Abstract
Limited antimicrobials remain active for treating severe infections due to KPC-producing pathogens, and optimal regimens have not been established. In murine thigh infections caused by nine KPC-producing clinical strains of Enterobacteriaceae (meropenem MICs, 1 to 4 μg/ml), we evaluated the activities of tigecycline, colistin, meropenem, rifampin, and gentamicin in single and combination regimens lasting for 24 h and 48 h. Rifampin, tigecycline, and gentamicin were the most effective monotherapies, reducing significantly the CFU counts yielded from thighs infected by 88.9 to 100%, 77.8 to 88.9%, and 66.7 to 88.9% of strains, respectively; meropenem and colistin alone exhibited considerably lower performance (significant CFU reduction in 33.3% and 22.2 to 33.3% of the strains, respectively). The addition of rifampin or gentamicin to tigecycline produced synergistic effect in most strains, while antagonism was observed in 33.3 to 44.4% of the strains when colistin was added to tigecycline and in 44.4 to 55.5% of the strains for meropenem combination with tigecycline. Tigecycline combinations with gentamicin or with rifampin caused higher CFU reductions than did tigecycline plus colistin or plus meropenem with almost all strains. Furthermore, tigecycline plus gentamicin was significantly more effective than tigecycline plus colistin or tigecycline plus meropenem in 33.3 to 44.4% and 55.5 to 66.7% of the strains, respectively, while tigecycline plus rifampin significantly outperformed tigecycline plus colistin and tigecycline plus meropenem in 33.3% and 66.7 to 77.8% of the strains, respectively. Overall, our in vivo study showed that tigecycline plus rifampin or plus gentamicin is a robust regimen against soft tissue infections caused by KPC-producing strains. The combinations of tigecycline with colistin or meropenem should be considered with caution in clinical practice.
INTRODUCTION
During the last decade, Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae have become common pathogens in many regions worldwide (1, 2). KPC enzymes confer various levels of resistance to all β-lactams, including carbapenems (2). Moreover, blaKPC genes are often linked with various non-β-lactam resistance determinants, further compromising the antibiotic alternatives that could be used for the treatment of clinically significant infections (2–4). It has been well documented from clinical reports that infections due to KPC producers are commonly associated with therapeutic failures and increased mortality rates (2, 3, 5, 6).
The optimal treatment for infections due to KPC-producing bacteria has not yet been well established (5). Susceptibility data suggest that the treatment of infections caused by KPC producers commonly requires the use of tigecycline, colistin, gentamicin, or meropenem as a last-resort drug (2, 5, 7). Regarding the therapeutic activity of tigecycline combinations against infections caused by Enterobacteriacae, data are scarce in the literature, which is derived mainly from in vitro studies on carbapenem-susceptible isolates and to a lesser degree from in vivo studies and human case reports (8–10). In particular, tigecycline combinations tested in vitro produced primarily an indifferent response (8). Nevertheless, in vitro synergy occurred when tigecycline was combined with rifampin against Enterobacter spp. and with amikacin against Enterobacter spp. or Klebsiella pneumoniae, while bactericidal synergisms occurred with tigecycline plus colistin against K. pneumoniae (11, 12). Data from clinical case reports, although still limited, displayed beneficial activity of tigecycline combined with colistin against K. pneumoniae bacteremia (12), while antagonism was extremely rare in vitro and was not reported in vivo (8). As to in vitro data from carbapenemase-producing Enterobacteriacae, we have previously reported that a degree of synergism may exist with tigecycline plus colistin but mainly at concentrations 4× MIC (13). Recent data for treatment outcomes of infections due to KPC producers suggested that antibiotic combinations in general proved superior to monotherapies (9, 14). Thus, the existing preliminary evidence suggests that tigecycline combinations with a second antimicrobial may prove useful for the treatment of infections due to KPC producers.
To ascertain the therapeutic value of tigecycline combinations against KPC infections, experimental animal studies would enable better delineation of antimicrobial effects. Previous in vitro data indicating that the combinations of tigecycline with either rifampin or an aminoglycoside were favorable led to a suggestion that they should be tested in animal infection models to establish their potential use in clinical situations (8). For this purpose, and based also on the available susceptibility results, we undertook experimental thigh infection studies using tigecycline, colistin, gentamicin, meropenem, and rifampin alone and in tigecycline combinations against KPC-producing Enterobacteriaceae clinical strains.
(This work was presented in part in the 52nd Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, CA, September 2012.)
MATERIALS AND METHODS
Bacterial strains and susceptibility testing.
The in vivo activities of tigecycline, colistin, gentamicin, meropenem, and rifampin alone and tigecycline combined with colistin, gentamicin, meropenem, and rifampin were tested against nine genetically confirmed KPC-producing clinical strains that included eight K. pneumoniae and one Escherichia coli strain, using the non-carbapenemase-producing E. coli strain ATCC 25922 as a control. The strains were selected from our collection of KPC producers, in order to be epidemiologically unrelated (they were isolated in four distinct hospitals located in north, central, and south Greece) and to exhibit relatively low meropenem MICs (≤4 μg/ml). Susceptibility of the strains to tigecycline, colistin, gentamicin, rifampin, meropenem, imipenem, and ertapenem was determined using the agar dilution method according to CLSI guidelines (15). PCR and sequencing assays showed that all microorganisms produced KPC-2 carbapenemase. The K. pneumoniae strains belonged to four different clonal types, as determined previously by pulsed-field gel electrophoresis.
Murine infection model.
The animal studies were approved by the Greek veterinary authorities and conformed to the Protocol for the Protection and Welfare of Animals. The thigh infection protocol was performed as described previously (16). In brief, 6-week-old, specific-pathogen-free, female Bagg inbred albino c-strain (BALB/c) mice (Harlan, Indianapolis, IN) weighing 23 to 27 g were used in each test group (17). Mice were rendered neutropenic (neutrophil count < 100/mm3) by injecting cyclophosphamide intraperitoneally on day 4 (150 mg/kg of body weight) and on day 1 (100 mg/kg) preinoculation (18, 19). Thigh infections with each strain were produced by injecting 0.1 ml of a bacterial suspension of 107 CFU/ml. The infections were done in triplicate using three mice for each 24-h regimen and another three mice for each 48-h regimen for each of the nine study strains as well as for the ATCC 25922 control. After mice were infected, they were administered subcutaneously tigecycline at 50 mg/kg/24 h (20), gentamicin at 5 mg/kg/12 h (21), colistin methanesulfate at 40 mg/kg/8 h (equivalent to approximately 25 mg/kg/8 h of colistin sulfate; reported doses ranged from 20 mg/kg/8 h of colistin methanesulfate to 40 mg/kg/8 h of colistin sulfate [22, 23]), and meropenem at 200 mg/kg/8 h (reported doses ranged from 100 mg/kg/12 h to 400 mg/kg/8 h [24, 25]) and intraperitoneally rifampin at 25 mg/kg/6 h (22), as described previously, or remained untreated. Mice were humanely euthanized at 24 h and 48 h. Thigh muscles were aseptically excised, homogenized in 10 ml of saline, serially diluted, and cultured quantitatively on antibiotic-free agar plates after serial dilutions, for CFU enumeration. The level of detection for this assay was 100 CFU/thigh.
The thigh CFU titer was expressed as log10 CFU/thigh muscle. A t test was used for statistical analysis. For all experiments, an antibiotic scheme (either monotherapy or combination) was considered effective when resulting in a statistically significant reduction of CFU counts (P < 0.05) compared with another scheme or without treatment. All statistical analyses were performed using Minitab software (version 13.31). A combination was considered to be synergistic when resulting in a higher reduction of CFU yielded from treated mice than with each of the drugs alone; antagonism was considered when monotherapy caused a higher log CFU reduction than the combination (26). CFU reductions of >3 Δlog were considered to indicate bactericidal levels of activity (10).
RESULTS
Susceptibility testing.
The agar dilution MICs of the nine KPC-producing strains used in the study are shown in Table 1. All nine strains had relatively low meropenem MICs (1 to 4 μg/ml) and susceptibility to colistin, gentamicin, and tigecycline, whereas they exhibited relatively high rifampin MICs (≥32 μg/ml).
Table 1.
Isolate | Agar dilution MIC (μg/ml) |
||||||
---|---|---|---|---|---|---|---|
MER | TIG | RIF | GEN | COL | IMP | ERT | |
K1 | 1 | 2 | >32 | 2 | 0.25 | 8 | 8 |
K2 | 2 | 2 | 32 | 2 | 0.5 | 4 | 8 |
K3 | 2 | 2 | >32 | 1 | 1 | 2 | 8 |
K4 | 1 | 1 | 32 | 2 | 2 | 4 | 8 |
K5 | 2 | 2 | >32 | 1 | 0.5 | 2 | 4 |
K6 | 4 | 2 | 32 | 4 | 1 | 4 | 16 |
K7 | 4 | 2 | >32 | 1 | 1 | 2 | 8 |
K8 | 4 | 2 | >32 | 1 | 2 | 2 | 16 |
E9 | 2 | 0.5 | 32 | 1 | 0.5 | 2 | 8 |
ATCC 25922 | 0.06 | 0.12 | 32 | 0.5 | 0.5 | 0.12 | 0.015 |
K, K. pneumoniae; E, E. coli; TIG, tigecycline; COL, colistin; GEN, gentamicin; MER, meropenem; RIF, rifampin; IMP, imipenem; ERT, ertapenem.
Activities of single regimens and tigecycline combinations compared with infection in untreated animals.
The performance of antibiotics used as monotherapies and in tigecycline combinations, reflected by a significant reduction of colonies in treated compared with untreated animals, is presented in Table 2. Rifampin monotherapy, although not recommended as a single agent (15), had the best performance at both 24 h (effective [P < 0.05] in 8 strains; 88.9%) and 48 h (100%), followed by tigecycline at 24 h (77.8%) and gentamicin at 48 h (88.9%). Colistin and meropenem as monotherapies exhibited considerably lower activity, treating effectively (P < 0.05) 33.3% or a lower proportion of mice at 24 h and 48 h. Regarding tigecycline combinations, tigecycline with gentamicin and tigecycline with rifampin were the most effective ones (100% at 24 and 48 h), followed by tigecycline with meropenem (88.9% at 24 h) and tigecycline with colistin (77.8%). The combinations of tigecycline with colistin and tigecycline with meropenem at 48 h exhibited reduced efficacy (66.7%). Also, the combinations of tigecycline with rifampin and tigecycline with gentamicin resulted in higher reductions in bacterial densities grown from infected animals than did the other two combinations used. In particular, the activity of tigecycline with rifampin reached bactericidal levels (>3 Δlog reduction) against all KPC producers, while tigecycline with gentamicin was considered bactericidal in 66.7% of strains at 24 h and 88.9% at 48 h.
Table 2.
Isolate | TIG vs no treatment |
COL vs no treatment |
GEN vs no treatment |
MER vs no treatment |
RIF vs no treatment |
TIG + COL vs no treatment |
TIG + GEN vs no treatment |
TIG + MER vs no treatment |
TIG + RIF vs no treatment |
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Δlog | P valueb | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | |
24 h of treatment | ||||||||||||||||||
K 1/4089 | 0.16 | 0.644 | −0.001 | 0.990 | −0.05 | 0.590 | 0.02 | 0.558 | −2.41 | 0.002 | −0.16 | 0.071 | −1.87 | 0.003 | −1.36 | 0.004 | −4.81 | <0.001 |
K 2/4179 | −0.24 | 0.137 | −0.11 | 0.068 | −0.39 | 0.256 | −0.24 | 0.137 | −2.25 | <0.001 | −3.14 | 0.023 | −4.42 | 0.007 | −1.81 | 0.077 | −3.87 | 0.007 |
K 3/3268 | −2.51 | 0.032 | 0.85 | 0.002 | −1.51 | 0.002 | −0.15 | 0.182 | −2.55 | 0.002 | −2.75 | 0.109 | −4.72 | 0.004 | −1.38 | 0.004 | −4.54 | <0.001 |
K4/3712 | −5.23 | <0.001 | 0.41 | 0.414 | −1.38 | 0.046 | −0.60 | 0.080 | −2.66 | 0.002 | −3.45 | 0.001 | −4.20 | <0.001 | −2.03 | 0.002 | −5.84 | <0.001 |
K5/2458 | −2.98 | 0.003 | −0.52 | 0.052 | −1.02 | <0.001 | −1.33 | 0.003 | −3.03 | 0.001 | −3.80 | 0.003 | −4.86 | <0.001 | −3.56 | 0.001 | −5.17 | <0.001 |
K6/3189 | −5.48 | <0.001 | −1.11 | 0.015 | −1.36 | 0.019 | −1.12 | 0.003 | −3.48 | <0.001 | −4.74 | 0.001 | −5.86 | <0.001 | −4.87 | <0.001 | −5.71 | <0.001 |
K7/2868 | −2.62 | 0.006 | −1.47 | 0.023 | −0.88 | 0.112 | −0.27 | 0.341 | −0.98 | 0.143 | −2.38 | 0.025 | −3.09 | <0.001 | −2.68 | <0.001 | −3.45 | 0.008 |
K8/3342 | −1.55 | 0.006 | −0.44 | 0.184 | −1.96 | 0.004 | −0.84 | 0.112 | −1.55 | 0.011 | −1.04 | 0.001 | −1.90 | 0.027 | −1.27 | 0.038 | −4.20 | <0.001 |
E1/1098 | −2.60 | 0.001 | 0.12 | 0.588 | −1.52 | 0.029 | −1.77 | 0.004 | −2.44 | 0.004 | −3.11 | 0.015 | −2.52 | 0.005 | −2.67 | 0.029 | −3.10 | <0.001 |
ATCC 25922 | −2.37 | 0.003 | −0.62 | 0.394 | −0.77 | 0.305 | −4.15 | 0.001 | −1.63 | 0.004 | −4.05 | 0.015 | −5.04 | <0.001 | −6.05 | <0.001 | −4.81 | <0.001 |
48 h of treatment | ||||||||||||||||||
K 1/4089 | 0.26 | −0.844 | −0.10 | 0.224 | −0.24 | 0.558 | −0.67 | 0.053 | −2.10 | 0.035 | −0.18 | 0.275 | −4.28 | 0.003 | −0.11 | 0.105 | −4.47 | <0.001 |
K 2/4179 | 0.101 | 0.233 | −0.79 | 0.262 | −0.87 | 0.003 | −0.15 | 0.195 | −1.60 | 0.027 | −3.13 | 0.018 | −4.58 | 0.014 | −0.98 | 0.08 | −5.60 | <0.001 |
K 3/3268 | −3.70 | <0.001 | −0.76 | 0.182 | −1.15 | 0.001 | −0.58 | 0.337 | −1.75 | 0.014 | −1.90 | 0.169 | −3.58 | 0.022 | −3.38 | <0.001 | −3.76 | 0.001 |
K4/3712 | −5.54 | <0.001 | −5.22 | <0.001 | −3.02 | 0.001 | 0.01 | 0.913 | −2.27 | 0.014 | −6.10 | <0.001 | −7.08 | <0.001 | −5.91 | 0.001 | −7.11 | <0.001 |
K5/2458 | −5.67 | 0.002 | −1.67 | 0.026 | −1.90 | 0.004 | −1.24 | <0.001 | −3.90 | 0.002 | −5.89 | 0.001 | −6.77 | <0.001 | −5.70 | <0.001 | −6.19 | <0.001 |
K6/3189 | −3.94 | <0.001 | −0.21 | 0.495 | −2.90 | 0.028 | −1.37 | 0.011 | −3.76 | 0.002 | −4.09 | 0.015 | −5.65 | <0.001 | −2.70 | <0.001 | −6.49 | <0.001 |
K7/2868 | −2.96 | 0.010 | −2.18 | 0.014 | −2.72 | 0.036 | −0.23 | 0.161 | −2.78 | 0.003 | −2.31 | 0.015 | −4.62 | <0.001 | −1.44 | 0.025 | −4.66 | <0.001 |
K8/3342 | −0.24 | 0.054 | −0.91 | 0.266 | −2.77 | <0.001 | −0.52 | 0.217 | −3.22 | 0.002 | −1.22 | 0.015 | −2.51 | 0.003 | −1.54 | 0.058 | −3,98 | 0.002 |
E1/1098 | −4.57 | 0.008 | 0.27 | 0.399 | −1.86 | 0.027 | −2.12 | 0.007 | −4.55 | <0.001 | −0.68 | 0.457 | −4.10 | 0.010 | −3.95 | 0.001 | −5.16 | <0.001 |
ATCC 25922 | −3.95 | 0.002 | −0.99 | 0.029 | −1.40 | 0.121 | −3.85 | <0.001 | −2.80 | 0.024 | −5.56 | 0.002 | −6.27 | <0.001 | −6.23 | <0.001 | −6.00 | <0.001 |
K, K. pneumoniae; E, E. coli; TIG, tigecycline; COL, colistin; GEN, gentamicin; MER, meropenem; RIF, rifampin.
A P value of <0.05 indicates significance and is shown in bold.
Activity of single regimens compared with other monotherapies.
The direct comparison of antibiotic monotherapies with each other after 24 h and 48 h of treatment is presented in Table 3. Tigecycline was significantly more effective (P < 0.05) at 24 h and 48 h than were colistin (in 77.8% and 66.7% of strains at each time point, respectively), meropenem (66.7% and 77.8%, respectively), and gentamicin (66.7% at 24 h and 48 h). Tigecycline was similarly effective with rifampin at 24 h (more effective than rifampin [P < 0.05] in 33.3% of strains and less effective also in 33.3% of strains) and more effective than rifampin at 48 h (more effective than rifampin in 33.3% of strains, while rifampin was more effective in 11.1% of strains). Rifampin was more effective than colistin (88.9% of strains), gentamicin (66.7%), and meropenem (88.9%) at 24 h, but at 48 h rifampin lost a part of its activity; still, it remained more effective than colistin (44.4% of strains), meropenem (66.7%), and gentamicin (33.3%).
Table 3.
Isolate | TIG vs COL |
TIG vs GEN |
TIG vs MER |
TIG vs RIF |
COL vs GEN |
COL vs MER |
COL vs RIF |
GEN vs MER |
GEN vs RIF |
MER vs RIF |
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---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Δlog | P valueb | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | |
24 h of treatment | ||||||||||||||||||||
K 1/4089 | 0.14 | 0.723 | 0.19 | 0.070 | 0.11 | 0.746 | 1.55 | 0.023 | 0.05 | 0.451 | −0.02 | 0.638 | 2.41 | 0.001 | −0.07 | 0.426 | 2.36 | 0.001 | 2.43 | 0.001 |
K 2/4179 | −0.10 | 0.262 | 0.24 | 0.627 | 0.04 | 0.423 | 1.06 | 0.005 | 0.28 | 0.379 | 0.07 | 0.246 | 2.13 | <0.001 | −0.20 | 0.546 | 1.86 | 0.009 | 2.06 | 0.001 |
K 3/3268 | −3.31 | 0.025 | −2.12 | 0.048 | −2.31 | 0.049 | −0.27 | 0.126 | 2.36 | <0.001 | 1.00 | <0.001 | 3.40 | 0.001 | −1.36 | 0.002 | 1.04 | 0.066 | 2.40 | 0.002 |
K4/3712 | −4.56 | <0.001 | −3.57 | 0.010 | −3.92 | 0.001 | −2.16 | 0.002 | 1.79 | 0.010 | 1.04 | 0.034 | 3.07 | 0.004 | −0.79 | 0.070 | 1.28 | 0.090 | 2.07 | 0.007 |
K5/2458 | −2.46 | 0.001 | −1.95 | 0.008 | −1.65 | 0.003 | 0.05 | 0.90 | 0.50 | 0.055 | 0.80 | 0.006 | 2.51 | 0.002 | 0.30 | 0.140 | 2.00 | 0.001 | 1.70 | 0.010 |
K6/3189 | −4.37 | <0.001 | −4.12 | 0.001 | −4.36 | 0.001 | −1.99 | 0.001 | 0.25 | 0.071 | 0.01 | 0.977 | 2.37 | 0.001 | −0.24 | 0.580 | 2.12 | 0.004 | 2.36 | 0.002 |
K7/2868 | −2.25 | 0.017 | −0.51 | 0.487 | −1.58 | 0.017 | −1.85 | 0.002 | 1.94 | 0.021 | 1.17 | 0.072 | 2.44 | 0.022 | −0.82 | 0.247 | 2.19 | 0.008 | 0.47 | 0.362 |
K8/3342 | −1.70 | 0.022 | −1.22 | 0.021 | −1.55 | 0.023 | 1.15 | 0.004 | −0.08 | 0.658 | 0.85 | 0.096 | 2.88 | 0.006 | −0.58 | 0.461 | 1.44 | 0.017 | 2.14 | 0.042 |
E1/1098 | −3.10 | 0.005 | −2.87 | 0.001 | −1.90 | 0.052 | −0.15 | 0.430 | 0.29 | 0.362 | 1.94 | 0.001 | 0.84 | 0.094 | −2.11 | 0.036 | 0.63 | 0.092 | 1.93 | 0.009 |
ATCC 25922 | −1.75 | 0.114 | −1.61 | 0.145 | 1.77 | 0.007 | −0.74 | 0.079 | 0.14 | 0.878 | 3.52 | 0.024 | 1.01 | 0.170 | 3.38 | 0.014 | 0.87 | 0.278 | −2.51 | 0.003 |
48 h of treatment | ||||||||||||||||||||
K 1/4089 | −1.26 | 0.177 | −1.12 | 0.346 | −0.69 | 0.422 | 0.387 | 0.707 | 0.14 | 0.727 | 0.56 | 0.60 | 2.00 | 0.033 | 0.42 | 0.467 | 1.86 | 0.002 | 1.44 | 0.120 |
K 2/4179 | 0.90 | 0.187 | 0.98 | 0.001 | 0.26 | 0.094 | 1.70 | 0.019 | 0.08 | 0.892 | −0.64 | 0.371 | 0.80 | 0.316 | −0.72 | 0.030 | 0.73 | 0.181 | 1.45 | 0.030 |
K 3/3268 | −2.69 | 0.032 | −2.50 | 0.002 | −2.82 | 0.039 | −2.10 | 0.020 | 0.39 | 0.436 | −0.18 | 0.091 | 0.99 | 0.276 | −0.57 | 0.345 | 0.60 | 0.197 | 1.17 | 0.247 |
K4/3712 | 0.55 | 0.211 | −1.76 | 0.042 | −4.65 | 0.003 | −2.44 | 0.049 | −2.19 | 0.004 | −5.23 | <0.001 | −2.95 | 0.006 | −3.04 | 0.003 | −0.76 | 0.236 | 2.28 | 0.014 |
K5/2458 | −4.01 | 0.015 | −3.76 | 0.008 | −4.43 | 0.004 | −1.77 | 0.132 | 0.24 | 0.721 | −0.42 | 0.369 | 2.24 | 0.001 | −0.66 | 0.073 | 1.99 | 0.046 | 2.66 | 0.007 |
K6/3189 | −3.73 | 0.001 | −1.05 | 0.255 | −2.57 | 0.002 | −0.18 | 0.613 | 2.68 | 0.009 | 1.16 | <0.001 | 3.55 | 0.007 | −1.52 | 0.050 | 0.87 | 0.426 | 2.39 | 0.018 |
K7/2868 | −2.75 | 0.015 | −2.38 | 0.020 | −2.10 | 0.044 | −1.92 | 0.029 | −2.36 | 0.031 | −1.09 | 0.184 | 0.48 | 0.468 | −0.66 | 0.148 | 2.17 | 0.012 | 0.26 | 0.571 |
K8/3342 | −0.30 | 0.018 | −0.36 | 0.025 | −0.97 | 0.024 | −1.02 | 0.317 | −0.15 | 0.562 | 0.73 | 0.211 | 1.66 | 0.019 | −0.74 | 0.166 | 0.28 | 0.475 | 1.12 | 0.036 |
E1/1098 | −4.16 | 0.001 | −4.01 | 0.003 | −3.23 | 0.003 | −0.35 | 0.481 | 0.54 | 0.361 | 1.09 | 0.027 | −0.74 | 0.232 | −2.55 | 0.019 | 0.72 | 0.226 | 1.37 | 0.041 |
ATCC 25922 | −2.95 | 0.009 | −2.55 | 0.036 | −0.09 | 0.787 | −1.14 | 0.249 | 0.41 | 0.421 | 2.86 | 0.001 | 1.82 | 0.111 | 2.45 | 0.021 | 1.41 | 0.275 | −1.04 | 0.190 |
K, K. pneumoniae; E, E. coli; TIG, tigecycline; COL, colistin; GEN, gentamicin; MER, meropenem; RIF, rifampin.
A P value of <0.05 indicates significance and is shown in bold.
Activities of tigecycline combinations compared with monotherapies.
The comparison of the efficacies of tigecycline combinations in comparison with monotherapies after 24 and 48 h of treatment is presented in Table 4. In brief, the addition of colistin to tigecycline did not improve essentially tigecycline performance, while colistin antagonized tigecycline in as many as 44.4% of strains at 48 h. The addition of gentamicin to tigecycline improved tigecycline in 33.3% (24 h) to 44.4% (48 h) of strains. Further, tigecycline plus rifampin was more effective (P < 0.05) than tigecycline alone in 44.4% (24 h) to 66.7% (48 h) of strains. The addition of meropenem to tigecycline did not alter considerably the activity of tigecycline, while it antagonized tigecycline in 22.2% of strains at both 24 h and 48 h. Colistin, which was inactive as monotherapy, performed particularly well when combined with tigecycline (88.9% efficiency). Of interest, gentamicin and rifampin, which were effective as monotherapies, when combined with tigecycline exhibited a significantly improved performance after both 24 h and 48 h.
Table 4.
Isolate | TIG vs TIG + COL |
TIG vs TIG + GEN |
TIG vs TIG + MER |
TIG vs TIG + RIF |
COL vs TIG+COL |
GEN vs TIG + GEN |
MER vs TIG + MER |
RIF vs TIG + RIF |
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---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Δlog | P valueb | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | |
24 h of treatment | ||||||||||||||||
K 1/4089 | 0.27 | 0.534 | 1.98 | 0.048 | 1.48 | 0.038 | 2.83 | 0.002 | 0.63 | 0.036 | 1.79 | 0.004 | 1.35 | <0.001 | 1.28 | 0.006 |
K 2/4179 | 2.97 | 0.026 | 4.16 | 0.010 | 1.57 | 0.094 | 1.86 | 0.044 | 2.80 | 0.008 | 4.22 | 0.006 | 1.57 | 0.094 | 0.62 | 0.315 |
K 3/3268 | 1.00 | 0.081 | 2.21 | 0.095 | −1.13c | 0.148 | 1.72 | <0.001 | 3.87 | 0.013 | 3.21 | 0.001 | 1.14 | 0.001 | 1.98 | 0.001 |
K4/3712 | 0.05 | 0.918 | 0.80 | 0.074 | −1.37c | 0.002 | 0.62 | 0.125 | 5.46 | 0.001 | −0.93c | 0.054 | 0.47 | 0.135 | 2.77 | 0.002 |
K5/2458 | 0.83 | 0.307 | 1.88 | 0.027 | 0.58 | 0.364 | 2.19 | 0.011 | 3.28 | 0.008 | 3.83 | 0.001 | 2.23 | 0.009 | 2.14 | 0.001 |
K6/3189 | −0.74c | 0.108 | 0.38 | 0.136 | −0.61c | 0.014 | 0.22 | 0.065 | 3.63 | 0.001 | 4.50 | 0.001 | 3.75 | 0.001 | 2.22 | <0.001 |
K7/2868 | −0.24c | 0.515 | 0.46 | 0.199 | 0.07 | 0.843 | 0.48 | 0.182 | 2.01 | 0.063 | 0.98 | 0.100 | 1.65 | <0.001 | 0.44 | 0.502 |
K8/3342 | −0.51c | 0.186 | 0.35 | 0.614 | −0.28c | 0.597 | 2.38 | 0.008 | 1.69 | 0.003 | 1.57 | 0.061 | 1.27 | 0.024 | 2.02 | 0.002 |
E1/1098 | 0.52 | 0.090 | 0.17 | 0.559 | 0.56 | 0.390 | 0.18 | 0.398 | 3.25 | 0.005 | 2.80 | <0.001 | 2.31 | 0.035 | 2.96 | <0.001 |
ATCC 25922 | 1.68 | 0.155 | 2.67 | 0.006 | 3.68 | 0.010 | 2.43 | 0.005 | 3.43 | 0.027 | 4.28 | 0.006 | 1.90 | 0.037 | 3.17 | <0.001 |
48 h of treatment | ||||||||||||||||
K 1/4089 | −0.51c | 0.561 | 2.17 | 0.134 | −1.58c | 0.230 | 2.55 | 0.008 | 1.02 | 0.322 | 6.65 | 0.001 | −0.60c | 0.062 | 2.17 | 0.033 |
K2/4179 | 3.07 | 0.004 | 4.88 | 0.001 | 1.15 | 0.004 | 5.46 | <0.001 | 2.16 | 0.040 | 3.90 | 0.001 | 0.90 | 0.031 | 3.76 | 0.001 |
K 3/3268 | −0.90c | 0.429 | −1.21c | 0.848 | −0.18c | 0.349 | 0.80 | 0.016 | 1.60 | 0.282 | 1.84 | 0.074 | 9.91 | 0.015 | 2.89 | 0.008 |
K4/3712 | 0.97 | 0.110 | 1.28 | 0.153 | 0.24 | 0.611 | 1.56 | 0.001 | 0.42 | 0.053 | 3.16 | 0.001 | 5.04 | 0.002 | 3.95 | 0.003 |
K5/2458 | 0.22 | 0.787 | 1.10 | 0.155 | 0.03 | 0.964 | 0.52 | 0.435 | 4.23 | 0.009 | 4.87 | 0.001 | 4.45 | 0.001 | 2.29 | 0.008 |
K6/3189 | 0.15 | 0.871 | 1.71 | 0.001 | −1.25c | 0.001 | 2.55 | 0.001 | 3.87 | 0.014 | 2.76 | 0.038 | 1.32 | 0.012 | 2.73 | 0.003 |
K7/2868 | −0.64c | 0.342 | 1.67 | 0.031 | −1.52c | 0.003 | 3.88 | <0.001 | 2.11 | 0.019 | 4.05 | 0.001 | 0.58 | 0.305 | 2.48 | 0.031 |
K8/3342 | 0.90 | 0.076 | 2.19 | 0.002 | 1.21 | 0.083 | 0.36 | 0.541 | 1.20 | 0.024 | 2.55 | 0.002 | 1.58 | 0.054 | 3.22 | 0.005 |
E1/1098 | −3.72c | 0.002 | −0.29c | 0.495 | −0.43c | 0.241 | −0.28c | 0.612 | 0.45 | 0.370 | 3.72 | 0.001 | 2.81 | 0.001 | 3.68 | 0.002 |
ATCC 25922 | 1.62 | 0.134 | 2.33 | 0.007 | 2.28 | 0.018 | 2.06 | 0.038 | 4.58 | 0.006 | 4.88 | 0.007 | 2.38 | 0.002 | 3.20 | 0.007 |
K, K. pneumoniae; E, E. coli; TIG, tigecycline; COL, colistin; GEN, gentamicin; MER, meropenem; RIF, rifampin.
A P value of <0.05 indicates significance and is shown in bold.
Antagonistic combination.
Comparison of tigecycline combinations.
In Table 5, we compare the efficacies of tigecycline combinations after 24 and 48 h of treatment. In brief, the combination of tigecycline with gentamicin was superior to tigecycline with colistin against infections by 88.9% of strains and significantly more effective (P < 0.05) in 33.3% of strains after 24 h, while at 48 h it was superior in all strains and more effective in 44.4% of strains. The combination of tigecycline with gentamicin was superior to tigecycline with meropenem in 88.9% of strains and more effective (P < 0.05) in 66.7% of strains after 24 h, and at 48 h it was superior in 88.9% of strains and more effective in 55.5% of strains. The combination of tigecycline with rifampin was superior to tigecycline with colistin in 88.9% of strains and more effective (P < 0.05) in 33.3% of strains after 24 h, and at 48 h it was superior in all strains and more effective in 33.3% of strains. Finally, of note, the combination of tigecycline with rifampin was superior to tigecycline with meropenem in all strains and more effective (P < 0.05) in 77.8% of strains after 24 h, and at 48 h it was superior in all strains and more effective in 66.7% of strains. There was no clear difference between the activities of the two most effective combinations (tigecycline with gentamicin versus tigecycline with rifampin) and no clear difference between the two least active combinations (tigecycline with colistin versus tigecycline with meropenem) at 24 h and 48 h.
Table 5.
Isolate | TIG + COL vs TIG + GEN |
TIG + COL vs TIG + MER |
TIG + COL vs TIG + RIF |
TIG + GEN vs TIG + MER |
TIG + GEN vs TIG + RIF |
TIG + MER vs TIG + RIF |
||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Δlog | P valueb | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | Δlog | P value | |
24 h of treatment | ||||||||||||
K 1/4089 | 1.71 | 0.003 | 1.20 | 0.003 | 3.47 | 0.005 | −0.50 | 0.085 | 1.73 | 0.033 | 2.37 | 0.001 |
K 2/4179 | 1.70 | 0.127 | −1.19 | 0.202 | −0.07 | 0.933 | −2.88 | 0.004 | −1.76 | 0.081 | 1.12 | 0.308 |
K 3/3268 | 1.71 | 0.151 | −1.72 | 0.089 | 1.52 | 0.159 | −3.43 | <0.001 | −0.19 | 0.147 | 3.24 | 0.001 |
K4/3712 | 0.75 | 0.049 | −1.42 | 0.038 | 0.56 | 0.098 | −2.17 | 0.003 | −0.91 | 0.003 | 1.99 | 0.003 |
K5/2458 | 1.05 | 0.159 | −0.24 | 0.648 | 1.37 | 0.047 | −1.23 | 0.002 | 0.315 | 0.168 | 1.61 | 0.003 |
K6/3189 | 1.12 | 0.040 | 0.13 | 0.615 | 0.97 | 0.083 | −0.99 | 0.004 | −0.15 | 0.397 | 0.84 | 0.011 |
K7/2868 | 0.70 | 0.245 | 0.30 | 0.580 | 1.69 | 0.032 | −0.90 | 0.006 | −1.17 | 0.114 | 1.55 | 0.026 |
K8/3342 | 0.86 | 0.135 | 0.23 | 0.520 | 0.56 | 0.311 | −0.63 | 0.092 | 1.00 | 0.226 | 0.52 | 0.229 |
E1/1098 | −0.35 | 0.280 | 0.04 | 0.942 | 0.74 | 0.215 | 0.39 | 0.303 | −0.64 | 0.371 | 1.98 | 0.034 |
ATCC 25922 | 0.99 | 0.230 | 2.00 | 0.023 | 0.76 | 0.278 | 1.01 | 0.032 | −0.23 | 0.290 | −1.25 | 0.027 |
48 h of treatment | ||||||||||||
K 1/4089 | 2.68 | 0.013 | −1.06 | 0.295 | 3.03 | 0.058 | −3.82 | 0.002 | 0.38 | 0.378 | 4.20 | <0.001 |
K 2/4179 | 1.82 | 0.045 | −1.70 | 0.078 | 2.40 | 0.003 | −3.60 | 0.016 | 0.59 | 0.285 | 4.25 | 0.003 |
K 3/3268 | 0.64 | 0.151 | 1.14 | 0.217 | 2.29 | 0.074 | 0.50 | 0.515 | 1.65 | 0.115 | 1.15 | 0.017 |
K4/3712 | 0.17 | 0.588 | −0.99 | 0.110 | 0.98 | 0.038 | −1.16 | 0.188 | −1.15 | 0.058 | 1.19 | 0.092 |
K5/2458 | 0.88 | 0.067 | −0.20 | 0.635 | 0.30 | 0.519 | −1.08 | 0.005 | −0.88 | 0.035 | 0.50 | 0.133 |
K6/3189 | 1.57 | 0.174 | −1.40 | 0.187 | 2.40 | 0.063 | −2.96 | <0.001 | 0.84 | 0.003 | 3.80 | <0.001 |
K7/2868 | 2.31 | 0.023 | −0.87 | 0.166 | 1.98 | 0.041 | −3.18 | 0.002 | 0.36 | 0.711 | 2.76 | 0.033 |
K8/3342 | 1.29 | 0.068 | 0.31 | 0.694 | 0.96 | 0.133 | −0.98 | 0.170 | 1.46 | 0.137 | 0.87 | 0.132 |
E1/1098 | 3.42 | 0.003 | 3.28 | 0.005 | 1.27 | 0.208 | −0.14 | 0.612 | −0.79 | 0.165 | 2.79 | 0.009 |
ATCC 25922 | 0.71 | 0.251 | 0.66 | 0.191 | 0.44 | 0.186 | −0.05 | 0.802 | −0.27 | 0.364 | −0.23 | 0.313 |
K, K. pneumoniae; E, E. coli; TIG, tigecycline; COL, colistin; GEN, gentamicin; MER, meropenem; RIF, rifampin.
A P value of <0.05 indicates significance and is shown in bold.
DISCUSSION
Despite the increasing occurrence and the severity of infections due to carbapenemase producers, limited in vivo data exist on the efficacies of the available treatment schemes (8, 9). We performed thigh infections with KPC producers and used tigecycline, colistin, meropenem, rifampin, and gentamicin, alone or in tigecycline combinations, to investigate their in vivo activities and their applicabilities in the clinical setting.
When tigecycline was used as monotherapy, bactericidal activity was observed already from 24 h and was evident after 48 h of treatment. Bactericidal activity of tigecycline has been also reported previously in vivo against extended-spectrum-β-lactamase (ESBL)-producing bacteria but occurred after 72 h of treatment (10). The study reported in reference 10 supports the findings of the present study and is in contrast with previous observations for bacteriostatic antibiotics, such as tetracyclines (27). Rifampin, even exhibiting high MICs, was generally effective, being comparable to tigecycline and superior to colistin, gentamicin, and meropenem up to 24 h, while its activity was somewhat reduced at 48 h. Similar in vivo activity of rifampin has been reported previously against carbapenem-resistant Acinetobacter baumannii (28). Nevertheless, rifampin monotherapy is not recommended for the treatment of clinical infections (15). Gentamicin, colistin, and meropenem were rather ineffective as single agents, as was also suggested previously (14).
As for the combinations tested in this study, tigecycline combinations with rifampin or gentamicin exhibited a synergistic effect and were the most effective regimens against most strains. These observations support previous preliminary suggestions that tigecycline combined with either rifampin or an aminoglycoside is promising and could be clinically valuable if validated in animal models (8). In contrast, the addition of colistin and meropenem to tigecycline resulted in antagonism in a considerable proportion of strains, although these combinations, frequently used in clinical practice, were still active and clearly superior to colistin and meropenem used as single regimens. It should be noted that in vitro antagonism of tigecycline plus colistin was also observed previously against New Delhi metallo-β-lactamase (NDM)-1-producing Enterobacteriaceae (29), while a degree of in vitro synergy was observed previously against KPC producers but only up to 4 to 8 h of exposure when tigecycline plus colistin was used at 1× to 2× MIC (13). Regarding the combination of tigecycline with meropenem, previous in vitro results indicated suboptimal activity (13), while it was also reported to underperform in clinical infections (30). We could speculate based on this evidence that in clinical cases where the empirical combination of tigecycline plus colistin or a carbapenem was used successfully (14), the treatment outcome was mainly due to the efficiency of tigecycline rather than the antimicrobial activity added by the second agent.
In conclusion, it was observed that among the last-resort antibiotics tested as monotherapies, tigecycline exhibited the best activity, followed by rifampin. Among tigecycline combinations, the addition of rifampin improved considerably tigecycline activity, followed by the addition of gentamicin, while colistin and meropenem did not ameliorate particularly or even deteriorate tigecycline activity. We believe that these in vivo data, derived from a soft tissue infection model, substantiate the most active combinations and would be of value to clinicians treating infections due to KPC producers.
ACKNOWLEDGMENTS
This work was funded by an Investigator-Initiated Research Grant from Pfizer (WS1394235). The funding source was not involved in the study design, execution, or result interpretation or publication.
Footnotes
Published ahead of print 23 September 2013
REFERENCES
- 1. da Silva RM, Traebert J, Galato D. 2012. Klebsiella pneumoniae carbapenemase (KPC)-producing Klebsiella pneumoniae: a review of epidemiological and clinical aspects. Expert Opin. Biol. Ther. 12:663–671 [DOI] [PubMed] [Google Scholar]
- 2. Nordmann P, Cuzon G, Naas T. 2009. The real threat of Klebsiella pneumoniae carbapenemase-producing bacteria. Lancet Infect. Dis. 9:228–236 [DOI] [PubMed] [Google Scholar]
- 3. Bratu S, Landman D, Haag R, Recco R, Eramo A, Alam M. 2005. Rapid spread of carbapenem-resistant Klebsiella pneumoniae in New York City: a new threat to our antibiotic armamentarium. Arch. Intern. Med. 165:1430–1435 [DOI] [PubMed] [Google Scholar]
- 4. Tsakris A, Kristo I, Poulou A, Markou F, Ikonomidis A, Pournaras S. 2008. First occurrence of KPC-2-possessing Klebsiella pneumoniae in a Greek hospital and recommendation for detection with boronic acid disc tests. J. Antimicrob. Chemother. 62:1257–1260 [DOI] [PubMed] [Google Scholar]
- 5. Hirsch EB, Tam VH. 2010. Detection and treatment options for Klebsiella pneumoniae carbapenemases (KPCs): an emerging cause of multidrug-resistant infection. J. Antimicrob. Chemother. 65:1119–1125 [DOI] [PubMed] [Google Scholar]
- 6. Gasink LB, Edelstein PH, Lautenbach E, Synnestvedt M, Fishman NO. 2009. Risk factors and clinical impact of Klebsiella pneumoniae carbapenemase-producing K. pneumoniae. Infect. Control Hosp. Epidemiol. 30:1180–1185 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7. Pournaras S, Protonotariou E, Voulgari E, Kristo I, Dimitroulia E, Vitti D, Tsalidou M, Maniatis AN, Tsakris A, Sofianou D. 2009. Clonal spread of KPC-2 carbapenemase-producing Klebsiella pneumoniae in Greece. J. Antimicrob. Chemother. 64:348–352 [DOI] [PubMed] [Google Scholar]
- 8. Ertenza JM, Moreillon P. 2009. Tigecycline in combination with other antimicrobials: a review of in vitro, animal and case report studies. Int. J. Antimicrob. Agents 34:8.e1–8.e9. 10.1016/j.ijantimicag.2008.11.006 [DOI] [PubMed] [Google Scholar]
- 9. Qureshi ZA, Paterson DL, Potoski BA, Kilayko MC, Sandovsky G, Sordillo E, Polsky B, Adams-Haduch JM, Doi Y. 2012. Treatment outcome of bacteremia due to KPC-producing Klebsiella pneumoniae: superiority of combination antimicrobial regimens. Antimicrob. Agents Chemother. 56:2108–2113 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10. Tessier PR, Nicolau DP. 2013. Tigecycline displays in vivo bactericidal activity against extended spectrum-β-lactamase-producing Klebsiella pneumoniae after 72-hour exposure period. Antimicrob. Agents Chemother. 57:640–642 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11. Petersen PJ, Labthavikul P, Jones CH, Bradford PA. 2006. In vitro antibacterial activities of tigecycline in combination with other antimicrobial agents determined by chequerboard and time-kill kinetic analysis. J. Antmicrob. Chemother. 57:573–576 [DOI] [PubMed] [Google Scholar]
- 12. Cobo J, Morosini MI, Pintado V, Tato M, Samaranch N, Baquero F, Canton R. 2008. Use of tigecycline for the treatment of prolonged bacteremia due to a multiresistant VIM-1 and SHV-12 β-lactamase-producing Klebsiella pneumoniae epidemic clone. Diagn. Microbiol. Infect. Dis. 60:319–322 [DOI] [PubMed] [Google Scholar]
- 13. Pournaras S, Vrioni G, Neou E, Dendrinos J, Dimitroulia E, Poulou A, Tsakris A. 2011. Activity of tigecycline alone and in combination with colistin and meropenem against Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae strains by time-kill assay. Int. J. Antimicrob. Agents 37:244–247 [DOI] [PubMed] [Google Scholar]
- 14. Lee GS, Burgess DS. 2012. Treatment of Klebsiella pneumoniae carbapenemase (KPC) infections: a review of published case series and case reports. Ann. Clin. Microbiol. Antimicrob. 11:32–41 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15. Clinical and Laboratory Standards Institute 2010. Performance standards for antimicrobial susceptibility testing, 21th informational supplement. M100-S20. Clinical and Laboratory Standards Institute, Wayne, PA [Google Scholar]
- 16. Ikonomidis A, Michail G, Vasdeki A, Labrou M, Karavasilis V, Stathopoulos C, Maniatis A, Pournaras S. 2008. In vitro and in vivo evaluations of oxacillin efficiency against mecA-positive oxacillin-susceptible Staphylococcus aureus. Antimicrob. Agents Chemother. 52:3905–3908 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17. Hegde SS, Reyes N, Skinner R, Difuntorum S. 2008. Efficacy of telavancin in a murine model of pneumonia induced by methicillin-susceptible Staphylococcus aureus. J. Antimicrob. Chemother. 61:169–172 [DOI] [PubMed] [Google Scholar]
- 18. Andes D, van Ogtrop ML, Peng J, Craig WA. 2002. In vivo pharmacodynamics of a new oxazolidinone (linezolid). Antimicrob. Agents Chemother. 46:3484–3489 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19. LaPlante KL, Leonard SN, Andes DR, Craig WA, Rybak MJ. 2008. Activities of clindamycin, daptomycin, doxycycline, linezolid, trimethoprim-sulfamethoxazole, and vancomycin against community associated methicillin-resistant Staphylococcus aureus with inducible clindamycin resistance in murine thigh infection and in vitro pharmacodynamic models. Antimicrob. Agents Chemother. 52:2156–2162 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20. Barthold SW, Hodzic E, Imai DM, Feng S, Yang X, Luft BJ. 2010. Ineffectiveness of tigecycline against persistent Borrelia burgdorferi. Antimicrob. Agents Chemother. 54:643–651 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21. Lenz MA, Franklin GA, Fairweather M, McClintock ML, Jala VR, Peyton JC, Gardner SA, Cheadle WG. 2007. Endogenous IL-10 leads to impaired bacterial clearance and reduced survival in a murine model of chronic peritonitis. Cytokine 40:207–215 [DOI] [PubMed] [Google Scholar]
- 22. Pachón-Ibáñez ME, Docobo-Perez F, Lopez-Rojas R, Domínguez-Herrera J, Jiménez-Mejias ME, García-Curiel A, Pichardo C, Jiménez L, Pachón J. 2010. Efficacy of rifampin and its combinations with imipenem, sulbactam, and colistin in experimental models of infection caused by imipenem-resistant Acinetobacter baumannii. Antimicrob. Agents Chemother. 54:1165–1172 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23. Dudhani RV, Turnidge JD, Nation RL, Li J. 2010. fAUC/MIC is the most predictive pharmacokinetic/pharmacodynamic index of colistin against Acinetobacter baumannii in murine thigh and lung infection models. J. Antimicrob. Chemother. 65:1984–1990 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24. Tam VH, Ledesma KR, Schilling AN, Lim TP, Yuan Z, Ghose R, Lewis RE. 2009. In vivo dynamics of carbapenem-resistant Pseudomonas aeruginosa selection after suboptimal dosing. Diagn. Microbiol. Infect. Dis. 64:427–433 [DOI] [PubMed] [Google Scholar]
- 25. Morinaga Y, Yanagihara K, Nakamura S, Yamamoto K, Izumikawa K, Seki M, Kakeya H, Yamamoto Y, Yamada Y, Kohno S, Kamihira S. 2008. In vivo efficacy and pharmacokinetics of tomopenem (CS-023), a novel carbapenem, against Pseudomonas aeruginosa in a murine chronic respiratory tract infection model. J. Antimicrob. Chemother. 62:1326–1331 [DOI] [PubMed] [Google Scholar]
- 26. Rishi P, Preet S, Bharrhan S, Verma I. 2011. In vitro and in vivo synergistic effects of cryptdin 2 and ampicillin against Salmonella. Antimicrob. Agents Chemother. 55:4176–4182 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 27. Agwuh KN, MacGowan A. 2006. Pharmacokinetics and pharmacodynamics of the tetracyclines including glycylcyclines. J. Antimicrob. Chemother. 58:256–265 [DOI] [PubMed] [Google Scholar]
- 28. Montero A, Ariza J, Corbella X, Doménech A, Cabellos C, Ayats J, Tubau F, Borraz C, Gudiol F. 2004. Antibiotic combinations for serious infections caused by carbapenem-resistant Acinetobacter baumannii in a mouse pneumonia model. J. Antimicrob. Chemother. 56:1085–1091 [DOI] [PubMed] [Google Scholar]
- 29. Albur M, Noel A, Bowker K, MacGowan A. 2012. Bactericidal activity of multiple combinations of tigecycline and colistin against NDM-1-producing Enterobacteriaceae. Antimicrob. Agents Chemother. 56:3441–3443 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30. Weisenberg SA, Morgan DJ, Espinal-Witter R, Larone DH. 2009. Clinical outcomes of patients with Klebsiella pneumoniae carbapenemase-producing K. pneumoniae after treatment with imipenem or meropenem. Diagn. Microbiol. Infect. Dis. 64:233–235 [DOI] [PMC free article] [PubMed] [Google Scholar]