In vitro activity of the siderophore monosulfactam BAL30072 against contemporary Gram-negative pathogens from New York City,including multidrug-resistant isolates

The in vitro activity of BAL30072 was assessed against clinical isolates from NYC hospitals, including isolates from a citywide surveillance study and a collection of isolates with well-characterised resistance mechanisms. BAL30072 was the most active β-lactam against Pseudomonas aeruginosa (MIC_50/90, 0.25/1 μg/mL), Acinetobacter baumannii (MIC_50/90, 4/>64 μg/mL) and KPC-possessing Klebsiella pneumoniae (MIC_50/90, 4/>64 μg/mL). Combining BAL30072 with meropenem resulted in a ≥4-fold decrease in the BAL30072 MIC₉₀ both for A. baumannii and K. pneumoniae. For isolates with a BAL30072 MIC > 4 μg/mL, addition of a sub-MIC concentration of colistin resulted in a four-fold decrease in the BAL30072 MIC in 44% of P. aeruginosa, 82% of A. baumannii and 23% of K. pneumoniae. Using sub-MIC concentrations, BAL30072 plus colistin was bactericidal against 4 of 11 isolates in time–kill studies. BAL30072 MICs were frequently lower for P. aeruginosa and K. pneumoniae when tested using Mueller–Hinton agar versus Iso-Sensitest agar or Mueller–Hinton broth. Against the well-characterised isolates, reduced susceptibility to BAL30072 correlated with mexA and mexX expression (P. aeruginosa), adeB expression (A. baumannii) and presence of SHV-type ESBLs (A. baumannii and K. pneumoniae). BAL30072 shows promising activity against contemporary Gram-negatives, including MDR P. aeruginosa, A. baumannii and K. pneumoniae. Enhanced activity was often present when BAL30072 was combined with meropenem or colistin. BAL30072 MICs were influenced by the testing method, particularly for P. aeruginosa and K. pneumoniae. Further in vivo studies are warranted to determine the potential clinical utility of BAL30072 alone and combined with other agents.     

Diversity of amino acid substitutions in PmrCAB associated with colistin resistance in clinical isolates of Acinetobacter baumannii

This study aimed to investigate the mechanisms of colistin resistance in 64 Acinetobacter baumannii isolates obtained from patients with ventilator-associated pneumonia hospitalised in Greece, Italy and Spain. In total, 31 A. baumannii isolates were colistin-resistant. Several novel amino acid substitutions in PmrCAB were found in 27 colistin-resistant A. baumannii. Most substitutions were detected in PmrB, indicating the importance of the histidine kinase for colistin resistance. In two colistin-resistant isolates, 93 amino acid changes were observed in PmrCAB compared with A. baumannii ACICU, and homologous recombination across different clonal lineages was suggested. Analysis of gene expression revealed increased pmrC expression in isolates harbouring pmrCAB mutations. Complementation of A. baumannii ATCC 19606 and ATCC 17978 with a pmrAB variant revealed increased pmrC expression but unchanged colistin MICs, indicating additional unknown factors associated with colistin resistance. Moreover, a combination of PmrB and PmrC alterations was associated with very high colistin MICs, suggesting accumulation of mutations as the mechanism for high-level resistance. The pmrC homologue eptA was detected in 29 colistin-susceptible and 26 colistin-resistant isolates. ISAba1 was found upstream of eptA in eight colistin-susceptible and one colistin-resistant isolate and eptA was disrupted by ISAba125 in two colistin-resistant isolates. Whilst in most isolates an association of eptA with colistin resistance was excluded, in one isolate an amino acid substitution in EptA (R127L) combined with a point mutation in ISAba1 upstream of eptA contributed to elevated colistin MICs. This study helps to gain an insight into the diversity and complexity of colistin resistance in A. baumannii.     

Enhanced bacterial killing with colistin/sulbactam combination against carbapenem-resistant Acinetobacter baumannii

Aims: Polymyxin-based combination therapy is often used to treat carbapenem-resistant Acinetobacter baumannii (A. baumannii) infections. Although sulbactam is intrinsically active against A. baumannii, few studies have investigated colistin/sulbactam combinations against carbapenem-resistant A. baumannii.Methods: Whole genome sequencing was undertaken on eight carbapenem-resistant (colistin-susceptible) isolates of A. baumannii from Chinese patients. Bacterial killing of colistin and sulbactam, alone and in combination, was examined with checkerboard (all isolates) and static and dynamic time-kill studies (three isolates). In the dynamic studies, antibiotics were administered in various clinically-relevant dosing regimens that mimicked patient pharmacokinetics.Results: The eight isolates consisted of ST195, ST191 and ST208 belonging to clonal complex 208, which is the most epidemic clonal type of A. baumannii globally. All isolates possessed Acinetobacter-derived cephalosporinase (ADC-61 or ADC-78) and seven of eight isolates contained the carbapenem-resistance gene bla_OXA-23. The colistin/sulbactam combination was synergistic against two of eight isolates in checkerboard studies. In time-kill studies, rapid bacterial killing of ca. 3–6 log₁₀ CFU/mL was observed with colistin monotherapy, followed by steady regrowth. Sulbactam monotherapy was generally ineffective. Substantially enhanced bacterial killing was observed with colistin/sulbactam combinations in both static and dynamic models, especially with the higher sulbactam concentration (2 g) and/or longer sulbactam infusion time (2 hours) in the dynamic model.Conclusions: This study was the first to use a pharmacokinetics/pharmacodynamics model to investigate synergistic activity of colistin/sulbactam combinations against A. baumannii. It showed that clinically-relevant dosing regimens of colistin combined with sulbactam may substantially improve bacterial killing of multidrug-resistant and carbapenem-resistant A. baumannii.     

In vitro evaluation of ceftaroline alone and in combination with tobramycin against hospital-acquired meticillin-resistant Staphylococcus aureus (HA-MRSA) isolates

The aim of this study was to evaluate the in vitro activity of ceftaroline and its potential for synergy with tobramycin in comparison with vancomycin against a collection of hospital-acquired meticillin-resistant Staphylococcus aureus (HA-MRSA), including isolates with reduced susceptibility to glycopeptides. Ceftaroline, vancomycin, daptomycin and linezolid susceptibilities were determined for 200 HA-MRSA isolates. Four randomly selected strains [including one vancomycin-intermediate S. aureus (VISA) and one heteroresistant VISA (hVISA)] were evaluated in time–kill experiments with ceftaroline and vancomycin alone or combined with tobramycin at 0.25 and 0.5 times the minimum inhibitory concentration (MIC). MICs for 50% and 90% of the organisms (MIC₅₀ and MIC₉₀, respectively) were both 1 mg/L for ceftaroline and were 1 mg/L and 2 mg/L, respectively, for vancomycin. The same ceftaroline MIC ranges (0.25–2 mg/L) were observed for isolates recovered from respiratory tract samples, blood or skin. In time–kill experiments, no synergy was observed at 0.25× MIC against any tested isolates with either ceftaroline or vancomycin. In contrast, the combination of ceftaroline plus tobramycin at 0.5× MIC was synergistic against the two MRSA strains and the hVISA but was indifferent against the VISA isolate. In conclusion, ceftaroline demonstrated antimicrobial activity independently of the specimen source and exhibited lower MICs than vancomycin. Finally, at sub-MIC levels, ceftaroline plus tobramycin displayed significantly greater activity than vancomycin plus tobramycin against MRSA (P < 0.01).     

Colistin therapy for microbiologically documented multidrug-resistant Gram-negative bacterial infections: a retrospective cohort study of 258 patients

It is unclear whether the effectiveness of polymyxins depends on the site of infection, the responsible pathogen, dosage, and monotherapy vs. combination therapy. We investigated colistin therapy in a large, retrospective, single-centre, cohort study. Primary analysis outcomes were infection outcome, survival and nephrotoxicity. Over a 7-year period (October 2000 to October 2007), 258 patients received intravenous (i.v.) colistin for at least 72 h for microbiologically documented multidrug-resistant Gram-negative bacterial infections, comprising 170 (65.9%) Acinetobacter baumannii, 68 (26.4%) Pseudomonas aeruginosa, 18 (7.0%) Klebsiella pneumoniae, 1 (0.4%) Stenotrophomonas maltophilia and 1 (0.4%) Enterobacter cloacae. Cure of infection occurred in 79.1% of patients, nephrotoxicity in 10% and hospital survival in 65.1%. In the multivariate analysis, independent predictors of survival were colistin average daily dose [adjusted odds ratio (aOR) = 1.22, 95% confidence interval (CI) 1.05–1.42] and cure of infection (aOR = 9, 95% CI 3.6–23.1), whilst the proportion of creatinine change (aOR = 0.21, 95% CI 0.1–0.45), Acute Physiology and Chronic Health Evaluation (APACHE) II score (aOR = 0.89, 95% CI 0.84–0.95) and haematological disease (aOR = 0.23, 95% CI 0.08–0.66) were associated with mortality. Effectiveness of colistin was not dependent on the type of pathogen. No independent predictors for nephrotoxicity were observed. The findings of the largest cohort study to date on i.v. colistin show that colistin is a valuable antibiotic with acceptable nephrotoxicity and considerable effectiveness that depends on the daily dosage and infection site.     

Effect of colistin exposure and growth phase on the surface properties of live Acinetobacter baumannii cells examined by atomic force microscopy

The diminishing antimicrobial development pipeline has forced the revival of colistin as a last line of defence against infections caused by multidrug-resistant Gram-negative ‘superbugs’ such as Acinetobacter baumannii. The complete loss of lipopolysaccharide (LPS) mediates colistin resistance in some A. baumannii strains. Atomic force microscopy was used to examine the surface properties of colistin-susceptible and -resistant A. baumannii strains at mid-logarithmic and stationary growth phases in liquid and in response to colistin treatment. The contribution of LPS to surface properties was investigated using A. baumannii strains constructed with and without the lpxA gene. Bacterial spring constant measurements revealed that colistin-susceptible cells were significantly stiffer than colistin-resistant cells at both growth phases (P < 0.01), whilst colistin treatment at high concentrations (32 mg/L) resulted in more rigid surfaces for both phenotypes. Multiple, large adhesive peaks frequently noted in force curves captured on colistin-susceptible cells were not evident for colistin-resistant cells. Adhesion events were markedly reduced following colistin exposure. The cell membranes of strains of both phenotypes remained intact following colistin treatment, although fine topographical details were illustrated. These studies, conducted for the first time on live A. baumannii cells in liquid, have contributed to our understanding of the action of colistin in this problematic pathogen.     

Carbapenemase-producing Enterobacteriaceae in a tertiary hospital in Madrid,Spain: high percentage of colistin resistance among VIM-1-producing Klebsiella pneumoniae ST11 isolates

Here we describe the carbapenemase genes, genetic relatedness and antimicrobial susceptibility data of 123 carbapenemase-producing Enterobacteriaceae (CPE) clinical isolates recovered from 2010 to 2012, comprising Klebsiella pneumoniae (n = 79), Klebsiella oxytoca (n = 13), Serratia marcescens (n = 14), Enterobacter cloacae (n = 12), Enterobacter asburiae (n = 4) and Enterobacter aerogenes (n = 1). VIM-1 was the most common carbapenemase (n = 101) followed by KPC-2 (n = 19), OXA-48 (n = 2) and IMP-22 (n = 1). Among the K. pneumoniae isolates, nine sequence types (STs) were identified but two clones were dominant: ST11 (54/79) containing mainly VIM-1-producing isolates; and ST101 (13/79) constituted by KPC-2-producing strains. Pulsed-field gel electrophoresis (PFGE) showed a higher genetic diversity among the remaining Enterobacteriaceae. Amikacin and fosfomycin were the most active agents with 82.9% and 80.5% susceptibility, respectively. Non-susceptibility to tigecycline was detected in 36.5% of strains. Overall, colistin resistance was 24.7% and was as high as 47% in Enterobacter spp. An increase in colistin resistance from 13.5% to 31.7% was observed among K. pneumoniae isolates during the study period. Resistance was focused on ST11 since 83.3% of colistin-resistant strains belonged to this clone. The high level of colistin resistance observed in this study is worrying with respect to the already limited therapeutic options for infections caused by multidrug-resistant Gram-negative bacteria.     

Multicentre study of risk factors for mortality in patients with Acinetobacter bacteraemia receiving colistin treatment

Colistin remains a last-line antibiotic for the treatment of infections by multidrug-resistant Acinetobacter species. However, mortality rates are high in patients with Acinetobacter infection receiving colistin treatment. This multicentre study evaluated whether colistin susceptibility, additional antimicrobial agents or other prognostic factors influenced the clinical outcomes of patients receiving colistin treatment for Acinetobacter bacteraemia. This retrospective study enrolled 122 adults receiving colistin for monomicrobial Acinetobacter bacteraemia at six medical centres in the ACTION Study Group over an 8-year period. Clinical information, antimicrobial susceptibility and colistin resistance determinants were analysed. The primary outcome measure was 14-day mortality. Among 122 patients, 18 and 104 were infected with colistin-resistant (ColR) isolates [minimum inhibitory concentration (MIC) ≥4 mg/L] and colistin-susceptible (ColS) isolates (MIC ≤2 mg/L), respectively. Patients infected with ColR and ColS isolates did not differ significantly with regard to Charlson comorbidity index, invasive procedures, sources of bacteraemia, disease severity and 14-day mortality rate (44.4% vs. 34.6%; P = 0.592). No specific additional antimicrobial agent was independently associated with higher or lower mortality. Coronary artery disease, higher Acute Physiology and Chronic Health Evaluation (APACHE) II score and bacteraemia caused Acinetobacter baumannii were independent risk factors associated with 14-day mortality. Mechanisms of colistin resistance were associated with amino acid variants in the pmrCAB operon. Finally, previously unreported Acinetobacter nosocomialis amino acid variants related to colistin resistance were identified. In conclusion, colistin susceptibility and colistin combination antimicrobial treatment were not associated with decreased 14-day mortality in patients with Acinetobacter bacteraemia receiving colistin treatment.     

Activity of ceftobiprole against methicillin-resistant Staphylococcus aureus strains with reduced susceptibility to daptomycin,linezolid or vancomycin,and strains with defined SCCmec types

Ceftobiprole is a broad-spectrum cephalosporin with activity against methicillin-resistant Staphylococcus aureus (MRSA) and Gram-negative pathogens including Pseudomonas aeruginosa. The aim of this study was to evaluate the activity of ceftobiprole against MRSA isolates with decreased susceptibility to daptomycin, linezolid or vancomycin as well as isolates from staphylococcal chromosome cassette mec (SCCmec) types I, II, III and IV. Overall, ceftobiprole demonstrated high potency against the 216 isolates tested, with MIC₅₀ and MIC₉₀ values (minimum inhibitory concentrations required to inhibit 50% and 90% of the isolates, respectively) of 1 mg/L and 2 mg/L (97.2% susceptible). The MIC₉₀ for ceftobiprole was 2 mg/L against the linezolid-non-susceptible, daptomycin-non-susceptible and vancomycin-intermediate (VISA and hVISA) subsets and was 1 mg/L against the vancomycin-resistant (VRSA) strains. The MIC_50/90 values for ceftobiprole were 2/4, 1/2, 2/2 and 1/1 mg/L against SCCmec types I, II, III and IV, respectively. SCCmec type I strains had the highest MICs, with six strains exhibiting a ceftobiprole MIC of 4 mg/L and 15 strains at 2 mg/L. Ceftobiprole demonstrated potent activity against MRSA, including subsets of isolates with reduced susceptibility to daptomycin, linezolid and vancomycin. The activity of ceftobiprole against these resistant phenotypes indicates that it may have clinical utility in the treatment of infections caused by multidrug-resistant S. aureus and across strains from prevalent SCCmec types.     

Daptomycin activity tested against 164 457 bacterial isolates from hospitalised patients: Summary of 8 years of a Worldwide Surveillance Programme (2005–2012)

We report the results of 8 years (2005–2012) of the Daptomycin Surveillance Programme Worldwide. Consecutive non-duplicate bacterial isolates (prevalence design) were collected from patients with documented infections in 410 medical centres and were susceptibility tested by reference broth microdilution methods. A total of 164 457 Gram-positive isolates were evaluated, including 97 542 Staphylococcus aureus, 21 413 coagulase-negative staphylococci (CoNS), 29 619 enterococci and 15 883 β-haemolytic streptococci. The prevalence of daptomycin-non-susceptible isolates was extremely low for all species in all geographic regions. Overall, the highest occurrence of non-susceptible isolates was observed among CoNS (0.19%), followed by Enterococcus faecium (0.18%), S. aureus (0.05%), Enterococcus faecalis (0.02%) and β-haemolytic streptococci (0.00%). Moreover, no trend towards increased daptomycin resistance (non-susceptibility) was observed for any species in any geographic region during the study interval. Against S. aureus, the daptomycin MIC_50/90 was 0.25/0.5 mg/L in all geographic regions (99.95% susceptible overall). Only 53 daptomycin-non-susceptible S. aureus isolates were observed and the vast majority (49; 92.5%) had a daptomycin MIC value only 1 log₂ dilution above the published susceptible breakpoint. Daptomycin was also active against CoNS (MIC_50/90, 0.25/0.5 mg/L; 99.81% susceptible), E. faecalis (MIC_50/90, 1/2 mg/L; 99.98% susceptible), E. faecium (MIC_50/90, 2/4 mg/L; 99.82% susceptible) including vancomycin-non-susceptible isolates (4521 isolates; MIC_50/90, 2/2 mg/L; 99.76% susceptible), and β-haemolytic streptococci (MIC_50/90, ≤0.06/0.25 mg/L; 100.0% susceptible). In conclusion, daptomycin has remained very active against indicated species worldwide, and no significant year-to-year or regional variation in daptomycin activity has been detected.     

In vitro synergistic activity of NCL195 in combination with colistin against Gram-negative bacterial pathogens

In this study, the potential of using the novel antibiotic NCL195 combined with subinhibitory concentrations of colistin against infections caused by Gram-negative bacteria (GNB) was investigated. We showed synergistic activity of the combination NCL195 + colistin against clinical multidrug-resistant GNB pathogens with minimum inhibitory concentrations (MICs) for NCL195 ranging from 0.5–4 μg/mL for Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, whereas NCL195 alone had no activity. Transmission electron microscopy of the membrane morphology of E. coli and P. aeruginosa after single colistin or combination drug treatment showed marked ultrastructural changes most frequently in the cell envelope. Exposure to NCL195 alone did not show any change compared with untreated control cells, whereas treatment with the NCL195 + colistin combination caused more damage than colistin alone. Direct evidence for this interaction was demonstrated by fluorescence-based membrane potential measurements. We conclude that the synergistic antimicrobial activity of the combination NCL195 + colistin against GNB pathogens warrants further exploration for specific treatment of acute GNB infections.     

Colistin-based treatment for extensively drug-resistant Acinetobacter baumannii pneumonia

Data for treatment and outcomes of extensively drug-resistant Acinetobacter baumannii (XDR-AB) pneumonia are limited. A retrospective cohort study of 236 adult patients with XDR-AB pneumonia was conducted between January 2009 and December 2012. The median age of subjects was 70 years (range 17–95 years), 53% were male, 55% had ventilator-associated pneumonia and 42% had been admitted to the intensive care unit. All XDR-AB isolates were susceptible only to tigecycline and colistin; 52 (22%) of the 236 subjects did not receive an agent active against XDR-AB, with an associated 28-day survival of 0%. Colistin-based two-drug combination treatment was prescribed to 166 subjects (70%); regimens included (i) colistin and high-dose sulbactam (n = 93); (ii) colistin and tigecycline (n = 43); and (iii) colistin and high-dose prolonged infusion of a carbapenem (n = 30). The 28-day survival rate and mean length of hospital stay were not statistically different between these three regimens (65%, 53% and 60% and 39, 39 and 38 days, respectively). Predictors of mortality included Acute Physiology and Chronic Health Evaluation (APACHE) II score [adjusted odds ratio (aOR) = 1.11; P < 0.001 for each point increase], duration from infection onset to receipt of active regimen (aOR = 1.01; P = 0.002 for each hour delay), underlying malignancy (aOR = 3.46; P = 0.01) and chronic kidney disease (aOR = 2.85; P = 0.03). These findings suggest that the three colistin-based two-drug combination regimens may be treatment options for XDR-AB pneumonia.     

Bactericidal activity of daptomycin versus vancomycin in the presence of human albumin against vancomycin-susceptible but tolerant methicillin-resistant Staphylococcus aureus (MRSA) with daptomycin minimum inhibitory concentrations of 1–2 μg/mL

This study explored the influence of vancomycin tolerance and protein binding on the bactericidal activity of vancomycin versus daptomycin (protein binding 36.9% vs. 91.7%, respectively) against four vancomycin-tolerant methicillin-resistant Staphylococcus aureus (MRSA) [minimum inhibitory concentration/minimum bactericidal concentration (MIC/MBC) = 0.5/16, 1/32, 2/32 and 1/32 μg/mL for vancomycin and 1/1, 1/2, 2/2 and 2/4 μg/mL for daptomycin]. Killing curves were performed with vancomycin/daptomycin concentrations equal to serum peak concentrations (C_max) (65.70/98.60 μg/mL) and trough concentrations (C_min) (7.90/9.13 μg/mL) in the presence and absence of a physiological human albumin concentration (4 g/dL), controlled with curves with the theoretical free drug fraction of vancomycin/daptomycin C_max (41.45/8.18 μg/mL) and C_min (4.98/0.76 μg/mL). Vancomycin C_max and C_min concentrations, regardless of the media, showed a bacteriostatic profile not reaching a reduction of 99% or 99.9% of the initial inocula during the 24-h experimental time period. Daptomycin antibacterial profiles significantly differed when testing C_max and C_min. C_max was rapidly bactericidal (≤4 h) with >5 log₁₀ reduction in the initial inocula for all strains, regardless of the presence or not of albumin or the use of concentrations similar to free C_max. C_min exhibited similar final colony counts at 0 h and 24 h in curves with albumin, but with >3 log colony-forming units (CFU)/mL reduction at ≤4 h for strains with an MIC of 1 μg/mL and ca. 2 log CFU/mL reduction at ≤6 h for strains with an MIC of 2 μg/mL. This activity was significantly higher than the activity of the free C_min fraction. The results of this study reinforce the idea that pharmacodynamics using concentrations calculated using reported protein binding are unreliable. Daptomycin exhibited rapid antibacterial activity against vancomycin-tolerant MRSA isolates even against those with high daptomycin MICs in the presence of physiological albumin concentrations.     

Evaluation of dalbavancin,tigecycline, minocycline,tetracycline, teicoplanin and vancomycin against community-associated and multidrug-resistant hospital-associated meticillin-resistant Staphylococcus aureus

Dalbavancin is an investigational semisynthetic lipoglycopeptide that is structurally related to teicoplanin. We examined the activity of dalbavancin (DAL) compared with tigecycline (TIG), minocycline (MIN), tetracycline (TET), teicoplanin (TEC) and vancomycin (VAN) against community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) and multidrug-resistant hospital-associated meticillin-resistant S. aureus (MDR HA-MRSA). Two hundred and twenty clinical isolates of CA-MRSA and MDR HA-MRSA were utilised. Minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) were determined according to Clinical and Laboratory Standards Institute guidelines. Selective time–kill studies were performed against CA-MRSA and MDR HA-MRSA in triplicate. Overall, DAL exhibited low MIC values against all strains of MRSA. Time–kill studies with CA-MRSA demonstrated DAL = VAN > TIG > MIN = TEC > TET (P < 0.006), and with MDR HA-MRSA demonstrated DAL = VAN = TEC > TIG = MIN > TET (P > 0.05). DAL demonstrated potent activity against clinical strains of CA-MRSA and MDR HA-MRSA, including tetracycline-resistant S. aureus.     

Characterising the post-antifungal effects of micafungin against Candida albicans,Candida glabrata,Candida parapsilosis and Candida krusei isolates

In this study, we measured time–kills and post-antifungal effects (PAFEs) for micafungin against Candida albicans (n = 4), Candida glabrata (n = 3), Candida parapsilosis (n = 3) and Candida krusei (n = 2) isolates and further characterised the PAFEs. Minimum inhibitory concentrations (MICs) were 0.5–1.0 mg/L against C. parapsilosis and 0.008–0.125 mg/L against the other species. Micafungin caused kills >1 log at 1 × MIC, 4 × MIC (range 1.19–3.10 log) and 16 × MIC (2.27–3.68 log), achieving fungicidal levels (≥3 log) against nine isolates. One-hour drug exposure during PAFE experiments resulted in kills of 0.73–2.88 log and 1.72–3.55 log at 4× and 16× MIC, respectively, achieving fungicidal levels against five isolates. Isolates of each species collected 8 h after a 1-h exposure to micafungin (4× and 16× MIC) were hypersusceptible to sodium dodecyl sulphate (SDS) and Calcofluor White. Cells tested during the PAFE period demonstrated cell wall disturbances as evident on electron micrographs as well as significant reductions in adherence to epithelial cells. Phagocytosis by J774 macrophages was significantly enhanced for three PAFE isolates tested. Micafungin is fungicidal and exerts PAFEs that kill diverse Candida spp., disturb cell walls of viable organisms, reduce adherence and enhance susceptibility to phagocytosis.     

Efficacy of ceftolozane/tazobactam,alone and in combination with colistin,against multidrug-resistant Pseudomonas aeruginosa in an in vitro biofilm pharmacodynamic model

Objectives

Ceftolozane/tazobactam is a potential tool for infections caused by multidrug-resistant (MDR) Pseudomonas aeruginosa (P. aeruginosa), but its efficacy against some difficult-to-treat infections has not been well defined.

In vitro activity of rifaximin against isolates from patients with small intestinal bacterial overgrowth

Rifaximin, a non-absorbable rifamycin derivative, has published clinical efficacy in the alleviation of symptoms in patients with irritable bowel syndrome (IBS). Small intestinal bacterial overgrowth (SIBO) is associated with the pathogenesis of IBS. This study describes for the first time the antimicrobial effect of rifaximin against SIBO micro-organisms from humans. Fluid was aspirated from the third part of the duodenum from 567 consecutive patients; quantitative cultures diagnosed SIBO in 117 patients (20.6%). A total of 170 aerobic micro-organisms were isolated and the in vitro efficacy of rifaximin was studied by (i) minimum inhibitory concentration (MIC) testing by a microdilution technique and (ii) time–kill assays using bile to simulate the small intestinal environment. At a breakpoint of 32 μg/mL, rifaximin inhibited in vitro 85.4% of Escherichia coli, 43.6% of Klebsiella spp., 34.8% of Enterobacter spp., 54.5% of other Enterobacteriaceae spp., 82.6% of non-Enterobacteriaceae Gram-negative spp., 100% of Enterococcus faecalis, 100% of Enterococcus faecium and 100% of Staphylococcus aureus. For the time–kill assays, 11 E. coli, 15 non-E. coli Gram-negative enterobacteria and three E. faecalis isolates were studied. Rifaximin produced a >3 log₁₀ decrease in the starting inoculum against most of the tested isolates at 500 μg/mL after 24 h of growth. The results indicate that rifaximin has a potent effect on specific small bowel flora associated with SIBO. This conclusion should be regarded in light of the considerable time–kill effect at concentrations lower than those achieved in the bowel lumen after administration of conventional doses in humans.     

Pharmacokinetic/pharmacodynamic evaluation of sulbactam against Acinetobacter baumannii in in vitro and murine thigh and lung infection models

Acinetobacter baumannii is a pathogen that has become globally associated with nosocomial infections. Sulbactam, a potent inhibitor of β-lactamases, was previously shown to be active against A. baumannii strains in vitro and effective against A. baumannii infections. However, a pharmacokinetic/pharmacodynamic (PK/PD) analysis of sulbactam against A. baumannii infections has not yet been performed. This is necessary because optimisation of dosing regimens should be based on PK/PD analysis. Therefore, in vitro and in vivo PK/PD analyses of sulbactam were performed using murine thigh and lung infection models of A. baumannii to evaluate the pharmacokinetics and pharmacodynamics of sulbactam. Sulbactam showed time-dependent bactericidal activity in vitro against A. baumannii. The PK/PD index that best correlated with its in vivo effects was the time that the free drug concentration remained above the minimum inhibitory concentration (fT_>MIC) both in the thigh (R² = 0.95) and lung (R² = 0.96) infection models. Values of fT_>MIC for a static effect and 1, 2 and 3 log₁₀ kill, respectively, were 21.0%, 32.9%, 43.6% and 57.3% in the thigh infection model and 20.4%, 24.5%, 29.3% and 37.3% in the lung infection model. Here we report the in vitro and in vivo time-dependent activities of sulbactam against A. baumannii infection and demonstrate that sulbactam was sufficiently bactericidal when an fT_>MIC of >60% against A. baumannii thigh infection and >40% against A. baumannii lung infection was achieved.     

Application of pharmacokinetic/pharmacodynamic modelling and simulation for the prediction of target attainment of ceftobiprole against meticillin-resistant Staphylococcus aureus using minimum inhibitory concentration and time–kill curve based approaches

The purpose of this report was to compare two different methods for dose optimisation of antimicrobials. The probability of target attainment (PTA) was calculated using Monte Carlo simulation to predict the PK/PD target of fT_>MIC or modelling and simulation of time–kill curve data. Ceftobiprole, the paradigm compound, activity against two MRSA strains was determined, ATCC 33591 (MIC = 2 mg/L) and a clinical isolate (MIC = 1 mg/L). A two-subpopulation model accounting for drug degradation during the experiment adequately fit the time–kill curve data (concentration range 0.25–16× MIC). The PTA was calculated for plasma, skeletal muscle and subcutaneous adipose tissue based on data from a microdialysis study in healthy volunteers. A two-compartment model with distribution factors to account for differences between free serum and tissue interstitial space fluid concentration appropriately fit the pharmacokinetic data. Pharmacodynamic endpoints of fT_>MIC of 30% or 40% and 1- or 2-log kill were used. The PTA was >90% in all tissues based on the PK/PD endpoint of fT_>MIC >40%. The PTAs based on a 1- or 2-log kill from the time–kill experiments were lower than those calculated based on fT_>MIC. The PTA of a 1-log kill was >90% for both MRSA isolates for plasma and skeletal muscle but was slightly below 90% for subcutaneous adipose tissue (both isolates ca. 88%). The results support a dosing regimen of 500 mg three times daily as a 2-h intravenous infusion. This dose should be confirmed as additional pharmacokinetic data from various patient populations become available.     

Mefloquine enhances the activity of colistin against antibiotic-resistant Enterobacterales in vitro and in an in vivo animal study

Infections caused by carbapenem-resistant Enterobacterales are difficult to treat. Colistin is the last-resort drug for the treatment of these infections, however colistin resistance has emerged in animals and humans. This study investigated the in vitro efficacy of mefloquine in combination with colistin against 114 antibiotic-resistant Enterobacterales isolates including NDM-1, extended-spectrum β-lactamase (ESBL) and mcr-1 containing strains from a broad range of origins. The effect of the mefloquine and colistin combination was examined in vitro by chequerboard method and time–kill analysis and in vivo in a murine peritoneal infection model. The fractional inhibitory concentration index (FICI) of the combination indicated that synergy was detected for all NDM-1 and mcr-1 containing strains, 87.5% of ESBL producing Escherichia coli and 97.9% of ESBL producing Klebsiella pneumoniae strains. Time–kill curves demonstrated significant synergistic activity with low concentrations of colistin that were boosted by mefloquine. The combination showed enhanced activity against infection with NDM-1- or mcr-1 containing Enterobacteriaceae in mice at 4 h and 6 h after treatment. These findings suggest that the combination of mefloquine and colistin has the potential for rejuvenating the activity of colistin against multidrug-resistant Enterobacterales.