Efficacy of sulbactam and its combination with imipenem, colistin and tigecycline in an experimental model of carbapenem-resistant Acinetobacter baumannii sepsis

Colistin potentiation in multidrug-resistant Acinetobacter baumannii by a non-cytotoxic guanidine derivative of silver

A novel nano-formulation (NF) that sensitizes Acinetobacter baumannii (AB) to otherwise ineffective colistin is described in the present study. Infections due to multidrug resistant (MDR) AB represent a major therapeutic challenge, especially in situations of pre-existing colistin resistance (colR). Subsequently, boosting the effectiveness of colistin would be a better alternative tactic to treat AB infections rather than discovering a new class of antibiotics. We have previously demonstrated an NF comprising self-assembled guanidinium and ionic silver nanoparticles [AD-L@Ag(0)] to have anti-biofilm and bactericidal activity. We report NF AD-L@Ag(0) for the very first time for the potentiation of colistin in Gram-negative colistin-resistant bacteria. Our results implied that a combination of clinically relevant concentrations of colistin and AD-L@Ag(0) significantly decreased colistin-resistant AB bacterial growth and viability, which otherwise was elevated in the presence of only colistin. In this study, we have described various combinations of minimum inhibitory concentration (MIC) of colistin (MICcol, ^1/2 MICcol, and ^1/4 MICcol) and that of AD-L@Ag(0) [MICAD-L@Ag(0), ^1/2 MICAD-L@Ag(0), and ^1/4 MICAD-L@Ag(0)] and tested them against MDR AB culture. The results (in broth as well as in solid media) signified that AD-L@Ag(0) was able to potentiate the anti-microbial activity of colistin at sub-MIC concentrations. Furthermore, the viability and metabolic activity of bacterial cells were also measured by CTC fluorescence assay and ATP bioluminescence assay. The results of these assays were in perfect concordance with the scores of cultures (colony forming unit and culture turbidity). In addition, quantitative real-time PCR (qRT-PCR) was performed to unveil the expression of selected genes, DNAgyrA, DNAgyrB, and dac. These genes introduce negative supercoiling in the DNA, and hence are important for basic cellular processes. These genes, due to mutation, modified the Lipid A of bacteria, further resisting the uptake of colistin. Therefore, the expression of these genes was upregulated when AB was treated with only colistin, substantiating that AB is resistant to colistin, whereas the combinations of MICcol + MICAD-L@Ag(0) downregulated the expression of these genes, implying that the developed formulation can potentiate the efficiency of colistin. In conclusion, AD-L@Ag(0) can potentiate the proficiency of colistin, further enhancing colistin-mediated death of AB by putatively disrupting the outer membrane (OM) and facilitating bacterial death.

Activity of Omadacycline Alone and in Combination against Carbapenem-Nonsusceptible Acinetobacter baumannii with Varying Minocycline Susceptibility

Tetracycline-based combinations are increasingly used for serious carbapenem-nonsusceptible Acinetobacter baumannii (CNSAb) infections given their potent in vitro activity, synergism with other agents, and acceptable toxicity profile. Omadacycline is a novel aminomethylcycline with activity against minocycline-resistant pathogens, once daily oral dosing, and favorable pharmacokinetic properties. Given these potential advantages, the in vitro potency and antibacterial activity of omadacycline were evaluated alone and in combination against CNSAb with varying minocycline susceptibility. Broth microdilution testing of 41 CNSAb revealed that omadacycline (MIC50/90: 4/8 mg/L) inhibited 68.3% (28/41) of isolates at ≤4 mg/L and its activity was unaffected by minocycline nonsusceptibility (MIC50/90: 4/8 mg/L; 74.2% [23/31] inhibited at ≤4 mg/L). Ten (5 minocycline susceptible and 5 nonsusceptible) of the 41 CNSAb isolates were then evaluated in time-kill analyses against omadacycline and comparator agents alone and in dual- and triple-drug combinations at the free maximum concentration of drug in serum (fCmax). Amikacin, meropenem, and polymyxin B alone were each bactericidal against 4 of 10 (40%) isolates while omadacycline and sulbactam were bactericidal against 0 (0%) and 1 (10%), respectively. In dual-drug combinations with omadacycline, synergy was observed against 80% of isolates with sulbactam followed by 30% with amikacin or polymyxin B and 0% with meropenem or rifampin. The triple-drug combination of omadacycline, sulbactam, and polymyxin B achieved synergy against just one additional strain over the omadacycline-sulbactam dual combination but significantly reduced the time to 99.9% kill by more than 6 h (4.6 ± 2.8 h vs. 11.3 ± 5.9 h, P < 0.01). These results support the continued investigation into tetracycline-based combinations against CNSAb, particularly those including sulbactam, and suggest that omadacycline may have in vitro advantages over existing tetracycline-derivatives. IMPORTANCE Treatment of infections due to Acinetobacter baumannii often involves the use of multiple antibiotics simultaneously as combination therapy, but it is unknown which antibiotics are best used together. Tetracycline agents such as minocycline and tigecycline maintain good activity against A. baumannii and are often used with one or more other agents to achieve better killing of the bacteria. Omadacycline is a new tetracycline that may have a role in the treatment of A. baumannii, but no data are available evaluating its interaction with other commonly used drugs such as polymyxin B and sulbactam. Therefore, the purpose of this study was to investigate the antibacterial activity of omadacycline when combined with one or more other agents against carbapenem-resistant strains of A. baumannii. These findings may then be used to design confirmatory studies that could help decide what drugs work best together and what combination of agents should be used for patients.

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.

Losing the Battle but Winning the War: Can Defeated Antibacterials Form Alliances to Combat Drug-Resistant Pathogens?

Despite the recent development of antibacterials that are active against multidrug-resistant pathogens, drug combinations are often necessary to optimize the killing of difficult-to-treat organisms. Antimicrobial combinations typically are composed of multiple agents that are active against the target organism; however, many studies have investigated the potential utility of combinations that consist of one or more antibacterials that individually are incapable of killing the relevant pathogen. The current review summarizes in vitro, in vivo, and clinical studies that evaluate combinations that include at least one drug that is not active individually against Pseudomonas aeruginosa, Klebsiella pneumoniae, Acinetobacter baumannii, or Staphylococcus aureus. Polymyxins were often included in combinations against all three of the Gram-negative pathogens, and carbapenems were commonly incorporated into combinations against K. pneumoniae and A. baumannii. Minocycline, sulbactam, and rifampin were also frequently investigated in combinations against A. baumannii, whereas the addition of ceftaroline or another β-lactam to vancomycin or daptomycin showed promise against S. aureus with reduced susceptibility to vancomycin or daptomycin. Although additional clinical studies are needed to define the optimal combination against specific drug-resistant pathogens, the large amount of in vitro and in vivo studies available in the literature may provide some guidance on the rational design of antibacterial combinations.

Sepsis and Septic Shock; Current Treatment Dilemma and Role of Stem Cell Therapy in Pediatrics

Context: Sepsis’s primary therapy consists of antibiotics therapy, supportive therapies, and source control of infection. The failure rate of this approach is about 20 - 40%. The widespread use of antibiotics has caused multiple drug resistance in primary etiological agents of sepsis in community-acquired and healthcare-associated infections. In the absence of new antibiotic options, alternative treatment modalities seem necessary. Evidence Acquisition: Herein, we have reviewed and discussed current problems with sepsis management and stem cell therapy in sepsis, preclinical, experimental studies, and early-phase clinical trials using stem cells to treat sepsis. In the preparation of the paper, PubMed, Web of Science Core Collection (Clarivate), Scopus, and the web address (www.clinicaltrials.gov) were searched by the keywords (sepsis and cell therapy, septic shock, and cell therapy). Results: After the inclusion of criteria, we reviewed 301 original articles. Few articles were found for phase II and phase III clinical trials. Eighty-three articles were included in the current review article. Besides problems with infection source control, the host immune response to the infection enumerated for primary underlying pathophysiologic dysregulation of sepsis and complicated the treatment. Mesenchymal stem cells (MSCs) therapy offers a promising treatment option for sepsis. Indeed, immunomodulatory properties, antimicrobial activity, the capacity of protection against organ failure, enhance the resolution of tissue injury, tissue repair, and restoration after sepsis confer MSCs with a significant advantage to treat the immune and inflammatory dysfunctions associated with severe sepsis and septic shock. Conclusions: It seems that MSCs therapy exhibits an appropriate safety index. Future trials should focus on strengthening study quality, reporting MSCs’ therapeutic effects and adverse events. Although early clinical trials seem promising and have beneficial effects, we need more controlled clinical studies, especially in phases II and III.

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

AIMS

Polymyxin-based combination therapy is often used to treat carbapenem-resistant Acinetobacter baumannii (A. baumannii) infections. Although sulbactam is intrinsically active against A. baumannii, few studies have investigated colistin/sulbactam combinations against carbapenem-resistant A. baumannii.

METHODS

Whole genome sequencing was undertaken on eight carbapenem-resistant (colistin-susceptible) isolates of A. baumannii from Chinese patients. Bacterial killing of colistin and sulbactam, alone and in combination, was examined with checkerboard (all isolates) and static and dynamic time-kill studies (three isolates). In the dynamic studies, antibiotics were administered in various clinically-relevant dosing regimens that mimicked patient pharmacokinetics.

RESULTS

The eight isolates consisted of ST195, ST191 and ST208 belonging to clonal complex 208, which is the most epidemic clonal type of A. baumannii globally. All isolates possessed Acinetobacter-derived cephalosporinase (ADC-61 or ADC-78) and seven of eight isolates contained the carbapenem-resistance gene bla_OXA-23. The colistin/sulbactam combination was synergistic against two of eight isolates in checkerboard studies. In time-kill studies, rapid bacterial killing of ca. 3-6 log₁₀ CFU/mL was observed with colistin monotherapy, followed by steady regrowth. Sulbactam monotherapy was generally ineffective. Substantially enhanced bacterial killing was observed with colistin/sulbactam combinations in both static and dynamic models, especially with the higher sulbactam concentration (2 g) and/or longer sulbactam infusion time (2 hours) in the dynamic model.

CONCLUSIONS

This study was the first to use a pharmacokinetics/pharmacodynamics model to investigate synergistic activity of colistin/sulbactam combinations against A. baumannii. It showed that clinically-relevant dosing regimens of colistin combined with sulbactam may substantially improve bacterial killing of multidrug-resistant and carbapenem-resistant A. baumannii.

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.

Antibiotic resistance of pathogenic Acinetobacter species and emerging combination therapy

The increasing antibiotic resistance of Acinetobacter species in both natural and hospital environments has become a serious problem worldwide in recent decades. Because of both intrinsic and acquired antimicrobial resistance (AMR) against last-resort antibiotics such as carbapenems, novel therapeutics are urgently required to treat Acinetobacter-associated infectious diseases. Among the many pathogenic Acinetobacter species, A. baumannii has been reported to be resistant to all classes of antibiotics and contains many AMR genes, such as bla _ADC (Acinetobacter-derived cephalosporinase). The AMR of pathogenic Acinetobacter species is the result of several different mechanisms, including active efflux pumps, mutations in antibiotic targets, antibiotic modification, and low antibiotic membrane permeability. To overcome the limitations of existing drugs, combination theraphy that can increase the activity of antibiotics should be considered in the treatment of Acinetobacter infections. Understanding the molecular mechanisms behind Acinetobacter AMR resistance will provide vital information for drug development and therapeutic strategies using combination treatment. Here, we summarize the classic mechanisms of Acinetobacter AMR, along with newly-discovered genetic AMR factors and currently available antimicrobial adjuvants that can enhance drug efficacy in the treatment of A. baumannii infections.

Epidemiology of multidrug-resistant Acinetobacter baumannii strains in Iran: a systematic review and meta-analysis

Acinetobacter baumannii is an important opportunistic pathogen that causes major public health concern especially in hospitalized patients due to the acquisition of multidrug resistance (MDR). The aim of this study was to systematically review published data about the prevalence rate of MDR-A. baumannii (MDR-AB) from different parts of Iran and provide an overall relative frequency (RF) using meta-analysis. All available national and international databanks were searched to find published studies up to June 2016. Quality of studies was assessed by STROB and PRISMA forms. Because of the significant heterogeneity observed, random effects model was used to combine the results. STATA SE version 11.2 was used for statistical analysis. Out of the 9646 results, 37 suitable articles were extracted according to inclusion and exlusion criteria. The pooled prevalence of MDR-AB was estimated 72% annually. Relative frequency of MDR-AB in different studies varied from 22.8 to 100%. Since the prevalence of MDR-AB is higher than many other countries, measures should be taken to keep the emergence and transmission of these strains to a minimum.

Synergistic effects of sulbactam in multi-drug-resistant Acinetobacter baumannii

Acinetobacter baumannii is a frequently isolated etiologic agent of nosocomial infections, especially in intensive care units. With the increase in multi-drug resistance of A. baumannii isolates, finding appropriate treatment alternatives for infections caused by these bacteria has become more difficult, and available alternate treatments include the use of older antibiotics such as colistin or a combination of antibiotics. The current study aimed to evaluate the in vitro efficacy of various antibiotic combinations against multi-drug resistant A. baumannii strains. Thirty multi-drug and carbapenem resistant A. baumannii strains isolated at the Ankara Training and Research Hospital between June 2011 and June 2012 were used in the study. Antibiotic susceptibility tests and species-level identification were performed using conventional methods and the VITEK 2 system. The effects of meropenem, ciprofloxacin, amikacin, tigecycline, and colistin alone and in combination with sulbactam against the isolates were studied using Etest (bioMérieux) in Mueller-Hinton agar medium. Fractional inhibitory concentration index (FIC) was used to determine the efficacy of the various combinations. While all combinations showed a predominant indifferent effect, a synergistic effect was also observed in 4 of the 5 combinations. Synergy was demonstrated in 43% of the isolates with the meropenem-sulbactam combination, in 27% of the isolates with tigecycline-sulbactam, and in 17% of the isolates with colistin-sulbactam and amikacin-sulbactam. No synergy was detected with the sulbactam-ciprofloxacin combination and antagonism was detected only in the sulbactam-colistin combination (6.66% of the isolates). Antibiotic combinations can be used as an alternative treatment approach in multi-drug resistant A. baumannii infections.

Activity of Colistin in Combination with Meropenem, Tigecycline, Fosfomycin, Fusidic Acid, Rifampin or Sulbactam against Extensively Drug-Resistant Acinetobacter baumannii in a Murine Thigh-Infection Model

Few effective therapeutic options are available for treating severe infections caused by extensively drug-resistant Acinetobacter baumannii (XDR-AB). Using a murine thigh-infection model, we examined the in vivo efficacy of colistin in combination with meropenem, tigecycline, fosfomycin, fusidic acid, rifampin, or sulbactam against 12 XDR-AB strains. Colistin, tigecycline, rifampin, and sulbactam monotherapy significantly decreased bacterial counts in murine thigh infections compared with those observed in control mice receiving no treatment. Colistin was the most effective agent tested, displaying bactericidal activity against 91.7% of strains at 48 h post-treatment. With strains showing a relatively low minimum inhibitory concentration (MIC) for meropenem (MIC ≤ 32 mg/L), combination therapy with colistin plus meropenem caused synergistic inhibition at both 24 h and 48 h post-treatment. However, when the meropenem MIC was ≥64 mg/L, meropenem did not significantly alter the efficacy of colistin. The addition of rifampin and fusidic acid significantly improved the efficacy of colistin, showing a synergistic effect in 100% and 58.3% of strains after 24 h of treatment, respectively, while the addition of tigecycline, fosfomycin, or sulbactam did not show obvious synergistic activity. No clear differences in activities were observed between colistin-rifampin and colistin-fusidic acid combination therapy with most strains. Overall, our in vivo study showed that administering colistin in combination with rifampin or fusidic acid is more efficacious in treating XDR-AB infections than other combinations. The colistin-meropenem combination may be another appropriate option if the MIC is ≤32 mg/L. Further clinical studies are urgently needed to confirm the relevance of these findings.

In vitro synergy of colistin combinations against extensively drug-resistant Acinetobacter baumannii producing OXA-23 carbapenemase

Fifty extensively drug-resistant Acinetobacter baumannii (XDRAB) were isolated from patients. The chequerboard microdilution method was used to determine the in vitro activities of five colistin (COL)-based combinations including COL+fosfomycin (FOS), COL+rifampicin (RIF), COL+imipenem (IMP), COL+sulbactam (SUP) and COL+levofloxacin (LVX). The synergistic activity was evaluated by the fractional inhibitory concentration index (FICI). According to our results, the combination of COL was synergistic with FOS, RIF, IMP, SUP and LVX with the ratios of 50, 72, 88, 92 and 64%, respectively. When combined with COL, the other five agents showed increased antimicrobial activities. In addition, two of the combinations, COL+RIF and COL+IMP, were more active than the combinations of COL+FOS, COL+SUP and COL+LVX. More importantly, these combination regimens could exert synergistic effects at the sub-minimum inhibitory concentration (MIC) levels against XDRAB strains.

Antimicrobial Resistance of Acinetobacter baumannii to Imipenem in Iran: A Systematic Review and Meta-Analysis

Imipenem-resistant multi-drug resistant (IR-MDR) Acinetobacter baumannii has been emerged as a morbidity successful nosocomial pathogen throughout the world.To address imipenem being yet the most effective antimicrobial agent against A. baumannii to control outbreaks and treat patients, a systematic review and meta-analysis was performed to evaluate the prevalence of IR-MDR A. baumannii. We systematically searched Web of Science, PubMed, MEDLINE, Science Direct, EMBASE, Scopus, Cochrane Library, Google Scholar, and Iranian databases to identify studies addressing the antibiotic resistance of A. baumannii to imipenem and the frequency of MDR strains in Iran. Out of 58 articles and after a secondary screening using inclusion and exclusion criteria and on the basis of title and abstract evaluation, 51 studies were selected for analysis. The meta-analysis revealed that 55% [95% confidence interval (CI), 53.0–56.5] of A. baumannii were resistant to imipenem and 74% (95% CI, 61.3–83.9) were MDR. The MDR A. baumannii population in Iran is rapidly changing toward a growing resistance to imipenem. Our findings highlight the critical need for a comprehensive monitoring and infection control policy as well as a national susceptibility review program that evaluates IR-MDR A. baumannii isolates from various parts of Iran.

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.

In vitro and in vivo analysis of antimicrobial agents alone and in combination against multi-drug resistant Acinetobacter baumannii

Objective: To investigate the in vitro and in vivo antibacterial activities of tigecycline and other 13 common antimicrobial agents, alone or in combination, against multi-drug resistant Acinetobacter baumannii.

Methods: An in vitro susceptibility test of 101 A. baumannii was used to detect minimal inhibitory concentrations (MICs). A mouse lung infection model of multi-drug resistant A. baumannii, established by the ultrasonic atomization method, was used to define in vivo antimicrobial activities.

Results: Multi-drug resistant A. baumannii showed high sensitivity to tigecycline (98% inhibition), polymyxin B (78.2% inhibition), and minocycline (74.2% inhibition). However, the use of these antimicrobial agents in combination with other antimicrobial agents produced synergistic or additive effects. In vivo data showed that white blood cell (WBC) counts in drug combination groups C (minocycline + amikacin) and D (minocycline + rifampicin) were significantly higher than in groups A (tigecycline) and B (polymyxin B) (P < 0.05), after administration of the drugs 24 h post-infection. Lung tissue inflammation gradually increased in the model group during the first 24 h after ultrasonic atomization infection; vasodilation, congestion with hemorrhage were observed 48 h post infection. After 3 days of anti-infective therapy in groups A, B, C, and D, lung tissue inflammation in each group gradually recovered with clear structures. The mortality rates in drug combination groups(groups C and D) were much lower than in groups A and B.

Conclusion: The combination of minocycline with either rifampicin or amikacin is more effective against multi-drug resistant A. baumannii than single-agent tigecycline or polymyxin B. In addition, the mouse lung infection by ultrasonic atomization is a suitable model for drug screening and analysis of infection mechanism.

Colistin-resistant Acinetobacter baumannii: beyond carbapenem resistance

BACKGROUND

With an increase in the use of colistin methansulfonate (CMS) to treat carbapenem-resistant Acinetobacter baumannii infections, colistin resistance is emerging.

METHODS

Patients with infection or colonization due to colistin-resistant A. baumannii were identified at a hospital system in Pennsylvania. Clinical data were collected from electronic medical records. Susceptibility testing, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing (MLST) were performed. To investigate the mechanism of colistin resistance, lipid A was subjected to matrix-assisted laser desorption/ionization mass spectrometry.

RESULTS

Twenty patients with colistin-resistant A. baumannii were identified. Ventilator-associated pneumonia was the most common type of infection. Nineteen patients had received intravenous and/or inhaled CMS for treatment of carbapenem-resistant, colistin-susceptible A. baumannii infection prior to identification of colistin-resistant isolates. The 30-day all-cause mortality rate was 30%. The treatment regimen for colistin-resistant A. baumannii infection associated with the lowest mortality rate was a combination of CMS, a carbapenem, and ampicillin-sulbactam. The colistin-susceptible and -resistant isolates from the same patients were highly related by PFGE, but isolates from different patients were not, suggesting evolution of resistance during CMS therapy. By MLST, all isolates belonged to the international clone II, the lineage that is epidemic worldwide. Phosphoethanolamine modification of lipid A was present in all colistin-resistant A. baumannii isolates.

CONCLUSIONS

Colistin-resistant A. baumannii occurred almost exclusively among patients who had received CMS for treatment of carbapenem-resistant, colistin-susceptible A. baumannii infection. Lipid A modification by the addition of phosphoethanolamine accounted for colistin resistance. Susceptibility testing for colistin should be considered for A. baumannii identified from CMS-experienced patients.