In vitro activity of rifampin in combination with oxacillin against Staphylococcus aureus.

The in vitro activity of rifampin alone and in combination with oxacillin was determined for 75 Staphylococcus aureus strains (64 susceptible and 11 resistant to oxacillin). Minimal inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBCs) were determined by broth microdilution; antibiotic combinations were evaluated by microdilution checkerboard and time-kill studies. The 90% MIC of rifampin was less than or equal to 0.015 micrograms/ml after both 24 and 48 h of incubation. The 90% MBC of rifampin was less than or equal to 2.0 micrograms/ml on subculture at 24 h of incubation and less than or equal to 0.5 micrograms/ml on subculture at 48 h. MIC checkerboards with oxacillin-susceptible strains revealed an additive or indifferent effect in 35 strains (55%) and antagonism in 29 strains (45%). MBC checkerboards performed by subculture at 24 h demonstrated antagonism for all but one of the oxacillin-susceptible strains, with sub-MBCs of rifampin impairing the bactericidal activity of oxacillin. MBC checkerboards performed by 48-h subculture revealed antagonism with 37 strains (58%); in 26 additional strains (40%), a synergistic, additive, or indifferent effect was observed at low antibiotic concentrations, but antagonism was seen at higher concentrations. Time-kill studies tended to show indifference rather than antagonism with oxacillin plus rifampin. In checkerboards performed with oxacillin-resistant strains, the addition of rifampin did not improve oxacillin inhibitory or bactericidal activity to a clinically significant extent; however, the addition of oxacillin improved the bactericidal activity of rifampin at easily achievable serum concentrations.     

In vitro and in vivo evaluations of the activities of lauric acid monoester formulations against Staphylococcus aureus

Due to increasing mupirocin resistance, alternatives for Staphylococcus aureus nasal decolonization are needed. Lauric acid monoesters combined with lactic, mandelic, malic, or benzoic acid are being evaluated as possible alternatives. We determined the in vitro activity of 13 lauric acid monoester (LAM) formulations and mupirocin against 30 methicillin-susceptible S. aureus (MSSA) isolates and 30 methicillin-resistant S. aureus (MRSA) isolates. We then used a murine model of MRSA nasopharyngeal colonization to compare the in vivo activity of mupirocin with three LAM formulations. MSSA and MRSA MIC(90) values were 0.25 microg/ml for mupirocin and 相似文献   

Activity of oxacillin versus that of vancomycin against oxacillin-susceptible mecA-positive Staphylococcus aureus clinical isolates evaluated by population analyses, time-kill assays, and a murine thigh infection model

We compared the activity of dicloxacillin with that of vancomycin against 15 oxacillin-susceptible, methicillin-resistant Staphylococcus aureus (OS-MRSA) clinical isolates. By population analyses, we found that 6 OS-MRSA isolates were able to grow in the presence of up to 8 μg/ml dicloxacillin and 9 isolates were able to grow in 12 to >32 μg/ml dicloxacillin; all isolates grew in up to 2 μg/ml vancomycin. Both drugs exhibited similar bactericidal activities. In experimental infections, the therapeutic efficacy of dicloxacillin was significant (P < 0.05 versus untreated controls) in 10 OS-MRSA isolates and vancomycin was effective (P < 0.05) against 12 isolates; dicloxacillin had an efficacy that was comparable to that of vancomycin (P > 0.05) in 8 isolates. The favorable response to dicloxacillin treatment might suggest that antistaphylococcal penicillins could be used against OS-MRSA infections.     

In vitro evaluation of clindamycin in combination with oxacillin, rifampin, or vancomycin against Staphylococcus aureus

In a study of antibiotic combinations of clindamycin with rifampin, oxacillin, or vancomycin using the time kill-curve method, the combination of clindamycin and rifampin were sometimes synergistic (5 of 15 times), otherwise indifferent and always enhanced killing of fifteen tested Staphylococcus aureus isolates. In contrast, vancomycin and clindamycin or oxacillin and clindamycin were either indifferent or antagonistic (approximately 50%). Vancomycin alone, however, was generally as effective as the combinations of clindamycin and rifampin.     

Fatal sepsis caused by mecA-positive oxacillin-susceptible Staphylococcus aureus: First report in a tertiary hospital of southern Brazil

mecA-positive oxacillin phenotypically susceptible Staphylococcus aureus (OS-MRSA) is increasingly reported worldwide. This bacterium poses a therapeutic threat, as it can be misidentified as an oxacillin-susceptible organism by phenotypic methods that are routinely used in the majority of clinical microbiology laboratories. Herein, we report the first case of fatal sepsis in a 43-year-old female patient caused by an OS-MRSA SCCmec type IVa/ST1/CC1 in a tertiary hospital in southern Brazil, which highlights the difficulties involved in diagnosing this bacterium. Blood cultures and phenotypic susceptibility tests on admission yielded a penicillin-resistant S. aureus. Although vancomycin therapy was initiated, this antibacterial was replaced by oxacillin, based on the susceptibility result. However, the clinical conditions of the patient deteriorated rapidly evolving to fatal septic shock. Clinical microbiology laboratories should consider the use of additional tests to accurately distinguish between various antimicrobial phenotypes of S. aureus.     

Bactericidal activity of oxacillin against beta-lactamase-hyperproducing Staphylococcus aureus.

The bactericidal activity of oxacillin against beta-lactamase-hyperproducing strains of Staphylococcus aureus for which the MIC by MicroScan was 1 or 2 micrograms/ml after incubation for 24 h was evaluated by MBC studies and kill kinetics methods. MBC and kill kinetics tests were both performed using Mueller-Hinton broth (MHB), with and without 2% NaCl supplementation, and incubation at 30 and 35 degrees C. When MBC testing was performed with salt-supplemented MHB, the oxacillin MBC/MIC ratio was greater than 8 for 17 and 16 of 17 S. aureus isolates at 30 and 35 degrees C, respectively. With unsupplemented MHB, the MBC/MIC ratio was greater than 8 for nine and six strains at 30 and 35 degrees C, respectively. Five representative strains were selected for kill kinetics studies under the four different test conditions. Oxacillin appeared more bactericidal by the kill kinetics method than by MBC testing. Moreover, salt supplementation did not affect the results of kill kinetics studies as dramatically as it did the MBC results. Thus, bactericidal testing results are markedly influenced by the technique employed, and further in vivo studies are necessary to fully evaluate the efficiency of oxacillin against beta-lactamase-hyperproducing strains of S. aureus.     

In vitro activity of pristinamycin against methicillin-resistant Staphylococcus aureus

An agar dilution technique was used to determine the MIC of pristinamycin for 124 clinical isolates of methicillin-resistant Staphylococcus aureus. All were inhibited by less than or equal to 0.5 mg/1, quite similar to the sensitivity of a number of methicillin-sensitive strains. MICs obtained using a microdilution method were also comparable. Antagonism was seen with the combination of pristinamycin and erythromycin.     

In vitro activity of TD-6424 against Staphylococcus aureus

TD-6424, a rapidly bactericidal agent with multiple mechanisms of action, is more potent in vitro and more rapidly bactericidal than currently available agents against methicillin-susceptible and methicillin-resistant Staphylococcus aureus. TD-6424 produces a postantibiotic effect with a duration of 4 to 6 h against these organisms. The results suggest potential efficacy against susceptible and resistant strains of S. aureus.     

青霉素体外诱导金黄色葡萄球菌耐药的实验研究

目的探讨在实验室条件下青霉素诱导金黄色葡萄球菌产生耐药性所需要的时间及浓度。方法将标准菌株接种到含不同浓度青霉素的营养肉汤培养液中,35℃孵育,每24 h更换1次培养液,每周做1次菌落计数及药敏试验,观察其药敏变化情况,直至培养生长菌株全部耐苯唑西林为止。结果青霉素对ATCC25923的最低抑菌浓度为0.313μg/mL。培养1周后各青霉素浓度培养液中均有菌株生长,且药敏试验都出现双圈,0.625～20μg/mL6个青霉素浓度的培养液中分别在2～7周后无菌生长;0.313、0.156、0.078μg/mL 3个青霉素浓度培养液中的生长菌株分别在6、7、3周全部耐苯唑西林。0.156μg/mL青霉素浓度诱导的耐药菌株,去除药物进行传代培养5周仍耐药;而0.313、0.078μg/mL2个浓度诱导的耐药菌株传代3周即恢复了对青霉素、苯唑西林的敏感性。结论青霉素浓度大于或等于0.625μg/mL为杀菌浓度,0.313、0.156、0.078μg/mL3个浓度易诱导金黄色葡萄球菌产生耐药,0.313及0.078μg/mL青霉素浓度诱导的耐药菌株耐药性不稳定,0.156μg/mL诱导的耐药菌株耐药性较稳定。     

In vitro and in vivo studies of three antibiotic combinations against gram-negative bacteria and Staphylococcus aureus.

The activities of azlocillin, cefotaxime, and amikacin alone and in combination were evaluated in in vitro checkerboard studies, in infected neutropenic mice, and in human volunteers. The combination of cefotaxime plus amikacin was more synergistic in vitro than the others against the Enterobacteriaceae tested, and the combination of azlocillin plus amikacin was more synergistic against Pseudomonas aeruginosa and Staphylococcus aureus. Survival of neutropenic mice infected with Escherichia coli and Klebsiella pneumoniae, respectively, was greater with azlocillin plus amikacin (24 of 40 and 11 of 40) and with cefotaxime plus amikacin (21 of 40 and 17 of 40) than with azlocillin plus cefotaxime (22 of 40 and 3 of 40; P less than 0.05). Median serum bactericidal activity in volunteers receiving these antibiotics alone and in combination was greater than or equal to 1:8 with most agents and with all combinations tested against 10 strains each of E. coli, K. pneumoniae, P. aeruginosa, and S. aureus. These data suggest that clinical trials with combinations of azlocillin or cefotaxime plus amikacin deserve further study in febrile neutropenic patients.     

pH-dependent oxacillin tolerance of Staphylococcus aureus

A proportion of clinical isolates of Staphylococcus aureus exhibit resistance to bactericidal activity of certain antibiotics, despite normal susceptibility to inhibition. This phenomenon is termed "tolerance." The methodology used to determine tolerance varies greatly. To clarify the relationship between laboratory methodology and tolerance, we determined the minimal bactericidal concentrations and minimal inhibitory concentrations for 20 clinical isolates by two methods. Inocula were prepared by either 3-h growth of the organism in Mueller-Hinton broth or overnight (22 to 24 h) cultures in Trypticase soy broth (BBL Microbiology Systems, Cockeysville, Md.). All inocula were plated for colony counts and tested for pH. An American Type Culture Collection (Rockville, Md.) reference strain was included in all tests for standardization. Tolerance was defined by the strictest criterion, i.e., a minimal bactericidal concentration/minimal inhibitory concentration ratio of greater than or equal to 100. With the first method, none of the 20 isolates displayed tolerance (mean inoculum pH, 7.15). When inocula were grown in Trypticase soy broth with overnight incubation, 35% (7 of 20) showed tolerance (mean inoculum pH, 6.22). There was a significant association between the decreased bactericidal capacity at high oxacillin concentrations and overnight incubation in Trypticase soy broth (P less than 0.01). We suggest that tolerance in staphylococci is in some way related to the pH value of the inoculating culture. Such pH-induced tolerance may have a clinical corollary in sequestered infections where the pH is acidic.     

In vitro activity of novel rifamycins against rifamycin-resistant Staphylococcus aureus

We describe novel rifamycin derivatives (new chemical entities [NCEs]) that retain significant activity against a comprehensive collection of Staphylococcus aureus strains that are resistant to rifamycins. This collection of resistant strains contains 21 of the 26 known single-amino-acid alterations in RpoB, the target of rifamycins. Some NCEs also demonstrated a lower frequency of resistance development than rifampin and rifalazil in S. aureus as measured in a resistance emergence test. When assayed for activity against the strongest rifamycin-resistant mutants, several NCEs had MICs of 2 microg/ml, in contrast to MICs of rifampin and rifalazil, which were 512 microg/ml for the same strains. The properties of these NCEs therefore demonstrate a significant improvement over those of earlier rifamycins, which have been limited primarily to combination therapy due to resistance development, and suggest a potential use of these NCEs for monotherapy in several clinical indications.     

In vitro and in vivo activity of coumermycin and other antibacterial agents against methicillin-resistant strains of Staphylococcus aureus

The in vitro activity of coumermycin, fusidic acid, cotrimoxazole, and vancomycin was determined by broth microdilution assay against 33 methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates from the Detroit Receiving Hospital, Detroit, Mich. Coumermycin was the most active drug tested, while fusidic acid, vancomycin, and cotrimoxazole also had good activity. The four antimicrobials were tested in vivo against 7 strains of MRSA employing the mouse protection model. Again, coumermycin was the most active, followed by vancomycin, cotrimoxazole, and fusidic acid. Coumermycin was very active, while vancomycin and fusidic acid were inactive in neutropenic mice infected with an MRSA strain. Coumermycin retained activity when given 18 h before an MRSA infection, while vancomycin activity was lost. Coumermycin was active in a local thigh infection while vancomycin was inactive. The results indicate that coumermycin is potent against MRSA with activity equal or superior to comparable agents in various experimental mouse infections.     

Synergy of daptomycin with oxacillin and other beta-lactams against methicillin-resistant Staphylococcus aureus

We previously observed marked synergy between daptomycin and both rifampin and ampicillin against vancomycin-resistant enterococci (VRE). Because the synergy between daptomycin and ampicillin was observed for 100% of VRE strains with high-level ampicillin resistance (ampicillin MIC of > or =128 microg/ml), we looked for synergy between daptomycin and other beta-lactams against 18 strains of methicillin-resistant Staphylococcus aureus (MRSA) by employing a time-kill method using Mueller-Hinton broth supplemented to 50 mg of Ca2+/liter. All strains were resistant to oxacillin (16 of 18 strains were resistant at drug concentrations of > or =256 microg/ml), and all strains were susceptible to daptomycin (the MIC at which 90% of the tested isolates were inhibited was 1 microg/ml). Daptomycin was tested at concentrations of 2, 1, 0.5, 0.25, 0.125, and 0.0625 microg/ml alone or in combination with oxacillin at a fixed concentration of 32 microg/ml. Synergy was found for all 18 strains with daptomycin at one-half the MIC in combination with 32 microg of oxacillin/ml, and synergy was found for 11 of 18 strains (61%) with daptomycin at one-fourth the MIC or less in combination with oxacillin. At 24 h, the daptomycin-oxacillin combination with daptomycin at one-half the MIC showed bactericidal activity against all 18 strains, and the combination with one-fourth the daptomycin MIC showed bactericidal activity against 9 of 18 strains. We also used a novel screening method to look for synergy between daptomycin and other beta-lactams. In this approach, daptomycin was incorporated into Ca(2+)-supplemented Mueller-Hinton agar at subinhibitory concentrations, and synergy was screened by comparing test antibiotic Kirby-Bauer disks on agar with and without daptomycin. By this method, daptomycin with ampicillin-sulbactam, ticarcillin-clavulanate, or piperacillin-tazobactam showed synergy comparable to or greater than daptomycin with oxacillin. For seven of the eight strains tested, time-kill studies confirmed synergy between daptomycin and ampicillin-sulbactam with ampicillin in the range of 2 to 8 microg/ml. The combination of daptomycin and beta-lactams may be useful for the treatment of MRSA infection, but further studies are needed to elucidate the mechanisms and to determine the in vivo efficacy of the combination.     

In vitro and in vivo efficacy of the combination trimethoprim-sulfamethoxazole against clinical isolates of methicillin-resistant Staphylococcus aureus.

The in vitro susceptibilities of 16 independent, geographically distinct clinical isolates of methicillin-resistant Staphylococcus aureus to trimethoprim (TMP) in combination with sulfamethoxazole (SMX) were evaluated. Although methicillin-resistant S. aureus strains appear to be universally resistant to SMX, the combination TMP-SMX was found to be synergistic in vitro (in combination, the MICs of both drugs decreased 6- to 25-fold) as well as in vivo (5- to 6-fold reduction in TMP at 50% effective doses).