Comparative bactericidal activity of representative ß-lactams against Enterobacterales, Acinetobacter baumannii and Pseudomonas aeruginosa.

BACKGROUND: There is surprisingly little comparative published data on the bactericidal action of different sub-classes of ß-lactams against aerobic Gram-negative rods, and the assumption is that all behave in the same way. OBJECTIVES: To describe a systematic investigation of a representative penicillin, cephalosporin, monobactam and carbapenem against Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii and Pseudomonas aeruginosa. METHODS: Concentration-time-kill curves (TKC) were determined for three strains each of E. coli, K. pneumoniae, A. baumannii and P. aeruginosa. All strains were susceptible to the agents used. The antibiotics were piperacillin/tazobactam, ceftazidime, aztreonam and meropenem. The initial inoculum was 106 cfu/mL and TKC were determined over 48 h. The area-under-the-bacterial-kill curve to 24 h (AUBKC 24 log cfu·h/mL) and 48 h (AUBKC 48) were used to measure antibacterial effect (ABE). Population profiles before and after antibiotic exposure were recorded. RESULTS: Against E. coli and K. pneumoniae meropenem had a maximal ABE at ≥MIC × 1 concentrations while piperacillin/tazobactam and ceftazidime had maximal effect at ≥MIC × 4 and aztreonam at ≥MIC × 8 concentrations. Ceftazidime, aztreonam and meropenem had less ABE against K. pneumoniae than E. coli. Against P. aeruginosa, meropenem was most bactericidal, with a maximum ABE at 8×/16 × MIC. Other ß-lactams had notably less ABE. In contrast, against A. baumannii, ceftazidime and meropenem had the greatest ABE, with a maximal effect at ≥MIC × 4, concentration changes in population profiles were least apparent with E. coli. CONCLUSIONS: ß-Lactam sub-classes (penicillins, cephalosporins, monobactams and carbapenems) have different antibacterial effects against E. coli, K. pneumoniae, A. baumannii and P. aeruginosa. Extrapolation of in vitro pharmacodynamic findings from one species to another or one sub-class of ß-lactam to another is not justified.

Limited phylogenetic overlap between fluoroquinolone-resistant Escherichia coli isolated on dairy farms and those causing bacteriuria in humans living in the same geographical region.

BACKGROUND: Our primary aim was to test whether cattle-associated fluoroquinolone-resistant (FQ-R) Escherichia coli found on dairy farms are closely phylogenetically related to those causing bacteriuria in humans living in the same 50 × 50 km geographical region suggestive of farm-human sharing. Another aim was to identify risk factors for the presence of FQ-R E. coli on dairy farms. METHODS: FQ-R E. coli were isolated during 2017-18 from 42 dairy farms and from community urine samples. Forty-two cattle and 489 human urinary isolates were subjected to WGS, allowing phylogenetic comparisons. Risk factors were identified using a Bayesian regularization approach. RESULTS: Of 489 FQ-R human isolates, 255 were also third-generation-cephalosporin-resistant, with strong genetic linkage between aac(6')Ib-cr and blaCTX-M-15. We identified possible farm-human sharing for pairs of ST744 and ST162 isolates, but minimal core genome SNP distances were larger between farm-human pairs of ST744 and ST162 isolates (71 and 63 SNPs, respectively) than between pairs of isolates from different farms (7 and 3 SNPs, respectively). Total farm fluoroquinolone use showed a positive association with the odds of isolating FQ-R E. coli, while total dry cow therapy use showed a negative association. CONCLUSIONS: This work suggests that FQ-R E. coli found on dairy farms have a limited impact on community bacteriuria within the local human population. Reducing fluoroquinolone use may reduce the on-farm prevalence of FQ-R E. coli and this reduction may be greater when dry cow therapy is targeted to the ecology of resistant E. coli on the farm.

Clinical diagnosis and treatment of common respiratory tract infections in relation to microbiological profiles in rural health facilities in China: implications for antibiotic stewardship.

BACKGROUND: This paper tries to describe prevalence and patterns of antibiotics prescription and bacteria detection and sensitivity to antibiotics in rural China and implications for future antibiotic stewardship. METHODS: The study was implemented in one village clinic and one township health center in each of four rural residential areas in Anhui Province, China. It used mixed-methods comprising non-participative observations, exit-survey and microbiological study. Observations were conducted to record clinical diagnosis and antibiotic prescription. Semi-structured questionnaire survey was used to collect patient's sociodemographic information and symptoms. Sputum and throat swabs were collected for bacterial culture and susceptibility testing. RESULTS: A total of 1068 (51.0% male vs 49.0% female) patients completed the study with diagnosis of respiratory tract infection (326,30.5%), bronchitis/tracheitis (249,23.3%), pharyngitis (119,11.1%) and others (374, 35.0%). They provided 683 sputum and 385 throat swab specimens. Antibiotics were prescribed for 88% of the RTI patients. Of all the specimens tested, 329 (31%) were isolated with bacteria. The most frequently detected bacteria were K. pneumonia (24% in all specimens), H. influenza (16%), H. parainfluenzae (15%), P. aeruginosa (6%), S.aureus (5%), M. catarrhalis (3%) and S. pneumoniae (2%). CONCLUSIONS: The study establishes the feasibility of conducting microbiological testing outside Tier 2 and 3 hospitals in rural China. It reveals that prescription of antibiotics, especially broad-spectrum and combined antibiotics, is still very common and there is a clear need for stewardship programs aimed at both reducing the number of prescriptions and promoting single and narrow-spectrum antibiotics.

The pharmacodynamics of fosfomycin against Staphylococcus aureus studied in an in vitro model of infection.

OBJECTIVES: The pharmacodynamics of intravenous fosfomycin have not been described for Gram-positive pathogens such as Staphylococcus aureus (S. aureus). This paper described the dominant pharmacodynamic index for fosfomycin against S. aureus and its size for antibacterial effect. METHODS: A single-compartment dilutional in vitro pharmacokinetic model was used to provide fosfomycin exposures against S. aureus, three methicillin-susceptible S. aureus (MSSA), two methicillin-resistant S. aureus (MRSA); fosfomycin MICs were 2 mg/L (one strain), 4 mg/L (one strain), 8 mg/L (two strains) and 16 mg/L (one strain). For all simulations, a fosfomycin half-life of 2.5 hours was modelled. Cmax/MICs from 0-74.8, AUC/MICs from 0-750 and T>MIC 0-100% were simulated. The primary end-points were changes in bacterial load after 24 hours and changes in population profiles after 48 hours. RESULTS: Log AUC/MIC R2 = 0.55 and log Cmax/MIC R2 = 0.66 were related to S. aureus log reduction in viable count at 24 hours; T>MIC was poorly related. Cmax/MIC for a 24 hour static, -1 log drop and -2 log drop were 3.0 ± 1.7, 4.6 ± 2.4 and 6.6 ± 3.8, respectively. AUC/MIC for a 24 hour static, -1 log drop and -2 log drop were 26.4 ± 11.8, 42.8 ± 21.8 and 66.6 ± 39.1. Emergence of resistance as indicated by > 2 log growth on MICx8 recovery media was maximal at AUC/MIC ratios of 10-40 and was suppressed at AUC/MIC ratios of ≥ 250. CONCLUSIONS: The dominant pharmacodynamic index for fosfomycin against S. aureus was Cmax/MIC in terms of reduction of bacterial load and AUC/MIC in terms of suppressing emergence of resistance. AUC/MIC ratios of 20-75 were associated with a -1 log reduction in bacterial load and AUC/MIC of 10-40 maximally increased emergence of resistance.

Characterization of cefotaxime-resistant urinary Escherichia coli from primary care in South-West England 2017-18.

OBJECTIVES: Third-generation cephalosporin-resistant Escherichia coli from community-acquired urinary tract infections are increasingly reported worldwide. We sought to determine and characterize the mechanisms of cefotaxime resistance employed by urinary E. coli obtained from primary care, over 12 months, in Bristol and surrounding counties in South-West England. METHODS: Cefalexin-resistant E. coli isolates were identified from GP-referred urine samples using disc susceptibility testing. Cefotaxime resistance was determined by subsequent plating onto MIC breakpoint plates. ß-Lactamase genes were detected by PCR. WGS was performed on 225 isolates and analyses were performed using the Center for Genomic Epidemiology platform. Patient information provided by the referring general practices was reviewed. RESULTS: Cefalexin-resistant E. coli (n=900) isolates were obtained from urines from 146 general practices. Following deduplication by patient approximately 69% (576/836) of isolates were cefotaxime resistant. WGS of 225 isolates identified that the most common cefotaxime-resistance mechanism was blaCTX-M carriage (185/225), followed by plasmid-mediated AmpCs (pAmpCs) (17/225), AmpC hyperproduction (13/225), ESBL blaSHV variants (6/225) or a combination of both blaCTX-M and pAmpC (4/225). Forty-four STs were identified, with ST131 representing 101/225 isolates, within which clade C2 was dominant (54/101). Ciprofloxacin resistance was observed in 128/225 (56.9%) of sequenced isolates, predominantly associated with fluoroquinolone-resistant clones ST131 and ST1193. CONCLUSIONS: Most cefalexin-resistant E. coli isolates were cefotaxime resistant, predominantly caused by blaCTX-M carriage. The correlation between cefotaxime resistance and ciprofloxacin resistance was largely attributable to the high-risk pandemic clones ST131 and ST1193. Localized epidemiological data provide greater resolution than regional data and can be valuable for informing treatment choices in the primary care setting.

Comparative evaluation of eight in vitro pharmacodynamic models of infection: Activity of moxifloxacin against Escherichia coli and Streptococcus pneumoniae as an exemplary example.

Use of pharmacodynamic in vitro models provides more clinically relevant information about the activities of antibiotics than static endpoints. Several models are used to simulate pharmacokinetics by dilution of the medium. It is discussed whether this procedure would result in a washout of bacteria, particularly if profiles with a short half-life are simulated. Methods have been developed to minimise the washout of bacteria. Bacteria are retained in the system either by centrifugation and resuspension, use of filters, a capillary unit, dialysis tubing or mathematical correction, versus systems with an unprotected outflow allowing a continuous washout of bacteria. None of these eight models has been directly compared with another. Therefore, an interlaboratory study was performed to address the question of whether or not washout matters. All laboratories used identical batches of media, bacteria, antibiotics and simulated pharmacokinetic profiles with a short or long half-life. Values of area under the bacterial kill-time curve (AUBKC), single-point kill rate and time to 3-log10 reduction of inoculum were calculated. These parameters did not differ significantly between the models. Differences were noted if the inoculum was prepared from the early logarithmic growth phase compared with the late logarithmic or stationary growth phase, resulting either in a pronounced or reduced antibacterial activity. Thus, preparation of inocula affects the results generated, whereas washout of bacteria has apparently a negligible impact on antibacterial activities.

Pathways to optimising antibiotic use in rural China: identifying key determinants in community and clinical settings, a mixed methods study protocol.

INTRODUCTION: This study aims to investigate patterns of antibiotic treatment-seeking, describe current levels of and drivers for antibiotic use for common infections (respiratory tract and urinary tract infections) and test the feasibility of determining the prevalence and epidemiology of antimicrobial resistance (AMR) in rural areas of Anhui province, in order to identify potential interventions to promote antibiotic stewardship and reduce the burden of AMR in China. METHODS AND ANALYSIS: We will conduct direct observations, structured and semistructured interviews in retail pharmacies, village clinics and township health centres to investigate treatment-seeking and antibiotic use. Clinical isolates from 1550 sputum, throat swab and urine samples taken from consenting patients at village and township health centres will be analysed to identify bacterial pathogens and ascertain antibiotic susceptibilities. Healthcare records will be surveyed for a subsample of those recruited to the study to assess their completeness and accuracy. ETHICS AND DISSEMINATION: The full research protocol has been reviewed and approved by the Biomedical Ethics Committee of Anhui Medical University (reference number: 20170271). Participation of patients and doctors is voluntary and written informed consent is sought from all participants. Findings from the study will be disseminated through academic routes including peer-reviewed publications and conference presentations, via tailored research summaries for health professionals, health service managers and policymakers and through an end of project impact workshop with local and regional stakeholders to identify key messages and priorities for action.

Pharmacodynamics of plazomicin and a comparator aminoglycoside, amikacin, studied in an in vitro pharmacokinetic model of infection.

The new aminoglycoside plazomicin shows in vitro potency against multidrug-resistant Enterobacteriales. The exposure-response relationship of plazomicin and the comparator aminoglycoside amikacin was determined for Escherichia coli, while for Klebsiella pneumoniae only plazomicin was tested. An in vitro pharmacokinetic model was used. Five E. coli strains (two meropenem-resistant) and five K. pneumoniae strains (two meropenem-resistant) with plazomicin MICs of 0.5-4 mg/L were used. Antibacterial effect was assessed by changes in bacterial load and bacterial population profile. The correlation between change in initial inoculum after 24 h of drug exposure and the AUC/MIC ratio was good (plazomicin R2 ≥ 0.8302; amikacin R2 ≥ 0.9520). Escherichia coli plazomicin AUC/MIC ratios for 24-h static, -1, -2 and -3 log drop were 36.1 ± 18.4, 39.3 ± 20.9, 41.2 ± 21.9 and 44.8 ± 24.3, respectively, and for amikacin were 49.5 ± 12.7, 55.7 ± 14.8, 64.1 ± 19.2 and 73.3 ± 25.3. Klebsiella pneumoniae plazomicin AUC/MIC ratios for 24-h static, -1, -2 and -3 log drop were 34.0 ± 15.2, 46.8 ± 27.8, 67.4 ± 46.5 and 144.3 ±129.8. Plazomicin AUC/MIC ratios >66 and amikacin AUC/MIC ratios >57.7 were associated with suppression of E. coli growth on 4 × or 8 × MIC recovery plates. The equivalent plazomicin AUC/MIC to suppress resistance emergence with K. pneumoniae was >132. The plazomicin AUC/MIC for 24-h static effect and -1 log reduction in E. coli and K. pneumoniae bacterial load was in the range 30-60. Plazomicin AUC/MIC targets aligned with those of amikacin for E. coli.

Antibacterial effect of imipenem/relebactam on aerobic Gram-negative bacilli: in vitro simulations of 7 or 14 day human exposures.

OBJECTIVES: We assessed the antibacterial effect of human simulations of dosing with imipenem/relebactam (with or without amikacin) on Enterobacteriaceae or Pseudomonas aeruginosa over 7 or 14 day antibiotic exposures. METHODS: An in vitro pharmacokinetic model was used to assess changes in bacterial load and population profiles. RESULTS: Imipenem/relebactam produced an initial >4 log drop in viable counts followed by suppression for 7 days for Enterobacteriaceae whether the strain was WT, produced KPC enzymes or produced an AmpC enzyme with porin loss. Similarly, with the P. aeruginosa strains, there was an initial >4 log clearance over the first 24 h irrespective of whether the strain was WT, hyperexpressed AmpC or had OprD mutation with porin loss. However, with three of four strains there was modest regrowth over the 7 days. There were no changes in imipenem/relebactam MICs over the 7 days. Addition of amikacin in 7 day simulations resulted in more suppression of pseudomonal growth. In 14 day simulations with P. aeruginosa there was regrowth to 8 log10 by 14 days with imipenem/relebactam alone and associated increases in MICs. Addition of amikacin resulted in clearance from the model and prevented changes in population profiles. CONCLUSIONS: Imipenem/relebactam was highly effective at reducing the bacterial load of Enterobacteriaceae and there was no emergence of resistance. Against P. aeruginosa, the initial bacterial burden was also rapidly reduced, but there was subsequent regrowth, especially after 7 days of exposure. Addition of amikacin increased the clearance of P. aeruginosa and prevented emergence of resistance.

Antibacterial effect of ceftolozane/tazobactam in combination with amikacin against aerobic Gram-negative bacilli studied in an in vitro pharmacokinetic model of infection.

Objectives: To use a pre-clinical infection model to assess the antibacterial effect of human simulations of dosing with ceftolozane/tazobactam (with or without amikacin) or meropenem against Enterobacteriaceae and Pseudomonas aeruginosa. Methods: An in vitro pharmacokinetic model was used to assess changes in bacterial load and profiles after exposure to mean human serum concentrations over 168 h. Changes in area under the bacterial kill curve (AUBKC; log cfu/mL·h) and growth on 4 × MIC recovery plates were the co-primary outcome measures. Results: Simulations of ceftolozane/tazobactam at 1 g/0.5 g or 2 g/1 g q8h or meropenem 2 g q8h all produced a >4 log reduction in bacterial load of Escherichia coli. Meropenem had smaller AUBKC values, indicating greater reduction in bacterial load than ceftolozane/tazobactam. Meropenem was also more effective than ceftolozane/tazobactam against Klebsiella pneumoniae strains. All regimens were equally effective in reducing P. aeruginosa bacterial load measured by AUBKC but growth on 4 × MIC recovery plates and changes in population profiles were only seen with meropenem. Addition of amikacin at 15 mg/kg q24h or 7.5 mg/kg q12h to 2 g/1 g of ceftolozane/tazobactam produced greater reductions in bacterial load but generated changes in amikacin population profiles with the 7.5 mg/kg q12h amikacin simulation. Conclusions: The doses of ceftolozane/tazobactam simulated were highly effective in reducing the bacterial load of E. coli (MIC ≤0.25 mg/L), but less so for K. pneumoniae (MIC 4 mg/L). For both species, meropenem produced an overall greater reduction in pathogen load. Ceftolozane/tazobactam and meropenem were equally effective as monotherapy against P. aeruginosa but emergence of resistance occurred with meropenem. Addition of amikacin to ceftolozane/tazobactam reduced the bacterial load of P. aeruginosa at the expense of emergence of resistance to amikacin.

Pharmacodynamics of inhaled amikacin (BAY 41-6551) studied in an in vitro pharmacokinetic model of infection.

Background: The pharmacodynamics of inhaled antimicrobials are poorly studied. Amikacin is being developed for inhalational therapy as BAY 41-6551. Objectives: We employed an in vitro pharmacokinetic model to study the pharmacokinetics/pharmacodynamics of amikacin. Methods: A dose-ranging design was used to establish fAUC/MIC and fCmax/MIC targets for static, -1 log drop and -2 log drop effects for strains of Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa. We then modelled epithelial lining fluid (ELF) concentration associated with inhaled amikacin (400 mg every 12 h), over 5 days using mean human concentrations. Results: The 24 h static effect fAUC/MIC targets and -1 log drop targets were 51.0 ±âŸ26.7 and 71.6 ±âŸ27.6 for all species of aerobic Gram-negative bacilli. fAUC/MIC targets for static effect, -1 log drop or -2 log drop were smaller than the 24 h values at 12 h and larger at 48 h. Emergence of resistance occurred maximally with E. coli in the fAUC/MIC range 12-60; K. pneumoniae 0-60 (48 h) and P. aeruginosa 12-80. When human ELF concentrations were modelled for strains with MIC ≤8 mg/L, there was rapid clearance and no regrowth. For strains with MIC ≥32 mg/L, there was initial clearance followed by regrowth. If MIC values were related to bacterial clearance then at least a static effect or -1 log drop in count would be expected for bacterial strains with MICs of ≤180 mg/L (static effect) or ≤148 mg/L (-1 log drop effect). Conclusions: An fAUC/MIC amikacin target of 50-80 is appropriate for aerobic Gram-negative bacilli and mean ELF concentrations of BAY 41-6551 would produce a static to -1 log clearance with strains up to 128 mg/L.

Prediction of Fluoroquinolone Susceptibility Directly from Whole-Genome Sequence Data by Using Liquid Chromatography-Tandem Mass Spectrometry To Identify Mutant Genotypes.

Fluoroquinolone resistance in Gram-negative bacteria is multifactorial, involving target site mutations, reductions in fluoroquinolone entry due to reduced porin production, increased fluoroquinolone efflux, enzymes that modify fluoroquinolones, and Qnr, a DNA mimic that protects the drug target from fluoroquinolone binding. Here we report a comprehensive analysis, using transformation and in vitro mutant selection, of the relative importance of each of these mechanisms for fluoroquinolone nonsusceptibility using Klebsiella pneumoniae as a model system. Our improved biological understanding was then used to generate 47 rules that can predict fluoroquinolone susceptibility in K. pneumoniae clinical isolates. Key to the success of this predictive process was the use of liquid chromatography-tandem mass spectrometry to measure the abundance of proteins in extracts of cultured bacteria, identifying which sequence variants seen in the whole-genome sequence data were functionally important in the context of fluoroquinolone susceptibility.

Pharmacodynamics of minocycline against Acinetobacter baumannii studied in a pharmacokinetic model of infection.

Minocycline (MNO) is an old antibiotic that may have an important role in the treatment of multidrug-resistant Gram-negative bacterial infections as the burden of such infections increases. In this study, a single-compartment dilutional pharmacokinetic model was used to determine the relationship between MNO exposure and antibacterial effect, including the risk of resistance emergence, against strains of Acinetobacter baumannii. The mean ± standard deviation area under the unbound drug concentration-time curve to minimum inhibitory concentration ratio (fAUC/MIC) associated with a 24-h bacteriostatic effect was 16.4 ± 2.6 and with a -1 log reduction in bacterial load at 24 h was 23.3 ± 3.7. None of the strains reached a -2 log reduction over 48 h. Changes in population profiles were noted for two of the three strains studied, especially at fAUC/MIC ratios of >5-15. A reasonable translational pharmacodynamic target for MNO against A. baumannii could be an fAUC/MIC of 20-25. However, if maximum standard 24-h doses of intravenous MNO are used (400 mg/day), many strains would be exposed to MNO concentrations likely to change population profiles and associated with the emergence of resistance. Either MNO combination therapy or an increased MNO dose (>400 mg/day) should be considered when treating A. baumannii infections.

The pharmacodynamics of avibactam in combination with ceftaroline or ceftazidime against ß-lactamase-producing Enterobacteriaceae studied in an in vitro model of infection.

Objectives: Pharmacodynamics of ß-lactamase inhibitors are an area of intense interest as new ß-lactam/ß-lactamase inhibitor combinations enter clinical development and clinical practice. Avibactam, a non-ß-lactam ß-lactamase inhibitor, has been combined with ceftaroline or ceftazidime but these two combinations have not been directly compared. Methods: Using an in vitro pharmacokinetic model we simulated human drug concentration-time courses associated with ceftaroline 600 mg every 8 h and ceftazidime 2000 mg every 8 h. Avibactam was given by continuous infusion at a range of concentrations up to 10 mg/L and antibacterial effect assessed against a CTX-M-producing Escherichia coli , AmpC-hyperproducing Enterobacter cloacae and KPC-producing Klebsiella pneumoniae. Simulations were performed over 72 h. Results: Avibactam, at a concentration of 1-2 mg/L, produced maximum bacterial clearance over 72 h for the E. coli and E. cloacae strains with both ceftaroline and ceftazidime. Avibactam (4 mg/L) was required for maximum reduction in bacterial load with the KPC-producing K. pneumoniae. A series of dose fractionation experiments were performed with avibactam against each of the three strains and AUC, C max or T > avibactam concentration of 1, 2 or 4 mg/L related to antibacterial effect as measured by change in bacterial count at 24 h. AUC or C max were best related to 24 h antibacterial effect for avibactam though there was no consistent pattern favouring one over the other. Conclusions: As AUC is a much easier and more reliable pharmacokinetic measure than C max , it would be useful to explore how AUC-based indices for avibactam exposures could be used for translating the results of the present study into patients' therapy.

Differences in the pharmacodynamics of ceftaroline against different species of Enterobacteriaceae studied in an in vitro pharmacokinetic model of infection.

OBJECTIVES: Dose-ranging experiments were performed to study the pharmacodynamics of ceftaroline against Enterobacteriaceae. METHODS: A range of fT>MIC values (0%-100%) were simulated over 96 h using a single-compartment dilutional in vitro pharmacokinetic model using Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Citrobacter koseri and Serratia marcescens (n = 16). Antibacterial effect was assessed by change in viable count and population profiles by growth on ceftaroline MIC ×2, ×4 and ×8 agar plates. The fT>MIC (%) was related to antibacterial effect using a sigmoid Emax model. RESULTS: The 24 h bacteriostatic effect fT>MIC was 39.7% ±â€Š15.7% and 43.2% ±â€Š15.6% for a -1 log drop for all strains. E. coli required lower exposures than K. pneumoniae, i.e. 24 h fT>MIC for a -3 log drop in viable count was 40.0% ±â€Š9.6% and 84.8% ±â€Š15.2% for K. pneumoniae. Similarly at 96 h, fT>MIC was >100% for K. pneumoniae (for four of five strains), 27.2%-66.2% for E. coli and 16.2%-86.6% for P. mirabilis. Strain-to-strain variation within species in the fT>MIC for static and cidal effect was marked; the 24 h bacteriostatic range was 14.1%-73.4% for P. mirabilis, 34.2%-44.6% for E. coli and 42.2%-62.5% for K. pneumoniae. Changes in ceftaroline population analysis profiles were observed with E. coli, K. pneumoniae and C. koseri, especially at fT>MIC values just below the bacteriostatic effect exposures. CONCLUSIONS: The pharmacodynamics of ceftaroline against the species within the Enterobacteriaceae group are different. K. pneumoniae requires higher drug exposures than E. coli, and P. mirabilis strains are highly variable, which may have important clinical correlates. Translational extrapolations from preclinical observations using E. coli to other Enterobacteriaceae species may not be optimal.

Pharmacodynamics of Ceftolozane plus Tazobactam Studied in an In Vitro Pharmacokinetic Model of Infection.

Ceftolozane plus tazobactam is an antipseudomonal cephalosporin combined with tazobactam, an established beta-lactamase inhibitor, and has in vitro potency against a range of clinically important ß-lactamase-producing bacteria, including most extended-spectrum-ß-lactamase (ESBL)-positive Enterobacteriaceae. The pharmacodynamics of ß-lactam-ß-lactamase inhibitor combinations presents a number of theoretical and practical challenges, including modeling different half-lives of the compounds. In this study, we studied the pharmacodynamics of ceftolozane plus tazobactam against Escherichia coli and Pseudomonas aeruginosa using an in vitro pharmacokinetic model of infection. Five strains of E. coli, including three clinical strains plus two CTX-M-15 (one high and one moderate) producers, and five strains of P. aeruginosa, including two with OprD overexpression and AmpC ß-lactamases, were employed. Ceftolozane MICs (E. coli, 0.12 to 0.25 mg/liter, and P. aeruginosa, 0.38 to 8 mg/liter) were determined in the presence of 4 mg/liter tazobactam. Dose ranging of ceftolozane (percentage of time in which the free-drug concentration exceeds the MIC [fT>MIC], 0 to 100%) plus tazobactam (human pharmacokinetics) was simulated every 8 hours, with half-lives (t1/2) of 2.5 and 1 h, respectively. Ceftolozane and tazobactam concentrations were confirmed by high-performance liquid chromatography (HPLC). The ceftolozane-plus-tazobactam fT>MIC values at 24 h for a static effect and a 1-log and 2-log drop in initial inoculum for E. coli were 27.8% ± 5.6%, 33.0% ± 5.6%, and 39.6% ± 8.5%, respectively. CTX-M-15 production did not affect the 24-h fT>MIC for E. coli strains. The ceftolozane-plus-tazobactam fT>MIC values for a 24-h static effect and a 1-log and 2-log drop for P. aeruginosa were 24.9% ± 3.0%, 26.6% ± 3.9%, and 31.2% ± 3.6%. Despite a wide range of absolute MICs, the killing remained predictable as long as the MICs were normalized to the corresponding fT>MIC. Emergence of resistance on 4× MIC plates and 8× MIC plates occurred maximally at an fT>MIC of 10 to 30% and increased as time of exposure increased. The fT>MIC for a static effect for ceftolozane plus tazobactam is less than that observed with other cephalosporins against E. coli and P. aeruginosa and is more similar to the fT>MIC reported for carbapenems.

The combination of colistin and fosfomycin is synergistic against NDM-1-producing Enterobacteriaceae in in vitro pharmacokinetic/pharmacodynamic model experiments.

The chemotherapeutic options against NDM-1-producing Enterobacteriaceae infections are limited and therefore combination therapy is gaining momentum to counter the secondary resistance and potential suboptimal efficacy of monotherapy. Colistin and fosfomycin are two separate classes of antimicrobial agents that act on bacterial cells by different mechanisms. Hence, there is a potential for both synergy and antagonism. In this study, the antibacterial effects (ABEs) of colistin and fosfomycin were systematically investigated by time-kill curve studies over 48 h as well as in an in vitro pharmacokinetic model over 96 h against six well characterised strains of NDM-1-producing Enterobacteriaceae (three isolates resistant and three susceptible to fosfomycin) at a standard inoculum of 10(6)CFU/mL. Clinically achievable free serum concentrations of colistin sulphate and fosfomycin were used. In a single-chamber in vitro model, peak/trough concentrations (C(max)/C(min)) and the half-life (t(1/2)) for fosfomycin (250/40 mg/L and 2.7 h, respectively) and colistin sulphate (3.0/0.75 mg/L and 4 h, respectively) were used, along with a growth control. ABEs were measured by the decrease in viable bacterial counts (log kill), area under the bacterial kill curve (AUBKC) and population analysis profile (PAP). The combination of colistin and fosfomycin compared with either agent alone achieved increased bacterial killing and decreased the chance of emergence of resistance. Also, the ABEs of the combination were sustained for a longer duration and were evident both against fosfomycin-sensitive and -resistant strains. This study provides important information and support for the role of combination therapy against multidrug-resistant Gram-negative bacteria with limited therapeutic options.