Efficacy of daptomycin versus vancomycin in an experimental model of foreign-body and systemic infection caused by biofilm producers and methicillin-resistant Staphylococcus epidermidis.

Staphylococcus epidermidis is a frequent cause of device-associated infections. In this study, we compared the efficacy of daptomycin versus vancomycin against biofilm-producing methicillin-resistant S. epidermidis (MRSE) strains in a murine model of foreign-body and systemic infection. Two bacteremic biofilm-producing MRSE strains were used (SE284 and SE385). The MIC of daptomycin was 1 mg/liter for both strains, and the MICs of vancomycin were 4 and 2 mg/liter for SE284 and for SE385, respectively. The in vitro bactericidal activities of daptomycin and vancomycin were evaluated by using time-kill curves. The model of foreign-body and systemic infection of neutropenic female C57BL/6 mice was used to ascertain in vivo efficacy. Animals were randomly allocated into three groups (n = 15): without treatment (controls) or treated with daptomycin at 50 mg/kg/day or vancomycin at 440 mg/kg/day. In vitro, daptomycin showed concentration-dependent bactericidal activity, while vancomycin presented time-dependent activity. In the experimental in vivo model, daptomycin and vancomycin decreased liver and catheter bacterial concentrations (P < 0.05) and increased the survival and the number of sterile blood cultures (P < 0.05) using both strains. Daptomycin produced a reduction in the bacterial liver concentration higher than 2.5 log(10) CFU/g compared to vancomycin using both strains, with this difference being significant (P < 0.05) for infection with SE385. For the catheter bacterial concentrations, daptomycin reduced the concentration of SE284 3.0 log(10) CFU/ml more than did vancomycin (P < 0.05). Daptomycin is more effective than vancomycin for the treatment of experimental foreign-body and systemic infections by biofilm-producing methicillin-resistant S. epidermidis.

Activity of ciprofloxacin and levofloxacin in experimental pneumonia caused by Klebsiella pneumoniae deficient in porins, expressing active efflux and producing QnrA1.

The objective of this study was to evaluate the activities of ciprofloxacin and levofloxacin in a murine model of pneumonia caused by Klebsiella pneumoniae C2 (with altered GyrA, deficient in porins and expressing active efflux of quinolones) and the transconjugant C2pMG252 derived from it and expressing the qnrA1 determinant. MICs and MBCs of the two quinolones were determined according to CLSI guidelines. Time-kill curves (at 1x and 4x MIC) were also performed to assess bactericidal activity. An experimental model of pneumonia in mice was evaluated. Groups of 15 mice were infected with either strain and treated with ciprofloxacin (80 mg/kg/day) or levofloxacin (100 mg/kg/day). Control non-treated animals were also evaluated. In the case of strain C2, log(10) CFU/g of lung in non-treated animals was 9.16 +/- 2.16. This value was reduced to 3.53 +/- 1.04 (p <0.001) and 3.38 +/- 0.46 (p <0.001) in animals treated with ciprofloxacin or levofloxacin, respectively. Percentages of surviving mice were 26.7% (control group) and 100% (both ciprofloxacin and levofloxacin; p <0.001 vs. controls). Bacterial counts (log(10) CFU/g) in lungs of animals infected with strain C2pMG252 were 9.65 +/- 2.49 in non-treated animals and 7.74 +/- 2.67 and 7.57 +/- 3.84 for those treated with ciprofloxacin or levofloxacin, respectively (p >0.05 vs. control group). Of non-treated animals infected with strain C2pMG252, 14.3% survived. Ciprofloxacin and levofloxacin improved the survival in these mice (53.3% for both antimicrobials, p 0.03). In conclusion, the expression of qnrA1 in K. pneumoniae with additional mechanisms of resistance causes decreased efficacy of fluoroquinolones in a pneumonia model in mice.

Efficacy of amoxycillin-clavulanate in an experimental model of murine pneumonia caused by AmpC-non-hyperproducing clinical isolates of Escherichia coli resistant to cefoxitin.

The algorithms included in most automated systems used for antimicrobial susceptibility testing (e.g., Vitek 2) consider that Escherichia coli isolates resistant to cefoxitin are AmpC-hyperproducers and, consequently, resistant also to amoxycillin-clavulanate. However, a recent study revealed that 30% of E. coli clinical isolates resistant to cefoxitin remained susceptible in vitro to amoxycillin-clavulanate. The aim of the present study was to evaluate the in-vivo efficacy of amoxycillin-clavulanate in the treatment of an experimental model of pneumonia, using two clonally related isolates (with identical repetitive extragenic palindromic sequence (REP)-PCR patterns) of AmpC-non-hyperproducing and OmpF-lacking E. coli (Ec985 and Ec571) that were resistant to cefoxitin and susceptible to cefotaxime and amoxycillin-clavulanate. MICs were determined using a microdilution technique, and in-vitro bactericidal activity was tested using time-kill assays. The in-vivo efficacy of amoxycillin, amoxycillin-clavulanate and cefotaxime against both isolates was tested in a murine pneumonia model using immunocompetent C57BL/6 mice. Ec571 (a TEM-1/2 producer) was resistant to amoxycillin, whereas Ec985 (a TEM-1/2 non-producer) was susceptible. Amoxycillin, amoxycillin-clavulanate and cefotaxime were bactericidal for Ec985, and amoxycillin-clavulanate and cefotaxime were bactericidal for Ec571 at different concentrations and time-points, as determined using time-kill assays. Treatment with amoxycillin, amoxycillin-clavulanate and cefotaxime reduced the bacterial lung concentration of Ec985 compared with non-treated controls (p <0.05), whereas amoxycillin-clavulanate and cefotaxime showed efficacy against Ec571 when compared with the control and amoxycillin groups (p <0.05). Regardless of the exact underlying mechanism(s) of resistance, amoxycillin-clavulanate was effective in the experimental murine model in the treatment of pneumonia caused by AmpC-non-hyperproducing strains of E. coli resistant to cefoxitin.

Pharmacokinetic/pharmacodynamic assessment of the in-vivo efficacy of imipenem alone or in combination with amikacin for the treatment of experimental multiresistant Acinetobacter baumannii pneumonia.

A guinea-pig pneumonia model involving imipenem-susceptible and imipenem-resistant strains of Acinetobacter baumannii was developed to assess the in-vitro and in-vivo activities of imipenem, alone or in combination with amikacin, and the pharmacokinetic and pharmacodynamic parameters. Serum levels were measured by bioassay (imipenem) or immunoassay (amikacin), followed by calculation of pharmacokinetic and pharmacodynamic parameters (Cmax, AUC, t1/2, Cmax/MIC, AUC/MIC, and Deltat/MIC). In-vivo efficacy was evaluated by comparing bacterial counts in the lungs of treatment groups with end-of-therapy controls by anova and post-hoc tests. Decreases in the Cmax (13.4%), AUC (13%), t1/2 (25%) and Deltat/MIC (11.8-32.2%) of imipenem were observed when it was administered with amikacin, compared with administration of imipenem alone. Similarly, decreases in the Cmax (34.5%), AUC (11.6%), Cmax/MIC (34.5%) and AUC/MIC (11.7%) of amikacin were observed when it was administered with imipenem. Bacterial counts in lungs were reduced by imipenem (p 0.004) with the imipenem-susceptible strain, and by amikacin (p 0.001) with the imipenem-resistant strain. The combination of imipenem plus amikacin was inferior to imipenem alone with the imipenem-susceptible strain (p 0.01), despite their in-vitro synergy, and was inferior to amikacin alone with the imipenem-resistant strain (p < 0.0001). In summary, combined use of imipenem with amikacin was less efficacious than monotherapy, probably because of a drug-drug interaction that resulted in decreased pharmacokinetic and pharmacodynamic parameters for both antimicrobial agents.

Activity of cefepime and carbapenems in experimental pneumonia caused by porin-deficient Klebsiella pneumoniae producing FOX-5 beta-lactamase.

The in-vivo activities of cefepime, imipenem and meropenem against the porin-deficient strain Klebsiella pneumoniae C2 and its derivative K. pneumoniae C2(pMG252) coding for the AmpC-type beta-lactamase FOX-5 were determined. Bactericidal activities were determined with the kill-curve method. A pneumonia model in guinea-pigs was developed, and Cmax, t(1/2) and DeltaT/MIC were calculated for the three agents tested. Animals were treated for 72 h with sterile saline (control group) or with cefepime, imipenem or meropenem (240 mg/kg/day, intramuscularly, three times daily). Bacterial counts in lungs (log10 CFU/g tissue) were determined by serial dilution. MICs (mg/L) of cefepime, imipenem and meropenem against K. pneumoniae C2/K. pneumoniae C2(pMG252), determined by macrodilution, were: 0.5/4, 0.5/0.5 and 0.25/0.5, respectively. Bacterial counts in the lungs of animals infected with K. pneumoniae C2 and treated with antimicrobial agents were always lower than in the control group (cefepime, 4.4 +/- 0.5; imipenem, 4.6 +/- 0.4; meropenem, 4.7 +/- 0.5; control group, 5.6 +/- 0.8; p <0.01). No significant differences were observed among the groups receiving therapy (p >0.05). Bacterial lung clearance was higher in treated animals than in control animals following infection with K. pneumoniae C2(pMG252) (cefepime, 4.5 +/- 0.4; imipenem, 4.0 +/- 0.3; meropenem, 4.6 +/- 0.4; control group, 6.1 +/- 0.6; p <0.01), with imipenem producing better clearance than either cefepime or meropenem (p <0.05). Thus, in the guinea-pig pneumonia model, cefepime, imipenem and meropenem were each effective against the porin-deficient K. pneumoniae strain C2 and its derivative expressing the plasmid-mediated AmpC type beta-lactamase FOX-5.

Colistin efficacy in an experimental model of Acinetobacter baumannii endocarditis.

The in-vivo activity of colistin was evaluated in an experimental rabbit model of Acinetobacter baumannii endocarditis with a strain susceptible to colistin and intermediate to imipenem. Compared to a control group, colistin was effective (p < 0.05) in bacterial clearance from blood and in the sterilisation of blood cultures, but was not effective in clearing A. baumannii from vegetations. Thus, although colistin may be effective in treating bacteraemia caused by susceptible strains of A. baumannii, it may not be a suitable treatment for endocarditis, perhaps because of poor penetration into vegetations and a low C(max)/MIC ratio in tissue.

Sulbactam efficacy in experimental models caused by susceptible and intermediate Acinetobacter baumannii strains.

Sulbactam and imipenem were compared in an experimental pneumonia model in immunocompetent mice, using a susceptible strain of Acinetobacter baumannii, and in an experimental endocarditis model in rabbits, using an intermediately susceptible strain. In the former, sulbactam was as efficacious as imipenem in terms of survival, sterility of lungs and in the bacterial clearance from lungs and blood, provided that the t > MIC for sulbactam (1.84 h) was similar to that for imipenem (2.01 h). In the endocarditis model, imipenem (t > MIC, 2.12 h) was more efficacious than sulbactam (t > MIC, 1.17 h) in bacterial clearance from vegetations. These results show the efficacy of sulbactam in infections caused by susceptible strains of A. baumannii, with an MIC up to 4 mg/L, provided that doses reach a t > MIC similar to that of imipenem. The activity of sulbactam was time dependent.

Imipenem, doxycycline and amikacin in monotherapy and in combination in Acinetobacter baumannii experimental pneumonia.

Acinetobacter baumannii is a common cause of nosocomial pneumonia and other nosocomial infections. Multiresistant A. baumannii has also a high prevalence, which can make effective treatment difficult. We designed a new model of A. baumannii experimental pneumonia using C57BL/6 immunocompetent mice. This model was used to compare the efficacy of imipenem, doxycycline and amikacin in monotherapy, and the combination of imipenem plus amikacin and doxycycline plus amikacin. Doxycycline plus amikacin were synergic in vitro after 24 h incubation, whereas imipenem plus amikacin showed no in vitro synergy. The number of sterile lungs and the lung clearance of A. baumannii were greater in the group treated with imipenem than in those treated with amikacin or doxycycline in monotherapy (P < 0.05). The combination of imipenem plus amikacin and doxycycline plus amikacin was no more effective than imipenem alone in the clearance of organisms from lungs (2.42 +/- 1.46 cfu/g versus 2.7 +/- 1.5 cfu/g versus 1.23 +/- 1.02 cfu/g). These results suggest that the addition of amikacin does not improve the results obtained by imipenem monotherapy. Doxycycline plus amikacin is an alternative to imipenem in the therapy of A. baumannii pneumonia.