Combined activity of metronidazole and gentamicin on Bacteroides fragilis in vivo and in vitro

A mixed aerobic/anaerobic infection in mice was established by co-injecting Bacteroides fragilis and Escherichia coli into a thigh muscle. Metronidazole administration 6 h after this inoculation resulted in a dose-dependent decrease of Bact. fragilis at 18 h after drug administration. Co-administration of gentamicin with metronidazole resulted in a significant decrease of the numbers of E. coli but did not markedly influence the effect of metronidazole on Bact. fragilis.     

Levofloxacin plus metronidazole administered once daily versus moxifloxacin monotherapy against a mixed infection of Escherichia coli and Bacteroides fragilis in an in vitro pharmacodynamic model

Moxifloxacin has been suggested as an option for monotherapy of intra-abdominal infections. Recent data support the use of a once-daily metronidazole regimen. The purpose of this study was to investigate the activity of levofloxacin (750 mg every 24 h [q24h]) plus metronidazole (1,500 mg q24h) compared with that of moxifloxacin (400 mg q24h) monotherapy in a mixed-infection model. By using an in vitro pharmacodynamic model in duplicate, Escherichia coli and Bacteroides fragilis were exposed to peak concentrations of 8.5 mg of levofloxacin/liter q24h, 32 mg of metronidazole/liter q24h, and 2 mg for moxifloxacin/liter q24h for 24 h. The activities of levofloxacin, metronidazole, moxifloxacin, and levofloxacin plus metronidazole were evaluated against E. coli, B. fragilis, and E. coli plus B. fragilis. The targeted half-lives of levofloxacin, metronidazole, and moxifloxacin were 8, 8, and 12 h, respectively. Time-kill curves were analyzed for time to 3-log killing, slope, and regrowth. Pre- and postexposure MICs were determined. The preexposure levofloxacin, metronidazole, and moxifloxacin MICs for E. coli and B. fragilis were 0.5 and 1, >64 and 0.5, and 1 and 0.25 mg/liter, respectively. Levofloxacin and moxifloxacin achieved a 3-log killing against E. coli and B. fragilis in all experiments, as did metronidazole against B. fragilis. Metronidazole did not decrease the starting inoculum of E. coli. The area under the concentration-time curve/MIC ratios for E. coli and B. fragilis were 171.7 and 85.9, respectively, for levofloxacin and 26 and 103.9, respectively, for moxifloxacin. Levofloxacin plus metronidazole exhibited the fastest rates of killing. The levofloxacin and moxifloxacin MICs for B. fragilis increased 8- to 16-fold after the organism was exposed to moxifloxacin. No other changes in the postexposure MICs were found. Levofloxacin plus metronidazole administered once daily exhibited activity similar to that of moxifloxacin against the mixed E. coli and B. fragilis infection. A once-daily regimen of levofloxacin plus metronidazole looks promising for the treatment of intra-abdominal infections.     

Comparison of the bactericidal activities of piperacillin-tazobactam, ticarcillin-clavulanate, and ampicillin-sulbactam against clinical isolates of Bacteroides fragilis, Enterococcus faecalis, Escherichia coli, and Pseudomonas aeruginosa.

Owing to the broad spectrum of activity afforded by beta-lactam-beta-lactamase inhibitor preparations, these agents are frequently selected as empiric therapy for the treatment of mixed infections such as intra-abdominal and diabetic foot infections, either alone or in combination with an aminoglycoside. Twelve healthy volunteers were enrolled in a randomized, open-label, four-way crossover trial comparing the bactericidal activities of piperacillin-tazobactam, ticarcillin-clavulanate, and ampicillin-sulbactam against microorganisms commonly isolated from mixed infections. Subjects received the following regimes: (i) 3.375 g of piperacillin-tazobactam intravenously (i.v.) every 6 h (q6h) (ii) 4.5 g of piperacillin-tazobactam i.v. q8h, (iii) 3.1 g of ticarcillin-clavulanate i.v. q6h, and (iv) 3.0 g of ampicillin-sulbactam i.v. q6h. Serum bactericidal titers were determined and used to calculate the duration of measurable bactericidal activity over the dosing interval of each of the regimens against two clinical isolates of Bacillus fragilis, Escherichia coli, Enterococcus faecalis, and Pseudomonas aeruginosa. The percentage of the dosing interval over which drug concentrations in serum remained above the MIC for each organism was determined and compared with the observed duration of bactericidal activity was noted (r = 0.78; P < 0.001). All of the regimens demonstrated good activity against B. fragilis and E. coli. Against E. faecalis and P. aeruginosa, however, all of the regimens provided bactericidal activity for less than 50% of the respective dosing intervals. These data suggest that use of shorter dosing intervals or continuous-infusion regimens should be considered in combination with an aminoglycoside to improve the bactericidal profiles of these agents for E. faecalis and P. aeruginosa.     

In vitro activity of trovafloxacin against Bacteroides fragilis in mixed culture with either Escherichia coli or a vancomycin- resistant strain of Enterococcus faecium determined by an anaerobic time-kill technique

To determine the efficacy of trovafloxacin as a possible treatment for intra-abdominal abscesses, we have developed an anaerobic time-kill technique using different inocula to study the in vitro killing of Bacteroides fragilis in pure culture or in mixed culture with either Escherichia coli or a vancomycin-resistant strain of Enterococcus faecium (VREF). With inocula of 5 x 10(5) CFU/ml and trovafloxacin concentrations of /=6.1 (log(10) CFU/ml) was attained with all pure and mixed cultures within 24 h. With inocula of 10(8) CFU/ml, a similar E(max) and a similar concentration to produce 50% of E(max) (EC(50)) for B. fragilis were found in both pure cultures and mixed cultures with E. coli. However, to produce a similar killing of B. fragilis in the mixed cultures with VREF, a 14-fold increase in the concentration of trovafloxacin was required. A vancomycin-susceptible strain of E. faecium and a trovafloxacin-resistant strain of E. coli were also found to confer a similar "protective" effect on B. fragilis against the activity of trovafloxacin. Using inocula of 10(9) CFU/ml, the activity of trovafloxacin was retained for E. coli and B. fragilis and was negligible against VREF. We conclude that this is a useful technique to study the anaerobic killing of mixed cultures in vitro and may be of value in predicting the killing of mixed infections in vivo. The importance of using mixed cultures and not pure cultures is clearly shown by the difference in the killing of B. fragilis in the mixed cultures tested. Trovafloxacin will probably be ineffective in the treatment of infections involving large numbers of enterococci. However, due to its ability to retain activity against large cultures of B. fragilis and E. coli, trovafloxacin could be beneficial in the treatment of intra-abdominal abscesses.     

In vivo efficacy of trovafloxacin against Bacteroides fragilis in mixed infection with either Escherichia coli or a vancomycin-resistant strain of Enterococcus faecium in an established-abscess murine model

The pharmacodynamic and pharmacokinetic properties of trovafloxacin were studied in a standardized murine model of established subcutaneous abscesses. Daily dosing regimens of 37.5 to 300 mg/kg every 8 h (q8h) or every 24 h (q24h) were started 3 days after inoculation with mixtures containing either Bacteroides fragilis-Escherichia coli-autoclaved cecal contents (ACC) or B. fragilis-vancomycin-resistant Enterococcus faecium (VREF)-ACC. Treatment was continued for 3 or 5 days. The efficacy of treatment was determined by the decrease in abscess bacterial counts and abscess weights, as well as by the reduction in inflammation (biodistribution of (99m)Tc-HYNIC immunoglobulin G) compared to saline-treated controls. Trovafloxacin showed a significant dose-response effect on the bacterial counts, weight, and inflammation of B. fragilis-E. coli abscesses after 3 and/or 5 days of treatment. A maximum 3.4 and 3.1 log(10) reduction in CFU/abscess in the respective B. fragilis and E. coli bacterial counts was attained after 5 days of treatment with daily doses of 300 mg/kg. The peak serum concentration was more predictive for effect than the area under the concentration-time curve. The C(max) was the pharmacodynamic index most predictive for success, and the efficacy of the q24h regimens was significantly better than the q8h regimens. The antibiotic was ineffective against the VREF in mixed infection with B. fragilis, while the killing of the anaerobe in the same combination was significantly less than in the E. coli combination (P < 0.05). We conclude that this is a useful model for studying the activity of antimicrobials for the treatment of small (<1-cm), undrainable, mixed-infection abscesses. In addition, we have shown for the first time that a decrease in bacterial numbers also leads to a reduction in both abscess weight and inflammation.     

甲磺酸加替沙星等3种抗菌药物的体外杀菌作用研究

目的：研究甲磺酸加替沙星体外杀菌曲线特点。方法：采用菌落计数法对1／4、1、4、8、16、64最低抑菌浓度(MIC)系列浓度的3种抗菌药物(甲磺酸加替沙星、阿米卡星、哌拉西林)进行体外杀菌实验。结果：甲磺酸加替沙星与阿米卡星随着抗菌药物浓度的升高。其杀菌时间明显缩短。而4MIC哌拉西林较1MIC哌拉西林的杀菌时间有所缩短，但较8MIC、16MIC、64MIC哌拉西林的杀菌时间及杀菌程度无统计学差异。结论：甲磺酸加替沙星与阿米卡星的杀菌曲线显示了浓度依赖性．哌拉西林则显示了非浓度依赖性的杀菌曲线。     

Bactericidal activity of gentamicin against Enterococcus faecalis in vitro and in vivo

The activity of gentamicin at various concentrations against two strains of Enterococcus faecalis was investigated in vitro and in a rabbit model of aortic endocarditis. In vitro, gentamicin at 0.5 to 4 times the MIC failed to reduce the number of bacteria at 24 h. Rabbit or human serum dramatically increased gentamicin activity, leading to a >/=3-log(10) CFU/ml decrease in bacterial counts when the drug concentration exceeded the MIC. Susceptibility testing in the presence of serum was predictive of in vivo activity, since gentamicin alone significantly reduced the number of surviving bacteria in the vegetations if the peak-to-MIC ratio was greater than 1. However, gentamicin selected resistant mutants in rabbits. The intrinsic activity of gentamicin should be taken into account in evaluation of combinations of gentamicin and cell wall-active agents against enterococci.     

Inactivation of cefuroxime and cefotaxime by Bacteroides fragilis in vitro, and its influence on the treatment of an experimental Escherichia coli/Bacteroides fragilis mixed infection

The activity of amoxycillin/clavulanic acid was compared with that of cefuroxime and cefotaxime, alone and combined with metronidazole, in preventing the development of infections in the mouse caused by subcutaneous injection of a beta-lactamase-producing strain, Escherichia coli E96, or a mixed inoculum of E. coli E96 and Bacteroides fragilis VPI 8908. Amoxycillin/clavulanic acid, cefuroxime and cefotaxime were equally efficacious in preventing development of the E. coli E96 monoinfection at clinically achievable concentrations. However, the activity of cefuroxime against E. coli E96 in the mixed infection was markedly reduced as was, to a lesser extent, that of cefotaxime. Co-administration of metronidazole improved slightly the activity of cefuroxime against E. coli E96 in the mixed infection, but had no such enhancing effect on cefotaxime. In contrast, amoxycillin/clavulanic acid effectively prevented development of the mixed infection in all treated mice. Results of in-vitro studies showed that cefuroxime and cefotaxime were stable in a culture of E. coli E96, and were rapidly bactericidal against this strain. In contrast, both cephalosporins were hydrolysed in a mixed culture of E. coli E96/Bact. fragilis VPI 8908, which resulted in diminished bactericidal activity, particularly of cefuroxime, which was not restored by addition of metronidazole. Amoxycillin/clavulanic acid was stable in the mixed culture and caused a significant reduction in numbers of both organisms. These in-vitro data explain the findings of the in-vivo studies, and establish that the beta-lactamase activity of Bact. fragilis VPI 8908 was responsible for the diminished activity of cefuroxime and cefotaxime, combined with metronidazole, against a mixed E. coli/Bact. fragilis infection.     

The in-vitro activity of metronidazole against Enterobacteriaceae alone and in mixed cultures with Bacteroides fragilis

The in-vitro antimicrobial action of therapeutic concentrations of metronidazole against Bacteroides fragilis and six different strains of Enterobacteriaceae in pure and mixed cultures have been studied. Under anaerobic conditions, metronidazole suppressed the growth of pure cultures of the Enterobacteriaceae. A reduction in the viable counts from 10(9) cfu/ml, in the 24 h controls, to 10(8), 10(7) and 10(5) cfu/ml in the presence of 10, 50 and 100 mg/l of metronidazole respectively, was observed. These concentrations of drug produced a marked bactericidal effect against B. fragilis, as expected. The antimicrobial activity of metronidazole on mixed cultures of B. fragilis and each one of the Enterobacteriaceae studied was greater against both micro-organisms than the corresponding effect on their respective pure cultures, under the same experimental conditions.     

In vitro bactericidal activity of antimicrobial agents against enterohaemorrhagic Escherichia coli

OBJECTIVES: In vitro bactericidal activity of four antimicrobial agents was determined against nine strains of enterohaemorrhagic Escherichia coli. METHODS: Pulsed-field gel electrophoresis was carried out with the Bio-Rad Gene Path system. Each antimicrobial agent was added to logarithmic phase of enterohaemorrhagic E. coli (four strains of E. coli O157:H7, two of E. coli O26, two of E. coli O111, and one of E. coli O165) in broth to obtain a concentration of 10 or 50 mg/L, and viable cells were counted after 1, 2, 6 and 24 h. RESULTS: All nine strains were confirmed to differ in their DNA pattern by pulsed-field gel electrophoresis. Norfloxacin at concentrations of 10 and 50 mg/L had bactericidal effects on all nine strains of enterohaemorrhagic E. coli. However, cefoperazone, kanamycin and fosfomycin had no bactericidal effects on some strains. In particular, after addition of 10 mg/L fosfomycin or kanamycin, four of the nine strains showed proliferation. CONCLUSIONS: Norfloxacin had marked bactericidal effects on enterohaemorrhagic E. coli. This information could be of value in planning randomized clinical trials of antimicrobial agents as treatment for enterohaemorrhagic E. coli infection.     

In vitro activity of the two principal oxidative metabolites of metronidazole against Bacteroides fragilis and related species.

Metronidazole and its two principal oxidative metabolites were tested in vitro against 20 clinical isolates of the Bacteroides fragilis group. Both metabolites were bactericidal, and the exhibited 65 and 5%, respectively, of the inhibitory effect of metronidazole. Additive or weak synergistic effects resulted in combination with the parent compound.     

Synergism between penicillin, clindamycin, or metronidazole and gentamicin against species of the Bacteroides melaninogenicus and Bacteroides fragilis groups

Clinical isolates of the Bacteroides melaninogenicus and Bacteroides fragilis groups were tested for in vitro and in vivo susceptibility to penicillin, clindamycin, and metronidazole, used singly or in combination with gentamicin. The in vitro tests consisted of determinations of minimal inhibitory concentrations (MICs) carried out with or without constant amounts of gentamicin. When used alone, gentamicin had negligible effects on the bacteria but significantly reduced the MICs of penicillin, clindamycin, and metronidazole against 11, 10, and 3, of the 15 strains of the B. melaninogenicus group, respectively. The 15 strains of the B. fragilis group were all beta-lactamase producers and were highly resistant to penicillin or the combination of penicillin and gentamicin. However, gentamicin reduced the MICs of clindamycin and metronidazole against 1 and 7 strains of this group, respectively. The in vivo tests were carried out in mice and consisted of measurements of the effects of the antimicrobial agents on the sizes and bacterial content of abscesses induced by subcutaneous injection of bacterial suspensions. The results of the in vivo tests were generally consistent with those obtained in vitro with strains of the B. melaninogenicus group. Synergism between gentamicin and penicillin, clindamycin, or metronidazole was shown in 13, 10, and 3 strains of this group, respectively. In vivo synergism was not clearly demonstrated with the strains of the B. fragilis group, possibly because clindamycin and metronidazole used alone were highly efficacious. We suggest that the synergistic effect of gentamicin is due to its increased transport into the bacterial cell in the presence of penicillin and, possibly, other antimicrobial agents. The newly recognized in vitro and in vivo synergism between penicillin and other antimicrobial agents and an aminoglycoside in B. melaninogenicus may have clinical implications that deserve to be investigated.     

Pharmacodynamics of pulse dosing versus standard dosing: in vitro metronidazole activity against Bacteroides fragilis and Bacteroides thetaiotaomicron

Pulse dosing is a novel approach to dosing that produces escalating antibiotic levels early in the dosing interval followed by a prolonged dose-free period. Antibiotic is frontloaded by means of four sequential bolus injections, after which antibiotic levels are allowed to diminish until the next dose. This study compares standard thrice-daily dosing and pulse dosing of metronidazole against Bacteroides spp. in an in vitro model. Two American Type Culture Collection Bacteroides fragilis isolates (metronidazole MIC for each organism = 1 mg/liter) were exposed to metronidazole for 48 or 96 h. Human pharmacokinetics were simulated for an oral 500-mg dose given every 8 h (maximum concentration of drug [C(max)] = 12 mg/liter; half-life = 8 h; area under the curve [AUC] = 294 mg . h/liter) and for pulse dosing. Pulses, each producing an increase in metronidazole concentration of 9 mg/liter, were administered at times 0, 2, 4, and 6 h of each 24-h cycle, with a targeted half-life of 8 h (AUC = 347 mg . h/liter). A metronidazole-resistant B. fragilis strain (metronidazole MIC = 32 mg/liter) was exposed to both dosing regimens and, additionally, to a regimen of 1,500 mg administered once daily (C(max) = 36 mg/liter; AUC = 364 mg . h/liter). Furthermore, regimens against one B. fragilis isolate and one B. thetaiotaomicron isolate corresponding to one-fourth and one-eighth of the thrice-daily and pulse dosing regimens, mimicking peak metronidazole concentrations achieved in abscesses, were simulated in 48-h experiments (metronidazole MIC = 1 mg/liter). Time-kill curves were generated for each experiment and analyzed for bactericidal activity, defined as a bacterial burden reduction >/= 3 log(10) CFU/ml. The results of paired (Wilcoxon matched-pair signed-rank test) and nonpaired (Mann-Whitney test) statistical analyses conducted on time to 3 log(10) kill data and area under the kill curve data from each of the thrice-daily dosing experiments versus each of the pulse dosing experiments were considered not significant for a given isolate-dosing regimen combination. The thrice-daily dosing, pulse dosing, and once-daily dosing regimens all exhibited bactericidal activity. Metronidazole administered in standard or pulse dosing fashion was highly active against both susceptible and resistant strains of Bacteroides spp.     

Effect of metronidazole on Escherichia coli in the presence of Bacteroides fragilis: an investigation in mice

The observation that, in rats, metronidazole exhibited antimicrobial activity against resistant Escherichia coli (when this was accompanied by susceptible Bacteroides fragilis) prompted us to attempt to reproduce this phenomenon in another species and under other experimental conditions. In experiment 1, mice injected intraperitoneally with an E. coli/B. fragilis mixture were treated with metronidazole, 250 mg/kg given by mouth at 0 and 10 h, or left untreated. At 24 h, viable counts of bacteria in blood and peritoneal washings were determined. In experiment 2, mice with 5-day-old subcutaneous abscesses containing E. coli and B. fragilis were also either given metronidazole as above or left untreated. At 24 h, viable counts of bacteria in pus were determined. Metronidazole affected neither the frequency with which E. coli persisted at the three sites, nor the viability of E. coli at these sites. This was so despite the fact that, in each of the sites, a significant B. fragilis kill was registered. Thus, pending elucidation of the mechanism by which the phenomenon operates, its non-appearance in these experiments cautions against the extrapolation of the original observations beyond the circumstances under which they were first made.     

头孢米诺等抗菌药物对大肠埃希菌、肺炎克雷伯菌及拟杆菌属的体外抗菌活性

目的 比较头孢米诺等抗菌药物对临床分离大肠埃希菌、肺炎克雷伯菌、拟杆菌属的体外抗菌活性.方法 琼脂稀释法测定16种抗菌药物对来自全国15家教学医院的945株大肠埃希菌和588株肺炎克雷伯菌的MIC值以及4种抗菌药物对50株拟杆菌属的MIC值.WHONET 5.4软件进行药敏数据统计分析.结果 1 533株大肠埃希菌和肺炎克雷伯菌中,不产超广谱β内酰胺酶(extended spectrum beta lactamases,ESBLs)和AmpC 628株,837株仅产ESBLs,68株产AmpC.头孢米诺对不产ESBLs或单产ESBLs的大肠埃希菌和肺炎克雷伯菌敏感率均高于90%,其MIC50较头孢美唑低2～4倍,较头孢西丁低8～16倍;MIC90较头孢美唑低2～8倍,较头孢西丁低8～16倍.对单产ESBLs的菌株,头孢米诺体外抗菌活性优于第三、四代头孢菌素、头孢哌酮/舒巴坦、氨曲南、左氧氟沙星和阿米卡星,劣于碳青霉烯类药物,活性与哌拉丙林/三唑巴坦相仿.但对产AmpC的菌株,头孢米诺的敏感率低于20%.头孢米诺对拟杆菌属的敏感率为90%,高于头孢美唑(50%～70%)和青霉素(0%),活性与甲硝唑相仿.结论 头孢米诺对产ESBLs及非产ESBLs的大肠埃希菌和肺炎克雷伯菌以及拟杆菌属有良好的体外抗菌活性,提示头孢米诺可为临床治疗此类菌株感染提供一种选择.     

头孢米诺等抗菌药物对大肠埃希菌、肺炎克雷伯菌及拟杆菌属的体外抗菌活性

目的 比较头孢米诺等抗菌药物对临床分离大肠埃希菌、肺炎克雷伯菌、拟杆菌属的体外抗菌活性.方法 琼脂稀释法测定16种抗菌药物对来自全国15家教学医院的945株大肠埃希菌和588株肺炎克雷伯菌的MIC值以及4种抗菌药物对50株拟杆菌属的MIC值.WHONET 5.4软件进行药敏数据统计分析.结果 1 533株大肠埃希菌和肺炎克雷伯菌中,不产超广谱β内酰胺酶(extended spectrum beta lactamases,ESBLs)和AmpC 628株,837株仅产ESBLs,68株产AmpC.头孢米诺对不产ESBLs或单产ESBLs的大肠埃希菌和肺炎克雷伯菌敏感率均高于90%,其MIC_(50)较头孢美唑低2～4倍,较头孢西丁低8～16倍;MIC90较头孢美唑低2～8倍,较头孢西丁低8～16倍.对单产ESBLs的菌株,头孢米诺体外抗菌活性优于第三、四代头孢菌素、头孢哌酮/舒巴坦、氨曲南、左氧氟沙星和阿米卡星,劣于碳青霉烯类药物,活性与哌拉丙林/三唑巴坦相仿.但对产AmpC的菌株,头孢米诺的敏感率低于20%.头孢米诺对拟杆菌属的敏感率为90%,高于头孢美唑(50%～70%)和青霉素(0%),活性与甲硝唑相仿.结论 头孢米诺对产ESBLs及非产ESBLs的大肠埃希菌和肺炎克雷伯菌以及拟杆菌属有良好的体外抗菌活性,提示头孢米诺可为临床治疗此类菌株感染提供一种选择.     

Efficacy of amoxycillin/clavulanic acid in experimental Bacteroides fragilis/Escherichia coli mixed infections

The efficacy of amoxycillin/clavulanic acid was compared with those of metronidazole, cefuroxime, metronidazole/ampicillin, metronidazole/gentamicin and metronidazole/cefuroxime, in experimental mixed infections produced in mice by subcutaneous inoculation of amoxycillin-resistant strains of Bacteroides fragilis and Escherichia coli. The combination of metronidazole/ampicillin failed to inhibit the growth of E. coli, and exerted only a transient effect on the numbers of Bact. fragilis in the groin abscesses. In contrast, amoxycillin/clavulanic acid prevented the development of the infection, eliminating both organisms. Metronidazole and cefuroxime, alone and in combination, were less effective than amoxycillin/clavulanic acid in inhibiting the growth of the infecting organisms. These results demonstrate the clinical potential of amoxycillin/clavulanic acid in prophylaxis, or in the therapy of mixed aerobe/anaerobe infections.     

Comparison of the bactericidal activity of clindamycin and metronidazole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group.

Time-kill kinetic methodology was used to evaluate the bactericidal activity of cefoxitin, cefotetan, clindamycin, and metronidizole against cefoxitin-susceptible and cefoxitin-resistant isolates of the Bacteroides fragilis group. Overall, metronidazole was the most bactericidal agent, with all isolates being killed with less than or equal to 4 micrograms/ml at 24 hr. Clindamycin was the next most bactericidal agent, with 20 of 26 isolates being killed with less than 16 micrograms/ml. Six isolates with clindamycin MICs greater than or equal to 64 micrograms/ml were not killed at 24 hr, with concentrations as high as 256 micrograms/ml. Cefoxitin and cefotetan were the least bactericidal agents tested. Seven isolates with MICs of greater than or equal to 64 micrograms/ml to each agent demonstrated a lack of killing at 24 hr, with concentrations of the respective agent as high as 256 micrograms/ml. At concentrations with either agent of 32 micrograms/ml, the remaining 19 isolates were killed at 24 hr. Of the six B. fragilis isolates resistant to clindamycin, four were also resistant to both cefoxitin and cefotetan. We conclude that in hospitals with cefoxitin-resistant B. fragilis group isolates, metronidazole would provide appropriate therapy.     

In vitro synergism between daptomycin and fosfomycin against Enterococcus faecalis isolates with high-level gentamicin resistance.

Daptomycin and fosfomycin are two agents which inhibit different steps in peptidoglycan synthesis. We studied the in vitro activities of these drugs, alone and in combination, by time-kill techniques against 21 clinical isolates of Enterococcus (Streptococcus) faecalis demonstrating high-level resistance to gentamicin. Combinations of fosfomycin and daptomycin exhibited synergistic bactericidal activity (100-fold decrease in CFU per milliliter at 24 h compared with daptomycin alone) against all strains (mean +/- standard deviation of increment in killing = 2.7 +/- 0.7 log10 CFU/ml). In a subgroup of strains against which daptomycin (5 micrograms/ml) alone was bactericidal (greater than 3 log10 killing), synergistic activity was demonstrable only when the concentration of daptomycin was lowered to 0.25 to 0.5 microgram/ml. A 50% dilution of human serum diminished the bactericidal activity of daptomycin alone at 24 h but did not affect killing observed with the daptomycin-fosfomycin combination. The inhibition of peptidoglycan synthesis by the combination was greater than the inhibition observed with either drug alone. The combination of daptomycin and fosfomycin exhibited consistent synergistic bactericidal activity against strains of E. faecalis possessing high-level resistance to gentamicin. This synergism may be the result of sequential inhibition of early steps in peptidoglycan synthesis.