but was inactive against methicillin-resistantStaphylococcus aureus, Enterococcus faecalis andBacteroides fragilis group.