Mutant prevention concentrations of fluoroquinolones for clinical isolates of Streptococcus pneumoniae

Effects of Comparative Killing by Pradofloxacin and Seven Other Antimicrobials Against Varying Bacterial Densities of Swine Isolates of Pasteurella multocida

Bacterial killing is important for recovering from infection. Pasteurella multocida is a key bacterial pathogen causing swine respiratory disease and is associated with substantial mortality. Antimicrobial therapy remains an important therapeutic intervention for treating infected animals. Pradofloxacin (fluoroquinolone) is the most recently approved antimicrobial agent for treating pigs with swine respiratory disease. We compared in vitro killing of swine P. multocida strains by pradofloxacin in comparison to ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin, and tulathromycin over a range of bacterial densities and four clinically relevant drug concentrations. Pradofloxacin killed 92-96.9% of cells across 106-108 cfu/mL densities at the mutant prevention drug concentration following 2-24 h of drug exposure, 96.9-98.9% of cells across 106-109 cfu/mL at the maximum serum drug concentration following 30 min of drug exposure, increasing to 99.9-100% kill following 12-24 h of drug exposure. At the maximum tissue drug concentration and against bacterial densities of 106-109 cfu/mL, pradofloxacin killed 91.3-99.8% of cells following 2 h of drug exposure, which increased to 99.9-100% kill following 12-24 h of drug exposure. Pradofloxacin was rapidly bactericidal across a range of bacterial densities and at clinically relevant drug concentrations. Pradofloxacin will be an important antibiotic for treating pigs with swine respiratory disease and where clinically indicated.

Comparison of the Minimum Inhibitory and Mutant Prevention Drug Concentrations for Pradofloxacin and 7 Other Antimicrobial Agents Tested Against Swine Isolates of Actinobacillus pleuropneumoniae and Pasteurella multocida

Pradofloxacin is a dual targeting, bactericidal fluoroquinolone recently approved for treating bacteria causing swine respiratory disease. Currently, an abundance of in vitro data does not exist for pradofloxacin. We determined the minimum inhibitory concentration (MIC) and mutant prevention concentrations (MPC) of pradofloxacin compared to ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin and tulathromycin against swine isolates of Actinobacillus pleuropneumoniae and Pasteurella multocida. Overall, pradofloxacin had the lowest MIC and MPC values as compared to the other agents tested. For example, pradofloxacin MIC values for 50%, 90% and 100% of A. pleuropneumoniae strains were ≤0.016 µg/mL, ≤0.016 µg/mL and ≤0.016 µg/mL and for P. multocida were ≤0.016 µg/mL, ≤0.016 µg/mL and 0.031 µg/mL, respectively. The MPC values for 50%, 90% and 100% of A. pleuropneumoniae strains were 0.031 µg/mL, 0.063 µg/mL and 0.125 µg/mL and for P. multocida were ≤0.016 µg/mL, 0.031 µg/mL and 0.0.063 µg/mL, respectively. By MPC testing, all strains were at or below the susceptibility breakpoint. Based on MPC testing, pradofloxacin appears to have a low likelihood for resistance selection. This study represents the most comprehensive in vitro comparison of the above noted drugs and the first report for pradofloxacin and tildipirosin.

In vitro killing of drug susceptible and multidrug resistant bacteria by amikacin considering pulmonary drug concentrations based on an inhaled formulation

Nosocomial infections with drug resistant bacteria impact morbidity and mortality, length of therapy and stay and the overall cost of treatment. Key pathogens with ventilator associated pneumonia may be drug-susceptible or multi-drug resistant and inhaled amikacin has been investigated as an adjunctive therapy option. High pulmonary drug concentrations (epithelial lining fluid [ELF]) along with minimal systemic toxicity is seen as an advantage to inhaled formulations. In vitro killing of bacteria using clinically relevant drug concentrations provide insight on bug-drug interactions. The aim of this study was to measure killing of clinical isolates of Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus and methicillin-susceptible S. aureus using the minimum inhibitory concentration (MIC), mutant prevention concentration (MPC) and median (976 µg/ml) ELF drug concentration for amikacin. Overall killing took longer at the MIC drug concentration and was inconsistent amongst the pathogens tested with the percentage of bacteria killed following 180 min of drug exposure ranging from growth in the presence of the drug to 95% kill. At the MPC drug concentrations, killing ranged from 55-88% for all pathogens following 30 min of drug exposure and increased to 99-100% following 180 min of drug exposure. At the ELF amikacin tested, killing was 81-100% following 20 min and 94-100% by 30 min of drug exposure. Rapid killing against MDR respiratory pathogens by amikacin ELF drug concentrations is encouraging.

In Vitro Resistance-Predicting Studies and In Vitro Resistance-Related Parameters-A Hit-to-Lead Perspective

Despite the urgent need for new antibiotics, very few innovative antibiotics have recently entered clinics or clinical trials. To provide a constant supply of new drug candidates optimized in terms of their potential to select for resistance in natural settings, in vitro resistance-predicting studies need to be improved and scaled up. In this review, the following in vitro parameters are presented: frequency of spontaneous mutant selection (FSMS), mutant prevention concentration (MPC), dominant mutant prevention concentration (MPC-D), inferior-mutant prevention concentration (MPC-F), and minimal selective concentration (MSC). The utility of various adaptive laboratory evolution (ALE) approaches (serial transfer, continuous culture, and evolution in spatiotemporal microenvironments) for comparing hits in terms of the level and time required for multistep resistance to emerge is discussed. We also consider how the hit-to-lead stage can benefit from high-throughput ALE setups based on robotic workstations, do-it-yourself (DIY) continuous cultivation systems, microbial evolution and growth arena (MEGA) plates, soft agar gradient evolution (SAGE) plates, microfluidic chips, or microdroplet technology. Finally, approaches for evaluating the fitness of in vitro-generated resistant mutants are presented. This review aims to draw attention to newly emerged ideas on how to improve the in vitro forecasting of the potential of compounds to select for resistance in natural settings.

In Vitro Antibacterial Activities of Fosfomycin against Escherichia coli Isolates from Canine Urinary Tract Infection

Fosfomycin is a bactericidal drug recommended as an alternative treatment for canine bacterial cystitis, particularly in cases involving multidrug-resistant (MDR) infections when no other options are available. In this study, minimum inhibitory concentration (MIC) and mutant prevention concentration (MPC) of fosfomycin were determined against 79 clinical E. coli isolates using the agar dilution method. The susceptibility rate of E. coli to fosfomycin was 86.06%, with MIC50 and MIC90 values of 4 mg/L and 96 mg/L, respectively. MPC50 and MPC90 values were 64 mg/L and 192 mg/L. Using pharmacokinetic (PK) data from dogs given a single 80 mg/kg oral dose of fosfomycin, the area under the curve per MIC50 (AUC0-24/MIC50) was 85.79 with time above MIC50 (T > MIC50) exceeding 50%. In urine, the AUC0-24/MIC50 was 10,694.78, and the AUC0-24/MPC90 was 222.81, with T > MPC90 extending beyond 24 h. Therefore, fosfomycin exhibited significant antibacterial activity against canine uropathogenic E. coli, including MDR strains, at concentrations below the susceptible MIC breakpoint. However, the high MPC values, especially the MPC90, indicate the critical importance of performing susceptibility testing for fosfomycin and maintaining ongoing resistance monitoring.

Comparative In Vitro Killing by Pradofloxacin in Comparison to Ceftiofur, Enrofloxacin, Florfenicol, Marbofloxacin, Tildipirosin, Tilmicosin and Tulathromycin against Bovine Respiratory Bacterial Pathogens

Pradofloxacin is the newest of the veterinary fluoroquinolones to be approved for use in animals-initially companion animals and most recently food animals. It has a broad spectrum of in vitro activity, working actively against Gram-positive/negative, atypical and some anaerobic microorganisms. It simultaneously targets DNA gyrase (topoisomerase type II) and topoisomerase type IV, suggesting a lower propensity to select for antimicrobial resistance. The purpose of this study was to determine the rate and extent of bacterial killing by pradofloxacin against bovine strains of Mannheimia haemolytica and Pasteurella multocida, in comparison with several other agents (ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin and tulathromycin) using four clinically relevant drug concentrations: minimum inhibitory and mutant prevention drug concentration, maximum serum and maximum tissue drug concentrations. At the maximum serum and tissue drug concentrations, pradofloxacin killed 99.99% of M. haemolytica cells following 5 min of drug exposure (versus growth to 76% kill rate for the other agents) and 94.1-98.6% of P. multocida following 60-120 min of drug exposure (versus growth to 98.6% kill rate for the other agents). Statistically significant differences in kill rates were seen between the various drugs tested depending on drug concentration and time of sampling after drug exposure.

Comparative Minimum Inhibitory and Mutant Prevention Drug Concentrations for Pradofloxacin and Seven Other Antimicrobial Agents Tested against Bovine Isolates of Mannheimia haemolytica and Pasteurella multocida

Pradofloxacin-a dual-targeting fluoroquinolone-is the most recent approved for use in food animals. Minimum inhibitory and mutant prevention concentration values were determined for pradofloxacin, ceftiofur, enrofloxacin, florfenicol, marbofloxacin, tildipirosin, tilmicosin, and tulathromycin. For M. haemolytica strains, MIC50/90/100 values were ≤0.016/≤0.016/≤0.016 and MPC50/90/100 values were 0.031/0.063/0.063; for P. multocida strains, the MIC50/90/100 values ≤0.016/≤0.016/0.031 and MPC50/90/100 ≤ 0.016/0.031/0.063 for pradofloxacin. The pradofloxacin Cmax/MIC90 and Cmax/MPC90 values for M. haemolytica and P. multocida strains, respectively, were 212.5 and 53.9 and 212.5 and 109.7. Similarly, AUC24/MIC90 and AUC24/MPC90 for M. haemolytica were 825 and 209.5, and for P. multocida, they were 825 and 425.8. Pradofloxacin would exceed the mutant selection window for >12-16 h. Pradofloxacin appears to have a low likelihood for resistance selection against key bovine respiratory disease bacterial pathogens based on low MIC and MPC values.

Impact of Environmental Sub-Inhibitory Concentrations of Antibiotics, Heavy Metals, and Biocides on the Emergence of Tolerance and Effects on the Mutant Selection Window in E. coli

Bacteria's ability to withstand the detrimental effects of antimicrobials could occur as resistance or tolerance with the minimum inhibitory concentration, the mutant prevention concentration, and the mutant selection window as salient concepts. Thus, this study assessed the impact of exposure to extremely high doses of ampicillin on the level of persistence and tolerance development in isolates previously exposed to different concentrations of selected antibiotics, biocides, and heavy metals. These isolates were previously exposed to oxytetracycline (OXYTET), amoxicillin (AMX), copper (Cu), zinc (Zn), benzalkonium chloride (BAC) 10, dimethylammonium chloride (DADMAC) 12 and a combination of all the individual pollutants (ALL). The isolates were exposed to very high concentrations (25 × MIC) of ampicillin, and their tolerance was calculated as the time required to kill 99.9% of the bacterial population (MDK99.9). The MDK99.9 increased by 30 to 50% in test isolates (DADMAC, OXYTET, Zinc = 28 h; BAC, Copper = 30 h; amoxycillin, ALL = 26 h) compared to the untreated control. BAC-exposed isolates decreased from 2.5 × 108 CFU/mL to 2.5 × 104 CFU/mL on the second day, displaying the highest tolerance increase. The tolerance appeared to originate from two sources, i.e., stochastic persistence and genetic-induced persistence, involving multiple genes with diverse mechanisms. The mutant selection window of the isolates to ampicillin, amoxicillin, and oxytetracycline also slightly increased compared to the control, indicating the selective survival of persister cells during the 30-day exposure. These findings indicate that bacterial exposure to sub-inhibitory concentrations of environmental chemical stressors may not always result in the development of antimicrobial resistance but could initiate this process by selecting persisters that could evolve into resistant isolates.

Antimicrobial activity and pathogen mutation prevention of originator and generics of cefepime, linezolid and piperacillin/tazobactam against clinical isolates of Staphylococcus aureus

OBJECTIVES

Although generic medicinal products are required to have the same qualitative and quantitative composition of the active substance as their reference originator product, patients and health care professionals express concerns about their interchangeability and safety. Therefore, the present study investigated the antimicrobial activity and pathogen mutation prevention of original and generic cefepime, linezolid and piperacillin/tazobactam against Staphylococcus aureus.

METHODS

Two generic formulations of cefepime, linezolid and piperacillin/tazobactam were tested against their respective originator products. Susceptibility testing was performed with twenty-one clinical isolates of S. aureus and ATCC-29213 using broth microdilution. Time kill curves (TKC) were performed with ATCC-29213 at drug concentrations above and below the respective minimum inhibitory concentrations (MIC). Mutation prevention concentration was determined for each drug formulation against ATCC-29213. All experiments were performed in triplicate. Mutant colonies from mutation prevention concentration (MPC) experiments were genotypically tested by sequence analysis.

RESULTS

MIC ratios between contiguous originator and generic drugs were similar for each isolate. No visual differences were observed in TKCs between originator and generic substances. The MPC did not differ between different formulations of the same substance. Although sequence analysis of mutant colonies revealed genomic differences compared with the original ATCC-29213, no differences in mutation frequencies were observed between clinical isolates and ATCC-29213 treated with originator or generic substances.

CONCLUSIONS

Similar antimicrobial activity and pathogen mutation prevention was observed between contiguous substances. These results support the interchangeability of generic and originator drug formulations with the same active ingredient.

Mutant Prevention Concentrations of Ciprofloxacin and Levofloxacin and Target Gene Mutations of Fluoroquinolones in Elizabethkingia anophelis

Fluoroquinolones are potentially effective against Elizabethkingia anophelis. We investigated the MIC, mutant prevention concentration (MPC), and target gene mutations of fluoroquinolones in E. anophelis. Eighty-five E. anophelis isolates were collected from five hospitals in Taiwan. The MIC and MPC of ciprofloxacin and levofloxacin were examined for all E. anophelis except 17 isolates, in which ciprofloxacin MPC could not be determined due to drug precipitation caused by overly high drug concentration. Mutations in the quinolone resistance-determining regions of DNA gyrase (GyrA and GyrB) and topoisomerase IV (ParC and ParE) in the clinical isolates and fluoroquinolone-selected mutants were examined. Overall, 23.5% and 71.8% of the isolates tested were susceptible to ciprofloxacin and levofloxacin, respectively. The MPC₅₀ of ciprofloxacin was 128 mg/L, and the MPC₅₀ of levofloxacin was 51.2 mg/L. The MPC₅₀/MIC₅₀ ratio for ciprofloxacin was 64, whereas that for levofloxacin was 25.6. The coefficient of determination between the MPC and MIC for ciprofloxacin and levofloxacin was 0.72 and 0.56, respectively, in the linear regression analysis. Preexisting mutations in GyrA (S83I, S83R, and D87Y) were identified in 18 clinical isolates, all of which were resistant to both ciprofloxacin and levofloxacin. Additional amino acid substitutions in GyrA were identified in all ciprofloxacin- and levofloxacin-selected mutants. Furthermore, GyrB alterations (D431N or D431H) were found in nine levofloxacin-treated isolates. Given that maintaining the serum concentrations of fluoroquinolones above MPCs is impossible under presently recommended doses, the selection of mutant E. anophelis strains seems inevitable.

The in vitro activity of danofloxacin plus ceftiofur combination: implications for antimicrobial efficacy and resistance prevention

Due to the high prevalence of multi-drug resistant bacteria, combination therapy is an efficient choice for treatment of infections caused by highly resistant strains. In this study, the efficacy of ceftiofur plus danofloxacin combination was investigated against resistant Escherichia coli. The interaction between the two drugs was determined by checkerboard tests and time-kill assays. The combination was defined as bactericidal or bacteriostatic based on the minimum bactericidal concentration test results. Mutant prevention concentration test was used to evaluate the resistance tendency suppression potential of the combination. The combination had a synergistic effect against 83.00% of the isolates as verified by the checkerboard and time-kill assays. The combination was defined as bactericidal against all E. coli strains, since minimum bactericidal concentration: minimum inhibitory concentration ratios were below four thresholds and also markedly reduced mutant prevention concentration values of ceftiofur up to 4000-fold compared to its single use. Ceftiofur plus danofloxacin combination inhibited growth of E. coli strains which were resistant to ceftiofur or newer generation of fluoroquinolones. Our results suggest that ceftiofur plus danofloxacin combination has a bactericidal characteristic and can be an important alternative for the treatment of infections caused by resistant E. coli.

Comparing the minimum inhibitory and mutant prevention concentrations of selected antibiotics against animal isolates of Pasteurella multocida and Salmonella typhimurium

Historically, the use of antibiotics was not well regulated in veterinary medicine. The emergence of antibiotic resistance (ABR) in pathogenic bacteria in human and veterinary medicine has driven the need for greater antibiotic stewardship. The preservation of certain antibiotic classes for use exclusively in humans, especially in cases of multidrug resistance, has highlighted the need for veterinarians to reduce its use and redefine dosage regimens of antibiotics to ensure efficacy and guard against the development of ABR pathogens. The minimum inhibitory concentration (MIC), the lowest concentration of an antibiotic drug that will prevent the growth of a bacterium, is recognised as a method to assist in antibiotic dosage determination. Minimum inhibitory concentrations sometimes fail to deal with first-step mutants in bacterial populations; therefore dosing regimens based solely on MIC can lead to the development of ABR. The mutant prevention concentration (MPC) is the minimum inhibitory antibiotic concentration of the most resistant first-step mutant. Mutant prevention concentration determination as a complementary and sometimes preferable alternative to MIC determination for veterinarians when managing bacterial pathogens. The results of this study focused on livestock pathogens and antibiotics used to treat them, which had a MIC value of 0.25 µg/mL for enrofloxacin against all 27 isolates of Salmonella typhimurium. The MPC values were 0.50 µg/mL, with the exception of five isolates that had MPC values of 4.00 µg/mL. The MPC test yielded 65.52% (18 isolates) Salmonella isolates with florfenicol MICs in the sensitive range, while 11 isolates were in the resistant range. Seventeen isolates (58.62%) of Pasteurella multocida had MIC values in the susceptible range and 41.38% (12 isolates) had an intermediate MIC value. Mutant prevention concentration determinations as done in this study is effective for the antibiotic treatment of bacterial infections and minimising the development of resistance. The MPC method can be used to better control to prevent the development of antibiotic drug resistance used in animals.

Nemonoxacin enhances antibacterial activity and anti-resistant mutation ability of vancomycin against methicillin-resistant Staphylococcus aureus in an in vitro dynamic pharmacokinetic/pharmacodynamic model

Reduced susceptibility and emergence of resistance to vancomycin in methicillin-resistant Staphylococcus aureus (MRSA) have led to the development of various vancomycin based combinations. Nemonoxacin is a novel nonfluorinated quinolone with antibacterial activity against MRSA. The present study aimed to investigate the effects of nemonoxacin on antibacterial activity and the anti-resistant mutation ability of vancomycin for MRSA and explore whether quinolone resistance genes are associated with a reduction in the vancomycin minimal inhibitory concentration (MIC) and mutant prevention concentration (MPC) when combined with nemonoxacin. Four isolates, all with a vancomycin MIC of 2 μg/mL, were used in a modified in vitro dynamic pharmacokinetic/pharmacodynamic model to investigate the effects of nemonoxacin on antibacterial activity (M04, M23 and M24) and anti-resistant mutation ability (M04, M23 and M25, all with MPC ≥19.2 μg/mL) of vancomycin. The mutation sites of gyrA, gyrB, parC, and parE of 55 clinical MRSA isolates were sequenced. We observed that in M04 and M23, the combination of vancomycin (1g q12h) and nemonoxacin (0.5g qd) showed a synergistic bactericidal activity and resistance enrichment suppression. All clinical isolates resistant to nemonoxacin harbored gyrA (S84→L) mutation; gyrA (S84→L) and parC (E84→K) mutations were the two independent risk factors for the unchanged vancomycin MPC in combination. Nemonoxacin enhances the bactericidal activity and suppresses resistance enrichment ability of vancomycin against MRSA with a MIC of 2 μg/mL. Our in vitro data support the combination of nemonoxacin and vancomycin for the treatment of MRSA infection with a high MIC.

Comparative Minimal Inhibitory and Mutant Prevention Concentration of Eight Antimicrobial Agents Against Klebsiella pneumoniae

Purpose: With the emergence of multidrug-resistant and pan-resistant strains, Klebsiella pneumoniae (K. pneumoniae) shows higher treatment failure rates and mortality in clinics. It is more important to develop an effective method for treating K. pneumonia infections. The main objectives of this study were to determine the minimal inhibitory concentration (MIC) and the mutant prevention concentration (MPC) for eight antimicrobial agents against K. pneumoniae isolated from different hosts and compare the emergence of resistant mutants between animal strains and human strains. Materials and Methods: A total of 72 nonduplicate K. pneumoniae isolates and 8 antimicrobial agents (amikacin, azithromycin, levofloxacin, doxycycline, nitrofurantoin, colistin, tigecycline, and imipenem) were used. The MIC and MPC values were determined using agar plate assays. The values of the selection index (SI) were calculated with MPC₉₀/MIC₉₀. Pharmacodynamic parameters were calculated using published plasma pharmacokinetic variables. Results: For human isolate strains, the MPC_50/90 (μg/mL) values were as follows: amikacin, 32/128; azithromycin, 64/128; levofloxacin, 4/16; doxycycline, 32/32; nitrofurantoin, 128/512; colistin, 4/8; tigecycline, 8/16; and imipenem, 4/8. The value of SI was 8 for azithromycin, doxycycline, and tigecycline; 16 for amikacin, levofloxacin, and nitrofurantoin; 4 for imipenem; and 2 for colistin. For animal isolate strains, the MPC₉₀ values were 128 μg/mL for azithromycin and doxycycline, 64 μg/mL for amikacin, 32 μg/mL for levofloxacin, 512 μg/mL for nitrofurantoin, 8 μg/mL for colistin and tigecycline, 4 μg/mL for imipenem. The value of SI was 2 for colistin and imipenem, 8 for tigecycline, 16 for amikacin, and 32 for the other four agents. In combination with pharmacokinetic parameters, these findings indicated that the plasma concentrations of the seven antibiotics except imipenem were below the MPC for the entire dosing interval. Conclusion: The ability of eight antibiotics to prevent resistant mutants of K. pneumoniae was different between animal strains and human strains. Higher doses than those currently approved should be required to prevent the enrichment of mutants of drug-resistant bacteria in the clinics.

In Vitro Killing of Canine Urinary Tract Infection Pathogens by Ampicillin, Cephalexin, Marbofloxacin, Pradofloxacin, and Trimethoprim/Sulfamethoxazole

Urinary tract infections are common in dogs, necessitating antimicrobial therapy. We determined the speed and extent of in vitro killing of canine urinary tract infection pathogens by five antimicrobial agents (ampicillin, cephalexin, marbofloxacin, pradofloxacin, and trimethoprim/sulfamethoxazole) following the first 3 h of drug exposure. Minimum inhibitory and mutant prevention drug concentrations were determined for each strain. In vitro killing was determined by exposing bacteria to clinically relevant drug concentrations and recording the log10 reduction and percent kill in viable cells at timed intervals. Marbofloxacin and pradofloxacin killed more bacterial cells, and faster than other agents, depending on the time of sampling and drug concentration. Significant differences were seen between drugs for killing Escherichia coli, Proteus mirabilis, Enterococcus faecalis, and Staphylococcus pseudintermedius strains. At the maximum urine drug concentrations, significantly more E. coli cells were killed by marbofloxacin than by ampicillin (p < 0.0001), cephalexin (p < 0.0001), and TMP/SMX (p < 0.0001) and by pradofloxacin than by cephalexin (p < 0.0001) and TMP/SMX (p < 0.0001), following 5 min of drug exposure. Rapid killing of bacteria should inform thinking on drug selection for short course therapy for uncomplicated UTIs, without compromising patient care, and is consistent with appropriate antimicrobial use and stewardship principles.

The Role of PK/PD Analysis in the Development and Evaluation of Antimicrobials

Pharmacokinetic/pharmacodynamic (PK/PD) analysis has proved to be very useful to establish rational dosage regimens of antimicrobial agents in human and veterinary medicine. Actually, PK/PD studies are included in the European Medicines Agency (EMA) guidelines for the evaluation of medicinal products. The PK/PD approach implies the use of in vitro, ex vivo, and in vivo models, as well as mathematical models to describe the relationship between the kinetics and the dynamic to determine the optimal dosing regimens of antimicrobials, but also to establish susceptibility breakpoints, and prevention of resistance. The final goal is to optimize therapy in order to maximize efficacy and minimize side effects and emergence of resistance. In this review, we revise the PK/PD principles and the models to investigate the relationship between the PK and the PD of antibiotics. Additionally, we highlight the outstanding role of the PK/PD analysis at different levels, from the development and evaluation of new antibiotics to the optimization of the dosage regimens of currently available drugs, both for human and animal use.

Evaluation of Fluoroquinolone Resistance in Clinical Avian Pathogenic Escherichia coli Isolates from Flanders (Belgium)

Fluoroquinolones are frequently used antimicrobials for the treatment of avian pathogenic Escherichia coli (APEC) infections. However, rapid development and selection of resistance to this class of antimicrobial drugs is a significant problem. The aim of this study was to investigate the occurrence and mechanisms of antimicrobial resistance against enrofloxacin (ENRO) in APEC strains in Flanders, Belgium. One hundred and twenty-five APEC strains from broilers with clinical colibacillosis were collected in Flanders from November 2017 to June 2018. The minimum inhibitory concentration (MIC) of all strains and the mutant prevention concentration (MPC) of a sample of sensitive isolates were determined using a commercial gradient strip test and via the agar dilution method, respectively. Non-wild type (NWT) isolates were further characterized using polymerase chain reaction (PCR), gel electrophoresis and gene sequencing. Forty percent of the APEC strains were NWT according to the epidemiological cut-off (ECOFF) measure (MIC > 0.125 μg/mL). With respect to clinical breakpoints, 21% were clinically intermediate (0.5 ≤ MIC ≤ 1 μg/mL) and 10% were clinically resistant (MIC ≥ 2). The MPC values of the tested strains ranged from 0.064 to 1 μg/mL, resulting in MPC/MIC ratios varying from 4 to 32. The majority (92%) of the NWT strains carried one or two mutations in gyrA. Less than a quarter (22%) manifested amino acid substitutions in the topoisomerase IV parC subunit. Only three of the NWT strains carried a mutation in parE. Plasmid mediated quinolone resistance (PMQR) associated genes were detected in 18% of the NWT strains. In contrast to the relatively large number of NWT strains, only a small percentage of APEC isolates was considered clinically resistant. The most common MPC value for sensitive strains was 0.125 μg/mL. Some isolates showed higher values, producing wide mutant selection windows (MSW). Chromosomal mutations in DNA gyrase and topoisomerase IV were confirmed as the main source of decreased antimicrobial fluoroquinolone susceptibility, de-emphasizing the role of PMQR mechanisms.

Association between antimicrobial drug class selection for treatment and retreatment of bovine respiratory disease and health, performance, and carcass quality outcomes in feedlot cattle

Treatment and control of bovine respiratory disease (BRD) is predicated on the use of two categories of antimicrobials, namely bacteriostatic drugs that inhibit bacterial growth and replication (STATIC), and bactericidal drugs that kill bacteria in in vitro culture systems (CIDAL). Recently, we reported that initial BRD treatment with a STATIC antimicrobial followed by retreatment with a CIDAL antimicrobial was associated with a higher frequency of multidrug-resistant bacteria isolated from field cases of BRD submitted to a veterinary diagnostic laboratory. The present study was conducted to test the hypothesis that calves administered the same class of antimicrobial for first and second BRD treatment (i.e., CIDAL-CIDAL or STATIC-STATIC) would have improved health and performance outcomes at the feedlot compared to calves that received a different antimicrobial class for retreatment (i.e., STATIC-CIDAL or CIDAL-STATIC). The association between antimicrobial treatments and health, performance, and carcass quality outcomes were determined by a retrospective analysis of 4,252 BRD treatment records from a commercial feedlot operation collected from 2001 to 2005. Data were compared using generalized linear mixed statistical models that included gender, season, and arrival weight as covariates. The mean (±SE) probability of BRD cases identified as requiring four or more treatments compared to three treatments was greater in calves that received STATIC-CIDAL (73.58 ± 2.38%) or STATIC-STATIC (71.32 ± 2.52%) first and second antimicrobial treatments compared to calves receiving CIDAL-CIDAL (50.35 ± 3.46%) first and second treatments (P < 0.001). Calves receiving CIDAL-CIDAL first and second treatments also had an increased average daily gain (1.11 ± 0.03 kg/d) compared to calves receiving STATIC-CIDAL (0.95 ± 0.03 kg/d) and STATIC-STATIC (0.84 ± 0.02 kg/d) treatments (P < 0.001). Furthermore, CIDAL-CIDAL-treated calves had a higher probability of a choice quality grade at slaughter (36.44 ± 4.80%) compared to STATIC-CIDAL calves (28.09 ± 3.88%) (P = 0.037). There was no effect of antimicrobial treatment combination on BRD mortality (P = 0.855) or yield grade (P = 0.240) outcomes. These observations suggest that consideration should be given to antimicrobial pharmacodynamics when selecting drugs for retreatment of BRD. These findings have implications for developing BRD treatment protocols that address both post-treatment production and antimicrobial stewardship concerns.

In Vitro Synergistic Effect and Mutant Prevention Concentrations of Cefepime Alone or in Combination with Sulbactam Against OXA-48-positive Klebsiella pneumoniae Isolates

The aim of this study is to investigate the combination of cefepime and sulbactam. Sulbactam, when administered , will effectively inhibit all Extended-spectrum beta lactamases (ESBLs) of the microorganism, while cefepime will inhibit the growth of the resistant microorganisms since it will not be hydrolyzed by OXA-48. Forty OXA-48-producing K. pneumoniae strains were investigated for their Minimum inhibitory concentrations (MICs) for carbapenems, cefepime, and cefepime + sulbactam by broth microdilution method. Also, the mutant prevention concentration (MPC)s of cefepime alone or in combination with sulbactam was determined. Additionally, the bactericidal activities of cefepime and cefepime + sulbactam were evaluated by the time-kill curve (TKC) assay against selected strains. Also, the in vitro synergistic activity of cefepime + sulbactam combination was determined by TKC. Based on MIC results, up to 35/40 and 34/40 of the strains were resistant to carbapenems and cefepime, respectively. Cefepime + sulbactam MIC range was lower than those for cefepime alone against all the studied isolates. Moreover, cefepime + sulbactam combination presented lower MPC values than cefepime alone. The synergistic interactions of cefepime + sulbactam were also achieved against studied strains at 24 h. No antagonism was observed against studied K. pneumoniae strains. The findings of this study displayed that cefepime + sulbactam combination had synergistic or additive effect against OXA-48-producing K. pneumoniae strains. Additionally, it was first observed that this combination could display a lower MPC than cefepime alone. Further investigations may be helpful for understanding the effectiveness of cefepime + sulbactam combinations for OXA-48-positive carbapenem-resistant K. pneumoniae isolates.

Methicillin-resistant Staphylococcus aureus replication in the presence of high (≥32 µg/ml) drug concentration of vancomycin as seen by electron microscopy

Methicillin-resistant Staphylococcus aureus (MRSA) has unfortunately become a common pathogen in many healthcare facilities. In many institutions, vancomycin remains the preferred agent for treating serious MRSA infections including bacteraemia with or without endocarditis. The mutant prevention concentration (MPC) testing ≥10⁹ colony forming units of bacteria, describes the antimicrobial drug concentration blocking the growth of the least susceptible cell from high density bacterial populations. With blood culture isolates of MRSA, we discovered strains with MPC values ≥32 µg/ml and viable cells could be readily recovered from agar plates containing 32 µg/ml of vancomycin. To investigate MRSA strains surviving in high concentrations of vancomycin on drug containing agar plates, we utilized electron microscopy to measure cell wall thickness as this has been previously reported as a potential mechanism of resistance¹ along with septum thickening. Our data shows MRSA replication from high density bacterial populations in the presence of ≥32 µg/ml of vancomycin. Such observations may explain vancomycin failure in some patients and/or persistent bacteraemia and could potentially question the use of this drug in some critically ill patients in favour of an alternative agent.

Evolution of antibiotic cross-resistance and collateral sensitivity in Staphylococcus epidermidis using the mutant prevention concentration and the mutant selection window

In bacteria, evolution of resistance to one antibiotic is frequently associated with increased resistance (cross-resistance) or increased susceptibility (collateral sensitivity) to other antibiotics. Cross-resistance and collateral sensitivity are typically evaluated at the minimum inhibitory concentration (MIC). However, these susceptibility changes are not well characterized with respect to the mutant prevention concentration (MPC), the antibiotic concentration that prevents a single-step mutation from occurring. We measured the MIC and the MPC for Staphylococcus epidermidis and 14 single-drug resistant strains against seven antibiotics. We found that the MIC and the MPC were positively correlated but that this correlation weakened if cross-resistance did not evolve. If any type of resistance did evolve, the range of concentrations between the MIC and the MPC tended to shift right and widen. Similar patterns of cross-resistance and collateral sensitivity were observed at the MIC and MPC levels, though more symmetry was observed at the MIC level. Whole-genome sequencing revealed mutations in both known-target and nontarget genes. Moving forward, examining both the MIC and the MPC may lead to better predictions of evolutionary trajectories in antibiotic-resistant bacteria.

Pharmacodynamics of Ceftiofur Selected by Genomic and Proteomic Approaches of Streptococcus parauberis Isolated from the Flounder, Paralichthys olivaceus

We employed an integrative strategy to present subtractive and comparative metabolic and genomic-based findings of therapeutic targets against Streptococcus parauberis. For the first time, we not only identified potential targets based on genomic and proteomic database analyses but also recommend a new antimicrobial drug for the treatment of olive flounder (Paralichthys olivaceus) infected with S. parauberis. To do that, 102 total annotated metabolic pathways of this bacterial strain were extracted from computational comparative metabolic and genomic databases. Six druggable proteins were identified from these metabolic pathways from the DrugBank database with their respective genes as mtnN, penA, pbp2, murB, murA, coaA, and fni out of 112 essential nonhomologous proteins. Among these hits, 26 transmembrane proteins and 77 cytoplasmic proteins were extracted as potential vaccines and drug targets, respectively. From the FDA DrugBank, ceftiofur was selected to prevent antibiotic resistance as it inhibited our selected identified target. Florfenicol is used for treatment of S. parauberis infection in flounder and was chosen as a comparator drug. All tested strains of fish isolates with S. parauberis were susceptible to ceftiofur and florfenicol with minimum inhibitory concentrations (MIC) of 0.0039-1 μg/mL and 0.5-8 μg/mL, IC50 of 0.001-0.5 μg/mL and 0.7-2.7 μg/mL, and minimum biofilm eradication concentrations (MBEC) of 2-256 μg/mL and 4-64 μg/mL, respectively. Similar susceptibility profiles for ceftiofur and florfenicol were found, with ceftiofur observed as an effective and potent antimicrobial drug against both planktonic and biofilm-forming strains of the fish pathogen Streptococcus parauberis, and it can be applied in the aquaculture industry. Thus, our predictive approach not only showed novel therapeutic agents but also indicated that marketed drugs should also be tested for efficacy against newly identified targets of this important fish pathogen.

In vitro killing of canine strains of Staphylococcus pseudintermedius and Escherichia coli by cefazolin, cefovecin, doxycycline and pradofloxacin over a range of bacterial densities

Background

Bacterial densities likely fluctuate during infection and may exceed the bacterial density used in susceptibility testing. As such, investigation of bacterial killing by antibiotics over a range of varying bacterial densities may provide important differences between compounds and could impact drug selection for therapy.

Hypothesis/Objectives

To measure killing of clinical isolates of Staphylococcus pseudintermedius and Escherichia coli by cefazolin, cefovecin, doxycycline and pradofloxacin at clinically relevant (minimum inhibitory, mutant prevention, maximum serum and maximum tissue) drug concentrations against varying densities of bacteria.

Animals/Materials

Bacterial strains collected from dogs with urinary tract infections were studied.

Methods and materials

High bacterial densities ranging from 10⁶ to 10⁹ colony forming units (cfu)/mL were exposed to minimum inhibitory, mutant prevention, blood and tissue drug concentrations, and the percentages (log₁₀) of viable cells killed following 30 min, 1, 2, 4, 6, 12 and 24 h of drug exposure were quantified.

Results

Doxycycline exhibited bacteriostatic properties with less killing than the other three agents. For example, at a 10⁷ cfu/mL density of S. pseudintermedius, more cells were killed by pradofloxacin (P < 0.0001) and cefovecin (P = 0.0014) but not cefazolin when compared to doxycycline at the maximum serum drug concentration following 12 h of drug exposure.

Conclusions and clinical importance

Differences were seen between some drugs in the speed and extent of bacterial killing; this could be clinically important and may impact drug selection and length of therapy.

Background – Bacterial densities likely fluctuate during infection and may exceed the bacterial density used in susceptibility testing. As such, investigation of bacterial killing by antibiotics over a range of varying bacterial densities may provide important differences between compounds and could impact drug selection for therapy. Hypothesis – To measure killing of clinical isolates of Staphylococcus pseudintermedius and Escherichia coli by cefazolin, cefovecin, doxycycline and pradofloxacin at clinically relevant (minimum inhibitory, mutant prevention, maximum serum and maximum tissue) drug concentrations against varying densities of bacteria. Conclusions and clinical importance – Differences were seen between some drugs in the speed and extent of bacterial killing; this could be clinically important and may impact drug selection and length of therapy.

In Vitro Ceftaroline Resistance Selection of Methicillin-Resistant Staphylococcus aureus Involves Different Genetic Pathways

The pathways in the development of ceftaroline resistance of methicillin-resistant Staphylococcus aureus (MRSA) isolates belonging to the ST8, ST239, and ST228 were evaluated. Ceftaroline-resistant derivatives were isolated through selection during 40 passages. Ceftaroline MIC measurements and whole-genome sequencing were performed after 5, 20, and 40 passages. In two ST8 derivative isolates, ceftaroline MIC increased up to 128 mg/L. Mutations were acquired in gdpP and graS in one isolate after 20 passages and in gdpP in another after 40 passages. MIC for two ST239 derivatives increased to 128 mg/L. Substitutions in Pbp4 and polymorphisms in the upstream region of pbp4 were identified in both derivatives after 40 passages. In one isolate, additional mutation in gdpP and deletion in graR were detected. In an ST228 derivative, MIC increased to 32 mg/L with one mutation in penicillin-binding protein 2a (Y446N) detected after five passages and a second (E447K) after 20 passages. Three pathways in the development of ceftaroline resistance were identified. For ST8 and ST239 derivatives mutations were detected in gdpP and pbp4, respectively, whereas in ST228 - in mecA. Most derivatives harbored additional mutations whose potential role in the development of resistance has not been determined.

Mutant Prevention Concentration of Ciprofloxacin against Klebsiella pneumoniae Clinical Isolates: An Ideal Prognosticator in Treating Multidrug-Resistant Strains

Background

Fluoroquinolone-resistant Klebsiella pneumoniae poses a therapeutic challenge when implicated in urinary tract infections, pyelonephritis, pneumonia, skin infections, osteomyelitis, and respiratory infections. The mutant prevention concentration (MPC) represents a concentration threshold above which increase of resistant mutants occurs rarely. The aim of the present study is to determine the MPC among ciprofloxacin-resistant K. pneumoniae clinical isolates.

Materials and Methods

A total of 240 clinical isolates of K. pneumoniae were collected from a tertiary care hospital. The MPCs were determined for 24 selected strains using an inoculum of 10¹⁰ CFU/ml in Müller-Hinton agar plates with serial/various concentrations (0.003-100 μg) of ciprofloxacin. In addition to the MPC, phenotypic screening for ESBL, AmpC, and carbapenemase was performed. The detection of qnr genes for 24 isolates and DNA sequencing for six isolates were performed.

Results

Ciprofloxacin resistance was observed in 19.6% of the K. pneumoniae clinical isolates. Among the ciprofloxacin-resistant isolates, 14 isolates showed an MPC value of more than 100 μg. The MPC ranged between 100 μg and 20 μg for ciprofloxacin-resistant isolates. ESBL producers and qnr gene-producing strains had a high MPC. 11 isolates showed the presence of either qnrB or qnrS genes. None of the samples showed the presence of the qnrA gene.

Conclusion

From our study, we infer that ESBL producers and qnr gene-possessing strains are frequently resistant to ciprofloxacin. Estimation of the MPC in the case of multidrug-resistant isolates in the clinical setup may help in treating these drug-resistant strains.

Implications for dosing regimen of enrofloxacin administered concurrently with dexamethasone in febrile buffalo calves

The objective of the study was to determine the influence of dexamethasone (DXM) on pharmacokinetics (PK) and pharmacodynamics (PD) of enrofloxacin (ENR) for dosage optimization following concurrent administration of ENR and DXM in febrile buffalo calves. A 2 μg/kg intravenous dosage of lipopolysaccharide derived from Escherichia coli was used to induce fever in calves. After inducing fever, ENR was administered at the dose rate of 12 mg/kg, IM followed by IM injection of DXM (0.05 mg/kg) in calves. Minor alterations in PK of ENR were observed following the administration of ENR + DXM. The PK parameters were t_1/2K₁₀ = 6.34 h, Cl/F = 0.729 L/kg/h, and MRT_0-∞ = 10.5 h. Antibacterial activity (MIC, MBC, ex vivo time-kill kinetics) of ENR for P. multocida was not affected by DXM. But MPC of ENR against P. multocida was lessened in presence of DXM. Using PK-PD-modeled AUC_0-24h/MIC values for bactericidal effect against P. multocida, daily dosages of ENR administered in combination with DXM were 4.02 mg/kg and 16.1 mg/kg, respectively, for MIC_90s of 0.125 μg/ml and 0.50 μg/ml. A dose of 5.38 mg/kg was determined for ENR for frequently occurring P. multocida infections having ≤ MIC₉₀ of 0.125 μg/ml and PK-PD modeled dose was comparable with the recommended ENR dose of 5 mg/kg for bovines for mild infections. It is suggested that a recommended dosage of 5-12.5 mg/kg of ENR can be used effectively in combination with DXM to treat P. multocida associated infections in buffalo calves without any risk of resistance amplification.

A novel parameter to predict the effects of antibiotic combinations on the development of Staphylococcus aureus resistance: in vitro model studies at subtherapeutic daptomycin and rifampicin exposures

The search for optimal predictors of anti-mutant effects remains a pressing problem in studies of antibiotic-associated bacterial resistance. To relate the emergence of bacterial resistance with the antibiotic mutant prevention concentration (MPC), a novel integral parameter - the area around the resistance threshold, i.e. MPC level (AAMPC) is proposed. The AAMPC is the algebraic sum of the area under the antibiotic concentration-time curve that is above the MPC (positive area) and the area above the concentration-time curve that is under the MPC (negative area). To assess the predictive performance of AAMPC, the enrichment of resistant Staphylococcus aureus was studied by simulating treatment with daptomycin and rifampicin alone and in combination in an in vitro dynamic model. The enhanced anti-mutant effects of the antibiotic combinations were due to lowering the negative 24-h AAMPCs. These findings suggest that a novel MPC-related parameter is a reliable predictor of mutant enrichment.

Epistasis between antibiotic tolerance, persistence, and resistance mutations

Understanding the evolution of microorganisms under antibiotic treatments is a burning issue. Typically, several resistance mutations can accumulate under antibiotic treatment, and the way in which resistance mutations interact, i.e., epistasis, has been extensively studied. We recently showed that the evolution of antibiotic resistance in Escherichia coli is facilitated by the early appearance of tolerance mutations. In contrast to resistance, which reduces the effectiveness of the drug concentration, tolerance increases resilience to antibiotic treatment duration in a nonspecific way, for example when bacteria transiently arrest their growth. Both result in increased survival under antibiotics, but the interaction between resistance and tolerance mutations has not been studied. Here, we extend our analysis to include the evolution of a different type of tolerance and a different antibiotic class and measure experimentally the epistasis between tolerance and resistance mutations. We derive the expected model for the effect of tolerance and resistance mutations on the dynamics of survival under antibiotic treatment. We find that the interaction between resistance and tolerance mutations is synergistic in strains evolved under intermittent antibiotic treatment. We extend our analysis to mutations that result in antibiotic persistence, i.e., to tolerance that is conferred only on a subpopulation of cells. We show that even when this population heterogeneity is included in our analysis, a synergistic interaction between antibiotic persistence and resistance mutations remains. We expect our general framework for the epistasis in killing conditions to be relevant for other systems as well, such as bacteria exposed to phages or cancer cells under treatment.

Concentration-dependent enrichment of resistant Enterococcus faecium exposed to linezolid in an in vitro dynamic model

To explore the relationship between pharmacokinetic variables and enterococcal resistance to linezolid, a vancomycin-resistant strain whose mutant prevention concentration (MPC) exceeded the MIC by two fold was selected among six clinical isolates of Enterococcus faecium. The selected strain was exposed to simulated pharmacokinetics of twice-daily linezolid for five days. Mutants resistant to 2 × MIC of the antibiotic were enriched at ratios of the 24-h area under the concentration-time curve (AUC₂₄) to the MIC of 15 and 30 h but not at 60 and 120 h. These observations could be explained by the different times when antibiotic concentrations exceed the MPC (T_>MPC): 0 to 14, 63 and 100% of the dosing interval. Using the area under the bacterial mutant concentration-time curve (AUBC_M) determined in this study and in previous work with other E. faecium strains (MPC/MIC 4), a strain-independent T_>MPC relationship with mutant enrichment was established.

Variation in Mutant Prevention Concentrations

Objectives:Understanding how phenotypic traits vary has been a longstanding goal of evolutionary biologists. When examining antibiotic-resistance in bacteria, it is generally understood that the minimum inhibitory concentration (MIC) has minimal variation specific to each bacterial strain-antibiotic combination. However, there is a less studied resistance trait, the mutant prevention concentration (MPC), which measures the MIC of the most resistant sub-population. Whether and how MPC varies has been poorly understood. Here, we ask a simple, yet important question: How much does the MPC vary, within a single strain-antibiotic association? Using a Staphylococcus species and five antibiotics from five different antibiotic classes—ciprofloxacin, doxycycline, gentamicin, nitrofurantoin, and oxacillin—we examined the frequency of resistance for a wide range of concentrations per antibiotic, and measured the repeatability of the MPC, the lowest amount of antibiotic that would ensure no surviving cells in a 10¹⁰ population of bacteria.

Results: We found a wide variation within the MPC and distributions that were rarely normal. When antibiotic resistance evolved, the distribution of the MPC changed, with all distributions becoming wider and some multi-modal.

Conclusion: Unlike the MIC, there is high variability in the MPC for a given bacterial strain-antibiotic combination.

Synergistic activity of polymyxin B combined with vancomycin against carbapenem-resistant and polymyxin-resistant Acinetobacter baumannii: first in vitro study

PURPOSE

The effect of a combination of polymyxin B (PMB) and vancomycin (VAN) was assessed against six Acinetobacter baumannii clinical isolates belonging to six different clusters (three PMB-susceptible and three PMB-resistant).

METHODOLOGY

The synergistic effect of the PMB-VAN combination was determined with the checkerboard, time-kill, disk-diffusion and M.I.C.Evaluator assays. PMB-resistance was investigated with mcr-1 gene amplification and a mutant frequency assay.

RESULTS

In the checkerboard assay, all PMB-resistant isolates showed a synergistic effect. The time-kill assay demonstrated that the PMB-VAN combination had a bactericidal effect at 24 h against isolates with a high mutant rate for PMB, suggesting that this combination may block the hypermutation of some isolates. No antagonism was detected. All PMB-resistant isolates also showed synergism in the disk-diffusion test, and a significant decrease in VAN MICs in the M.I.C.Evaluator assay.

CONCLUSION

Our findings indicate that the PMB-VAN combination has a synergistic effect on A. baumannii, especially against PMB-resistant isolates.

Mutant prevention and minimum inhibitory concentration drug values for enrofloxacin, ceftiofur, florfenicol, tilmicosin and tulathromycin tested against swine pathogens Actinobacillus pleuropneumoniae, Pasteurella multocida and Streptococcus suis

Actinobacillus pleuropneumoniae, Pasteurella multocida and Streptococcus suis are prevalent bacterial causes of swine infections. Morbidity, mortality and positively impacting the financial burden of infection occurs with appropriate antimicrobial therapy. Increasing antimicrobial resistance complicates drug therapy and resistance prevention is now a necessity to optimize therapy and prolong drug life. Mutant bacterial cells are said to arise spontaneously in bacterial densities of 107-109 or greater colony forming units/ml. Antibiotic drug concentration inhibiting growth of the least susceptible cell in these high density populations has been termed the mutant prevention concentration (MPC). In this study MPC and minimum inhibitory concentration (MIC) values of ceftiofur, enrofloxacin, florfenicol, tilmicosin and tulathromycin were determined against the swine pathogens A. pleuropneumoniae, P.multocida and S. suis. The following MIC90/MPC90 values (mg/L) for 67 A. pleuropneumoniae and 73 P. multocida strains respectively were as follows: A. pleuropneumoniae 0.031/0.5, ≤0.016/0.5, 0.5/2, 4/32, 2/32; P. multocida 0.004/0.25, 0.016/0.125, 0.5/0.5, 8/16, 0.5/1. For 33 S. suis strains, MIC90 values (mg/L) respectively were as follows: 1, 0.25, 4, ≥8 and ≥8. A total of 16 S. suis strains with MIC values of 0.063-0.5 mg/L to ceftiofur and 0.25-0.5 mg/L to enrofloxacin were tested by MPC; MPC values respectively were 0.5 and 1 mg/L respectively. MPC concentrations provide a dosing target which may serve to reduce amplification of bacterial subpopulations with reduced antimicrobial susceptibility. Drug potency based on MIC90 values was ceftiofur > enrofloxacin >florfenicol = tulathromycin > tilmicosin; based on MPC90 values was enrofloxacin > ceftiofur > tulathromycin > florfenicol ≥ tilmicosin.

Comparative pharmacokinetics of danofloxacin in healthy and Pasteurella multocida infected ducks

Pasteurella multocida (P. multocida) infection causes substantial economic loss in the duck industry. Danofloxacin, a fluoroquinolone solely used in animals, shows good antibacterial activity against P. multocida. In this study, the in vitro pharmacodynamics of danofloxacin against P. multocida was studied. The serum and lung tissue pharmacokinetics of danofloxacin were studied in healthy and P. multocida infected ducks following oral administration of a single dose of 5 mg/kg body weight (b.w.). The MIC, MBC and MPC of danofloxacin against P. multocida (C48-1 ) were 0.25, 1 and 3.2 μg/ml, respectively. The Cmax was 0.34 μg/ml, attained at 2.03 hr in healthy ducks, and was 0.35 μg/ml, attained at 2.87 hr in diseased ducks. Compared to the serum pharmacokinetics of danofloxacin in healthy ducks, the absorption rate and extent were similar in healthy and diseased animals. In contrast, the elimination rate was slower, with an elimination half-life (T1/2β ) of 13.17 and 16.18 hr for healthy and infected animals, respectively; the AUCs in the two groups were 5.70 and 7.68 μg hr/ml, respectively, which means the total amount of drug in the circulation was increased in the infected ducks. The maximum concentration in lung tissues between healthy and infected animals was not significantly different (8.96 vs. 8.93 μg/g). However, the Tmax in healthy ducks was longer than that in infected ducks (4 hr vs. 1.75 hr), which means that the distribution rate of danofloxacin was slower in healthy ducks. The concentration of danofloxacin in lung tissues was approximately 24-fold higher than that in the serum. In the serum pharmacokinetic profiles, the ƒAUC0-24 hr /MIC was 18.19 in healthy ducks and was 25.04 in P. multocida infected ducks at the clinical recommended dose, which is far from the PK/PD target (125 hr) of fluoroquinolones. Danofloxacin, at a dose of 5 mg/kg b.w., seems to be insufficient for ducks infected with P. multocida, with an MIC equal to 0.25 μg/ml.

In Vivo Pharmacokinetic/Pharmacodynamic Profiles of Danofloxacin in Rabbits Infected With Salmonella typhimurium After Oral Administration

Salmonella typhimurium is a highly transmissible pathogen in rabbits that causes significant losses. Danofloxacin shows excellent efficacy against S. typhimurium infections. However, there are few reports of the pharmacokinetic/pharmacodynamic (PK/PD) modeling of danofloxacin against this pathogen. The aim of this study was to evaluate the in vivo PK/PD relationship of danofloxacin in rabbits infected with S. typhimurium. We used the reduction of bacterial burden in the blood, liver, spleen, and lung as the target PD endpoints, and determined the PK/PD indexes that best correlated with the efficacy and its corresponding magnitude. Danofloxacin was administrated orally to experimentally S. typhimurium-infected rabbits once daily for three successive days. The concentrations of danofloxacin in the serum and the bacterial burden in the blood, liver, spleen, and lung were determined. The PK/PD relationships of danofloxacin against S. typhimurium were evaluated using a Sigmoid E_max model. The results showed that the area under the concentration-time curve from 0 to 24 h/minimum inhibitory concentration (AUC_{24 h}/MIC) ratio correlated well with the in vivo antibacterial effectiveness in different organs, with an r² of 0.8971, 0.9186, 0.9581, and 0.8708 in the blood, liver, spleen, and lung, respectively. The AUC_{24 h}/MIC ratios for the bactericidal effect (3 × Log₁₀ colony forming units/mL reductions) were 121.30, 354.28, 216.64, and 228.66 in the blood, liver, spleen, and lung, respectively, indicating that the in vivo effectiveness of danofloxacin against S. typhimurium using bacterial reduction in different organs as PD endpoints was not identical. This study illustrated that the selection of the target organ for bacterial reduction determination had little effect on best PK/PD parameter determination, but is critical for parameter magnitude calculation in antimicrobial PK/PD modeling, and furthermore, has an impact on the rational dosage optimization process.

Mutant Prevention Concentration and Mutant Selection Window of Micafungin and Anidulafungin in Clinical Candida glabrata Isolates

We report the mutant prevention concentration (MPC) and mutant selection window (MSW) for micafungin and anidulafungin administered to treat Candida glabrata We also determine the mutation frequency. We studied 20 echinocandin-susceptible, fluconazole-intermediate, and FKS wild-type C. glabrata isolates. Adjusted inocula were stroked directly onto Sabouraud agar plates containing different concentrations of micafungin or anidulafungin and visually inspected daily for up to 5 days of incubation. Individual colonies growing on the plates containing echinocandins at 1 mg/liter were selected for antifungal susceptibility testing. The FKS genes of the resulting individual phenotypically resistant colonies were sequenced, and the MPC, MSW, and mutation frequency were determined. Biofilm was quantified, and the growth kinetics and virulence (Galleria mellonella model) of the resulting individual FKS mutant colonies were studied. For micafungin and anidulafungin, we found similar results for the MPC (0.06 to 2 mg/liter and 0.25 to 2 mg/liter, respectively), MSW (0.015 to 2 mg/liter for both echinocandins), and mutation frequency (3.7 × 10-8 and 2.8 × 10-8, respectively). A total of 12 isolates were able to grow at 1 mg/liter on echinocandin-containing plates, yielding a total of 32 phenotypically resistant colonies; however, FKS2 mutations (ΔF658, S663P, W715L, and E655A) were observed only in 21 colonies. We did not find differences in biofilm formation, the kinetic parameters studied, or the median survival of larvae infected by wild-type isolates and the resulting individual FKS2 mutant colonies. Echinocandin concentrations lower than 2 mg/liter can lead to selection of resistance mutations in C. glabrata isolates in vitro.

PK-PD Integration Modeling and Cutoff Value of Florfenicol against Streptococcus suis in Pigs

The aims of the present study were to establish optimal doses and provide an alternate COPD for florfenicol against Streptococcus suis based on pharmacokinetic-pharmacodynamic integration modeling. The recommended dose (30 mg/kg b.w.) were administered in healthy pigs through intramuscular and intravenous routes for pharmacokinetic studies. The main pharmacokinetic parameters of Cmax, AUC0-24h, AUC, Ke, t1/2ke, MRT, Tmax, and Clb, were estimated as 4.44 μg/ml, 88.85 μg⋅h/ml, 158.56 μg⋅h/ml, 0.048 h-1, 14.46 h, 26.11 h, 4 h and 0.185 L/h⋅kg, respectively. The bioavailability of florfenicol was calculated to be 99.14% after I.M administration. A total of 124 Streptococcus suis from most cities of China were isolated to determine the minimum inhibitory concentration (MIC) of florfenicol. The MIC50 and MIC90 were calculated as 1 and 2 μg/ml. A serotype 2 Streptococcus suis (WH-2), with MIC value similar to MIC90, was selected as a representative for an in vitro and ex vivo pharmacodynamics study. The MIC values of WH-2 in TSB and plasma were 2 μg/ml, and the MBC/MIC ratios were 2 in TSB and plasma. The MPC was detected to be 3.2 μg/ml. According to inhibitory sigmoid Emax model, plasma AUC0-24h/MIC values of florfenicol versus Streptococcus suis were 37.89, 44.02, and 46.42 h for the bactericidal, bacteriostatic, and elimination activity, respectively. Monte Carlo simulations the optimal doses for bactericidal, bacteriostatic, and elimination effects were calculated as 16.5, 19.17, and 20.14 mg/kg b.w. for 50% target attainment rates (TAR), and 21.55, 25.02, and 26.85 mg/kg b.w. for 90% TAR, respectively. The PK-PD cutoff value (COPD) analyzed from MCS for florfenicol against Streptococcus suis was 1 μg/ml which could provide a sensitivity cutoff value. These results contributed an optimized alternative to clinical veterinary medicine and showed that the dose of 25.02 mg/kg florfenicol for 24 h could have a bactericidal action against Streptococcus suis after I.M administration. However, it should be validated in clinical practice in the future investigations.

PK-PD Analysis of Marbofloxacin against Streptococcus suis in Pigs

Marbofloxacin is a fluoroquinolone antibiotic and highly effective treatment for respiratory diseases. Here we aimed to evaluate the ex vivo activity of marbofloxacin against Streptococcus suis in pig serum, as well as the optimal dosages scheme for avoiding the fluoroquinolone resistance development. A single dose of 8 mg/kg body weight (bw) was administrated orally to healthy pigs and serum samples were collected during the next 72 h. Serum marbofloxacin content was determined by high-performance liquid chromatography. We estimated the C_max (6.28 μg/ml), AUC_{0-24 h} (60.30 μg.h/ml), AUC_0-∞ (88.94 μg.h/ml), T_1/2ke, (12.48 h), T_max (0.75 h) and Cl_b (0.104 L/h) of marbofloxacin in pigs, as well as the bioavailability of marbofloxacin (94.21%) after a single 8 mg/kg oral administration. We also determined the pharmacodynamic of marbofloxacin against 134 Streptococcus suis strains isolated from Chinese cities in TSB and serum. These isolated strains had a MIC₉₀ of 1 μg/ml. HB2, a virulent, serotype 2 isolate of SS, was selected for having antibacterial activity in TSB and serum to marbofloxacin. We determined the minimum inhibitory concentration (MIC, 1 μg/ml in TSB, 2 μg/ml in serum), minimum bactericidal concentration (MBC, 4 μg/ml in TSB, 4 μg/ml in serum), and mutant prevention concentration (2.56 μg/ml in TSB) for marbofloxacin against Streptococcus suis (HB2). In serum, by inhibitory sigmoid E_max modeling, the AUC_0-24h/MIC values for marbofloxacin against HB2 were 25.23 (bacteriostatic), 35.64 (bactericidal), and 39.71 (elimination) h. Based on Monte Carlo simulations, the predicted optimal oral doses of marbofloxacin curing Streptococcus suis were 5.88 (bacteriostatic), 8.34 (bactericidal), and 9.36 (elimination) mg/kg.bw for a 50% target attainment ratio, and 8.16 (bacteriostatic), 11.31 (bactericidal), and 12.35 (elimination) mg/kg.bw for a 90% target attainment ratio. The data presented here provides optimized dosage information for clinical use; however, these predicted dosages should also be validated in clinical practice.

Pharmacokinetic and Pharmacodynamic Evaluation of Marbofloxacin and PK/PD Modeling against Escherichia coli in Pigs

The aim of this study was to evaluate the activity of marbofloxacin and establish the optimal dose regimens for decreasing the development of fluoroquinolone resistance in pigs against Escherichia coli with ex vivo pharmacokinetic/pharmacodynamic (PK/PD) modeling. The recommended dose (2 mg/kg body weight) of marbofloxacin was orally administered in healthy pigs. The ileum content and plasma were both collected for the determination of marbofloxacin. The main parameters of C_max, AUC_{0-24 h}, AUC, Ke, t_1/2ke, MRT and Cl_b were 11.28 μg/g, 46.15, 77.81 μg⋅h/g, 0.001 h^-1, 69.97 h, 52.45 h, 0.026 kg/h in ileum content, and 0.55 μg/ml, 8.15, 14.67 μg⋅h/ml, 0.023 h^-1, 30.67 h, 34.83 h, 0.14 L/h in plasma, respectively In total, 218 E. coli strains were isolated from most cities of China. The antibacterial activity in vitro and ex vivo of marbofloxacin against E. coli was determined following CLSI guidance. The MIC₉₀ of sensitive strains (142) was calculated as 2 μg/ml. The minimum inhibitory concentration (MIC) of HB197 was 2 and 4 μg/ml in broth and ileum fluids, respectively. In vitro mutant prevention concentration, growth and killing-time in vitro and ex vivo of marbofloxacin against selected HB197 were assayed for pharmacodynamic studies. According to the inhibitory sigmoid E_max modeling, the value of AUC_{0-24 h}/MIC produced in ileum content was achieved, and bacteriostatic, bactericidal activity, and elimination were calculated as 16.26, 23.54, and 27.18 h, respectively. Based on Monte Carlo simulations to obtain 90% target attainment rate, the optimal doses to achieve bacteriostatic, bactericidal, and elimination effects were 0.85, 1.22, and 1.41 mg/kg.bw for 50% target, respectively, and 0.92, 1.33, and 1.53 mg/kg.bw for 90% target, respectively, after oral administration. The results in this study provided a more optimized alternative for clinical use and demonstrated that the dosage 2 mg/kg of marbofloxacin by oral administration could have an effect on bactericidal activity against E. coli.

In Vitro Resistance Selection in Shigella flexneri by Azithromycin, Ceftriaxone, Ciprofloxacin, Levofloxacin, and Moxifloxacin

Shigella flexneri continues to be a major cause of diarrhea-associated illness, and increasing resistance to first-line antimicrobials complicates the treatment of infections caused by this pathogen. We investigated the pharmacodynamics of current antimicrobial treatments for shigellosis to determine the likelihood of resistance promotion with continued global antimicrobial use. The mutant prevention concentration (MPC) and mutant selection window (MSW) were determined for azithromycin, ceftriaxone, ciprofloxacin, levofloxacin, and moxifloxacin against a wild-type strain of S. flexneri (ATCC 12022) and an isogenic gyrA mutant (m-12022). Time-kill assays were performed to determine antimicrobial killing. Concentrations of approved doses of ciprofloxacin, levofloxacin, and moxifloxacin are predicted to surpass the MPC for a majority of the dosage interval against ATCC 12022. However, against m-12022, concentrations of all fluoroquinolones are predicted to fall below the MPC and remain in the MSW for a majority of the dosage interval. Concentrations of ceftriaxone fall within the MSW for the majority of the dosage interval for both strains. All agents other than azithromycin displayed bactericidal activity in time-kill assays. Results of pharmacodynamic analyses suggest that all tested fluoroquinolones would achieve a favorable area under the concentration-time curve (AUC)/MPC ratio for ATCC 12022 and would restrict selective enrichment of mutants but that mutant selection in m-12022 would be likely if ciprofloxacin were used. Based on pharmacodynamic analyses, azithromycin and ceftriaxone are predicted to promote mutant selection in both strains. Confirmation of these findings and examination of novel treatment regimens using in vivo studies are warranted.

Cj1199 Affect the Development of Erythromycin Resistance in Campylobacter jejuni through Regulation of Leucine Biosynthesis

The aim of this study was to reveal the biological function of Cj1199 which was overexpressed in the laboratory induced erythromycin resistant strains. The Cj1199 deletion mutant (ΦCj1199) was constructed via insertional inactivation from its parent strain Campylobacter jejuni NCTC11168. The ΦCj1199 and NCTC11168 were then subjected to microarray and real-time PCR to find gene pathway of Cj1199. The antimicrobial susceptibility, antimicrobial resistance development, growth characteristics and leucine metabolism were examined to confirm the biological function of Cj1199. Our result showed that a total of 20 genes were down-regulated in ΦCj1199. These genes were mainly involved in leucine biosynthesis, amino acid transport and periplasmic/membrane structure. Compared to NCTC11168, ΦCj1199 was difficult to acquire higher-level erythromycin resistance during the in vitro step-wise selection. The competition growth and leucine-dependent growth assays demonstrated that ΦCj1199 imposed a growth disadvantage under pressure of erythromycin and in the leucine-free medium. In conclusion, Cj1199 gene may directly regulate the leucine biosynthesis and transport and indirectly affect the development of erythromycin resistance in C. jejuni.

Human Lysozyme Synergistically Enhances Bactericidal Dynamics and Lowers the Resistant Mutant Prevention Concentration for Metronidazole to Helicobacter pylori by Increasing Cell Permeability

Metronidazole (MNZ) is an effective agent that has been employed to eradicate Helicobacter pylori (H. pylori). The emergence of broad MNZ resistance in H. pylori has affected the efficacy of this therapeutic agent. The concentration of MNZ, especially the mutant prevention concentration (MPC), plays an important role in selecting or enriching resistant mutants and regulating therapeutic effects. A strategy to reduce the MPC that can not only effectively treat H. pylori but also prevent resistance mutations is needed. H. pylori is highly resistant to lysozyme. Lysozyme possesses a hydrolytic bacterial cell wall peptidoglycan and a cationic dependent mode. These effects can increase the permeability of bacterial cells and promote antibiotic absorption into bacterial cells. In this study, human lysozyme (hLYS) was used to probe its effects on the integrity of the H. pylori outer and inner membranes using as fluorescent probe hydrophobic 1-N-phenyl-naphthylamine (NPN) and the release of aspartate aminotransferase. Further studies using a propidium iodide staining method assessed whether hLYS could increase cell permeability and promote cell absorption. Finally, we determined the effects of hLYS on the bactericidal dynamics and MPC of MNZ in H. pylori. Our findings indicate that hLYS could dramatically increase cell permeability, reduce the MPC of MNZ for H. pylori, and enhance its bactericidal dynamic activity, demonstrating that hLYS could reduce the probability of MNZ inducing resistance mutations.

In vitro profiling of antimethicillin-resistant Staphylococcus aureus activity of thymoquinone against selected type and clinical strains

This study explores antimethicillin-resistant Staphylococcus aureus (MRSA) activity of a bioactive phytochemical constituent, thymoquinone obtained from the medicinal herb, Nigella sativa Linn. Based on initial assessment on crude extract of seeds of Nigella sativa Linn, the pure active constituent was employed in the study. A total of 99 MRSA strains which comprised of 40 types and 59 clinical strains were selected for the study. Minimum inhibitory concentration (MIC), bactericidal activity, postantibiotic effect (PAE) and propensity to select resistant mutants were determined using standard protocols. Results revealed that thymoquinone exhibited MIC in the range of 8-16 μg ml(-1) and MIC90 of 16 μg ml(-1) against MRSA strains. It was bactericidal to MRSA by demonstrating >3 log kill. It showed a longer PAE of 3·2 ± 0·2 h. Upon exposure to high-density inoculum of MRSA, it did not select resistant mutants. Transmission electron microscopy of thymoquinone-treated MRSA showed no lysis but damage to cell wall and cell membrane which corroborated well with the salt tolerance and bacteriolysis assays. In conclusion, MIC90 , bactericidal property, longer PAE, absence of resistant mutant selection and damages in cell membrane and cell wall imply a promising anti-MRSA activity of thymoquinone.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first detailed report on anti-MRSA activity of thymoquinone. The assessment was made with both type and clinical strains. Thymoquinone may be a potential lead compound which can be further optimized to discover novel anti-MRSA agents.