In vitro pharmacodynamics of colistin against Acinetobacter baumannii clinical isolates

Comparison of in vitro synergy between polymyxin B or colistin in combination with 16 antimicrobial agents against multidrug-resistant Acinetobacter baumannii isolates

PURPOSES

This study determined the synergy of polymyxin B (POLB) and colistin (COL) with 16 other tested antimicrobial agents in the inhibition of multidrug-resistant Acinetobacter baumannii (MDR-AB).

METHODS

We used chequerboard assays to determine synergy between the drugs against 50 clinical MDR-AB from a tertiary hospital in the Zhejiang province in 2019, classifying combinations as either antagonistic, independent, additive, or synergistic. The efficacy of hit combinations which showed highest synergistic rate were confirmed using time-kill assays.

RESULTS

Both POLB and COL displayed similar bactericidal effects when used in combination with these 16 tested drugs. Antagonism was only observed for a few strains (2%) exposed to a combination of POLB and cefoperazone/sulbactam (CSL). A higher percentage of synergistic combinations with POLB and COL were observed with rifabutin (RFB; 90%/96%), rifampicin (RIF; 60%/78%) and rifapentine (RFP; 56%/76%). Time-kill assays also confirmed the synergistic effect of POLB and rifamycin class combinations. 1/2 MIC rifamycin exposure can achieve bacterial clearance when combined with 1/2 MIC POLB or COL.

CONCLUSION

Nearly no antagonism was observed when combining polymyxins with other drugs by both chequerboard and time-kill assays, suggesting that polymyxins may be effective in combination therapy. The combinations of POLB/COL with RFB, RIF, and RFP displayed neat synergy, with RFB showing the greatest effect.

Targeted dosing for susceptible heteroresistant subpopulations may improve rational dosage regimen prediction for colistin in broiler chickens

The dosage of colistin for the treatment of enteric E. coli in animals necessitates considering the heteroresistant (HR) nature of the targeted inoculum, described by the presence of a major susceptible population (S1, representing 99.95% of total population) mixed with an initial minor subpopulation of less susceptible bacteria (S2). Herein, we report the 1-compartment population pharmacokinetics (PK) of colistin in chicken intestine (jejunum and ileum) and combined it with a previously established pharmacodynamic (PD) model of HR in E. coli. We then computed probabilities of target attainment (PTA) with a pharmacodynamic target (AUC24h/MIC) that achieves 50% of the maximal kill of bacterial populations (considering inoculums of pure S1, S2 or HR mixture of S1 + S2). For an MIC of 1 mg/L, PTA > 95% was achieved with the registered dose (75,000 IU/kg BW/day in drinking water) for the HR mixture of S1 + S2 E. coli, whether they harboured mcr or not. For an MIC of 2 mg/L (ECOFF), we predicted PTA > 90% against the dominant susceptible sub-population (S1) with this clinical dose given (i) over 24 h for mcr-negative isolates or (ii) over 6 h for mcr-positive isolates (pulse dosing). Colistin clinical breakpoint S ≤ 2 mg/L (EUCAST rules) should be confirmed clinically.

99mTc-Labeled, Colistin Encapsulated, Theranostic Liposomes for Pseudomonas aeruginosa Infection

Infectious diseases are still the major issue not only due to antibiotic resistance but also causing deaths if not diagnosed at early-stages. Different approaches including nanosized drug delivery systems and theranostics are researched to overcome antibiotic resistance, decrease the side effects of antibiotics, improve the treatment response, and early diagnose. Therefore, in the present study, nanosized, radiolabeled with ^99mTc, colistin encapsulated, neutral and cationic liposome formulations were prepared as the theranostic agent for Pseudomonas aeruginosa infections. Liposomes exhibited appropriate physicochemical properties thanks to their nano-particle size (between 173 and 217 nm), neutral zeta potential value (about - 6.5 and 2.8 mV), as well as encapsulation efficiency of about 75%. All liposome formulations were radiolabeled with over 90% efficiency, and the concentration of stannous chloride was found as 1 mg.mL^-1 to obtain maximum radiolabeling efficiency. In alamar blue analysis, neutral liposome formulations were found more biocompatible compared with the cationic formulations. Neutral colistin encapsulated liposomes were found to be more effective against P. aeruginosa strain according to their time-dependent antibacterial effect, in addition to their highest bacterial binding capacity. As conclusion, theranostic, nanosized, colistin encapsulated, neutral liposome formulations were found as promising agents for the imaging and treating of P. aeruginosa infections.

Treatment of Acinetobacter baumannii severe infections

Acinetobacter baumannii is a Gram-negative, multidrug-resistant (MDR) pathogen that causes nosocomial infections, especially in intensive care units (ICUs) and immunocompromised patients. A. baumannii has developed a broad spectrum of antimicrobial resistance, associated with a higher mortality rate among infected patients compared with other non-baumannii species. In terms of clinical impact, resistant strains are associated with increases in both in-hospital length of stay and mortality. A. baumannii can cause a variety of infections, especially ventilator-associated pneumonia, bacteremia, and skin wound infections, among others. The most common risk factors for the acquisition of MDR A. baumannii are previous antibiotic use, mechanical ventilation, length of ICU and hospital stay, severity of illness, and use of medical devices. Current efforts are focused on addressing all the antimicrobial resistance mechanisms described in A. baumannii, with the objective of identifying the most promising therapeutic scheme.

Quantitative Pharmacodynamic Characterization of Resistance versus Heteroresistance of Colistin in E. coli Using a Semimechanistic Modeling of Killing Curves

Heteroresistance corresponds to the presence, in a bacterial isolate, of an initial small subpopulation of bacteria characterized by a significant reduction in their sensitivity to a given antibiotic. Mechanisms of heteroresistance versus resistance are poorly understood. The aim of this study was to explore heteroresistance in mcr-positive and mcr-negative Escherichia coli strains exposed to colistin by use of modeling killing curves with a semimechanistic model. We quantify, for a range of phenotypically (susceptibility based on MIC) and genotypically (carriage of mcr-1 or mcr-3 or mcr-negative) different bacteria, a maximum killing rate (E_max) of colistin and the corresponding potency (EC₅₀), i.e., the colistin concentrations corresponding to E_max/2. Heteroresistant subpopulations were identified in both mcr-negative and mcr-positive E. coli as around 0.06% of the starting population. Minority heteroresistant bacteria, both for mcr-negative and mcr-positive strains, differed from the corresponding dominant populations only by the maximum killing rate of colistin (differences for E_max by a factor of 12.66 and 3.76 for mcr-negative and mcr-positive strains, respectively) and without alteration of their EC₅₀s. On the other hand, the resistant mcr-positive strains are distinguished from the mcr-negative strains by differences in their EC₅₀, which can reach a factor of 44 for their dominant population and 22 for their heteroresistant subpopulations. It is suggested that the underlying physiological mechanisms differ between resistance and heteroresistance, with resistance being linked to a decrease in the affinity of colistin for its site of action, whereas heteroresistance would, rather, be linked to an alteration of the target, which will be more difficult to be further changed or destroyed.

The Effect of Different Colistin Dosing Regimens on Nephrotoxicity: A Cohort Study

(1) Background: It is not known whether different daily dosing schemes have different effects on colistin nephrotoxicity. We examined the effect of once- versus twice- or thrice-daily doses of colistin on renal function. (2) Methods: We performed a multicenter retrospective cohort study of hospitalized patients with a baseline glomerular filtration rate ≥ 50 mL/min who received intravenously the same colistin dose once (regimen A), twice (regimen B) or thrice daily (regimen C). The primary endpoint was acute kidney injury (AKI), defined as fulfilment of any of the RIFLE (Risk-Injury-Failure-Loss-End stage renal disease) criteria. (3) Results: We included 306 patients; 132 (43.1%) received regimen A, 151 (49.3%) regimen B, and 23 (7.5%) regimen C. Ninety-nine (32.4%) patients developed AKI; there was no difference between regimen A vs. B and C [45 (34.1%) vs. 54 (31.0%), p = 0.57]. In a propensity score−matched cohort, AKI was similar in patients receiving Regimen A, Regimen B, and Regimen C (31.6% vs. 33.3%, p = 0.78). On logistic regression analysis, diabetes was an independent predictor of AKI (OR = 4.59, 95% CI 2.03−10.39, p = 0.001) while eGFR > 80 mL/min (OR = 0.50, 95% CI 0.25−0.99, p = 0.048) was inversely associated with AKI. (4) Conclusions: Colistin once daily is not more nephrotoxic than the standard colistin regimens. The only independent predictor of nephrotoxicity was diabetes mellitus, while eGFR > 80 mL/min had a protective effect.

Heteroresistance Is Associated With in vitro Regrowth During Colistin Treatment in Carbapenem-Resistant Klebsiella pneumoniae

Polymyxins including polymyxin B and colistin (polymyxin E) are considered the last resort for treating infections caused by carbapenem-resistant gram-negative bacteria. However, in vitro regrowth with the emergence of resistance during treatment is common. Polymyxin heteroresistance, particularly in Acinetobacter baumannii and Klebsiella pneumoniae, has been widely reported. This study was primarily performed to evaluate the prevalence of colistin heteroresistance in carbapenem-resistant K. pneumoniae (CR-KP) and the association between in vitro regrowth and heteroresistance. The mechanisms of colistin resistance and the ability of combination therapies to suppress resistance selection were further investigated. A population analysis profile (PAP) analysis showed that 69 (71.9%) of 96 CR-KP strains had colistin heteroresistance. Time-kill assays revealed that the colistin monotherapy could quickly eliminate the bacterial cells in strains without heteroresistance within the first 6 h. Conversely, it could initially reduce the number of cells in heteroresistant strains, but then regrowth occurred rapidly. Resistance screening at 12 and 24 h in the time-kill assays indicated that susceptible populations were killed, and regrowth was the exact result of the continued growth of resistant subpopulations. Colistin resistance in the regrowth subpopulations was mainly due to the overexpression of phoPQ and pmrD. Colistin combined with tetracyclines (tigecycline or minocycline) or aminoglycosides (amikacin or gentamicin) could effectively suppress the resistance selection and significantly elicit in vitro synergistic effects. These findings suggested that the combination therapy can be used to treat infections caused by CR-KP with colistin heteroresistance. Nevertheless, further in vivo studies considering drugs pharmacokinetics/pharmacodynamics are needed to confirm these findings.

Use of colistin with rifampicin, trimethoprim-sulfamethoxazole and teicoplanin in acinetobacter mouse infection model

Background: Infections with multidrug-resistant Gram-negative bacteria such as Acinetobacter baumannii are major cause of morbidity and mortality. Colistin is used commonly to treat these infections. In this study, we evaluated the efficacy of different colistin combinations in a A. baumannii infection mouse model. Materials & methods: An A. baumannii mouse infection model was developed in 150 experimental animals. Treatment groups were as follows: colistin, colistin + rifampicin, colistin + trimethoprim/sulfamethoxazole, colistin + teicoplanin and a control group. The outcome was bacterial burden in the lung and liver tissues. The treatment groups were subdivided into 24-, 48- and 72-h groups. Results: Colistin and combinations reduce the A. baumannii burden significantly in lung and liver tissues compared with the control group. Compared with colistin alone colistin + rifampicin and colistin + TMP-SMX provided significantly better reduction in the bacterial burden. Conclusion: These results may suggest that rifampicin and TMP-SMX combination with colistin may have a potential role in the treatment of A. baumannii infections.

Pharmacokinetics and pharmacodynamics of peptide antibiotics

Antimicrobial resistance is a major global health challenge. As few new efficacious antibiotics will become available in the near future, peptide antibiotics continue to be major therapeutic options for treating infections caused by multidrug-resistant pathogens. Rational use of antibiotics requires optimisation of the pharmacokinetics and pharmacodynamics for the treatment of different types of infections. Toxicodynamics must also be considered to improve the safety of antibiotic use and, where appropriate, to guide therapeutic drug monitoring. This review focuses on the pharmacokinetics/pharmacodynamics/toxicodynamics of peptide antibiotics against multidrug-resistant Gram-negative and Gram-positive pathogens. Optimising antibiotic exposure at the infection site is essential for improving their efficacy and minimising emergence of resistance.

Correlative proteomics identify the key roles of stress tolerance strategies in Acinetobacter baumannii in response to polymyxin and human macrophages

The opportunistic pathogen Acinetobacter baumannii possesses stress tolerance strategies against host innate immunity and antibiotic killing. However, how the host-pathogen-antibiotic interaction affects the overall molecular regulation of bacterial pathogenesis and host response remains unexplored. Here, we simultaneously investigate proteomic changes in A. baumannii and macrophages following infection in the absence or presence of the polymyxins. We discover that macrophages and polymyxins exhibit complementary effects to disarm several stress tolerance and survival strategies in A. baumannii, including oxidative stress resistance, copper tolerance, bacterial iron acquisition and stringent response regulation systems. Using the spoT mutant strains, we demonstrate that bacterial cells with defects in stringent response exhibit enhanced susceptibility to polymyxin killing and reduced survival in infected mice, compared to the wild-type strain. Together, our findings highlight that better understanding of host-pathogen-antibiotic interplay is critical for optimization of antibiotic use in patients and the discovery of new antimicrobial strategy to tackle multidrug-resistant bacterial infections.

In vitro synergistic activity of colistin and teicoplanin combination against multidrug-resistant Acinetobacter spp

Drug combinations may have a crucial role in treating infections due to multidrug resistant Acinetobacter spp. One suggested combination is colistin with teicoplanin. The effect of colistin on Acinetobacter spp. outer membrane can permit teicoplanin to its target in the cell wall. The aim of this study was to evaluate the synergistic activity of colistin and teicoplanin combination against 29 multidrug resistant isolates of Acinetobacter spp. The antimicrobial activity of colistin alone and in combination with teicoplanin was assessed using MIC and time–kill assays. The combination of 1 mg/l colistin and 10 mg/l teicoplanin showed in vitro synergism against all tested Acinetobacter isolates except one (Acinetobacter lowffii). The combination of 1 mg/l colistin and 10 mg/l teicoplanin was bactericidal at 6 h against 100% of Acinetobacter baumannii isolates with no bacterial regrowth at 24 h. The same combination was bactericidal against three out of seven non-baumannii Acinetobacter isolates. The increased concentration of teicoplanin (20 mg/l) was synergistic but still not bactericidal against the four remaining isolates. The combination of colistin and teicoplanin was synergistic against all tested Acinetobacter spp It is therefore recommended that clinical trials are conducted to clarify the therapeutic potential of the combination.

Colistin Interaction and Surface Changes Associated with mcr-1 Conferred Plasmid Mediated Resistance in E. coli and A. veronii Strains

Colistin, a polycationic antimicrobial peptide, is one of the last-resort antibiotics for treating infections caused by carbapenem-resistant Gram-negative bacteria. The antibacterial activity of colistin occurs through electrostatic interaction between the polycationic peptide group of colistin and the negatively charged phosphate groups of lipid A membrane. This study investigated the interaction of colistin with the outer membrane and surface constituents of resistant and susceptible strains of Escherichia coli and Aeromonas veronii harboring mcr-1 resistance gene. Bacterial membrane and lipopolysaccharide used in this study were isolated from susceptible as well as colistin-resistant strains of E. coli and A. veronii. Interaction of colistin with the bacterial surface was studied by deoxycholate and lysozyme sensitivity test, N-phenyl-1-naphthylamine (NPN) uptake assay, Atomic force microscopy (AFM), Zeta potential measurements and ¹H NMR. The binding affinity of colistin was found to be lower with outer membrane from resistant strains in comparison with the susceptible strains. Colistin exposure enhances the outer membrane permeability of the susceptible strains to deoxycholate and lysozyme. However, on the other hand, colistin dose of 256 µg/mL did not permeabilize the outer membrane of resistant bacteria. The NPN permeability in resistant strains was greater in comparison with susceptible strains. Atomic force microscopy images depicted smooth, featherless and deformed membranes in treated susceptible cells. Contrary to the above, resistant treated cells displayed surface roughness topography even at 256 µg/mL colistin concentration. Surface charge alterations were confirmed by Zeta potential measurements as a function of the growth phase. Mid-logarithmic phase susceptible strains showed a greater negative charge than resistant strains upon exposure to colistin. However, there was no statistical variation in the Zeta potential measurements between resistant and susceptible strains at the stationary phase. NMR analysis revealed line broadening in susceptible strains with increasing colistin: LPS aggregates mass ratio. Moreover, resistant strains did not show line broadening for the outer membrane, even at the highest mass ratio. The findings of this study suggest that the resistant strains of E. coli and A. veronii can block the electrostatic contact between the cationic peptide and anionic lipid A component that drives the first phase of colistin action, thereby preventing hydrophobically driven second-tier action of colistin on the outer lipopolysaccharide layer.

Clinical Efficacy and Nephrotoxicity of the Loading Dose Colistin for the Treatment of Carbapenem-Resistant Acinetobacter baumannii in Critically Ill Patients

Carbapenem-resistant Acinetobacter baumannii (CRAB) is one of the most common causes of nosocomial infections in critically ill patients. Colistin methanesulfonate (CMS), an inactive prodrug, has been considered as a last-resort treatment for CRAB infection in critically ill patients. The objective of this study was to assess 30-day survival and nephrotoxicity in critically ill patients who received non-loading dose (LD) versus LD of CMS for CRAB infection treatment. Between 2012 and 2017, this retrospective cohort analysis was performed at Chiang Mai University Hospital (CMUH), focusing on critically ill patients with CRAB infection who received either non-LD or LD of CMS. A total of 383 patients met the criteria for inclusion. At the 30th day of treatment, the survival rate of patients in the LD CMS group was 1.70 times (adjusted HR) of those in the non-LD group (95% CI = 1.17-2.50, p = 0.006). Clinical response was significantly higher in the LD CMS group than non-LD CMS group (aHR, 1.35, 95% CI, 1.01-1.82, p = 0.046). In addition, a microbiological response-eradication of pre-treatment isolated pathogens in post-treatment cultures-in patients with LD CMS was 1.57 times that of patients with non-LD CMS (95% CI, 1.15-2.15, p = 0.004). Additionally, there was a significant difference in nephrotoxicity between LD CMS and non-LD CMS (aHR, 1.57, 95% CI, 1.14-2.17, p = 0.006). Based on these results, LD CMS should be used to increase the opportunity of patients to achieve favourable outcomes. However, LD CMS was found associated with an increase in nephrotoxicity, so renal function should be closely monitored when LD colistin was administered.

Safety and efficacy of colistin and fluoroquinolone in neonatal persistent late-onset sepsis

Growing resistance of microorganisms to antibiotics for the treatment of late-onset sepsis (LOS) in premature infants has led physicians to use antibiotics that are not well studied in neonatal populations. We aimed to determine the efficacy and safety of colistin and fluoroquinolone for the treatment of persistent LOS. We retrospectively reviewed infants with gram-negative LOS, who received either colistin or fluoroquinolone therapy, to determine if there was a significant difference in kidney and liver function tests and electrolyte levels before, during, and at the end of the treatment. Infants who received colistin and fluoroquinolone had 17 and 34 positive cultures with gram-negative organisms, respectively. Multi-drug resistant organisms were more common in infants who received colistin than in those who received fluoroquinolone. Microbiological clearance was found to be higher in infants treated with fluoroquinolone than in those treated with colistin. In both the groups, the median levels of kidney and liver function tests and electrolytes showed a significant increase during the treatment. The prescription of colistin and fluoroquinolones should be reserved for cases with no other safe and effective alternatives.

Current Update on Intrinsic and Acquired Colistin Resistance Mechanisms in Bacteria

Colistin regained global interest as a consequence of the rising prevalence of multidrug-resistant Gram-negative Enterobacteriaceae. In parallel, colistin-resistant bacteria emerged in response to the unregulated use of this antibiotic. However, some Gram-negative species are intrinsically resistant to colistin activity, such as Neisseria meningitides, Burkholderia species, and Proteus mirabilis. Most identified colistin resistance usually involves modulation of lipid A that decreases or removes early charge-based interaction with colistin through up-regulation of multistep capsular polysaccharide expression. The membrane modifications occur by the addition of cationic phosphoethanolamine (pEtN) or 4-amino-l-arabinose on lipid A that results in decrease in the negative charge on the bacterial surface. Therefore, electrostatic interaction between polycationic colistin and lipopolysaccharide (LPS) is halted. It has been reported that these modifications on the bacterial surface occur due to overexpression of chromosomally mediated two-component system genes (PmrAB and PhoPQ) and mutation in lipid A biosynthesis genes that result in loss of the ability to produce lipid A and consequently LPS chain, thereafter recently identified variants of plasmid-borne genes (mcr-1 to mcr-10). It was hypothesized that mcr genes derived from intrinsically resistant environmental bacteria that carried chromosomal pmrC gene, a part of the pmrCAB operon, code three proteins viz. pEtN response regulator PmrA, sensor kinase protein PmrAB, and phosphotransferase PmrC. These plasmid-borne mcr genes become a serious concern as they assist in the dissemination of colistin resistance to other pathogenic bacteria. This review presents the progress of multiple strategies of colistin resistance mechanisms in bacteria, mainly focusing on surface changes of the outer membrane LPS structure and other resistance genetic determinants. New handier and versatile methods have been discussed for rapid detection of colistin resistance determinants and the latest approaches to revert colistin resistance that include the use of new drugs, drug combinations and inhibitors. Indeed, more investigations are required to identify the exact role of different colistin resistance determinants that will aid in developing new less toxic and potent drugs to treat bacterial infections. Therefore, colistin resistance should be considered a severe medical issue requiring multisectoral research with proper surveillance and suitable monitoring systems to report the dissemination rate of these resistant genes.

Rescuing the Last-Line Polymyxins: Achievements and Challenges

Antibiotic resistance is a major global health challenge and, worryingly, several key Gram negative pathogens can become resistant to most currently available antibiotics. Polymyxins have been revived as a last-line therapeutic option for the treatment of infections caused by multidrug-resistant Gram negative bacteria, in particular Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Polymyxins were first discovered in the late 1940s but were abandoned soon after their approval in the late 1950s as a result of toxicities (e.g., nephrotoxicity) and the availability of "safer" antibiotics approved at that time. Therefore, knowledge on polymyxins had been scarce until recently, when enormous efforts have been made by several research teams around the world to elucidate the chemical, microbiological, pharmacokinetic/pharmacodynamic, and toxicological properties of polymyxins. One of the major achievements is the development of the first scientifically based dosage regimens for colistin that are crucial to ensure its safe and effective use in patients. Although the guideline has not been developed for polymyxin B, a large clinical trial is currently being conducted to optimize its clinical use. Importantly, several novel, safer polymyxin-like lipopeptides are developed to overcome the nephrotoxicity, poor efficacy against pulmonary infections, and narrow therapeutic windows of the currently used polymyxin B and colistin. This review discusses the latest achievements on polymyxins and highlights the major challenges ahead in optimizing their clinical use and discovering new-generation polymyxins. To save lives from the deadly infections caused by Gram negative "superbugs," every effort must be made to improve the clinical utility of the last-line polymyxins. SIGNIFICANCE STATEMENT: Antimicrobial resistance poses a significant threat to global health. The increasing prevalence of multidrug-resistant (MDR) bacterial infections has been highlighted by leading global health organizations and authorities. Polymyxins are a last-line defense against difficult-to-treat MDR Gram negative pathogens. Unfortunately, the pharmacological information on polymyxins was very limited until recently. This review provides a comprehensive overview on the major achievements and challenges in polymyxin pharmacology and clinical use and how the recent findings have been employed to improve clinical practice worldwide.

Intravenous colistin for the management of multidrug-resistant bacterial infections in Indian patients

Multidrug-resistant Gram-negative bacterial infection is a serious global concern and especially in low and middle-income countries (LMIC) such as India. Colistin, an antimicrobial once abandoned following reports of organ toxicity, has re-emerged as an essential therapeutic agent in the management of these infections. A retrospective review of 162 inpatients was done, focusing on culture-proven multidrug-resistant infections requiring colistin. The overall clinical outcome in 58% of patients was found to be good, with nephrotoxicity and neurotoxicity occurring only in 8 (5%) and 4 (2.5%) patients, respectively. Multivariate analysis revealed an elevated lactate and raised urea to be independent factors associated with poor clinical response. In conclusion, there appears to be strong evidence supporting the use of colistin in the management of multidrug-resistant Gram-negative bacterial infections.

Bi-Functional Alginate Oligosaccharide-Polymyxin Conjugates for Improved Treatment of Multidrug-Resistant Gram-Negative Bacterial Infections

The recent emergence of resistance to colistin, an antibiotic of last resort with dose-limiting toxicity, has highlighted the need for alternative approaches to combat infection. This study aimed to generate and characterise alginate oligosaccharide (“OligoG”)–polymyxin (polymyxin B and E (colistin)) conjugates to improve the effectiveness of these antibiotics. OligoG–polymyxin conjugates (amide- or ester-linked), with molecular weights of 5200–12,800 g/mol and antibiotic loading of 6.1–12.9% w/w, were reproducibly synthesised. In vitro inflammatory cytokine production (tumour necrosis factor alpha (TNFα) ELISA) and cytotoxicity (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) of colistin (2.2–9.3-fold) and polymyxin B (2.9–27.2-fold) were significantly decreased by OligoG conjugation. Antimicrobial susceptibility tests (minimum inhibitory concentration (MIC), growth curves) demonstrated similar antimicrobial efficacy of ester- and amide-linked conjugates to that of the parent antibiotic but with more sustained inhibition of bacterial growth. OligoG–polymyxin conjugates exhibited improved selectivity for Gram-negative bacteria in comparison to mammalian cells (approximately 2–4-fold). Both OligoG–colistin conjugates caused significant disruption of Pseudomonas aeruginosa biofilm formation and induced bacterial death (confocal laser scanning microscopy). When conjugates were tested in an in vitro “time-to-kill” (TTK) model using Acinetobacter baumannii, only ester-linked conjugates reduced viable bacterial counts (~2-fold) after 4 h. Bi-functional OligoG–polymyxin conjugates have potential therapeutic benefits in the treatment of multidrug-resistant (MDR) Gram-negative bacterial infections, directly reducing toxicity whilst retaining antimicrobial and antibiofilm activities.

In vitro Synergistic Activity of Antimicrobial Combinations Against bla KPC and bla NDM-Producing Enterobacterales With bla IMP or mcr Genes

Carbapenemase-producing Enterobacterales have become a severe public health concern because of their rapidly transmissible resistance elements and limited treatment options. The most effective antimicrobial combinations against carbapenemase-producing Enterobacterales are currently unclear. Here, we aimed to assess the therapeutic effects of seven antimicrobial combinations (colistin-meropenem, colistin-tigecycline, colistin-rifampicin, colistin-erythromycin, meropenem-tigecycline, meropenem-rifampicin, and meropenem-tigecycline-colistin) against twenty-four carbapenem-producing Enterobacterales (producing bla_KPC, bla_NDM, coexisting bla_NDM and bla_IMP, and coexisting mcr-1/8/9 and bla_NDM genes) and one carbapenem-susceptible Enterobacterales using the checkerboard assay, time-kill curves, and scanning electron microscopy. None of the combinations were antagonistic. The combination of colistin-rifampicin showed the highest synergistic effect of 76% (19/25), followed by colistin-erythromycin at 60% (15/25), meropenem-rifampicin at 24% (6/25), colistin-meropenem at 20% (5/25), colistin-tigecycline at 20% (5/25), and meropenem-tigecycline at 4% (1/25). The triple antimicrobial combinations of meropenem-tigecycline-colistin had a synergistic effect of 100%. Most double antimicrobial combinations were ineffective on isolates with coexisting bla_NDM and bla_IMP genes. Meropenem with tigecycline showed no synergistic effect on isolates that produced different carbapenemase genes and were highly resistant to meropenem (92% meropenem MIC ≥ 16 mg/mL). Colistin-tigecycline showed no synergistic effect on Escherichia coli producing bla_NDM–1 and Serratia marcescens. Time-kill curves showed that antimicrobial combinations achieved an eradication effect (≥ 3 log₁₀ decreases in colony counts) within 24 h without regrowth, based on 1 × MIC of each drug. The synergistic mechanism of colistin-rifampicin may involve the colistin-mediated disruption of bacterial membranes, leading to severe alterations in their permeability, then causes more rifampicin to enter the cell and induces cell death. In conclusion, the antimicrobial combinations evaluated in this study may facilitate the successful treatment of patients infected with carbapenemase-producing pathogens.

In vivo bactericidal effect of colistin-linezolid combination in a murine model of MDR and XDR Acinetobacter baumannii pneumonia

Recently, paradoxical combinations of colistin with anti-Gram-positive bacterial agents were introduced as a treatment alternative for multidrug-resistant Acinetobacter baumannii (MDRAB) infection. We assessed the therapeutic efficacy of the colistin–linezolid combination regimen in vitro and in a murine model of Acinetobacter baumannii pneumonia. A multidrug-resistant clinical strain (MDRAB31) and an extensively drug-resistant clinical strain (XDRAB78) were used in this study. The survival rates of mice and bacterial counts in lung tissue were used to assess the effects of colistin–linezolid combination. The survival rates of colistin–linezolid combination groups significantly increased compared with colistin groups for MDRAB31 (72% versus 32%, P = 0.03) and for XDRAB78 (92% versus 68%, P = 0.031). The colistin–linezolid combination groups significantly reduced the bacterial counts in lung tissue compared with colistin groups for MDRAB31 and for XDRAB78 (P < 0.05). The colistin–linezolid combination had a bactericidal and synergistic effect compared with colistin alone in time-kill assay and in murine model of pneumonia. Our data demonstrated the synergistic effect of colistin–linezolid combination regimen as a treatment alternative for the severe pulmonary infection caused by MDRAB and XDRAB.

A Comparison of Colistin versus Colistin Plus Meropenem for the Treatment of Carbapenem-Resistant Acinetobacter baumannii in Critically Ill Patients: A Propensity Score-Matched Analysis

Carbapenem-resistant Acinetobacter baumannii (CRAB), an important nosocomial pathogen, occurs particularly in the intensive care unit (ICU). Thus, the aim of this study was to compare the efficacy and safety of documented treatment with colistin monotherapy versus colistin plus meropenem in critically ill patients with CRAB infections at Chiang Mai University Hospital (CMUH). We conducted a retrospective cohort study of critically ill patients with CRAB infections in an ICU from 2015 to 2017, who received colistin monotherapy versus colistin plus meropenem. After propensity score matching, an adjusted odds ratio (aOR) of a 30-day mortality rate in patients who received colistin plus meropenem was 0.43 compared to those who received colistin monotherapy (95% CI, 0.23–0.82, p = 0.01). aORs of clinical response and microbiological response were also higher in patients who received colistin plus meropenem (1.81, 95% CI 1.01–3.26, p = 0.048 and 2.08, 95% CI 1.11–3.91, p = 0.023, respectively). There was no significant difference in nephrotoxicity (aOR, 0.76, 95% CI, 0.43–1.36, p = 0.363) between colistin monotherapy and colistin plus meropenem. In conclusion, the addition of meropenem to colistin caused a reduction in 30-day mortality, higher clinical and microbiological responses, and did not increase nephrotoxicity compared to colistin monotherapy. Furthermore, 30-day mortality was significantly related with age, receiving vasopressor, having malignancy, and the APACHE II score.

Colistin heteroresistance in Acinetobacter spp.: systematic review and meta-analysis of the prevalence and discussion of the mechanisms and potential therapeutic implications

BACKGROUND

Colistin is one of the few remaining options for carbapenem-resistant Acinetobacter baumannii (A. baumannii); however, emergence of resistance from heteroresistant populations is possible. This review aimed to systematically search and consolidate the literature on the prevalence, mechanisms and therapeutic implications of colistin heteroresistance in Acinetobacter spp.

METHODS

A systematic search was conducted in PubMed and Scopus. The pooled prevalence of colistin heteroresistance was calculated using meta-analysis of proportions with the Freeman-Tukey transformation and the random-effects (DerSimonian and Laird) method.

RESULTS

Based on 15 studies the prevalence of colistin heteroresistance was 33% (95% CI 16-53%) but considerable heterogeneity was observed (I² = 96%, P < 0.001). Prior exposure to colistin was associated with a higher proportion of resistant subpopulations. Colistin heteroresistance may result from chromosomal mutations in resistant subpopulations (predominantly in PmrAB and lpx genes) resulting in lipopolysaccharide modification or loss, or overexpression of efflux pumps. No dosage scheme of colistin monotherapy can prevent the emergence of resistant subpopulations in vitro, but few studies have reported in vivo emergence of resistance from heteroresistant A. baumannii during treatment, and studies examining the correlation between heteroresistance and clinical/microbiological outcomes are lacking. Several colistin-based combinations have been shown in vitro to prevent the emergence of the resistant subpopulations but none have been translated so far into clinical benefit. Reasons for this discrepancy are discussed.

CONCLUSIONS

Colistin heteroresistance was common but highly variable between studies. The impact of colistin heteroresistance (frequency of emergent resistance during treatment and correlation with treatment outcomes) requires further study.

Synergistic activity of an OmpA inhibitor and colistin against colistin-resistant Acinetobacter baumannii: mechanistic analysis and in vivo efficacy

Objectives

Preventing bacterial contact with host cells can provide an additional approach to tackling MDR Acinetobacter baumannii. Recently, we identified AOA-2 as a potential blocker of A. baumannii outer membrane protein A without presenting bactericidal activity. Here, we aimed to study whether AOA-2 can increase the activity of colistin against colistin-resistant A. baumannii in vitro and in vivo.

Methods

Reference and clinical A. baumannii strains susceptible and resistant to colistin (CST-S and CST-R) were used. Microdilution and time-kill curve assays were performed to determine the synergy between AOA-2 and colistin. SDS-PAGE assays with CST-S and CST-R outer membrane proteins and MALDI-TOF-TOF (MS-MS/MS) analysis were performed to determine the AOA-2 and colistin synergy mechanism. In a murine peritoneal sepsis model, the therapeutic efficacy of AOA-2 (10 mg/kg/24 h) in combination with a sub-optimal dose of colistin (10 mg/kg/24 h) against CST-R was evaluated by determining the bacterial load in tissues and blood, and mouse survival.

Results

We showed that AOA-2 increased the in vitro colistin susceptibility of reference and clinical CST-S and CST-R strains. This combination also enhanced their killing activity after 24 h of drug exposure. This synergy is mediated by the overexpression of Omp25. In vivo, the combination of AOA-2 with colistin significantly reduced the bacterial load in tissues and blood, and increased mouse survival, compared with colistin monotherapy.

Conclusions

We identified a novel class of antimicrobial agents that has proven to be effective in combination with colistin in an experimental model of severe infection by CST-R A. baumannii.

In vitro Pharmacodynamics and PK/PD in Animals

In the last decade, considerable advancements have been made to identify the pharmacokinetic/pharmacodynamic (PK/PD) index that defines the antimicrobial activity of polymyxins. Dose-fractionation studies performed in hollow-fiber models found that altering the dosing schedule had little impact on the killing or suppression of resistance emergence, alluding to AUC/MIC as the pharmacodynamic index that best describes polymyxin's activity. For in vivo efficacy, the PK/PD index that was the most predictive of the antibacterial effect of colistin against P. aeruginosa and A. baumannii was ƒAUC/MIC.

Dose-Dependent Synergistic Interactions of Colistin with Rifampin, Meropenem, and Tigecycline against Carbapenem-Resistant Klebsiella pneumoniae Biofilms

Carbapenem-resistant Klebsiella pneumoniae (CR-Kp) can cause biofilm-related bloodstream infections associated with significant morbidity and mortality worldwide. We investigated the bactericidal activities of colistin (CST), rifampin (RIF), meropenem (MEM), gentamicin (GEN), and tigecycline (TGC) alone and that of CST in combination with RIF, MEM, GEN, or TGC against CR-Kp mature biofilms. Twenty CR-Kp blood isolates were derived from an equal number of bloodstream infections in adult patients. Biofilm formation was assessed by staining with 0.4% crystal violet and measuring the optical density spectrophotometrically at 545 nm. Biofilm damage was measured as the percent reduction of metabolic activity by an XTT [2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide salt] assay. The MIC₅₀ for biofilms was determined as the minimum concentration that caused ≥50% bacterial damage compared to that for untreated controls. Antibacterial drug interactions were analyzed by the Bliss independence model. Four of the 20 CR-Kp isolates were biofilm producers. Biofilm MIC₅₀s of CST, RIF, MEM, GEN, and TGC for these isolates were 64, 8, >256, 128, and 8 mg/liter, respectively. Synergistic interactions were observed at 32 to 64 mg/liter of CST combined with 0.25 to 4 mg/liter of RIF, at 32 mg/liter of CST combined with 0.007 to 0.25 mg/liter of MEM, and at 16 to 32 mg/liter of CST combined with 16 to 64 mg/liter of TGC. The synergy was highest for CST plus RIF, with a mean ΔE ± standard error (SE) of 49.87% ± 9.22%, compared to 29.52% ± 4.97% for CST plus MEM (P < 0.001) and 32.44% ± 6.49% for CST plus TGC (P < 0.001). Indifferent results were exhibited by CST plus GEN. None of the combinations exhibited antagonism. These drug interaction findings, especially those for CST with RIF, may be of importance in the treatment of biofilm-related CR-Kp infections.

Rational Combinations of Polymyxins with Other Antibiotics

Combinations of antimicrobial agents are often used in the management of infectious diseases. Antimicrobial agents used as part of combination therapy are often selected empirically. As regrowth and the emergence of polymyxin (either colistin or polymyxin B) resistance has been observed with polymyxin monotherapy, polymyxin combination therapy has been suggested as a possible means by which to increase antimicrobial activity and reduce the development of resistance. This chapter provides an overview of preclinical and clinical investigations of CMS/colistin and polymyxin B combination therapy. In vitro data and animal model data suggests a potential clinical benefit with many drug combinations containing clinically achievable concentrations of polymyxins, even when resistance to one or more of the drugs in combination is present and including antibiotics normally inactive against Gram-negative organisms. The growing body of data on the emergence of polymyxin resistance with monotherapy lends theoretical support to a benefit with combination therapy. Benefits include enhanced bacterial killing and a suppression of polymyxin resistant subpopulations. However, the complexity of the critically ill patient population, and high rates of treatment failure and death irrespective of infection-related outcome make demonstrating a potential benefit for polymyxin combinations extremely challenging. Polymyxin combination therapy in the clinic remains a heavily debated and controversial topic. When combinations are selected, optimizing the dosage regimens for the polymyxin and the combinatorial agent is critical to ensure that the benefits outweigh the risk of the development of toxicity. Importantly, patient characteristics, pharmacokinetics, the site of infection, pathogen and resistance mechanism must be taken into account to define optimal and rational polymyxin combination regimens in the clinic.

Synergistic Activity of Niclosamide in Combination With Colistin Against Colistin-Susceptible and Colistin-Resistant Acinetobacter baumannii and Klebsiella pneumoniae

Colistin is among the few antibiotics effective against multidrug-resistant Acinetobacter baumannii and Klebsiella pneumoniae clinical isolates. However, in the last few years, colistin-resistant A. baumannii and K. pneumoniae strains have emerged. Therefore, combination therapies, between colistin and other old drugs, restoring the activity of colistin are required. The main objective of this study was to analyse the activity of niclosamide, an anthelmintic drug, in combination with colistin against colistin-susceptible (Col-S) and colistin-resistant (Col-R) A. baumannii and K. pneumoniae. The MIC were determined by microdilution assay and the time-kill curves were performed. The zeta potential of Col-S and Col-R of A. baumannii and K. pneumoniae in presence of niclosamide was assessed. Niclosamide in combination with colistin showed improved activity against Col-S and Col-R A. baumannii and K. pneumoniae. Time-killing curves showed synergic activity between niclosamide and colistin against Col-S and Col-R A. baumannii and K. pneumoniae, especially when niclosamide or colistin was added for second time at 4 h of the 24 h killing curve. Col-R A. baumannii and K. pneumoniae in presence of niclosamide exhibited a greater negative charge (-34.95 ± 0.35 mV and -38.85 ± 0.92 mV; P < 0.05) than Col-R A. baumannii and K. pneumoniae in absence of niclosamide (-26.85 ± 3.65 mV and -35.27 ± 0.72 mV). These data suggest that niclosamide might be combined with colistin, being a potential alternative for treatment of Col-R Gram-negative bacilli infections.

A Comparison of Adjusted Versus Unadjusted Doses of Polymyxin B Based on Renal Function and Incidence of Acute Kidney Injury

Purpose:

To determine whether or not polymyxin B needs dose adjustments based on renal function by comparing the incidence of acute kidney injury (AKI) in patients whose polymyxin B doses were adjusted versus not adjusted according to renal function.

Methods:

This was a single-center, prospective study with a retrospective cohort taking place in an acute care community hospital. Forty-two patients treated with polymyxin B were evaluated between April 2012 and December 2015. The primary outcome was incidence of AKI at day 7 after initiation of polymyxin B therapy with secondary outcomes including microbiological cure, clinical cure, and 30-day mortality.

Results:

There was no difference in the incidence of AKI at day 7 in patients with polymyxin B doses adjusted according to renal function versus patients without polymyxin B dose adjustment (20.0% vs 18.2%; P = .882). There were no differences between groups in occurrence of microbiological cure, clinical cure, or 30-day mortality (27.8% vs 57.1%; P = .065, 70.0% vs 72.7%; P = .845, 40.0% vs 31.8%; P = .581, respectively).

Conclusion:

The results from this study support the use of polymyxin B without any dose adjustment in the setting of renal impairment.

Insight into Acinetobacter baumannii: pathogenesis, global resistance, mechanisms of resistance, treatment options, and alternative modalities

Acinetobacter baumannii, once considered a low-category pathogen, has emerged as an obstinate infectious agent. The scientific community is paying more attention to this pathogen due to its stubbornness to last resort antimicrobials, including carbapenems, colistin, and tigecycline, its high prevalence of infections in the hospital setting, and significantly increased rate of community-acquired infections by this organism over the past decade. It has given the fear of pre-antibiotic era to the world. To further enhance our understanding about this pathogen, in this review, we discuss its taxonomy, pathogenesis, current treatment options, global resistance rates, mechanisms of its resistance against various groups of antimicrobials, and future therapeutics.

Synergistic Killing of Polymyxin B in Combination With the Antineoplastic Drug Mitotane Against Polymyxin-Susceptible and -Resistant Acinetobacter baumannii: A Metabolomic Study

Polymyxins are currently used as the last-resort antibiotics against multidrug-resistant Acinetobacter baumannii. As resistance to polymyxins emerges in A. baumannii with monotherapy, combination therapy is often the only remaining treatment option. A novel approach is to employ the combination of polymyxin B with non-antibiotic drugs. In the present study, we employed metabolomics to investigate the synergistic mechanism of polymyxin B in combination with the antineoplastic drug mitotane against polymyxin-susceptible and -resistant A. baumannii. The metabolomes of four A. baumannii strains were analyzed following treatment with polymyxin B, mitotane and the combination. Polymyxin B monotherapy induced significant perturbation in glycerophospholipid (GPL) metabolism and histidine degradation pathways in polymyxin-susceptible strains, and minimal perturbation in polymyxin-resistant strains. Mitotane monotherapy induced minimal perturbation in the polymyxin-susceptible strains, but caused significant perturbation in GPL metabolism, pentose phosphate pathway and histidine degradation in the LPS-deficient polymyxin-resistant strain (FADDI-AB065). The polymyxin B – mitotane combination induced significant perturbation in all strains except the lipid A modified polymyxin-resistant FADDI-AB225 strain. For the polymyxin-susceptible strains, the combination therapy significantly perturbed GPL metabolism, pentose phosphate pathway, citric acid cycle, pyrimidine ribonucleotide biogenesis, guanine ribonucleotide biogenesis, and histidine degradation. Against FADDI-AB065, the combination significantly perturbed GPL metabolism, pentose phosphate pathway, citric acid cycle, and pyrimidine ribonucleotide biogenesis. Overall, these novel findings demonstrate that the disruption of the citric acid cycle and inhibition of nucleotide biogenesis are the key metabolic features associated with synergistic bacterial killing by the combination against polymyxin-susceptible and -resistant A. baumannii.

Detection of colistin heteroresistance in Acinetobacter baumannii from blood and respiratory isolates

Polymyxins are one of the last-line antibiotics for multidrug-resistant Acinetobacter baumannii. Reports have demonstrated the emergence of colistin heteroresistance in A. baumannii, which can complicate assessment of minimum inhibitory concentrations and promote resistance to colistin. We aimed to determine the presence of colistin heteroresistance in A. baumannii isolates and correlate the results with clinical and microbiological outcomes via a retrospective study of 24 adult patients: 12 blood and 12 invasive respiratory cultures positive for colistin-susceptible A. baumannii between 1 January 2013 and 31 July 2015. Heteroresistance testing was performed by plating a 100-μL bacterial cell suspension on Mueller-Hinton agar plates containing 0, 1, 2, and 4 μg/mL colistin, and assessing for growth at 24 and 48 h. Colistin heteroresistance was exhibited in 83% of isolates. Median age was 56 [43-65] years, 10 (42%) patients resided at a facility prior to admission, 5 (21%) had a chronic tracheostomy, 18 (75%) were in the intensive care unit at the time of culture collection, and median infection-related length of stay was 12 [7-15] days. Clinical and microbiological cures were achieved in 75% of patients. Overall infection-related mortality was 21%. Our study demonstrated a high rate of colistin heteroresistance in clinical isolates of colistin-susceptible A. baumannii, although this was not associated with suboptimal clinical outcomes due to the use of aggressive colistin dosing and combination therapy. Further studies are needed to establish the association between in vitro colistin heteroresistance and clinical and microbiological outcomes.

Polymyxins: Antibacterial Activity, Susceptibility Testing, and Resistance Mechanisms Encoded by Plasmids or Chromosomes

Polymyxins are well-established antibiotics that have recently regained significant interest as a consequence of the increasing incidence of infections due to multidrug-resistant Gram-negative bacteria. Colistin and polymyxin B are being seriously reconsidered as last-resort antibiotics in many areas where multidrug resistance is observed in clinical medicine. In parallel, the heavy use of polymyxins in veterinary medicine is currently being reconsidered due to increased reports of polymyxin-resistant bacteria. Susceptibility testing is challenging with polymyxins, and currently available techniques are presented here. Genotypic and phenotypic methods that provide relevant information for diagnostic laboratories are presented. This review also presents recent works in relation to recently identified mechanisms of polymyxin resistance, including chromosomally encoded resistance traits as well as the recently identified plasmid-encoded polymyxin resistance determinant MCR-1. Epidemiological features summarizing the current knowledge in that field are presented.

Can Pharmacokinetic and Pharmacodynamic Principles Be Applied to the Treatment of Multidrug-Resistant Acinetobacter?

OBJECTIVE:

To discuss treatment options that can be used for treatment of Acinetobacter infections.

DATA SOURCES:

A MEDLINE search (1966-November 2010) was conducted to identify English-language literature on pharmacotherapy of Acinetobacter and the bibliographies of pertinent articles. Programs and abstracts from infectious diseases meetings were also searched. Search terms included Acinetobacter, multidrug resistance, pharmacokinetics, pharmacodynamics, Monte Carlo simulation, nosocomial pneumonia, carbapenems, polymyxins, sulbactam, aminoglycosides, tetracyclines, tigecycline, rifampin, and fluoroquinolones.

DATA SELECTION AND DATA EXTRACTION:

All articles were critically evaluated and all pertinent information was included in this review.

DATA SYNTHESIS:

Multidrug resistant (MDR) Acinetobacter, defined as resistance to 3 or more antimicrobial classes, has increased over the past decade. The incidence of carbapenem-resistant Acinetobacter is also increasing, leading to an increased use of dose optimization techniques and/or alternative antimicrobials, which is driven by local susceptibility patterns. However, Acinetobacter infections that are resistant to all commercially available antibiotics have been reported. General principles are available to guide dose optimization of aminoglycosides, β-lactams, fluoroquinolones, and tigecycline for infections due to gram-negative pathogens. Unfortunately, data specific to patients with Acinetobacter infections are limited. Recent pharmacokinetic-pharmacodynamic information has shed light on colistin dosing. The dilemma with colistin is its concentration-dependent killing, which makes once-daily dosing seem like an attractive option, but its short postantibiotic effect limits a clinician's ability to extend the dosing interval. Localized delivery of antimicrobials is also an attractive option due to the ability to increase drug concentration at the infection site while minimizing systemic adverse events, but more data are needed regarding this approach.

CONCLUSIONS:

Increased reliance on dosage optimization, combination therapy, and localized delivery of antimicrobials are methods to pursue positive clinical outcomes in MDR Acinetobacter infections since novel antimicrobials will not be available for several years. Well-designed clinical trials with MDR Acinetobacter are needed to define the best treatment options for these patients.

Antibiotic-non-antibiotic combinations for combating extremely drug-resistant Gram-negative 'superbugs'

The emergence of antimicrobial resistance of Gram-negative pathogens has become a worldwide crisis. The status quo for combating resistance is to employ synergistic combinations of antibiotics. Faced with this fast-approaching post-antibiotic era, it is critical that we devise strategies to prolong and maximize the clinical efficacy of existing antibiotics. Unfortunately, reports of extremely drug-resistant (XDR) Gram-negative pathogens have become more common. Combining antibiotics such as polymyxin B or the broad-spectrum tetracycline and minocycline with various FDA-approved non-antibiotic drugs have emerged as a novel combination strategy against otherwise untreatable XDR pathogens. This review surveys the available literature on the potential benefits of employing antibiotic-non-antibiotic drug combination therapy. The apex of this review highlights the clinical utility of this novel therapeutic strategy for combating infections caused by 'superbugs'.

Pharmacokinetics/pharmacodynamics of colistin and polymyxin B: are we there yet?

The polymyxin antibiotics [colistin and polymyxin B (PMB)] are increasingly used as a last-line option for the treatment of infections caused by extensively drug-resistant Gram-negative bacteria. Despite having similar structures and antibacterial activity in vitro, the two clinically available polymyxins have very different pharmacological properties, as colistin (polymyxin E) is intravenously administered to patients in the form of an inactive prodrug colistin methanesulphonate (sodium). This review will discuss recent progress in the pharmacokinetics/pharmacodynamics and toxicity of colistin and PMB, the factors that affect their pharmacological profiles, and the challenges for the effective use of both polymyxins. Strategies are proposed for optimising their clinical utility based upon the recent pharmacological studies in vitro, in animals and patients. In the 'Bad Bugs, No Drugs' era, polymyxins are a critically important component of the antibiotic armamentarium against difficult-to-treat Gram-negative 'superbugs'. Rational approaches to the use of polymyxins must be pursued to increase their effectiveness and to minimise resistance and toxicity.

Transcriptomic Analysis of the Activity of a Novel Polymyxin against Staphylococcus aureus

S. aureus is currently one of the most pervasive multidrug-resistant pathogens and commonly causes nosocomial infections. Clinicians are faced with a dwindling armamentarium to treat infections caused by S. aureus, as resistance develops to current antibiotics. This accentuates the urgent need for antimicrobial drug discovery. In the present study, we characterized the global gene expression profile of S. aureus treated with FADDI-019, a novel synthetic polymyxin analogue. In contrast to the concentration-dependent killing and rapid regrowth in Gram-negative bacteria treated with polymyxin B and colistin, FADDI-019 killed S. aureus progressively without regrowth at 24 h. Notably, FADDI-019 activated several vancomycin resistance genes and significantly downregulated the expression of a number of virulence determinants and enterotoxin genes. A synergistic combination with sulfamethoxazole was predicted by pathway analysis and demonstrated experimentally. This is the first study revealing the transcriptomics of S. aureus treated with a novel synthetic polymyxin analog.

Polymyxin B and colistin are exclusively active against Gram-negative pathogens and have been used in the clinic as a last-line therapy. In this study, we investigated the antimicrobial activity of a novel polymyxin, FADDI-019, against Staphylococcus aureus. MIC and time-kill assays were employed to measure the activity of FADDI-019 against S. aureus ATCC 700699. Cell morphology was examined with scanning electron microscopy (SEM), and cell membrane polarity was measured using flow cytometry. Transcriptome changes caused by FADDI-019 treatment were investigated using transcriptome sequencing (RNA-Seq). Pathway analysis was conducted to examine the mechanism of the antibacterial activity of FADDI-019 and to rationally design a synergistic combination. Polymyxin B and colistin were not active against S. aureus strains with MICs of >128 mg/liter; however, FADDI-019 had a MIC of 16 mg/liter. Time-kill assays revealed that no S. aureus regrowth was observed after 24 h at 2× to 4× MIC of FADDI-019. Scanning electron microscopy (SEM) and flow cytometry results indicated that FADDI-019 treatment had no effect on cell morphology but caused membrane depolarization. The vancomycin resistance genes vraRS, as well as the VraRS regulon, were activated by FADDI-019. Virulence determinants controlled by SaeRS and the expression of enterotoxin genes yent2, sei, sem, and seo were significantly downregulated by FADDI-019. Pathway analysis of transcriptomic data was predictive of a synergistic combination comprising FADDI-019 and sulfamethoxazole. Our study is the first to examine the mechanism of the killing of a novel polymyxin against S. aureus. We also show the potential of transcriptomic and pathway analysis as tools to design synergistic antibiotic combinations.

IMPORTANCES. aureus is currently one of the most pervasive multidrug-resistant pathogens and commonly causes nosocomial infections. Clinicians are faced with a dwindling armamentarium to treat infections caused by S. aureus, as resistance develops to current antibiotics. This accentuates the urgent need for antimicrobial drug discovery. In the present study, we characterized the global gene expression profile of S. aureus treated with FADDI-019, a novel synthetic polymyxin analogue. In contrast to the concentration-dependent killing and rapid regrowth in Gram-negative bacteria treated with polymyxin B and colistin, FADDI-019 killed S. aureus progressively without regrowth at 24 h. Notably, FADDI-019 activated several vancomycin resistance genes and significantly downregulated the expression of a number of virulence determinants and enterotoxin genes. A synergistic combination with sulfamethoxazole was predicted by pathway analysis and demonstrated experimentally. This is the first study revealing the transcriptomics of S. aureus treated with a novel synthetic polymyxin analog.

Colistin Use in Patients With Reduced Kidney Function

Colistin (polymyxin E) is a mainly concentration-dependent bactericidal antimicrobial active against multidrug-resistant Gram-negative bacteria. After being abandoned over the past 30 years due to its neuro- and nephrotoxicity, colistin has been reintroduced recently as a last-resort drug for the treatment of multidrug-resistant Gram-negative bacteria infections in combination with other antimicrobials. Unfortunately, although renal toxicity is a well-known dose-related adverse effect of colistin, relatively few studies are currently available on its peculiar pharmacodynamic/pharmacokinetic properties in clinical settings at high risk for drug accumulation, such as acute or chronic kidney disease. In these specific contexts, the risk for underdosing is also substantial because colistin can be easily removed by dialysis/hemofiltration, especially when the most efficient modalities of renal replacement therapy (RRT) are used in critically ill patients. For this reason, recent recommendations in patients undergoing RRT have shifted toward higher dosing regimens, and therapeutic drug monitoring is advised. This review aims to summarize the main issues related to chemical structure, pharmacodynamics/pharmacokinetics, and renal toxicity of colistin. Moreover, recent data and current recommendations concerning colistin dosing in patients with reduced kidney function, with special regard to those receiving RRT such as dialysis or hemofiltration, are also discussed.

Anthelmintic closantel enhances bacterial killing of polymyxin B against multidrug-resistant Acinetobacter baumannii

Polymyxins, an old class of antibiotics, are currently used as the last resort for the treatment of multidrug-resistant (MDR) Acinetobacter baumannii. However, recent pharmacokinetic and pharmacodynamic data indicate that monotherapy can lead to the development of resistance. Novel approaches are urgently needed to preserve and improve the efficacy of this last-line class of antibiotics. This study examined the antimicrobial activity of novel combination of polymyxin B with anthelmintic closantel against A. baumannii. Closantel monotherapy (16 mg l(-1)) was ineffective against most tested A. baumannii isolates. However, closantel at 4-16 mg l(-1) with a clinically achievable concentration of polymyxin B (2 mg l(-1)) successfully inhibited the development of polymyxin resistance in polymyxin-susceptible isolates, and provided synergistic killing against polymyxin-resistant isolates (MIC ⩾4 mg l(-1)). Our findings suggest that the combination of polymyxin B with closantel could be potentially useful for the treatment of MDR, including polymyxin-resistant, A. baumannii infections. The repositioning of non-antibiotic drugs to treat bacterial infections may significantly expedite discovery of new treatment options for bacterial 'superbugs'.

Optimizing Polymyxin Combinations Against Resistant Gram-Negative Bacteria

Polymyxin combination therapy is increasingly used clinically. However, systematic investigations of such combinations are a relatively recent phenomenon. The emerging pharmacodynamic (PD) and pharmacokinetic (PK) data on CMS/colistin and polymyxin B suggest that caution is required with monotherapy. Given this situation, polymyxin combination therapy has been suggested as a possible way to increase bacterial killing and reduce the development of resistance. Considerable in vitro data have been generated in support of this view, particularly recent studies utilizing dynamic models. However, most existing animal data are of poor quality with major shortcomings in study design, while clinical data are generally limited to retrospective analysis and small, low-power, prospective studies. This article provides an overview of clinical and preclinical investigations of CMS/colistin and polymyxin B combination therapy.

Influence of Colistin Dose on Global Cure in Patients with Bacteremia Due to Carbapenem-Resistant Gram-Negative Bacilli

The increasing prevalence of multidrug-resistant (MDR) nosocomial infections accounts for increased morbidity and mortality of such infections. Infections with MDR Gram-negative isolates are frequently treated with colistin. Based on recent pharmacokinetic studies, current colistin dosing regimens may result in a prolonged time to therapeutic concentrations, leading to suboptimal and delayed effective treatment. In addition, studies have demonstrated an association between an increased colistin dose and improved clinical outcomes. However, the specific dose at which these outcomes are observed is unknown and warrants further investigation. This retrospective study utilized classification and regression tree (CART) analysis to determine the dose of colistin most predictive of global cure at day 7 of therapy. Patients were assigned to high- and low-dose cohorts based on the CART-established breakpoint. The secondary outcomes included microbiologic outcomes, clinical cure, global cure, lengths of intensive care unit (ICU) and hospital stays, and 7- and 28-day mortalities. Additionally, safety outcomes focused on the incidence of nephrotoxicity associated with high-dose colistin therapy. The CART-established breakpoint for high-dose colistin was determined to be >4.4 mg/kg of body weight/day, based on ideal body weight. This study evaluated 127 patients; 45 (35%) received high-dose colistin, and 82 (65%) received low-dose colistin. High-dose colistin was associated with day 7 global cure (40% versus 19.5%; P = 0.013) in bivariate and multivariate analyses (odds ratio [OR] = 3.40; 95% confidence interval [CI], 1.37 to 8.45; P = 0.008). High-dose colistin therapy was also associated with day 7 clinical cure, microbiologic success, and mortality but not with the development of acute kidney injury. We concluded that high-dose colistin (>4.4 mg/kg/day) is independently associated with day 7 global cure.

Polymyxin B versus colistin: an update

Polymyxin B and colistin (polymyxin E) are polypeptide antibiotics that were developed in the 1940s, but fell into disfavor due to their high toxicity rates. These two antibiotics were previously regarded to be largely equivalent, due to similarities in their chemical structure and spectrum of activity. In recent years, several pertinent differences, especially in terms of potency and disposition, have been revealed between polymyxin B and colistin. These differences are mainly attributed to the fact that polymyxin B is administered parenterally in its active form, while colistin is administered parenterally as an inactive pro-drug, colistimethate. In this review, we summarize the similarities and differences between polymyxin B and colistin. We also discuss the potential clinical implications of these findings, and provide our perspectives on how polymyxins should be employed to preserve their utility in this era of multi-drug resistance.

Evaluation of antibiotic efficacy against infections caused by planktonic or biofilm cultures of Pseudomonas aeruginosa and Klebsiella pneumoniae in Galleria mellonella

The lack of novel antibiotics for more than a decade has placed increased pressure on existing therapies to combat the emergence of multidrug-resistant (MDR) bacterial pathogens. This study evaluated the Galleria mellonella insect model in determining the efficacy of available antibiotics against planktonic and biofilm infections of MDR Pseudomonas aeruginosa and Klebsiella pneumoniae strains in comparison with in vitro minimum inhibitory concentration (MIC) determination. In general, in vitro analysis agreed with the G. mellonella studies, and susceptibility in Galleria identified different drug resistance mechanisms. However, the carbapenems tested appeared to perform better in vivo than in vitro, with meropenem and imipenem able to clear K. pneumoniae and P. aeruginosa infections with strains that had bla(NDM-1) and bla(VIM) carbapenemases. This study also established an implant model in G. mellonella to allow testing of antibiotic efficacy against biofilm-derived infections. A reduction in antibiotic efficacy of amikacin against K. pneumoniae and P. aeruginosa biofilms was observed compared with a planktonic infection. Ciprofloxacin was found to be less effective at clearing a P. aeruginosa biofilm infection compared with a planktonic infection, but no statistical difference was seen between K. pneumoniae biofilm and planktonic infections treated with this antibiotic (P>0.05). This study provides important information regarding the suitability of Galleria as a model for antibiotic efficacy testing both against planktonic and biofilm-derived MDR infections.

New pharmacokinetic/pharmacodynamic studies of systemically administered colistin against Pseudomonas aeruginosa and Acinetobacter baumannii in mouse thigh and lung infection models: smaller response in lung infection

OBJECTIVES

This study investigated the exposure-response relationships between unbound colistin in plasma and antibacterial activity in mouse thigh and lung infections.

METHODS

Dose fractionation studies (subcutaneous colistin sulphate at 1.25-160 mg/kg/day) were conducted in neutropenic mice in which infection (three strains of Pseudomonas aeruginosa and three strains of Acinetobacter baumannii) had been produced by intramuscular thigh injection or aerosol lung delivery. Bacterial burden was measured at 24 h after initiation of colistin treatment. Plasma protein binding was measured by rapid equilibrium dialysis and ultracentrifugation. The inhibitory sigmoid dose-effect model and non-linear least squares regression were employed to determine the relationship between exposure to unbound colistin and efficacy.

RESULTS

Plasma binding of colistin was constant over the concentration range ∼2-50 mg/L. The average ± SD percentage bound for all concentrations was 92.9 ± 3.3% by ultracentrifugation and 90.4 ± 1.1% by equilibrium dialysis. In the thigh model, across all six strains the antibacterial effect of colistin was well correlated with fAUC/MIC (R(2) = 0.82-0.94 for P. aeruginosa and R(2) = 0.84-0.95 for A. baumannii). Target values of fAUC/MIC for 2 log10 kill were 7.4-13.7 for P. aeruginosa and 7.4-17.6 for A. baumannii. In the lung model, for only two strains of P. aeruginosa and one strain of A. baumannii was it possible to achieve 2 log10 kill (fAUC/MIC target values 36.8-105), even at the highest colistin dose tolerated by mice. This dose was not able to achieve bacteriostasis for the other two strains of A. baumannii.

CONCLUSIONS

Colistin was substantially less effective in lung infection. The pharmacokinetic/pharmacodynamic target values will assist in the design of optimized dosage regimens.