In vitro bactericidal activity of piperacillin,gentamicin, and metronidazole in a mixed model containing Escherichia coli,Enterococcus faecalis,and Bacteroides fragilis

An anaerobic, mixed model assay was used to study the bactericidal activities of piperacillin, gentamicin, and metronidazole, alone and in double- and triple-antibiotic combinations against a polymicrobial suspension of E. coli, E. faecalis, and B. fragilis. Only slight differences were noted with the agents when tested against single (10(5) cfu/mL inoculum) versus polymicrobic suspensions (10(6) cfu/mL final inoculum) of susceptible and resistant organisms. Contrary to previous reports in the literature, metronidazole was not active against E. coli in an anaerobic environment (even in the presence of B. fragilis) nor was the activity of metronidazole reduced against B. fragilis in the presence of E. faecalis. Gentamicin demonstrated excellent activity against E. coli when tested in a Bactron anaerobic chamber (5% hydrogen, 5% CO(2,) 90% nitrogen). The pH of the media was only reduced to 6.3-6.7, considerably higher than the pH range of 5-6 needed to significantly reduce the activity of aminoglycosides.     

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.     

Evaluation of fosfomycin alone and in combination with ceftriaxone or vancomycin in an experimental model of meningitis caused by two strains of cephalosporin-resistant Streptococcus pneumoniae

OBJECTIVES: To study the in vitro and in vivo efficacy of fosfomycin, alone and in combination with ceftriaxone or vancomycin, against two strains of Streptococcus pneumoniae: HUB 2349 (fosfomycin and ceftriaxone, MICs 16 and 2 mg/L) and ATCC 51916 (MICs 4 and 32 mg/L). METHODS: Pharmacokinetics/pharmacodynamics data were collected from the study of eight infected animals after a single intravenous dose of 300 mg/kg of fosfomycin. Time-kill curves were plotted using CSF antibiotic concentrations achievable clinically. In the rabbit model, we studied the efficacy and effects on inflammation of treatment with fosfomycin 1200 mg/kg/day, ceftriaxone 100 mg/kg/day and vancomycin 30 mg/kg/day, over 26 h. RESULTS: Fosfomycin peak level in serum was 324.48 +/- 102.1 mg/L at 0.5 h; CSF penetration was 49.2%. Time-kill curves showed that fosfomycin was bactericidal against the ATCC 51916 strain and that the addition of fosfomycin to ceftriaxone or vancomycin was synergic against the HUB 2349 strain. Resistance to fosfomycin was detected both when fosfomycin was studied alone and in combination. In the rabbit model, fosfomycin showed bactericidal activity only against the ATCC 51916 strain. Combinations of fosfomycin with ceftriaxone or vancomycin were bactericidal against both strains; they improved efficacy and decreased CSF inflammatory parameters over monotherapies, without showing statistical differences in comparison with the combination of ceftriaxone and vancomycin. CONCLUSIONS: Fosfomycin in combination with ceftriaxone or vancomycin appeared to be effective for the treatment of experimental cephalosporin-resistant pneumococcal meningitis. These combinations are possible alternatives in cases of allergy or intolerance to first-line drugs or in rare meningitis caused by highly cephalosporin-resistant pneumococci.     

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.     

Activities of Fosfomycin,Tigecycline, Colistin,and Gentamicin against Extended-Spectrum-β-Lactamase-Producing Escherichia coli in a Foreign-Body Infection Model

Limited antimicrobial agents are available for the treatment of implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli. We compared the activities of fosfomycin, tigecycline, colistin, and gentamicin (alone and in combination) against a CTX-M15-producing strain of Escherichia coli (Bj HDE-1) in vitro and in a foreign-body infection model. The MIC and the minimal bactericidal concentration in logarithmic phase (MBC_log) and stationary phase (MBC_stat) were 0.12, 0.12, and 8 μg/ml for fosfomycin, 0.25, 32, and 32 μg/ml for tigecycline, 0.25, 0.5, and 2 μg/ml for colistin, and 2, 8, and 16 μg/ml for gentamicin, respectively. In time-kill studies, colistin showed concentration-dependent activity, but regrowth occurred after 24 h. Fosfomycin demonstrated rapid bactericidal activity at the MIC, and no regrowth occurred. Synergistic activity between fosfomycin and colistin in vitro was observed, with no detectable bacterial counts after 6 h. In animal studies, fosfomycin reduced planktonic counts by 4 log₁₀ CFU/ml, whereas in combination with colistin, tigecycline, or gentamicin, it reduced counts by >6 log₁₀ CFU/ml. Fosfomycin was the only single agent which was able to eradicate E. coli biofilms (cure rate, 17% of implanted, infected cages). In combination, colistin plus tigecycline (50%) and fosfomycin plus gentamicin (42%) cured significantly more infected cages than colistin plus gentamicin (33%) or fosfomycin plus tigecycline (25%) (P < 0.05). The combination of fosfomycin plus colistin showed the highest cure rate (67%), which was significantly better than that of fosfomycin alone (P < 0.05). In conclusion, the combination of fosfomycin plus colistin is a promising treatment option for implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli.     

Metronidazole and gentamicin prophylaxis in synergistic aerobe-anaerobe infections in mice

The efficacy of prophylaxis by intraperitoneal injection of gentamicin, metronidazole or both drugs combined was investigated in a mouse model of mixed infection with Escherichia coli and Bacteroides fragilis. A three dose prophylactic regimen given 12 hourly was studied starting simultaneously with bacterial challenge. Serum antibiotic concentrations lay within the human therapeutic ranges. Metronidazole alone eliminated B. fragilis from the developing infections, but at higher challenge doses the mice developed abscesses which contained E. coli, and mortality was no different from controls. Gentamicin alone eliminated both bacteria and deaths in all infections, but sterile abscesses occurred in two of five animals at the highest challenge dose. Mixed metronidazole + gentamicin abolished all effects of the challenge bacteria, aborting all infections for all challenge doses with no formation of sterile abscesses. It is proposed that, in abdominal surgery, although metronidazole or gentamicin may be effective given alone for low levels of contamination, for higher levels (e.g. colonic surgery) both drugs may be required.     

Penetration of cefixime into fibrin clots and in vivo efficacy against Escherichia coli, Klebsiella pneumoniae, and Staphylococcus aureus.

The experimental model of infected fibrin clots in rabbits was used to study the penetration and in vivo activity of cefixime against Klebsiella pneumoniae, Escherichia coli, and Staphylococcus aureus. The respective MICs of cefixime against these strains were 0.25, 2, and 8 micrograms/ml. The clots were infected with 10(6) to 10(8) CFU/g. Groups of four animals for each strain received an intravenous injection of 100 mg of cefixime per kg over 30 min. High peak levels were observed in serum (146.5 micrograms/ml) and clots (15.8 micrograms/g), and the antibiotic was still detectable in the clots (0.6 micrograms/g) 24 h after administration. The respective serum and clot elimination half-lives were 0.7 and 5.0 h. The mean serum protein binding was 23.8 +/- 3.8%. Cefixime was highly bactericidal against K. pneumoniae and E. coli and reduced, over a 24-h period, their respective colony counts by 7.8 log10 and 6.2 log10 CFU/g of fibrin. It was less effective against S. aureus but still reduced the bacterial counts by 2.8 log10 CFU/g of fibrin. The present results demonstrate that cefixime, a new broad-spectrum oral cephalosporin, has a long tissue half-life which ensured, at the dose given here, good in vivo bactericidal activity against both gram-positive and gram-negative bacteria up to 24 h after administration of the antibiotic.     

磷霉素氨丁三醇对尿标本中肠杆菌科细菌体外抗菌活性研究

目的了解磷霉素氨丁三醇对尿标本中肠杆菌科细菌的体外抗菌活性,指导临床合理用药。方法采用ATB系统对中段尿标本分离的1185株肠杆菌科细菌进行鉴定,纸片扩散法进行药敏试验和ESBI．S的检测,结果按CLSI2012年版标准判断;应用WHONET5．6软件分析病原菌的分布及耐药情况。结果1185株肠杆菌科细菌对磷霉素氨丁三醇的敏感率为90．8％,其中大肠埃希菌929株（78．4％）、肺炎克雷伯菌124株（10．5％）和奇异变形杆菌69株（5．8％）对磷霉素氨丁三醇的敏感率分别为93．1％、88．7％和79．7％。住院患者尿标本分离的肠杆菌科细菌对磷霉素氨丁三醇的敏感率低于门诊患者（87．0％对92．7％）。大肠埃希菌、肺炎克雷伯菌和奇异变形杆菌ESBLs的检出率分别为54．4％、38．7％和23．2％。产ESBLs的大肠埃希菌和肺炎克雷伯菌对磷霉素氨丁三醇的敏感率分别为89．5％和87．5％。结论尿标本培养获得的肠杆菌科细菌以大肠埃希菌为主,磷霉素氨丁三醇对尿标本分离的产ESBLs的大肠埃希菌和肺炎克雷伯菌的肠杆细菌科细菌具有良好的体外抗菌活性,可作为临床经验用药之一。     

Mutators among CTX-M beta-lactamase-producing Escherichia coli and risk for the emergence of fosfomycin resistance

OBJECTIVES: Fosfomycin is a possible oral treatment for lower urinary tract infections caused by Escherichia coli with CTX-M extended-spectrum beta-lactamases but is vulnerable to mutational resistance. Hypermutability among natural E. coli populations might facilitate the emergence of resistance to fosfomycin. We therefore examined the prevalence of mutators amongst urinary isolates of E. coli producing CTX-M beta-lactamases. METHODS: Urinary E. coli isolates with CTX-M beta-lactamases (n = 220) were screened for resistance to both rifampicin and fosfomycin, as well as a mutator phenotype, by rifampicin and fosfomycin disc assays. Mutation frequencies for 10 isolates, identified as mutators by the initial disc screen, were determined in triplicate on agar with rifampicin or fosfomycin at 4x MIC and with fosfomycin or nitrofurantoin at 256 mg/L. RESULTS: The disc screen identified 10 likely mutators and quantitative tests indicated that 9 of these had mutation frequencies of 8.0 x 10(-6)-1.5 x 10(-4) for fosfomycin and 0.1-2.3 x 10(-6) for rifampicin. These mutators were diverse in terms of PFGE type and 4 of the 10 were confirmed as strong mutators with rifampicin and fosfomycin. Only the strongest mutator isolate and hypermutable MutS(-) control strain consistently gave single-step mutants resistant to 256 mg/L fosfomycin. No nitrofurantoin-resistant mutants were selected from any isolate, although they could be selected from the hypermutable MutS(-) control strain. CONCLUSIONS: Mutator phenotypes were found among E. coli expressing CTX-M beta-lactamases and were independent of strain type. These had an increased propensity to fosfomycin resistance.     

Activities of Fosfomycin and Rifampin on Planktonic and Adherent Enterococcus faecalis Strains in an Experimental Foreign-Body Infection Model

Enterococcal implant-associated infections are difficult to treat because antibiotics generally lack activity against enterococcal biofilms. We investigated fosfomycin, rifampin, and their combinations against planktonic and adherent Enterococcus faecalis (ATCC 19433) in vitro and in a foreign-body infection model. The MIC/MBC_log values were 32/>512 μg/ml for fosfomycin, 4/>64 μg/ml for rifampin, 1/2 μg/ml for ampicillin, 2/>256 μg/ml for linezolid, 16/32 μg/ml for gentamicin, 1/>64 μg/ml for vancomycin, and 1/5 μg/ml for daptomycin. In time-kill studies, fosfomycin was bactericidal at 8× and 16× MIC, but regrowth of resistant strains occurred after 24 h. With the exception of gentamicin, no complete inhibition of growth-related heat production was observed with other antimicrobials on early (3 h) or mature (24 h) biofilms. In the animal model, fosfomycin alone or in combination with daptomycin reduced planktonic counts by ≈4 log₁₀ CFU/ml below the levels before treatment. Fosfomycin cleared planktonic bacteria from 74% of cage fluids (i.e., no growth in aspirated fluid) and eradicated biofilm bacteria from 43% of cages (i.e., no growth from removed cages). In combination with gentamicin, fosfomycin cleared 77% and cured 58% of cages; in combination with vancomycin, fosfomycin cleared 33% and cured 18% of cages; in combination with daptomycin, fosfomycin cleared 75% and cured 17% of cages. Rifampin showed no activity on planktonic or adherent E. faecalis, whereas in combination with daptomycin it cured 17% and with fosfomycin it cured 25% of cages. Emergence of fosfomycin resistance was not observed in vivo. In conclusion, fosfomycin showed activity against planktonic and adherent E. faecalis. Its role against enterococcal biofilms should be further investigated, especially in combination with rifampin and/or daptomycin treatment.     

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.     

Distribution and antimicrobial activity of fosfomycin in the interstitial fluid of human soft tissues

Fosfomycin is a broad-spectrum antibiotic which is established as therapy for uncomplicated lower urinary tract infections. In addition, preliminary data indicate that fosfomycin has a potential role in the treatment of soft tissue infections. However, the use of fosfomycin has not been established for this condition, and it is unclear whether the level of fosfomycin penetration into human soft tissues is high enough to eradicate relevant pathogens. To better characterize the antibiotic potential of fosfomycin, we applied a combined in vivo pharmacokinetic-in vitro pharmacodynamic model to human volunteers. For this purpose fosfomycin concentrations in vivo in the fluid of the interstitial space of human soft tissues were measured by microdialysis following intravenous infusion of 4 or 8 g of fosfomycin (n = 6). Subsequently, bacterial isolates with relevance for soft tissue infections were exposed to concentrations according to the in vivo pharmacokinetic profile in the interstitial space fluid obtained by microdialysis. Our experiments indicated a high degree of soft tissue penetration for fosfomycin, with ratios of the area under the concentration-time curve from 0 to 8 h for muscle (AUC(0-8(muscle)))/AUC(0-8(serum)) of 0.48+/-0.08 and 0.53+/-0.04 and ratios of AUC(0-8(adipose tissue))/AUC(0-8(serum)) of 0.74+/-0.12 and 0.71+/-0.11 following administration of 4 and 8 g, respectively. In corresponding in vitro simulation experiments with selected isolates of Staphylococcus aureus, Enterobacter cloacae, and Serratia marcescens for which MICs were 16 microg/ml, organisms were undetectable after a single dosing interval. Fosfomycin exhibits a strong ability to penetrate into the fluid of the interstitial space of soft tissues and reaches levels sufficient to substantially inhibit the growth of relevant bacteria at the target site. We therefore conclude that fosfomycin might qualify as an alternative candidate for the therapy of soft tissue infections.     

beta-Lactamase produced by a highly beta-lactam-resistant strain of Bacteroides fragilis: an obstacle to the chemotherapy of experimental mixed infections.

A strain of Bacteroides fragilis, which produces a metallo-beta-lactamase, was inoculated into pouches on the backs of rats together with a beta-lactamase-negative Escherichia coli highly sensitive to beta-lactam antibiotics. The mixed infection rat pouch model was treated with either flomoxef (susceptible to hydrolysis by the beta-lactamase produced by B. fragilis), or cefmetazole (relatively resistant to hydrolysis). In this model of mixed infection flomoxef showed weak in-vivo activity against E. coli, although showing the same strong activity in a model of single infection with E. coli. On the other hand, cefmetazole showed strong activity against E. coli, even in the model of mixed infection. The concentrations of both drugs in the pouches were decreased in infections with the strain of B. fragilis. There was a greater decrease in the concentration of flomoxef than of cefmetazole. Flomoxef was unstable whereas cefmetazole was relatively stable in the pouch exudates that had been infected with B. fragilis. These experimental data suggest that bacteria that produce a metallo-beta-lactamase decrease the in-vivo efficacy of beta-lactam antibiotics against other co-infecting bacteria. Thus, it is suggested that it is important in the chemotherapy of mixed bacterial infections that include these highly resistant beta-lactamase-producing bacteria to use antibiotics that are stable to hydrolysis by these enzymes.     

Effect of recombinant murine granulocyte colony-stimulating factor with or without fluoroquinolone therapy on mixed-infection abscesses in mice

The aim of the study was to determine if immunomodulation of host defense with recombinant murine granulocyte colony-stimulating factor (G-CSF) improves the efficacy of trovafloxacin or moxifloxacin in abscesses containing Bacillus fragilis ATCC 23745 and different Escherichia coli strains varying in virulence. Treatment of mice inoculated with 10(7) CFU B. fragilis and 10(5) CFU low-virulence E. coli with either trovafloxacin (150 mg/kg/day every 24 hours, days 3 to 7) or moxifloxacin (96 mg/kg/day every 12 hours, days 3 to 7), significantly reduced the number of B. fragilis to 6.9 +/- 0.35 and 5.8 +/- 0.10 and that of E. coli to 4.9 +/- 0.09 and 4.2 +/- 0.07 log CFU/abscess for trovafloxacin and moxifloxacin, respectively, compared to controls (B. fragilis 8.7 and E. coli 7.4 log CFU/abscess) on day 8. Also, moxifloxacin was more potent than trovafloxacin. Addition of G-CSF prophylaxis (1 mug once on day -1) or therapy (1 mug/day on days 3 to 7) to fluoroquinolone treatment did not improve the efficacy of fluoroquinolone therapy alone. The effect of moxifloxacin with or without G-CSF prophylaxis on abscesses with a virulent hemolytic E. coli strain was also studied. In moxifloxacin-treated mice, 75% survived infection compared to 10% of controls. Combining moxifloxacin with G-CSF prophylaxis significantly decreased survival (30%) compared to moxifloxacin alone. In addition, G-CSF prophylaxis resulted in a threefold (E. coli) to 100-fold (B. fragilis) increased outgrowth in the abscesses of surviving mice. In conclusion, the addition of G-CSF to a fluoroquinolone is not advisable since, depending on the virulence of the E. coli strains, this might detrimentally influence the outcome of therapy.     

Norfloxacin penetration into subcutaneous tissue cage fluid in rabbits and efficacy in vivo.

The activity of norfloxacin was studied in vivo with steel net cages implanted subcutaneously in rabbits. Four weeks after implantation, two of four cages in each animal were inoculated with a strain of Escherichia coli (seven animals) or Klebsiella pneumoniae (six animals). Four animals in each group received oral treatment with norfloxacin for 7 days. Treatment was started 18 h after inoculation of the cages. Peak concentrations above the in vitro minimal inhibitory concentrations for the strains used were achieved in the fluid of 14 of 16 of the infected cages after the first norfloxacin dose. The penetration of norfloxacin into both infected and uninfected tissue cage fluid was significantly higher on treatment days 3 and 7 than on treatment day 1. No difference was observed between the concentrations in uninfected and infected cage fluids or between cage fluids infected with different organisms. The viable counts of E. coli and K. pneumoniae decreased from 2 X 10(3) to 2 X 10(8) CFU/ml of cage fluid to less than 10 CFU/ml in 10 of the infected cage fluids 12 h after the last dose of norfloxacin. Fluid from four cages still containing low numbers of viable bacteria at that time became free from bacteria (less than 10 CFU/ml) 1 to 4 days later. No regrowth was found in any cage fluid 7 days after the treatment period. The viable counts of E. coli or K. pneumoniae in five untreated control animals did not decrease during 8 to 14 days after inoculation of cage fluid. In comparison with cephalosporins and aminoglycosides studied previously with the same experimental method, norfloxacin penetrated better into cage fluid and more effectively reduced the viable counts of the organisms.     

Effect of moxifloxacin versus imipenem/cilastatin treatment on the mortality of mice infected intravenously with different strains of Bacteroides fragilis and Escherichia coli

OBJECTIVES: To study the effect of moxifloxacin versus imipenem/cilastatin (hereafter referred to as imipenem) treatment on the mortality of mice infected intravenously with different strains of Bacteroides fragilis and Escherichia coli. METHODS: Groups of 20 mice each were infected intravenously with different strains of B. fragilis [moxifloxacin and imipenem susceptible or resistant, and enterotoxin (ET) positive or negative] and E. coli (moxifloxacin and imipenem susceptible). Twenty-four hours post-infection, intravenous therapy with either moxifloxacin (2.0 mg twice a day) or imipenem (2.4 mg three times a day) was started and continued for 3 days. Control groups were left untreated. Survival rates were recorded at day 7 post-infection. At that time, surviving mice were killed and numbers of bacteria in the liver and kidneys were determined. RESULTS: If compared with untreated animals, mice treated with either moxifloxacin or imipenem showed significantly improved survival (P < 0.001). There was no significant difference (P = 0.97) in the survival rates comparing the two treatment regimens irrespective of the ET positivity or the susceptibility to moxifloxacin or imipenem of the infective B. fragilis strain. However, there was a tendency that B. fragilis was recovered more often from the liver and kidneys of mice infected with ET positive strains. CONCLUSIONS: The data show that moxifloxacin was as efficacious as imipenem in reducing the mortality rate of mice suffering from a severe systemic aerobic/anaerobic infection.     

Comparative pharmacokinetics of tromethamine fosfomycin and calcium fosfomycin in young and elderly adults.

The pharmacokinetics of two oral forms of fosfomycin, tromethamine (trometamol) salt and calcium salt, were studied in five young (age, 29 +/- 3 [standard deviation] years) and eight elderly (age, 72 +/- 6 years) adults. The subjects received a single 40-mg/kg (body weight) (approximately equal to 3-g) calcium fosfomycin dose and a 25-mg/kg (approximately equal to 2-g) tromethamine fosfomycin dose in fosfomycin acid form. Blood and urine samples were collected for 24 h. Antibiotic concentrations in serum and urine were measured by microbiological assay. In all subjects, the peak levels of the calcium salt in serum were two- to fourfold lower than those of the tromethamine salt (6 to 7 and 18 to 22 micrograms/ml, respectively), indicating poor intestinal absorption of the calcium form. The elimination half-life of the two oral forms was about 5 h in young adults, and the half-life was only moderately longer in elderly subjects, with large individual variations: 8.28 +/- 5.51 h for tromethamine fosfomycin and 11.80 +/- 6.86 h for calcium fosfomycin. In elderly subjects, absorption of the tromethamine salt form was not modified, but the time to peak level was delayed for the calcium salt (2.58 +/- 0.54 h versus 1.41 +/- 0.67 h in young adults). Pharmacokinetic elimination of the two forms of fosfomycin was only moderately affected in elderly subjects; we observed lower urinary elimination, about 58 versus 28% of the dose in 24-h urines for the tromethamine salt and decreased renal clearance of both forms. However, the dosages of tromethamine and calcium fosfomycin need not be adjusted for elderly subjects who have endogenous creatinine clearances above 50 ml/min per 1.73 m2.     

High Activity of Fosfomycin and Rifampin against Methicillin-Resistant Staphylococcus aureus Biofilm In Vitro and in an Experimental Foreign-Body Infection Model

Increasing antimicrobial resistance reduces treatment options for implant-associated infections caused by methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the activity of fosfomycin alone and in combination with vancomycin, daptomycin, rifampin, and tigecycline against MRSA (ATCC 43300) in a foreign-body (implantable cage) infection model. The MICs of the individual agents were as follows: fosfomycin, 1 μg/ml; daptomycin, 0.125 μg/ml; vancomycin, 1 μg/ml; rifampin, 0.04 μg/ml; and tigecycline, 0.125 μg/ml. Microcalorimetry showed synergistic activity of fosfomycin and rifampin at subinhibitory concentrations against planktonic and biofilm MRSA. In time-kill curves, fosfomycin exhibited time-dependent activity against MRSA with a reduction of 2.5 log₁₀ CFU/ml at 128 × the MIC. In the animal model, planktonic bacteria in cage fluid were reduced by <1 log₁₀ CFU/ml with fosfomycin and tigecycline, 1.7 log₁₀ with daptomycin, 2.2 log₁₀ with fosfomycin-tigecycline and fosfomycin-vancomycin, 3.8 log₁₀ with fosfomycin-daptomycin, and >6.0 log₁₀ with daptomycin-rifampin and fosfomycin-rifampin. Daptomycin-rifampin cured 67% of cage-associated infections and fosfomycin-rifampin cured 83%, whereas all single drugs (fosfomycin, daptomycin, and tigecycline) and rifampin-free fosfomycin combinations showed no cure of MRSA cage-associated infections. No emergence of fosfomycin resistance was observed in animals; however, a 4-fold increase in fosfomycin MIC (from 2 to 16 μg/ml) occurred in the fosfomycin-vancomycin group. In summary, the highest eradication of MRSA cage-associated infections was achieved with fosfomycin in combination with rifampin (83%). Fosfomycin may be used in combination with rifampin against MRSA implant-associated infections, but it cannot replace rifampin as an antibiofilm agent.     

Pharmacokinetics and bacteriological efficacy of N-formimidoyl thienamycin in experimental Escherichia coli meningitis.

The pharmacokinetics and bacteriological efficacy of N-formimidoyl thienamycin were determined in rabbits infected with Escherichia coli K1. After a single intravenous dose of 25 mg/kg, a peak N-formimidoyl thienamycin concentration in cerebrospinal fluid (CSF) of 2.5 micrograms/ml was attained at 45 min. The penetration into CSF was calculated to be 31%. In animals that received continuous intravenous infusions of the drug for 9 h, the mean CSF concentration was 8.2 microgram/ml, and the CSF bactericidal titers against the E. coli K1 strain were from 1:16 to 1:32. This infusion produced a reduction in the numbers of E. coli in the CSF of 4 log10 colony-forming units per ml. N-Formimidoyl thienamycin might prove to be useful for therapy of meningitis caused by E. coli and other susceptible bacteria.     

In vitro activity of fosfomycin against extended-spectrum-beta-lactamase-producing Escherichia coli and Klebsiella pneumoniae: comparison of susceptibility testing procedures

The agar dilution, broth microdilution, and disk diffusion methods were compared to determine the in vitro susceptibility of 428 extended-spectrum-beta-lactamase (ESBL)-producing Escherichia coli and Klebsiella pneumoniae to fosfomycin. Fosfomycin showed very high activity against all ESBL-producing strains. Excellent agreement between the three susceptibility methods was found for E. coli, whereas marked discrepancies were observed for K. pneumoniae.