In vivo pharmacodynamic study of contezolid acefosamil, a prodrug of contezolid for oral and intravenous administration.

OBJECTIVES: Contezolid acefosamil is a novel O-acyl phosphoramidate prodrug of contezolid. In the current study, we aimed to systemically evaluate the efficacy of contezolid acefosamil against infections caused by multiple Gram-positive pathogens, and compare the efficacy of the prodrug by oral and intravenous administrations. METHODS: The in vivo pharmacodynamic efficacy of contezolid acefosamil was evaluated in mouse models of systemic (with five S. aureus, three S. pneumoniae and two S. pyogenes bacterial isolates) and thigh (with two S. aureus isolates) infections using linezolid as the reference agent. RESULTS: In both models, contezolid acefosamil administrated either orally or intravenously, demonstrated high antibacterial efficacy similar to linezolid, and the antibacterial efficacy of oral and intravenous contezolid acefosamil were comparable. CONCLUSIONS: The high aqueous solubility and great efficacy of contezolid acefosamil support its clinical development as an injectable and oral antibiotic suitable for serious Gram-positive infections.

Clinical and microbiological characteristics of hypervirulent Klebsiella pneumoniae (hvKp) in a hospital from North China.

INTRODUCTION: The clinical and molecular characteristics of hypervirulent Klebsiella pneumoniae (hvKp) in various provinces of China have been reported, however, there have been few reports in Hebei Province, North China. METHODOLOGY: The hvKp was identified by PCR amplification of hypervirulence-related genes, the hypermucoviscous phenotype was determined by the "string test", the drug susceptibility analysis was performed using the VITEK® 2 Compact Bacterial Identification and Monitoring System. Logistic regression was used to identify risk factors for hvKp infection. The molecular epidemiological characteristics of the strains were analyzed by pulsed-field gel electrophoresis (PFGE), and the capsular serotype of hvKp strain was detected by PCR. RESULTS: Overall, 52.21% (59/113) of K. pneumoniae isolates were hvKp, and the ratios of patients with older ages or a higher PMN cell count among hvKp infection were higher than those among classical Klebsiella pneumoniae (cKp) infection. hvKp are more susceptible to antibacterial drugs than cKp, and one ESBLs-producing hvKp strain was detected. The main capsular serotype of hvKp were K2, K57 and K1. PFGE indicated that the 59 strains of hvKp could be classified into 51 PFGE band types, forming 6 PFGE clusters. CONCLUSIONS: In this study, the detection rate of hvKp was 52.21% (59/113) identified by virulence genes. People with older ages or a higher PMN cell count are more likely to gain hvKp infection. ESBLs-producing hvKp is emerging, indicating the importance of epidemiologic surveillance and clinical awareness of this pathogen in this region.

Development and validation of a sensitive LC-MS/MS method for the quantitation of IMB-YH-4py5-2H, an antituberculosis candidate, and its application to the pharmacokinetic study.

(E)-N,N-dimethyl-4-oxo-4-(4-(pyridin-4-yl)phenyl)but-2-enamide hydrochloride (IMB-YH-4py5-2H) is a novel Protein Kinase B (PknB) inhibitor with potent activity against Mycobacterium tuberculosis strains. In the present study, a sensitive and specific liquid chromatography/tandem mass spectrometry (LC-MS/MS) method was developed and validated to determine IMB-YH-4py5-2H in rat plasma. Sample pretreatment was achieved by liquid-liquid extraction with ethyl acetate, and separation was performed on an XTerra MS C18 column (2.1×50 mm, 3.5 µm) with gradient elution (methanol and 0.1% formic acid) at a flow rate of 0.3 mL/min. Detection was performed in multiple reaction monitoring (MRM) mode. Linear calibration curves were obtained over a concentration range of 1-100 ng/mL. The intra-day and inter-day precisions were lower than 8.46%, and the accuracies ranged from -8.71% to 12.36% at all quality control levels. The extraction recoveries were approximately 70%, and the matrix effects were negligible. All quality control samples were stable under different storage conditions. The validated method was successfully applied to a preclinical pharmacokinetic study in Sprague-Dawley rats. IMB-YH-4py5-2H demonstrated improved pharmacokinetic properties (higher exposure level) compared with its leading compound. IMB-YH-4py5-2H was also distributed throughout the lung pronouncedly, especially inside alveolar macrophages, indicating its effectiveness against lower respiratory infections.

Microdialysis combined with liquid chromatography-tandem mass spectrometry for the quantitation of gemifloxacin and its application to a muscle penetration study in healthy and MRSA-infected rats.

Pharmacological efficacy is based on the drug concentration in target tissues, which usually cannot be represented by the plasma concentration. The purpose of this study was to compare the pharmacokinetic characteristics of gemifloxacin in plasma and skeletal muscle and evaluate its tissue penetration in both healthy and MRSA (methicillin-resistant Staphylococcus aureus)-infected rats. A microdialysis (MD) combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated to determine free gemifloxacin concentrations in rat plasma and skeletal muscle simultaneously. The in vivo recoveries of MD were 23.21% ± 3.42% for skeletal muscle and 20.62% ± 3.19% for plasma, and were concentration independent. We provided evidence that the method developed here meets FDA requirements. Additionally, this method was successfully applied to the determination of free gemifloxacin in rats. Muscle and blood dialysates were collected after an 18 mg/kg intravenous bolus dose. The mean areas under the concentration-time curves (AUCs) from 0 to 9 h for skeletal muscle and plasma were 3641.50 ± 915.65 h*ng/mL and 7068.32 ± 1964.19 h*ng/mL in MRSA-infected rats and 3774.72 ± 700.36 h*ng/mL and 6927.49 ± 1714.86 h*ng/mL in healthy rats, respectively. There was no significant difference (P>0.05) in gemifloxacin exposure between healthy rats and MRSA-infected rats for plasma or muscle. The low ratio of AUC0-9 muscle to AUC0-9 plasma suggested lower drug exposure in skeletal muscle than in plasma for both healthy and MRSA-infected rats. Our study suggested that the administration of gemifloxacin according to drug levels in plasma to treat local infection is unreasonable and might result in an inadequate dose regimen.

Structure analysis of transposons carrying the aac(6')-aph(2″) gene in Enterococcus faecalis isolated in Beijing, China, and comparison of their transfer efficiency.

Transfer of aac(6')-aph(2″) transposons mediating high-level gentamicin resistance (HLGR) in Enterococcus faecalis is a serious problem in the clinic. However, factors affecting the transfer of aac(6')-aph(2″) have not yet been elucidated. The current study aimed to examine the genetic and molecular basis of HLGR in E. faecalis strains isolated in Beijing (China) and to clarify the relationship between transfer efficiency of aac(6')-aph(2″) transposons and the transposon structure/location. A total of five transposon structures were identified by PCR mapping of the corresponding transposon regions, including a Tn5281-like non-truncated transposon and four truncated transposons. A plasmid location study of aac(6')-aph(2″) by Southern blot following S1-PFGE and filter mating conjugation experiments demonstrated that plasmid location rates correlated with conjugation-positive rates. Chromosome walking to identify the sequence upstream of a representative type III truncated transposon found a truncated aph(2″)-Ia region, and further PCR analysis of this region among strains from different groups revealed similar a positive rate trend as the transposon plasmid location rate and conjugation-positive rate. In conclusion, aac(6')-aph(2″) transposons were of different structures in E. faecalis strains from Beijing, with two new transposon structures that have not been reported elsewhere. Presence of the truncated aph(2″)-Ia region upstream of some truncated transposons suggests recombination between aminoglycoside-modifying enzyme genes. Possible links exist among plasmid location, conjugation and the presence of truncated aph(2″)-Ia upstream of the transposon.

Recombinant expression and biochemical characterization of Mycobacterium tuberculosis 3Fe-4S ferredoxin Rv1786.

Ferredoxins are iron-sulfur protein that mediate electron transfer in cytochrome P450 mono-oxygenase (CYP)-related catalytic reactions in a wide variety of organisms. Rv1786 is a putative ferredoxin, encoded by a gene located downstream of the gene encoding CYP143A1 in the Mycobacterium tuberculosis genome. However, the structure and function of Rv1786 have remained unclear. Here, the recombinant Mtb Rv1786 was expressed, purified as a His-tagged form and characterized with [3Fe-4S] clusters as its cofactors using a series of measurements including SDS-PAGE, western blot, UV/Visible, MALDI-TOF/TOF-MS, and electron paramagnetic resonance spectroscopic analysis. Based on the assessments of surface plasmon resonance (SPR) and steady state kinetic assays, Rv1786 was found to be able to couple with both ferredoxin reductase A (FdrA) and flavoprotein reductase A (FprA) as redox partner, but with a stronger binding to FprA and a better coupling activity to FdrA. Preliminary structural and biochemical characterization of Mtb Rv1786 as a redox partner is presented here.

PRECLINICAL PHARMACOKINETIC ANALYSIS OF (E)-METHYL-4-ARYL-4-OXABUT-2-ENOATE, A NOVEL SER/THR PROTEIN KINASE B INIBITOR, IN RATS.

(E)-Methyl-4-aryl-4-oxabut-2-enoate, designated YH-8, is a novel Serflhr protein kinase B (PknB) inhibitor, which is designed for the treatment of tuberculosis. The aim of this study was to investigate the pharmacokinetics, bioavailability, tissue distribution and excretion characteristics of YH-8 in rats and study its plasma protein binding in vitro. The pharmacokinetic properties were examined after intravenously injected YH-8 at 10 and 20 mg/kg and oral administrated YH-8 at 50, 100 and 200 mg/kg to rats. The concentrations of YH-8 in plasma were determined with LC-MS/MS, with a liquid-liquid extraction. The tissue distribution and urinary, fecal and -biliary excretion patterns of YH-8 were investigated following a single oral dosing of 100 mg/kg. The plasma protein binding rates of YH-8 were determined using ultra-filtration method. After intra- venous and oral administration, YH-8 showed dose-independent pharmacokinetic characteristics, with T(1/2) of approximately 5.5 h and 7.1 h, respectively. The oral absolute bioavailability of YH-8 was relatively low (about 12%). YH-8 was widely distributed in various tissues and showed substantial deposition in intestine, stomach, liver, lung and kidney. The drug was mainly eliminated via fecal excretion and its binding rate with plasma protein was concentration-dependent. In conclusion, this study as first provided the full pharmacokinetic characteristics of YH-8, which would be helpful for its further development and clinical application.

Design, synthesis and biological evaluation of monobactams as antibacterial agents against gram-negative bacteria.

A series of monobactam derivatives were prepared and evaluated for their antibacterial activities against susceptible and resistant Gram-negative strains, taking Aztreonam and BAL30072 as the leads. Six conjugates (12a-f) bearing PIH-like siderophore moieties were created to enhance the bactericidal activities against Gram-negative bacteria based on Trojan Horse strategy, and all of them displayed potencies against susceptible Gram-negative strains with MIC ≤ 8 µg/mL. SAR revealed that the polar substituents on the oxime side chain were beneficial for activities against resistant Gram-negative bacteria. Compounds 19c and 33a-b exhibited the promising potencies against ESBLs-producing E. coli and Klebsiella pneumoniae with MICs ranging from 2 µg/mL to 8 µg/mL. These results offered powerful information for further strategic optimization in search of the antibacterial candidates against MDR Gram-negative bacteria.

In vivo antibacterial activity of MRX-I, a new oxazolidinone.

MRX-I is a potent oxazolidinone antibiotic against Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant Streptococcus pneumoniae (PRSP), penicillin-intermediate S. pneumoniae (PISP), and vancomycin-resistant enterococci (VRE). In this study, the in vivo efficacy of orally administered MRX-I was evaluated using linezolid as a comparator. MRX-I showed the same or better efficacy than linezolid in both systemic and local infection models against the tested strains.

Toxicokinetic study of melamine in the presence and absence of cyanuric acid in rats.

Several lines of evidence show that the nephrotoxic effect of melamine (MEL) in animals is consistent with combined ingestion of MEL and cyanuric acid (CYA). The aim of the present study was to compare the toxicokinetics of MEL in the presence and absence of CYA, and to elucidate the correlation between toxicity and kinetic properties of MEL. Sprague-Dawley rats were administered a single oral dose of MEL (100 mg kg(-1) ) with or without CYA (100 mg kg(-1) ). Plasma and tissue samples were analyzed by liquid chromatography-tandem mass spectrometric (LC-MS/MS) assay. Significant changes in toxicokinetic parameters of MEL such as lower maximum concentration (7.4 ± 3.5 vs 78.0 ± 11.0 µg ml(-1) ) and area under curve (94.9 ± 53.5 vs 295.1 ± 93.7 µg h ml(-1) ), higher plasma elimination half-life (7.0 ± 3.3 vs 2.5 ± 0.3 h) and volume of distribution (11 505.5 ± 5030.3 vs 1312.7 ± 337.7 ml kg(-1) ), as well as significantly higher concentration of MEL in rat kidney (2.96-274.15 vs < 1 µg g(-1) ) were detected in the CYA co-administration group when compared with MEL alone group (P < 0.05). The differences in kinetic parameters between the two groups meant that CYA co-administration could lower absorption, slow excretion and induce tissue accumulation of MEL, which correlated well with the generation and development of renal toxicity. In conclusion, co-administration with CYA leads to the alteration of the kinetic characteristics of MEL, which provides an additional explanation for renal toxicity.

In vitro antibacterial activity of chinfloxacin, a new fluoroquinolone antibiotic.

BACKGROUND: Chinfloxacin is a novel synthetic fluoroquinolone with a structure similar to moxifloxacin. The in vitro activity of chinfloxacin was evaluated in the current study. METHOD: Chinfloxacin was tested against a total of 1,739 clinical isolates representing 23 species using the agar dilution method. Studies of bactericidal activity, including minimum bactericidal concentrations (MBC) and time-kill curve determinations, were conducted according to the recommendations of the Clinical and Laboratory Standards Institute. RESULTS: Minimum inhibitory concentrations (MIC)(50)s and MIC(90)s of chinfloxacin were found to be the same or 2-fold lower than those of moxifloxacin against gram-positive isolates except for Streptococcus pyogenes (against which chinfloxacin showed similar MIC(50) as moxifloxacin but 2-fold higher MIC(90)), and the same as or 2-fold higher than those of moxifloxacin against gram-negative isolates. Chinfloxacin showed potent bactericidal activity with MBC/MIC ratios in the range of 1-2 for almost all the isolates tested. Time-kill curves further demonstrated chinfloxacin as a concentration-dependent bactericidal agent usually effective at concentrations of 2 MIC or higher. CONCLUSION: Chinfloxacin showed similar in vitro activity as moxifloxacin.

In vivo antibacterial activity of chinfloxacin, a new fluoroquinolone antibiotic.

OBJECTIVES: To evaluate the in vivo antibacterial efficacy of chinfloxacin, a novel fluoroquinolone, in murine systemic and local infection models. METHODS: The efficacy of chinfloxacin in systemic infection was evaluated in a mouse peritonitis model using isolates of methicillin-susceptible Staphylococcus aureus (MSSA, n = 3), methicillin-resistant Staphylococcus aureus (MRSA; n = 1), penicillin-intermediate Streptococcus pneumoniae (PISP; n = 1), penicillin-resistant S. pneumoniae (PRSP; n = 2), vancomycin-susceptible Enterococcus faecalis (VSE; n = 1), vancomycin-resistant E. faecalis (VRE; n = 2), Escherichia coli (n = 3) and Klebsiella pneumoniae (n = 2). The local infections included mouse pulmonary infections caused by penicillin-susceptible S. pneumoniae (PSSP; n = 1), PRSP (n = 1) and K. pneumoniae (n = 2). RESULTS: In the mouse systemic infection model, chinfloxacin demonstrated potent activity against MSSA [50% effective dose (ED(50)) 2.28-4.15 mg/kg], MRSA (ED(50) 14.75 mg/kg), PISP (ED(50) 6.20 mg/kg), PRSP (ED(50) 3.51-5.03 mg/kg), VSE (ED(50) 25.02 mg/kg), VRE (ED(50) 5.18-15.39 mg/kg), E. coli (ED(50) 1.25-1.90 mg/kg) and K. pneumoniae (ED(50) 2.92-8.28 mg/kg). The therapeutic efficacy of chinfloxacin was generally similar to (P > 0.05) that of moxifloxacin, significantly higher (P < 0.01 or P < 0.05) than that of levofloxacin in Gram-positive isolate infections (MSSA, MRSA, PISP, PRSP, VSE and VRE), and less than that of levofloxacin against E. coli and K. pneumoniae infections (P < 0.01). In the mouse pulmonary infection model, chinfloxacin showed potent activity towards S. pneumoniae (higher than levofloxacin and ciprofloxacin) and K. pneumoniae (lower than levofloxacin and similar to or higher than ciprofloxacin) infections. CONCLUSIONS: The results validated the potent efficacy of chinfloxacin in vivo. The high efficacy of chinfloxacin in murine systemic and local infections warrants investigation of its clinical use.

Susceptibility of vertilmicin to modifications by three types of recombinant aminoglycoside-modifying enzymes.

The susceptibilities of vertilmicin and seven reference aminoglycosides to modifications by six recombinant aminoglycoside-modifying enzymes, AAC(6')-Ie, APH(2'')-Ia, AAC(6')-Ie-APH(2'')-Ia, ANT(2'')-Ia, AAC(6')-Ib, and AAC(6')-Ib-cr, were studied by coupled spectrophotometric assays in microtiter plates. In comparison to other aminoglycosides, the susceptibility of vertilmicin was 45.8- to 250.0-fold lower for AAC(6')-Ie acetylation, 39.2- to 116.7-fold lower for AAC(6')-Ie-APH(2'')-Ia acetylation, and 1.8- to 7.5-fold lower for ANT(2'')-Ia adenylation (except that shown by amikacin) while relatively comparable for AAC(6')-Ib acetylation, AAC(6')-Ib-cr acetylation, APH(2'')-Ia phosphorylation, and AAC(6')-Ie-APH(2'')-Ia phosphorylation.

In vivo antibacterial activity of nemonoxacin, a novel non-fluorinated quinolone.

OBJECTIVES: To evaluate the in vivo antibacterial efficacy of nemonoxacin, a novel C8-methoxy non-fluorinated quinolone in murine systemic and local infection models. METHODS: The efficacy of nemonoxacin in systemic infections was evaluated in mouse peritonitis models using isolates of methicillin-susceptible Staphylococcus aureus (MSSA, n=1), methicillin-resistant S. aureus (MRSA, n=1), methicillin- and levofloxacin-resistant Staphylococcus capitis (levofloxacin-resistant MRSC, n=1), penicillin-intermediate Streptococcus pneumoniae (PISP, n=1), penicillin-resistant S. pneumoniae (PRSP, n=2), Enterococcus faecalis (n=2, including 1 vancomycin-resistant Enterococcus, VRE) and Escherichia coli (n=3). The local infections included mouse pulmonary infections caused by PRSP (n=1), Klebsiella pneumoniae (n=1) and mouse ascending urinary tract infection caused by E. coli (n=1). RESULTS: In the mouse systemic infection model, nemonoxacin demonstrated potent activity against MSSA (ED(50) =2.08 mg/kg), MRSA (ED(50) =2.59 mg/kg), levofloxacin-resistant MRSC (ED(50) =2.52 mg/kg), PISP (ED(50) =5.47 mg/kg), PRSP (ED(50) =3.68-5.28 mg/kg) and E. coli (ED(50) =3.13-5.28 mg/kg), and moderate activity towards E. faecalis infection (ED(50) =8.48-15.16 mg/kg). The therapeutic efficacy of nemonoxacin was significantly higher (P<0.01) than that of levofloxacin in infections caused by Gram-positive isolates (MSSA, MRSA, levofloxacin-resistant MRSC, PISP, PRSP and E. faecalis), but less potent than that of levofloxacin against E. coli infection (P<0.01). Nemonoxacin in vivo efficacy results with Gram-positive isolates (2- to 5-fold ED(50) advantage over levofloxacin) are consistent with the MIC data (4- to 16-fold MIC advantage of nemonoxacin over levofloxacin). In the mouse pulmonary infection model, nemonoxacin showed potent activity towards PRSP (higher than levofloxacin) and K. pneumoniae (lower than levofloxacin) infections. In the mouse ascending urinary tract infection model, nemonoxacin exhibited potent activity against E. coli infection (lower than levofloxacin). CONCLUSIONS: The results validated the potent efficacy of nemonoxacin in vivo. The higher efficacy of nemonoxacin than of levofloxacin towards infections caused by Gram-positive cocci (especially MRSA, levofloxacin-resistant MRSC, PRSP and VRE) warrants investigation of its clinical use.

In vivo antibacterial activity of vertilmicin, a new aminoglycoside antibiotic.

Vertilmicin is a novel aminoglycoside antibiotic with potent activity against gram-negative and -positive bacteria in vitro. In this study, we further evaluated the efficacy of vertilmicin in vivo in systemic and local infection animal models. We demonstrated that vertilmicin had relatively high and broad-spectrum activities against mouse systemic infections caused by Escherichia coli, Klebsiella pneumoniae, Staphylococcus aureus, and Enterococcus faecalis. The 50% effective doses of subcutaneously administered vertilmicin were 0.63 to 0.82 mg/kg, 0.18 to 0.29 mg/kg, 0.25 to 0.99 mg/kg, and 4.35 to 7.11 mg/kg against E. coli, K. pneumoniae, S. aureus, and E. faecalis infections, respectively. The therapeutic efficacy of vertilmicin was generally similar to that of netimicin, better than that of gentamicin in all the isolates tested, and better than that of verdamicin against E. coli 9612 and E. faecalis HH22 infections. The therapeutic efficacy of vertilmicin was further confirmed in local infection models of rabbit skin burn infection and mouse ascending urinary tract infection.

In vitro antibacterial activity of vertilmicin and its susceptibility to modifications by the recombinant AAC6'-APH2'' enzyme.

Vertilmicin is a new semisynthetic aminoglycoside with a structure similar to that of netilmicin except for a methyl group at the C-6' position. In the present study, the in vitro antibacterial activity of vertilmicin was studied, and its susceptibility to modifications by the recombinant aminoglycoside bifunctional modifying enzyme AAC(6')-APH(2'') was compared with those of verdamicin and netilmicin. A total of 1,185 clinical isolates collected from hospitals in Beijing between 2000 and 2001 were subjected to the in vitro antibacterial activity evaluations, including MIC, minimum bactericidal concentration (MBC), and time-kill curve tests. The MICs were evaluated in non-gentamicin-resistant (gentamicin-susceptible and gentamicin-intermediate) strains and gentamicin-resistant strains, respectively. For most of the non-gentamicin-resistant bacteria (except for the isolates of Pseudomonas spp.), the MIC(90)s of vertilmicin were in the range of 0.5 to 8 microg/ml, comparable to those of the reference aminoglycosides. For the gentamicin-resistant isolates, the three semisynthetic aminoglycosides (vertilmicin, netilmicin, and amikacin) demonstrated low MIC(50)s and/or MIC(90)s, as well as high percent susceptibility values. Among the study drugs, vertilmicin showed the lowest MIC(90)s, 16 microg/ml, for the gram-positive gentamicin-resistant isolates of Staphylococcus aureus and Staphylococcus epidermidis. Meanwhile, vertilmicin was a potent bactericidal agent, with MBC/MIC ratios in the range of 1 to 2 for Escherichia coli, Klebsiella pneumoniae, and S. aureus and 1 to 4 for S. epidermidis. The time-kill curve determination further demonstrated that this effect was rapid and concentration dependent. In evaluations of susceptibility to modifications by the recombinant AAC(6')-APH(2'') with maximum rate of metabolism/K(m) measurements, vertilmicin exhibited susceptibilities to both acetylation and phosphorylation lower than those of netilmicin and verdamicin.

In vitro activity of recombinant lysostaphin against Staphylococcus aureus isolates from hospitals in Beijing, China.

Lysostaphin is a glycylglycine endopeptidase. It cleaves the pentaglycine cross-bridge structure unique to the staphylococcal cell wall and is considered to be a potential drug for Staphylococcus aureus. In the present study, the in vitro activity of recombinant lysostaphin was investigated in 257 S. aureus isolates collected from hospital patients in Beijing, China, by determination of MIC and minimum bactericidal concentration (MBC) and a time-kill curve test. An agar dilution method was used for MIC determination in all of the isolates and a macrobroth dilution method was employed to verify MIC values for a subset of the isolates. All of the S. aureus strains were sensitive to the recombinant lysostaphin with MICs ranging from 0.03 to 2 microg ml(-1) in the agar dilution assay. The antibacterial activity of lysostaphin was greater than that of vancomycin and other reference agents. For most of the isolates, the MICs from the agar dilution method were higher than those from the broth dilution method. The MBCs of lysostaphin in the test isolates were between 1- and 8-fold higher than their MIC values. Bactericidal activity (>99.9 % reduction) was observed after 2 h exposure of the isolates to lysostaphin at concentrations of > or =0.5 MIC. Lysostaphin showed a rapid bactericidal activity against the test strains of meticillin-susceptible S. aureus and meticillin-resistant S. aureus. Its activity at > or =0.5 MIC was sustained for at least 6 h. These results will be informative for the clinical application and evaluation of lysostaphin.