ABSTRACT
Introduction
Multidrug-resistant (MDR) Gram-negative infections complicate care of combat casualties. We describe the clinical characteristics, resistance patterns, and outcomes of Pseudomonas aeruginosa infections in combat casualties.
Methods
Combat casualties included in the Trauma Infectious Disease Outcomes Study with infections with and without P. aeruginosa isolation during initial hospitalization were compared. Pseudomonas aeruginosa from initial wound, blood, and serial isolates (≥7 days from previous isolate) collected from June 2009 through February 2014 was subjected to antimicrobial susceptibility testing, pulsed-field gel electrophoresis, and whole genome sequencing for assessing clonality. Multidrug resistance was determined using the CDC National Healthcare Safety Network definition.
Results
Of 829 combat casualties with infections diagnosed during initial hospitalization, 143 (17%) had P. aeruginosa isolated. Those with P. aeruginosa were more severely injured (median Injury Severity Score 33 [interquartile range (IQR) 27-45] vs 30 [IQR 18.5-42]; P < .001), had longer hospitalizations (median 58.5 [IQR 43-95] vs 38 [IQR 26-56] days; P < .001), and higher mortality (6.9% vs 1.5%; P < .001) than those with other organisms. Thirty-nine patients had serial P. aeruginosa isolation (median 2 subsequent isolates; IQR: 1-5), with decreasing antimicrobial susceptibility. Ten percent of P. aeruginosa isolates were MDR, associated with prior exposure to antipseudomonal antibiotics (P = .002), with amikacin and colistin remaining the most effective antimicrobials. Novel antimicrobials targeting MDR Gram-negative organisms were also examined, and 100% of the MDR P. aeruginosa isolates were resistant to imipenem/relabactam, while ceftazidime/avibactam and ceftolozane/tazobactam were active against 35% and 56% of the isolates, respectively. We identified two previously unrecognized P. aeruginosa outbreaks involving 13 patients.
Conclusions
Pseudomonas aeruginosa continues to be a major cause of morbidity, affecting severely injured combat casualties, with emergent antimicrobial resistance upon serial isolation. Among MDR P. aeruginosa, active antimicrobials remain the oldest and most toxic. Despite ongoing efforts, outbreaks are still noted, reinforcing the crucial role of antimicrobial stewardship and infection control.
INTRODUCTION
Combat casualty care advances during military operations in Iraq and Afghanistan, including rapid medical evacuation and forward deployment of surgical and medical assets, have contributed to increased survival among wounded personnel.1,2 With the decline in battlefield case-fatality rates,3 critically injured personnel face significant morbidity from infectious complications. High-energy blast trauma typical of combat can result in different infectious complications than civilian trauma, making it important to study infectious complications specific to military trauma.4 Although the focus on infection control measures in theater and throughout the evacuation process has increased (including enhanced multidrug-resistant [MDR] organism screening), Gram-negative wound infections in combat casualties, particularly Pseudomonas aeruginosa, remain a concern.5,6
Multiple studies have shown that Gram-negative organisms predominate in combat wounds, and P. aeruginosa has been historically identified as a common pathogen.1–3,7–11 Initial wound bacteriology in Vietnam demonstrated that although Gram-positive organisms accounted for approximately half of the admission isolates, Gram-negative organisms, specifically P. aeruginosa, were more commonly isolated later during hospitalization.12 This pattern remains true today, as 47.6% of the combat-related infections were attributed to Gram-negative bacteria compared to 24.0% for Gram-positives.7
Treatment of P. aeruginosa presents a challenge as it has multiple intrinsic mechanisms of resistance and the ability to acquire resistance rapidly upon antimicrobial exposure.13,14 Given the significance of P. aeruginosa as a nosocomial pathogen, prolonged hospitalizations of battlefield trauma patients with significant accumulation of antimicrobial exposures, and potential acquisition of antimicrobial resistance, examination of P. aeruginosa infections in this severely injured population is needed.6,15,16 We describe clinical and epidemiologic characteristics and outcomes associated with P. aeruginosa wound infections and bacteremia in combat casualties. Additionally, we identify P. aeruginosa resistance patterns and evaluate serial isolates for the development of antimicrobial resistance.
METHODS
Study Population and Definitions
Data were collected through the Trauma Infectious Disease Outcomes Study (TIDOS), which is an observational study evaluating short- and long-term infectious disease complications following deployment-related trauma.1 Criteria for inclusion in TIDOS required being active duty personnel or DoD beneficiaries ≥18 years of age, wounded during deployment, and requiring medical evacuation from the combat theater to Landstuhl Regional Medical Center (LRMC, Germany) before transition to participating military hospitals in the USA. Three patients with P. aeruginosa clinical cultures from infections at LRMC who did not transition to participating U.S. hospitals were included (data collection ended at discharge from LRMC). The participating U.S. hospitals were Walter Reed National Military Medical Center in the National Capital Region (Walter Reed Army Medical Center [WRAMC] and National Naval Medical Center before September 2011) and Brooke Army Medical Center in San Antonio, TX. Patients were eligible for inclusion in this retrospective analysis if they had P. aeruginosa isolated from cultures obtained by clinicians for evaluation of suspected infections between June 2009 and February 2014. This study was approved by the Institutional Review Board of the Uniformed Services University.
Patient demographics, characteristics of injury severity and circumstances, and surgical care were collected through the DoD Trauma Registry, while infection-related information (e.g., syndrome, microbiology, and antimicrobial treatment) were obtained through the TIDOS Infectious Disease module.1 Of patients diagnosed with infections during their initial hospitalization, clinical characteristics and outcomes of those with isolation of P. aeruginosa were compared with those with infections by other organisms (i.e., Gram-positive or Gram-negative other than Pseudomonas spp.).
Pseudomonas aeruginosa isolates were obtained during clinical workups and linked to infections classified in accordance with National Healthcare Safety Network (NHSN) definitions17 using a combination of clinical findings and laboratory and other diagnostic tests or at the discretion of a physician’s clinical diagnosis (in the absence of a priori definition), provided that targeted antimicrobial therapy was initiated and continued for ≥5 days for skin and soft tissue infections.1 Initial isolates were included if they were from a wound culture or blood source. Serial isolates collected during infectious clinical workups from any source ≥7 days after collection of the initial or previous serial isolate were evaluated for resistance development. Isolates obtained from surveillance swabs at hospital admission (i.e., colonization) were excluded. As per NHSN, MDR was defined as resistance to ≥3 classes of antibiotics (aminoglycosides, β-lactams, carbapenems, and/or fluoroquinolones).18 Difficult-to-treat resistance (DTR) was defined as per the Infectious Disease Society of America Guidance.19 When examining antibiotic selection pressure before P. aeruginosa isolation, antipseudomonal antibiotics were defined as ceftazidime, piperacillin/tazobactam, imipenem, meropenem, cefepime, ciprofloxacin, levofloxacin, aztreonam, and aminoglycosides (i.e., gentamicin, tobramycin, and amikacin).20
Microbiology
After initial identification and susceptibility testing performed at military hospitals, isolates were stored in a repository for future analyses. Initial unique P. aeruginosa wound and blood isolates, as well as serial isolates, from patients in TIDOS were examined for speciation and antimicrobial susceptibilities using BD Phoenix Automated Microbiology System (NMIC/ID-304 panel, BD Diagnostics, Sparks, MD). Isolates classified as MDR were subjected to additional susceptibility testing for colistin, ceftazidime/avibactam, ceftolozane/tazobactam, imipenem/relebactam, and cefiderocol using ETEST (bioMerieux, Durham, NC) or disk diffusion (Hardy Diagnostics, Santa Maria, CA). An antibiogram was constructed using the Clinical Laboratory and Standards Institute interpretation criteria.21 Pulsed-field gel electrophoresis was performed on all samples to assess for clonality. For isolates with evidence of clonality, whole-genome sequencing (WGS) was performed by the Walter Reed Army Institute of Research, Multidrug-Resistant Organism Repository and Surveillance Network.22
Statistical Analysis
Wilcoxon testing was performed for continuous variables, while categorical variables were analyzed with chi-squared and Fisher’s exact tests as appropriate. Statistical significance was defined as P < .05. SAS version 9.4 (SAS, Cary, NC) was used to complete all statistical analyses.
RESULTS
Characteristics of Patients With P. aeruginosa Infections
Among 2,618 trauma patients admitted to participating U.S. Military hospitals, 829 (32%) developed ≥1 bacterial infection following battlefield injury. Overall, 440 patients had Pseudomonas species isolated with >1,100 surveillance and clinical cultures in the TIDOS database. Of these, 62 had other Pseudomonas species isolated and 232 had P. aeruginosa isolates, which did not meet inclusion criteria (collected for surveillance, obtained <7 days from prior isolates or were from nonblood or wound sites). Ultimately, 146 (5% of 2,618 trauma patients and 17% of 829 patients with an infection) patients met inclusion criteria (3 patients transitioned to nonparticipating hospitals and were not included in the percentage calculation; n = 143), with 374 isolates being included in the analysis (239 initial and 135 serial isolates). Isolates were collected from wounds (205 initial isolates and 42 serial isolates), blood (34 initial isolates and 0 serial isolates), respiratory specimens (40 serial isolates), urine (32 serial isolates), and other sources (21 serial isolates). Four of the patients with initial P. aeruginosa isolates identified from wound cultures were not ultimately diagnosed with an infection; however, as they had clinical suspicion of an infection, they were included in the analysis. The comparator group included 690 patients with non-Pseudomonas infections, with the predominant etiologic agents being coagulase-negative staphylococci, Escherichia coli, Acinetobacter calcoaceticus-baumannii complex, Enterococcus faecium, and Enterobacter cloacae.
The patients in the study population were predominantly male (146 [100%] patients with P. aeruginosa clinical isolation and 674 [97.6%] patients with non-Pseudomonas infections; P = 0.082) and had a median of 24 years (interquartile range [IQR] of 21-28 for patients with P. aeruginosa clinical isolation and IQR of 22-29 for patients with non-Pseudomonas infections; P = 0.552). Patients with P. aeruginosa clinical isolation were more frequently injured in Afghanistan (97% vs 89%) via a blast mechanism (90% vs 73%), had critical injury severity (median Injury Severity Score [ISS]: 33 vs 30), and sustained a greater proportion of amputations (65% vs 38%; Table I) compared to patients with non-Pseudomonas infections. Moreover, patients with P. aeruginosa had greater transfusion requirements, intensive care unit (ICU) admissions, and longer duration of hospitalization (Table I).
TABLE I.
Characteristics of Patients with Clinical Isolation of P. aeruginosa vs Other Non-Pseudomonas Pathogens, No. (%)
| Characteristics | Patients with P. aeruginosa infectionsa (n = 146) | Patients with non-Pseudomonas infections (n = 690)b | P-value |
|---|---|---|---|
| Injured in | .012 | ||
| Afghanistan | 141 (96.5) | 614 (88.9) | |
| Iraq | 5 (3.4) | 45 (6.5) | |
| Injury mechanism | |||
| Blast injury | 132 (90.4) | 507 (73.4) | <.001 |
| Improvised explosive device | 129 (88.3) | 445 (64.4) | <.001 |
| Nonblast injury | .085 | ||
| Gunshot wound | 10 (6.8) | 86 (12.4) | |
| Othersc | 4 (2.7) | 95 (13.7) | |
| Injury while on foot patrold | 97 (66.4) | 379 (54.9) | .043 |
| Injury Severity Score, median IQR | 33 (27-45) | 30 (18.5-42) | <.001 |
| 1-9 (minor) | 3 (2.0) | 47 (6.8) | .001 |
| 10-15 (moderate) | 2 (1.3) | 51 (7.3) | |
| 16-25 (severe) | 28 (19.1) | 153 (22.1) | |
| ≥26 (critical) | 113 (77.3) | 437 (63.3) | |
| First documented shock index, median (IQR) | 1.07 (0.79-1.42) | 0.87 (0.68-1.18) | <.001 |
| Blood transfusion within 24 hours postinjury, median units (IQR)e | 17 (9.5-31) | 12 (5-20) | <.001 |
| ICU admission | <.001 | ||
| None | 15 (10.3) | 118 (17.1) | |
| LRMC only | 15 (10.3) | 111 (16.0) | |
| U.S. hospital ± LRMC | 113 (77.4) | 461 (66.8) | |
| Nonparticipating hospital | 3 (2.1) | 0 | |
| Mechanical ventilation | .262 | ||
| None | 43 (29.4) | 250 (36.2) | |
| LRMC only | 34 (23.2) | 173 (25.0) | |
| LRMC and U.S. hospital ≤1 week | 68 (46.5) | 262 (37.9) | |
| LRMC and U.S. hospital ≥2 weeks | 1 (0.6) | 5 (0.7) | |
| Admission SOFA at LRMC, median (IQR) | 7 (3-9) | 5 (2-8) | .002 |
| U.S. Hospital Admission SOFA, median (IQR) | 4 (1-8) | 2 (0-6) | .010 |
| Most severe injuryf | <.001 | ||
| Amputation | 95 (65.0) | 261 (37.8) | |
| Open fracture | 41 (28.0) | 263 (38.1) | |
| Open soft tissue wound | 8 (5.4) | 90 (13.0) | |
| No open injury | 2 (1.3) | 74 (10.7) | |
| MDRO colonization at LRMC | 22 (15.0) | 93 (13.4) | .612 |
| Inpatient hospitalization, median days (IQR) | 58.5 (43-95) | 38 (26-56) | <.001 |
| Death | 10 (6.9) | 10 (1.5) | <.001 |
Abbreviations: ICU = intensive care unit; IQR = interquartile range; LRMC = Landstuhl Regional Medical Center; MDRO = multidrug-resistant organisms; Pseudomonas aeruginosa = P. aeruginosa; SOFA = Sequential Organ Failure Assessment.
Includes 142 patients with infections and four patients who had clinical suspicion of an infection. Three patients with infections at LRMC transitioned to a nonparticipating hospital.
Predominant non-Pseudomonas infections include coagulase-negative staphylococci (21%), E. coli (20%), A. calcoaceticus-baumannii complex (15%), E. faecium (13%), and E. cloacae (12%).
Other injury mechanism includes motor vehicle crashes and falls.
Data on mounted status are missing for 128 patients (23 with P. aeruginosa infections and 105 with non-Pseudomonas infections).
Blood data are missing for 185 patients (10 patients with P. aeruginosa infections and 175 with non-Pseudomonas infections).
Data on most severe injury missing for two patients with a non-Pseudomonas infection.
A total of 259 infections were identified among patients with P. aeruginosa isolation, and the non-Pseudomonas patients had 1,320 infections with an alternate microbiologic etiology. Skin and soft tissue infections contributed the greater proportion of infections to patients with P. aeruginosa isolation (n = 150; 58%) and those with non-Pseudomonas infections (n = 596; 45%). Additional sources of infection for the P. aeruginosa and non-Pseudomonas infection patients, respectively, were bloodstream infections (n = 27 [10%] and n = 173 [13%]), osteomyelitis (n = 27 [10%] and n = 93 [7%]), urinary tract infections (n = 20 [8%] and n = 58 [4%]), pneumonia (n = 15 [6%] and 213 [16%]), sepsis (n = 14 [5%] and n = 45 [3%]), central nervous system infections (0 and n = 31 [2%]), and other/undifferentiated infections (n = 6 [2%] and n = 111 [8%]). Initial P. aeruginosa isolates were recovered with a median of 14 days postinjury with wound specimens being the primary source of initial culture (125 patients; 86%) followed by blood (19 patients; 13%). Two (1%) patients had an initial culture of P. aeruginosa from both blood and wound specimens.
Thirty-nine of the 146 (27%) patients had serial cultures meeting inclusion criteria. Serial isolation, as compared to patients with only initial isolation of P. aeruginosa, was associated with a higher median ISS (45 vs 33, P < .001), longer period of mechanical ventilation (median 12 vs 5 days, P < .001), longer hospital stays (median 100 vs 50 days, P < .001), and more frequent isolation of MDR P. aeruginosa (23% vs 5%, P = .001), but not with mortality.
Fourteen (10%) patients had MDR P. aeruginosa isolated during their hospitalization (total of 43 MDR isolates), of whom eight patients had initial MDR isolation and six patients developed MDR on serial isolation. All patients with MDR P. aeruginosa infections sustained blast injuries. When compared to patients with only recovery of non-MDR P. aeruginosa isolates, patients with MDR P. aeruginosa had a higher median ISS (P = .001), longer length of the stay in the ICU (P = .045), longer hospital stays (P < .001), and more ventilator days (P = .012; Table II). Seventy-eight (53%) patients received antipseudomonal antibiotics for >48 hours before clinical isolation of P. aeruginosa, with a significantly higher proportion of receipt among the patients with MDR P. aeruginosa compared to non-MDR P. aeruginosa isolation (93% vs 49%; P = .002; Table II). Specifically, patients with MDR isolation had a higher proportion of prior use of amikacin (29% vs 6%; P = .017), cefepime (14% vs 0.8%; P = .024), imipenem (21% vs 5%; P = .042), and meropenem (57% vs 30%; P = .042) compared to patients with non-MDR P. aeruginosa isolation. The use of aztreonam, ceftazidime, ciprofloxacin, levofloxacin, gentamicin, piperacillin-tazobactam, and tobramycin was comparable between the two groups. Median duration of antipseudomonal antibiotic usage before isolation was comparable between the two groups (median of 11 days for MDR isolation and 10 days for non-MDR isolation; P = .464; Table II).
TABLE II.
Characteristics of Patients with Isolation of P. aeruginosa, Median (IQR)
| Characteristics | Total patients (n = 146) | Patients with non-MDR (n = 132) | Patients with MDR (n = 14) | P-value |
|---|---|---|---|---|
| Age, years | 24 (21-28) | 24 (22-28) | 22 (21-26) | .169 |
| Injury Severity Score | 33 (27-45) | 33 (26-45) | 50 (38-66) | .001 |
| Blood units within 24 hours postinjury | 17 (9.5-31) | 31 (13-44) | 17 (9-29) | .185 |
| First documented shock index | 1.07 (0.79-1.42) | 1.28 (1.12-1.67) | 1.04 (0.77-1.42) | .073 |
| LRMC admission SOFA | 7 (3-9) | 8 (7-13) | 6 (3-9) | .020 |
| Hospital days | 58.5 (43-95) | 56 (41.5-81.5) | 140.5 (70-220) | <.001 |
| Ventilator days | 7 (3-13) | 6 (3-12) | 9.5 (7-37) | .012 |
| ICU length of stay, days | 11.5 (7-22) | 11 (7-20.5) | 37 (8-65) | .045 |
| Days between injury and initial Pseudomonas isolation | 14 (6-30) | 14 (6-28) | 21 (8-59) | .137 |
| Receipt of >48 hours of antipseudomonal antibiotic before P. aeruginosa isolation, No. (%)a | 78 (53.4) | 65 (49.2) | 13 (92.9) | .002 |
| Duration of antipseudomonal use, daysb | 10.5 (6-22) | 10 (6-19) | 11 (8-24) | .464 |
| Death, no. (%) | 10 (6.9) | 9 (6.8) | 1 (7.1) | 1.000 |
Abbreviations: ICU = intensive care unit; IQR = interquartile range; LRMC = Landstuhl Regional Medical Center; Pseudomonas aeruginosa = P. aeruginosa; SOFA = Sequential Organ Failure Assessment.
Antipseudomonal antibiotics are aminoglycosides (i.e., gentamicin, tobramycin, and amikacin), aztreonam, cefepime, ceftazidime, ciprofloxacin, imipenem, levofloxacin, meropenem, and piperacillin/tazobactam.
Restricted to 78 patients with >48 hours of antipseudomonal antibiotic use before initial P. aeruginosa isolation.
Antimicrobial Susceptibility Patterns
Initial susceptibilities were highest for aminoglycosides and antipseudomonal cephalosporins (Table III). All antimicrobials demonstrated decreased susceptibility with serial isolation; however, amikacin and tobramycin did retain 95% and 80% susceptibility, respectively. Most notable was low susceptibility to fluoroquinolones and carbapenems, particularly on serial isolation.
TABLE III.
Susceptibility of Initial and Subsequent Isolates From Those with Serial Cultures by Patient, No. (%)
| Antimicrobial | Initial isolates (n = 146) | Subsequent isolates (n = 40) | P-value |
|---|---|---|---|
| Levofloxacin | 108 (74) | 13 (33) | <.001 |
| Ciprofloxacin | 112 (77) | 17 (43) | <.001 |
| Imipenem | 98 (67) | 12 (30) | <.001 |
| Meropenem | 108 (74) | 10 (25) | <.001 |
| Piperacillin-tazobactam | 118 (81) | 19 (48) | <.001 |
| Ceftazidime | 131 (90) | 24 (60) | <.001 |
| Cefepime | 129 (88) | 23 (58) | <.001 |
| Aztreonam | 104 (71) | 13 (33) | <.001 |
| Gentamicin | 134 (92) | 24 (60) | <.001 |
| Tobramycin | 134 (92) | 32 (80) | .033 |
| Amikacin | 146 (100) | 38 (95) | .045 |
Of 43 MDR isolates, 100% were resistant to carbapenems and fluoroquinolones and 93%-95% were resistant to aztreonam, piperacillin-tazobactam, ceftazidime, and cefepime, consistent with classification as DTR P. aeruginosa. The highest susceptibility of the MDR P. aeruginosa isolates was observed in amikacin (93%), colistin (84%), and cefiderocol (79%). For gentamicin and tobramycin, 35% and 47% of the MDR P. aeruginosa isolates were susceptible, respectively. Of newer antimicrobials targeting MDR Gram-negatives (MDRGN), all isolates were resistant to imipenem/relabactam, whereas ceftazidime/avibactam and ceftolozane/tazobactam were active against 35% and 56% of MDR isolates, respectively.
Clonality of P. aeruginosa Isolates
The pulsed-field gel electrophoresis analysis revealed 64 different pulsed-field types (PFTs) and 80 isolates not identified as a known type. Two PFT subgroups (PFT 24 and PFT 31) had a significant number of isolates and were the only two subgroups identified from ≥3 patients. Specifically, PFT 24 was identified in 22 isolates between May 2 and September 12, 2011, from seven patients, with Camp Bastion (Afghanistan) as the first associated medical facility in the combat zone for 21 of 22 isolates. All patients were ultimately admitted to the ICU at National Naval Medical Center. The last of these isolates was the only one of this group to be identified as MDR.
Thirteen isolates from six patients identified as PFT 31 (seven wound, two urine, three respiratory, and one blood isolate) were assessed using WGS. The isolates were separated into two closely related groups (Branches 1 and 2) that differed by no more than 40 single-nucleotide polymorphisms (SNPs), and all were assigned to ST-111, a genotype identified as a high-risk clonal group of MDR and extensively drug-resistant P. aeruginosa. The six isolates identified as Branch 1 were collected from five patients over 10 months, with successive accumulation of up to eight SNPs over a 10-month period (Fig. 1A). These five patients had their first postinjury medical contact at a variety of facilities within both the Afghanistan and Iraq combat theaters, with all P. aeruginosa isolates collected during their subsequent hospitalizations at WRAMC. The seven isolates identified as Branch 2 were all from one patient who was evacuated from Kandahar (Fig. 1B) and collected over 4 months during hospitalization at Brooke Army Medical Center. Analysis of SNP accumulation within each branch suggests that SNP accumulation for this strain occurred at approximately 10 SNPs per year, with 30 to 40 SNPs difference between Branch 1 and Branch 2.
FIGURE 1.
Whole genome phylogeny and single-nucleotide polymorphism–based comparison of the 13 isolates identified with pulsed-field type 31. (A) Six isolates from Branch 1; (B) seven isolates from Branch 2.
DISCUSSION
Although the recent wars in Iraq and Afghanistan led to important advances in combat casualty care, infectious complications continue to be a primary driver of morbidity following medical evacuation. This study highlights the major role of P. aeruginosa as a common pathogen isolated in combat-related infections; however, reassuringly, isolation of MDR P. aeruginosa is uncommon.
Previous studies on traumatic wound infections demonstrate variable rates—from an incidence of 3.9% in Vietnam to 8.2% of noncombat trauma patients admitted to a contemporary military facility and as high as 37% in a civilian trauma population.23–25 Hardaway23 acknowledges that the infection rate is likely artificially low as many patients likely developed infections after evacuation from Vietnam. Our data show that 32% of patients developed at least one infection, and 17% were P. aeruginosa. Historically, P. aeruginosa has been recognized as a frequent combat and noncombat trauma-related wound pathogen—one of the three most commonly identified in Vietnam and the sixth most common pathogen implicated in healthcare-associated infections.26,27 A study examining 63 extremity wounds in Vietnam demonstrated that although isolation of P. aeruginosa on day 1 was rare (3/188 cultures), by day 5, it was the most often isolated pathogen (42/146 cultures).12
Infection with P. aeruginosa is frequent among immunocompromised patients and hospitalized patients and more common in ICU and burn patients, although there is geographic variability noted (with P. aeruginosa being a frequent pathogen associated with burns in the Middle East).28,29 As with civilian trauma, factors related to high injury severity (e.g., traumatic amputations, ISS, ICU admission, blast injury, and use of mechanical ventilation) are recognized as being associated with increased risk of developing battlefield-related infections.6,7,10,30,31 A prior TIDOS examination of wounded military personnel spanning 5 years (2009-2014) identified that 55% of patients sustained blast trauma with a median ISS of 10.10 Patients in the TIDOS population were typically more severely injured than those who transitioned to other U.S. Military hospitals (higher ISS, more blast injuries, and ICU admissions).6 With 90% sustaining blast trauma, a median ISS of 33, and a median of 12 days in the ICU, patients in our study were not only more severely injured than the general military trauma population but also at the high end of injury severity of the TIDOS population. Furthermore, isolation of P. aeruginosa in these severely injured, combat casualties later in hospitalization (median 14 days postinjury) is consistent with nosocomial pathogens affecting those with high healthcare and antimicrobial exposure—emphasizing the importance of antimicrobial stewardship and infection prevention in these severely injured patients.
When compared to patients with non-MDR infections, the healthcare burden (ventilation requirement and hospitalization duration) was significantly greater for combat casualties with MDR P. aeruginosa infections, further emphasizing its role as a nosocomial pathogen. Wounded military personnel with MDRGN infections also have substantial antimicrobial exposure continuing past the 1st week postinjury.8 Although previous studies have documented P. aeruginosa as a common war wound pathogen, MDR P. aeruginosa has been less frequently reported.6,8,10 In a prior TIDOS analysis of MDRGN bacilli in combat casualties, P. aeruginosa was identified; however, multidrug resistance was low (3.7% of MDRGN isolates).8 These prior and current findings indicate that clinicians likely do not need to consider empiric therapy for MDR P. aeruginosa in combat casualties. Campbell et al. also noted a significantly longer hospital stays for MDRGN-infected patients (median 53 days vs 18 days for non-MDRGN infections).8 Our population with non-MDR P. aeruginosa was similar to the MDRGN population previously described, potentially reflecting the intrinsic resistance pattern of P. aeruginosa.
In evaluating characteristics of MDR P. aeruginosa isolation, our data support the findings of Paramythiotou et al.,20 which demonstrated an association between MDR P. aeruginosa occurrence and prior receipt of antipseudomonal antibiotics. Furthermore, when examining individual antibiotics, they found that the duration of ciprofloxacin was significantly associated with the development of MDR P. aeruginosa.20 Similarly, in our small population, patients with MDR P. aeruginosa isolation had a significantly higher proportion of prior use of a broad range of antipseudomonal antibiotics, including amikacin, cefepime, imipenem, and meropenem.
Studies of nosocomial P. aeruginosa isolates have demonstrated trends toward antibiotic resistance, consistent with known tendencies to display multiple resistance mechanisms simultaneously.14,32 One study examining the activity of eight antipseudomonal antimicrobials, including ceftolozane-tazobactam, against >3,800 P. aeruginosa clinical isolates from 32 U.S. hospitals demonstrated that for susceptible isolates, amikacin, colistin, and ceftolozane-tazobactam had the highest susceptibilities (all >95%). MDR isolates’ susceptibilities remained high for all three antimicrobials, with 84% susceptibility to ceftolozane-tazobactam.32 Our data demonstrated a statistically significant decrease in antimicrobial susceptibility upon serial isolation for all antimicrobials tested. Only amikacin and colistin (two of our oldest and most toxic options for treatment of P. aeruginosa infections) retained fair susceptibility upon serial isolation, consistent with previously documented preserved aminoglycoside susceptibility.32 Notably, in our small MDR population, of the newer antimicrobials considered for treatment of DTR P. aeruginosa, only cefiderocol retained fair susceptibility (79%), whereas the other agents, especially imipenem-relebactam, had relatively disappointing results compared to other recent data.33–35
Even with improved infection control efforts throughout the military healthcare system, our data revealed two previously unidentified outbreaks. Likely, these were unnoticed because neither involved MDR organisms, except the final isolate in the PFT 24 subgroup.5 As the PFT 24 subgroup, accounting for the first outbreak, was consistent with nosocomial transmission and all isolates were collected over a 4-month period from seven patients (six evacuated from the same hospital), all of whom were ultimately hospitalized in the same ICU, the isolates were not sent for WGS. The six isolates of PFT 31 Branch 1 involved in the second outbreak were highly related as demonstrated by WGS. Five patients were evacuated from four different deployed facilities, with all isolates collected at WRAMC, suggesting that infection control challenges are not limited to the deployed setting. Although ST-111 is identified as a major “high-risk” clonal group of MDR strains worldwide, and all 13 of the PFT 31 isolates belong to that sequence type, none of these isolates were MDR. Overall, MDR P. aeruginosa epidemiology in combat-wounded patients appears to be driven by similar factors as previously reported for MDRGNs (nosocomial antimicrobial pressure rather than environmental or clinical outbreaks).5
Limitations of this study include its retrospective nature. Blast injuries were the primary mechanism of injury for these patients, so the results may not be generalizable to civilian trauma populations. It is also important to note that although all initial wound cultures were obtained given suspicion for infection, there is bias inherent in the collection of serial isolates. These patients were extremely ill, immunocompromised, and receiving multiple antimicrobials and had numerous trips to the operating room during which repeat cultures were obtained, making interpretation of colonization vs infection challenging—although these were defined as per NHSN definitions and targeted therapy and so represent more pragmatic, real-world interpretations by the treating clinicians. Finally, our MDR P. aeruginosa data come from a small number of isolates, as multidrug resistance was not common in our population.
Our data demonstrate that P. aeruginosa, as in past wartime experience, remains a major infectious complication, commonly affecting the most severely injured patients following blast injury. The majority of P. aeruginosa wound and bloodstream infections were not MDR; however, MDR isolation was associated with higher ISS, increased ICU days, mechanical ventilation requirement, prior antipseudomonal antibiotic exposures, and longer hospitalization. Development of antimicrobial resistance upon serial isolation was significant, and among MDR isolates, the best options for empiric treatment remain the oldest and most toxic antimicrobials—even when testing for susceptibility with the newest agents designed to target MDR Gram-negative infections. Two previously unknown outbreaks of P. aeruginosa were identified, again reinforcing the importance of both antimicrobial stewardship and infection control to minimize infectious complications in trauma patients.
ACKNOWLEDGMENTS
We are indebted to the Infectious Disease Clinical Research Program Trauma Infectious Disease Outcomes Study team of clinical coordinators, microbiology technicians, data managers, clinical site managers, and administrative support personnel for their tireless hours to ensure the success of this project. We would also like to acknowledge specifically Patrick McGann and LTC Mary Hinkle of the Walter Reed Army Institute of Research, Multidrug-Resistant Organisms Repository and Surveillance Network for their contributions with the whole genome sequencing.
Contributor Information
CPT Mary B Ford, Brooke Army Medical Center, JBSA Fort Sam Houston, TX 78234, USA.
Katrin Mende, Brooke Army Medical Center, JBSA Fort Sam Houston, TX 78234, USA; Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD 20817, USA.
Susan J Kaiser, Brooke Army Medical Center, JBSA Fort Sam Houston, TX 78234, USA; Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD 20817, USA.
Miriam L Beckius, Brooke Army Medical Center, JBSA Fort Sam Houston, TX 78234, USA.
Dan Lu, Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD 20817, USA.
Jason Stam, Multidrug-Resistant Organism Repository and Surveillance Network, Walter Reed Army Institute of Research, Silver Spring, MD 20910, USA.
Ping Li, Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD 20817, USA.
Laveta Stewart, Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA; Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc., Bethesda, MD 20817, USA.
David R Tribble, Infectious Disease Clinical Research Program, Department ofPreventive Medicine and Biostatistics, Uniformed Services University of the Health Sciences, Bethesda, MD 20814, USA.
Lt Col Dana M Blyth, Brooke Army Medical Center, JBSA Fort Sam Houston, TX 78234, USA.
FUNDING
Support for this work (IDCRP-024) was provided by the Infectious Disease Clinical Research Program (IDCRP), a Department of Defense program executed through the Uniformed Services University of the Health Sciences, Department of Preventive Medicine and Biostatistics through a cooperative agreement with The Henry M. Jackson Foundation for the Advancement of Military Medicine, Inc. (HJF). This project has been funded by the National Institute of Allergy and Infectious Diseases, National Institutes of Health, under Inter-Agency Agreement Y1-AI-5072, the Defense Health Program, U.S. DoD, under award HU0001190002, the Department of the Navy under the Wounded, Ill, and Injured Program, and the Military Infectious Diseases Research Program.
CONFLICT OF INTEREST STATEMENT
None declared.
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