Re-Evaluating Cross-Contamination: Additional Trials on Co-Ventilation.

BACKGROUND: Medical equipment can become scarce in disaster scenarios. Prior work has reported that four sheep could be ventilated together on a single ventilator. Others found that this maneuver is possible when needed, but no one has yet investigated whether cross-contamination occurs in co-ventilated individuals. OBJECTIVE: Our goal was to investigate whether an infection could spread between co-ventilated individuals. METHODS: Four 2-L anesthesia bags were connected to a sterilized ventilator circuit that used heat and moisture exchange filters and bacterial and viral filters, as would be expected in this dire scenario. Serratia marcescens was inoculated into "lung" no. 1. After running for 24 h, each lung and three additional points in the circuit were cultured to see whether S. marcescens had spread. These cultures were examined at 24 and 48 h to assess for cross-contamination. This entire procedure was performed three times. RESULTS: S. marcescens was not found in lung no. 2, 3, or 4 or the three additional sites on the expiratory limb at 24 and 48 h in all three trials. CONCLUSIONS: Cross-contamination does not occur within 24 h using the described ventilator circuit configuration.

Carbapenem-Resistant Klebsiella pneumoniae in Large Public Acute-Care Healthcare System, New York, New York, USA, 2016-2022.

Controlling the spread of carbapenem-resistant Enterobacterales is a global priority. Using National Healthcare Safety Network data, we characterized the changing epidemiology of carbapenem-resistant Klebsiella pneumoniae (CRKP) in a large public health system in New York, New York, USA. During 2016-2020, CRKP cases declined; however, during 2021-June 2022, a notable increase occurred. Of 509 cases, 262 (51%) were considered community-onset, including 149 in patients who were living at home. Of 182 isolates with proven or presumptive (ceftazidime/avibactam susceptible) enzymes, 143 were serine carbapenemases; most confirmed cases were K. pneumoniae carbapenemase. The remaining 39 cases were proven or presumptive metallo-ß-lactamases; all confirmed cases were New Delhi metallo-ß-lactamases. After 2020, a marked increase occurred in the percentage of isolates possessing metallo-ß-lactamases. Most patients with metallo-ß-lactamases originated from long-term care facilities. An aggressive and universal program involving surveillance and isolation will be needed to control the spread of CRKP in the city of New York.

Contribution of Iron-Transport Systems and ß-Lactamases to Cefiderocol Resistance in Clinical Isolates of Acinetobacter baumannii Endemic to New York City.

The development of resistance to cefiderocol among multidrug resistant Acinetobacter baumannii has been attributed to downregulation in iron transport systems and a variety of ß-lactamases. However, the precise contribution of each in clinical isolates remains to be determined. Sixteen clinical isolates with varying degrees of cefiderocol resistance were investigated. Susceptibility testing was performed with and without the presence of iron and avibactam. Expression of 10 iron transport systems and blaADC and blaOXA-51-type were analyzed by real time RT-PCR. The acquisition of a variety of ß-lactamases was also determined. In 2 isolates the impact of silencing the blaADC gene was achieved using a target specific group II intron. For most resistant isolates, MICS for cefiderocol were similar with or without the presence of iron, and there was an overall decrease in expression of receptors (including pirA and piuA) involved in ferric uptake. However, expression of the ferrous uptake system (faoA) persisted. The addition of avibactam (4 µg/mL) lowered most cefiderocol MICs to 2 to 4 µg/mL. Most isolates possessed ADC-25 or ADC-33. Cefiderocol resistance correlated with over-expression of blaADC; silencing of this ß-lactamase resulted in a ≥ 8-fold decrease in cefiderocol MICs. Over-expression of specific blaADC subtypes, in a background of generalized repression of ferric uptake systems, were consistent features in clinical isolates of cefiderocol-resistant A. baumannii.

Is remdesivir safe in patients with renal impairment? Experience at a large tertiary urban medical center.

PURPOSE: Remdesivir is FDA-approved for treatment of patients hospitalized with COVID-19 pneumonia, but not recommended in patients with severe renal failure. This study aims to evaluate the safety of remdesivir in this patient population. METHODS: This was a single-center, retrospective cohort study including patients ≥ 18 years old, admitted between May 1, 2020 and April 30, 2021 who received remdesivir. Patients were divided into two groups: estimated creatinine clearance (eCrCl) < 30 mL/min and eCrCl ≥ 30 ml/min. Primary outcomes were development of acute kidney injury (AKI) after remdesivir initiation and hepatotoxicity (alanine aminotransferase [ALT] or aspartate aminotransferase [AST] > 5 × upper limit of normal) both at end of treatment (EOT) or 5 days after EOT. Secondary outcomes were length of stay (days) and mortality. RESULTS: 513 patients were assessed with 416 patients included in the study (eCrCl < 30 mL/min, n = 55; eCrCl ≥ 30 mL/min n = 361). Incidence of AKI (eCrCl < 30 mL/min 11% vs eCrCl ≥ 30 mL/min 7%, OR 1.57, 95% CI 0.57, 4.3) and hepatotoxicity (ALT: 2% vs 4%, OR 0.47, 95% CI 0.05, 3.7 and AST: 2% vs 2%, OR 1.26, 95% CI 0.14, 11.04) were similar between the two groups. Length of stay was longer in the eCrCl < 30 mL/min group (mean 18.6 vs 11.9, difference 6.7, 95% CI 3.8, 9.6), and no difference in mortality was observed (21.8% vs 18.8%, OR 1.2, 95% CI 0.6, 2.4). CONCLUSION: Remdesivir was not associated with development of AKI or hepatotoxicity in patients with eCrCl < 30 mL/min compared to patients with eCrCl ≥ 30 mL/min, and warrants further investigation.

Relationship of TonB-dependent receptors with susceptibility to cefiderocol in clinical isolates of Pseudomonas aeruginosa.

OBJECTIVES: Cefiderocol maintains activity against most MDR Gram-negative pathogens including Pseudomonas aeruginosa. In laboratory-derived isolates, down-regulation of TonB-dependent siderophore receptors have been implicated in resistance to cefiderocol. METHODS: In this report, the expression of seven TonB-dependent siderophore receptors was examined in 10 clinical isolates with cefiderocol MICs ranging from ≤0.03-8 mg/L. In addition, genetic sequences of the siderophore receptors were analysed to identify potentially disruptive mutations. RESULTS: There was no clear association between expression of the receptors with cefiderocol susceptibility, including the receptors piuA/piuD and pirA previously implicated in cefiderocol uptake. In addition, there were no disabling mutations identified in the receptors. Acquired ß-lactamase activity also could not explain the range in cefiderocol susceptibility. CONCLUSIONS: The aetiology of reduced susceptibility to cefiderocol in clinical isolates of P. aeruginosa remains an enigma and worthy of further investigation.

Evaluating cross contamination on a shared ventilator.

BACKGROUND: Disasters have the potential to cause critical shortages of life-saving equipment. It has been postulated that during patient surge, multiple individuals could be maintained on a single ventilator. This was supported by a previous trial that showed one ventilator could support four sheep. The goal of our study is to investigate if cross contamination of pathological agents occurs between individuals on a shared ventilator with strategically placed antimicrobial filters. METHODS: A multipatient ventilator circuit was assembled using four sterile, parallel standard tubing circuits attached to four 2 L anaesthesia bags, each representing a simulated patient. Each 'patient' was attached to a Heat and Moisture Exchange filter. An additional bacterial/viral filter was attached to each expiratory limb. 'Patient-Lung' number 1 was inoculated with an isolate of Serratia marcescens, and the circuit was run for 24 hours. Each 'lung' and three points in the expiratory limb tubing were washed with broth and cultured. All cultures were incubated for 48 hours with subcultures performed at 24 hours. RESULTS: Washed cultures of patient 2, 3 and 4 failed to demonstrate growth of S. marcescens. Cultures of the distal expiratory tubing, expiratory limb connector and expiratory limb prefilter tubing yielded no growth of S. marcescens at 24 or 48 hours. CONCLUSION: Based on this circuit configuration, it is plausible to maintain four individuals on a single ventilator for 24 hours without fear of cross contamination.

Emergence of Delafloxacin-Resistant Staphylococcus aureus in Brooklyn, New York.

Delafloxacin is an option for infections due to methicillin-resistant Staphylococcus aureus. In 2017, 22% of isolates from 7 hospitals in Brooklyn, New York, were nonsusceptible to delafloxacin. Isolates belonging to ST105, a strain associated with healthcare-related infections, predominated. Resistance was also found in ST8, a strain (USA300) associated with community-associated infections.

Cefiderocol Resistance in Acinetobacter baumannii: Roles of ß-Lactamases, Siderophore Receptors, and Penicillin Binding Protein 3.

Cefiderocol is a siderophore cephalosporin active against many multidrug-resistant (MDR) Gram-negative pathogens. We examined the resistance mechanisms in 12 Acinetobacter baumannii strains with cefiderocol MICs ranging from ≤0.03 to >32 µg/ml. Cefiderocol resistance could not be explained by ß-lactamase activity. Cefiderocol resistance was associated with reduced expression of the siderophore receptor gene pirA Mutations involving PBP3 may have contributed to resistance in one strain. Additional studies are needed to assess the role of other siderophore receptors.

Activity of Meropenem with a Novel Broader-Spectrum ß-Lactamase Inhibitor, WCK 4234, against Gram-Negative Pathogens Endemic to New York City.

WCK 4234 is a novel diazabicyclooctane with potent inhibitory activity against class A and D carbapenemases and class C enzymes. We examined the in vitro activity of meropenem plus WCK 4234 (4 or 8 µg/ml) against Gram-negative pathogens from New York City. Three groups of isolates were analyzed: a contemporary collection of isolates, a collection of known carbapenem-resistant isolates, and a collection of isolates with defined resistance mechanisms. From the contemporary collection, we found (i) that all Enterobacteriaceae were susceptible to meropenem plus WCK 4234, (ii) that susceptibility rates for Acinetobacter baumannii were 56.5% for meropenem alone, 82.6% with 4 µg/ml WCK 4234, and 95.7% with 8 µg/ml WCK 4234, and (iii) that WCK 4234 had a modest effect on susceptibility of Pseudomonas aeruginosa Against a collection of carbapenem-resistant isolates, the addition of WCK 4234 to meropenem (i) restored meropenem susceptibility against Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae isolates, (ii) improved susceptibility against A. baumannii, and (iii) had a negligible effect against P. aeruginosa When tested against isolates with defined mechanisms of resistance, MICs of meropenem plus WCK 4234 were higher for K. pneumoniae with blaKPC albeit well below the susceptibility breakpoint; efflux systems or porins did not correlate with susceptibility. For A. baumannii, MICs of meropenem plus WCK 4234 did not correlate with efflux systems, outer membrane protein, blaampC, or blaoxa-51; however, MICs were higher in isolates with extended-spectrum ß-lactamases (ESBLs). For P. aeruginosa, isolates with relatively higher MICs of meropenem plus WCK 4234 had increased expression of ampC WCK 4234 is a potent ß-lactamase inhibitor that, when combined with meropenem, displays promising activity against multidrug-resistant pathogens.

Activity of cefepime/zidebactam (WCK 5222) against Enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter baumannii endemic to New York City medical centres.

BACKGROUND: The combination of cefepime and zidebactam (WCK5222), a novel ß-lactam enhancer, has demonstrated activity against a wide variety of Gram-negative pathogens and is currently under clinical evaluation. OBJECTIVES: To examine the activity of cefepime/zidebactam against: (i) a contemporary collection of Gram-negative isolates from New York City; (ii) a collection of carbapenem-resistant clinical isolates; and (iii) a collection of isolates with characterized resistance mechanisms. METHODS: Susceptibility tests were performed using broth microdilution for cefepime, zidebactam and cefepime/zidebactam (1:1). RESULTS: More than 99% of a contemporary collection of Escherichia coli, Klebsiella pneumoniae and Enterobacter spp. had cefepime/zidebactam MICs ≤2 mg/L, the susceptibility breakpoint for cefepime. For K. pneumoniae, the acquisition of blaKPC resulted in increased MICs, although MICs remained ≤2 mg/L for 90% of KPC-possessing isolates. Overall for Pseudomonas aeruginosa, 98% of isolates had MICs ≤8 mg/L and MICs were affected by increased expression of ampC. For carbapenem-resistant P. aeruginosa, 78% of isolates had cefepime/zidebactam MICs ≤8 mg/L. The activity of cefepime/zidebactam against Acinetobacter baumannii was lower, with 85% of all isolates and 34% of carbapenem-resistant isolates with MICs ≤8 mg/L (cefepime interpretative criteria). CONCLUSIONS: Cefepime/zidebactam demonstrated excellent activity against Enterobacteriaceae and P. aeruginosa, although activity was reduced in carbapenem-non-susceptible isolates. The activity against A. baumannii was reduced and studies examining the therapeutic efficacy in strains with high cefepime/zidebactam MICs are warranted.

Carbapenemases in New York City: the continued decline of KPC-producing Klebsiella pneumoniae, but a new threat emerges.

Objectives: Carbapenemase-producing Enterobacteriaceae are important nosocomial pathogens in many medical centres. Surveillance is needed to track trends and detect emergence of new carbapenemases. Methods: Single-patient isolates of Enterobacteriaceae were gathered from seven medical centres in New York City over a 3 month period in 2017. Susceptibility testing was performed and isolates were screened for selected carbapenemases. Additional isolates referred to our laboratory in 2018 were also tested. Results: KPC was found in 3/1911 (0.16%) isolates of Escherichia coli, 22/533 (4.1%) isolates of Klebsiella pneumoniae and 3/175 (1.7%) isolates of Enterobacter spp. Compared with prior surveillance studies performed over the past decade, there has been a persistent decline in the number of KPC-possessing K. pneumoniae. However, in 2018 two patients from the same skilled nursing facility admitted to two separate hospitals were found to harbour Enterobacteriaceae with NDM-5 and CTX-M-15. Conclusions: Since the height of the outbreak in 2006, there has been a decline in the number of KPC-possessing Enterobacteriaceae in New York City acute care medical centres. However continued vigilance is needed to detect the emergence of other carbapenemases.

In vitro and in vivo activity of single and dual antimicrobial agents against KPC-producing Klebsiella pneumoniae.

Objectives: Options for treatment of infections due to KPC-producing Klebsiella pneumoniae are limited and combination therapy is often recommended. In this report, the in vitro and in vivo activity of potential therapeutic agents and combinations was assessed against four KPC-producing K. pneumoniae isolates. Methods: Using clinically relevant concentrations, time-kill experiments and the Galleria mellonella model of infection were used to examine the activity of polymyxin B, ceftazidime/avibactam, meropenem, rifampicin and amikacin alone and in combination. Results: Two K. pneumoniae isolates were resistant to polymyxin B and had ceftazidime/avibactam MICs of 8/4 mg/L. When ceftazidime/avibactam was combined with either amikacin or meropenem, synergy was observed in vitro, and these combinations were associated with improved survival in the in vivo model. Improved survival was also observed using higher doses of ceftazidime/avibactam. The other two K. pneumoniae isolates were susceptible to polymyxin B and had lower (1/4 mg/L) MICs of ceftazidime/avibactam. For these two isolates, bactericidal activity was observed in vitro at ceftazidime/avibactam concentrations four times the MIC. At one-quarter of the MIC, synergy was observed when ceftazidime/avibactam was combined with meropenem. In the in vivo model with the two susceptible isolates, improved survival rates were observed following therapy with ceftazidime/avibactam monotherapy. For all four isolates, polymyxin B with or without rifampicin or meropenem performed poorly in the in vivo model. Conclusions: Pending clinical studies, combining ceftazidime/avibactam with another agent (e.g. a carbapenem) should be considered when treating serious infections due to these pathogens, particularly for isolates with ceftazidime/avibactam MICs near the susceptibility breakpoint.

Rise and fall of KPC-producing Klebsiella pneumoniae in New York City.

OBJECTIVES: The study objective was to examine the epidemiological trends of KPC-producing Klebsiella pneumoniae in New York City medical centres. PATIENTS AND METHODS: Single patient isolates of K. pneumoniae were collected from nine medical centres in New York City during a 3 month period from 2013 to 2014. Isolates were tested for the presence of blaKPC. Results were compared with similar surveillance studies conducted in 2006 and 2009. Infection control data, including utilization of medical devices, were analysed at a subset of hospitals. RESULTS: There was a progressive decline in the percentage of K. pneumoniae harbouring blaKPC from 2006 to 2013-14. For the nine hospitals that participated in all three surveillance studies, the percentages of isolates with blaKPC fell from 36% in 2006 to 25% in 2009 to 13% in 2013-14. Seven of the nine hospitals had marked declines in isolates with blaKPC, while two hospitals continued to struggle with this pathogen. These two hospitals were smaller and had longer lengths of patient stay. Device utilization rates were obtained from two hospitals that successfully controlled the spread of KPC-producing K. pneumoniae; both had ∼20%-25% reduction in the usage of urinary catheters. Changes in antibiotic usage at one hospital could not explain the decline in these pathogens. CONCLUSIONS: Over the past decade there has been a steady decline in KPC-producing K. pneumoniae in most New York City hospitals. The reason for the decline is probably multifactorial, involving a reduction in device (catheter) utilization and possibly an improvement in infection control practices.

Activity of eravacycline against Enterobacteriaceae and Acinetobacter baumannii, including multidrug-resistant isolates, from New York City.

Eravacycline demonstrated in vitro activity against a contemporary collection of more than 4,000 Gram-negative pathogens from New York City hospitals, with MIC50/MIC90 values, respectively, for Escherichia coli of 0.12/0.5 µg/ml, Klebsiella pneumoniae of 0.25/1 µg/ml, Enterobacter aerogenes of 0.25/1 µg/ml, Enterobacter cloacae 0.5/1 µg/ml, and Acinetobacter baumannii of 0.5/1 µg/ml. Activity was retained against multidrug-resistant isolates, including those expressing KPC and OXA carbapenemases. For A. baumannii, eravacycline MICs correlated with increased expression of the adeB gene.

Activity of Imipenem with Relebactam against Gram-Negative Pathogens from New York City.

Imipenem with relebactam was active against Escherichia coli, Klebsiella pneumoniae, and Enterobacter spp., including K. pneumoniae carbapenemase (KPC)-producing isolates. Loss of OmpK36 in KPC-producing K. pneumoniae isolates affected the susceptibility of this combination. Enhanced activity was evident against Pseudomonas aeruginosa, including isolates with depressed oprD and increased ampC expression. However, the addition of relebactam to imipenem did not provide added benefit against Acinetobacter baumannii. The combination of imipenem with relebactam demonstrated activity against KPC-producing Enterobacteriaceae and multidrug-resistant P. aeruginosa.

Activity of Meropenem Combined with RPX7009, a Novel ß-Lactamase Inhibitor, against Gram-Negative Clinical Isolates in New York City.

Multidrug-resistant Klebsiella pneumoniae carbapenemase (KPC)-producing Enterobacteriaceae are endemic to hospitals in New York City and other regions. RPX7009 is a novel ß-lactamase inhibitor with activity against serine carbapenemases. We tested the activity of meropenem plus RPX7009 against 4,500 recent Gram-negative clinical isolates from 11 New York City hospitals. The meropenem-RPX7009 combination was found to have excellent in vitro activity against Escherichia coli, K. pneumoniae, and Enterobacter spp., including multidrug-resistant (MDR) KPC-producing strains. Overall, 131/133 (98.5%) KPC-producing Enterobacteriaceae strains were inhibited by meropenem (≤1 µg/ml) plus RPX7009 (8 µg/ml). In a limited number of strains, the combination appeared to have reduced activity against KPC-producing K. pneumoniae isolates with diminished ompK35 and ompK36 expression. The addition of RPX7009 did not affect the activity of meropenem against Acinetobacter baumannii and Pseudomonas aeruginosa. The meropenem-RPX7009 combination shows promise as a novel agent against KPC-producing Enterobacteriaceae and deserves further study. Other approaches will be needed to address multidrug-resistant A. baumannii and P. aeruginosa, which typically possess different mechanisms of carbapenem resistance.

Do risk adjustment calculations used by governmental agencies adequately assess risk for infection after colon surgery?

BACKGROUND: Surgical site infections (SSIs) following colon surgery are associated with clinical and financial consequences. The Centers for Medicare and Medicaid Services (CMS) and the New York State Department of Health (NYSDOH) use risk adjustment variables to determine quality measure scores. METHODS: Among patients in a large public system, surgical risk variables were compared between patients with and without SSIs. Propensity score matching, using CMS and NYSDOH risk variables, created control groups. Receiver Operating Characteristics (ROC) curves were created using current and augmented risk adjustment variables. RESULTS: When matched using CMS risk variables, more patients with SSIs had contaminated/dirty wounds, longer duration of surgery, and emergency surgery. The addition of these variables significantly improved the CMS ROC curve. When matching NYSDOH variables, more SSI patients were male, had contaminated/dirty wounds, and tended to be younger. The addition of these variables to the current NYSDOH adjustment criteria did not significantly improve the ROC curve. DISCUSSION: The CMS adjustment criteria for colon SSIs do not adequately account for complicated surgeries. The inclusion of additional variables significantly improved the performance of CMS risk adjustment. CONCLUSIONS: Until more robust risk adjustment criteria are developed, the reporting of SSIs following colon surgery as a quality measure should be suspended.

Reduction in catheter-associated urinary tract infections following a diagnostic stewardship intervention.

Catheter-associated urinary tract infections (CAUTIs) are a frequent hospital-acquired infection and public health concern. In an attempt to reduce the number of CAUTIs, an intervention that emphasized the appropriate laboratory evaluation by ordering providers was implemented. This intervention supplemented ongoing standard bundle protocols. Compared to the 16 months before the intervention, there was a significant decrease in the number of CAUTIs during the 12-month intervention period.

Reportable infections following colon surgery in a large public healthcare system in New York City: The consequences of being a level 1 trauma center.

OBJECTIVES: To examine differences in risk factors and outcomes of patients undergoing colon surgery in level 1 trauma centers versus other hospitals and to investigate the potential financial impact of these reportable infections. DESIGN: Retrospective cohort study between 2015 and 2022. SETTING: Large public healthcare system in New York City. PARTICIPANTS: All patients undergoing colon surgery; comparisons were made between (1) all patients undergoing colon surgery at the level 1 trauma centers versus patients at the other hospitals and (2) the nontrauma and trauma patients at the level 1 trauma centers versus the nontrauma patients at other hospitals. RESULTS: Of 5,217 colon surgeries reported, 3,531 were at level 1 trauma centers and 1686 at other hospitals. Patients at level 1 trauma centers had significantly increased American Society of Anesthesiology (ASA) scores, durations of surgery, rates of delayed wound closure, and rates of class 4 wounds, resulting in higher SIRs (1.1 ± 0.15 vs 0.75 ± 0.18; P = .0007) compared to the other hospitals. Compared to the nontrauma patients at the other hospitals, both the nontrauma and trauma patients at the level 1 trauma centers had higher ASA scores, rates of delayed wound closure, and of class 4 wounds. The SIRs of the nontrauma patients (1.16 ± 1.29; P = .008) and trauma patients (1.26 ± 2.69; P = .066) at the level 1 trauma center were higher than the SIRs of nontrauma patients in the other hospitals (0.65 ± 1.18). CONCLUSIONS: Patients undergoing colon surgery at level 1 trauma centers had increased complexity of surgery compared to the patients in other hospitals. Until there is appropriate adjustment for these risk factors, the use of infections following colon surgery as a reportable quality measure should be re-evaluated.

Irreproducible and uninterpretable Polymyxin B MICs for Enterobacter cloacae and Enterobacter aerogenes.

Carbapenem-resistant Enterobacter species are emerging nosocomial pathogens. As with most multidrug-resistant Gram-negative pathogens, the polymyxins are often the only therapeutic option. In this study involving clinical isolates of E. cloacae and E. aerogenes, susceptibility testing methods with polymyxin B were analyzed. All isolates underwent testing by the broth microdilution (in duplicate) and agar dilution (in duplicate) methods, and select isolates were examined by the Etest method. Selected isolates were also examined for heteroresistance by population analysis profiling. Using a susceptibility breakpoint of ≤2 µg/ml, categorical agreement by all four dilution tests (two broth microdilution and two agar dilution) was achieved in only 76/114 (67%) of E. cloacae isolates (65 susceptible, 11 resistant). Thirty-eight (33%) had either conflicting or uninterpretable results (multiple skip wells, i.e., wells that exhibit no growth although growth does occur at higher concentrations). Of the 11 consistently resistant isolates, five had susceptible MICs as determined by Etest. Heteroresistant subpopulations were detected in eight of eight isolates tested, with greater percentages in isolates with uninterpretable MICs. For E. aerogenes, categorical agreement between the four dilution tests was obtained in 48/56 (86%), with conflicting and/or uninterpretable results in 8/56 (14%). For polymyxin susceptibility testing of Enterobacter species, close attention must be paid to the presence of multiple skip wells, leading to uninterpretable results. Susceptibility also should not be assumed based on the results of a single test. Until the clinical relevance of skip wells is defined, interpretation of polymyxin susceptibility tests for Enterobacter species should be undertaken with extreme caution.