Epidemiological Trends and Evolving Antibiotic Resistance Profiles of Pseudomonas aeruginosa in Burn Patients: A 3-Year Cross-Sectional Surveillance in Northern Iran
ABSTRACT
Background and Aim
Pseudomonas aeruginosa (P. aeruginosa) is a major cause of burn wound infections, posing a significant challenge due to its increasing antibiotic resistance. This study evaluated the prevalence and antibiotic resistance patterns of P. aeruginosa isolates from burn patients admitted to a referral burn center in Northern Iran over 3 years (March 2018–March 2021).
Methods
A retrospective cross-sectional study was conducted on 535 samples collected from burn patients. P. aeruginosa isolates were identified using standard microbiological methods and tested for susceptibility to various antibiotics through the disc diffusion method. Data analysis involved descriptive statistics to assess the prevalence and resistance patterns of the isolates.
Results
P. aeruginosa accounted for 36.1% (193/535) of bacterial isolates. The annual prevalence of P. aeruginosa varied from 19.6% to 44.4%, with the highest frequency observed among patients with 26%–50% total body surface area burns. Skin and soft tissue infections were the most common source (75.1%), followed by bloodstream infections (15%). Notably, 62.7% of isolates originated from intensive care unit (ICU) patients. High resistance was observed against ceftazidime (80%), ciprofloxacin (77.2%), imipenem (76.6%), and meropenem (76.1%). Conversely, amikacin (78.2%) and gentamicin (45%) exhibited the highest susceptibility.
Conclusion
The findings indicate a high prevalence of Pseudomonas with DTR, highlighting the need for stricter antibiotic stewardship and alternative treatment options. While amikacin showed higher susceptibility in our study, the recent CLSI 2023 guidelines suggest its use should be limited to urine isolates. In light of these updated recommendations, the use of aminoglycosides in treating P. aeruginosa infections requires cautious evaluation. Further multicenter studies with larger sample sizes are warranted to generalize these findings.
1 Introduction
Burn injuries, a prevalent and devastating form of trauma, demand immediate medical attention to prevent long-term disabilities and fatalities [1, 2]. Among the concerns for burn patients, wound infections pose a significant threat, with notable pathogens including Staphylococcus aureus, Enterobacteriales, Pseudomonas aeruginosa (P. aeruginosa), and Acinetobacter baumannii. Infections can manifest in various sites, such as wounds, the urinary tract, the bloodstream, the respiratory tract, and the central nervous system during hospitalization [1, 3, 4]. P. aeruginosa, an opportunistic gram-negative bacterium, is a significant cause of burn infections. The bacterium's virulence factors, including exotoxins, proteases, and hemolysins, contribute to wound pathogenesis [5, 6]. The intrinsic resistance of P. aeruginosa to diverse antibiotics and its ability to acquire new resistance mechanisms presents a formidable challenge in infection control [3]. Managing P. aeruginosa infections becomes intricate due to the limited available antimicrobial agents and the escalating emergence of multidrug-resistant (MDR) strains. Combining antimicrobial drugs becomes imperative to enhance bactericidal effects and mitigate resistance levels [7]. Carbapenems are crucial in treating MDR P. aeruginosa infections, but there is growing concern about the rise of Pseudomonas strains with difficult-to-treat resistance (DTR), particularly in burn patients [8, 9]. Regional variations, time dynamics, and healthcare settings contribute to diverse antibiotic resistance patterns, underscoring the necessity for contextual information. Local and regional surveillance studies are pivotal in comprehending global antibiotic resistance trends. Periodic evaluation enables physicians to discern resistance patterns, aiding in prudent antibiotic selection for experimental treatments [10]. This study delves into the prevalence and antimicrobial resistance patterns of P. aeruginosa strains isolated from burn infections in a referral burn center in Northern Iran over 3 years. We hypothesize that P. aeruginosa isolates in burn patients in this region exhibit significant resistance to carbapenems and other critical antibiotics, necessitating exploring alternative treatment options.
2 Methods
2.1 Study Design and Setting
This original article presents a retrospective cross-sectional study conducted at Velayat Burn Hospital in Northern Iran, spanning 3 years from March 2018 to March 2021. This study exclusively involved patients with burn injuries admitted to this specialized referral center for burn care. Skin and soft tissue infections (SSTIs) were defined as erythema, swelling, heat, or purulence around the wound site, accompanied by clinical signs of infection (fever, elevated white blood cell count). Colonization was defined as the presence of bacteria without clinical signs of infection.
2.2 Infection Control or Antibiotic Protocols
At our facility, infection control protocols include regular bathing of patients every 24–48 h as needed, frequent changes of central lines (every 72 h unless clinically indicated), and debridement performed upon admission and subsequently as needed. Carbapenems are used as part of the empiric sepsis regimen, particularly for ICU patients with suspected gram-negative infections. Prophylactic antibiotics are not routinely administered unless clinically indicated based on patient-specific factors.
2.3 Sample Collection and Identification
All samples were collected based on clinical presentation, with no surveillance cultures included. Samples were obtained from different anatomical sites; none were duplicate samples from the same patient simultaneously. In cases where multiple infections occurred, only the first sample from each infection type was included in the study to ensure nonduplication. All isolates of P. aeruginosa were identified using conventional standard microbiological methods, including Gram stain, catalase, oxidase, Simon's citrate agar, triple sugar iron (TSI) agar, Methyl Red-Voges Proskauer (MR-VP) test, pyocyanin pigment production, motility, and growth at 42°C [11]. Identification of P. aeruginosa isolates was confirmed through PCR amplification of the 16S rRNA gene [12]. The isolates were stored at −70°C for further processing.
2.4 Antibiotic Susceptibility Testing
The antimicrobial susceptibility of P. aeruginosa isolates was determined using the conventional disc diffusion method on Mueller–Hinton agar, following Clinical Laboratory Standards Institute (CLSI) guidelines [13]. The antibiotics tested included imipenem (10 μg), meropenem (10 μg), ceftazidime (30 μg), amikacin (30 μg), gentamicin (10 μg), tobramycin (10 μg), and ciprofloxacin (5 μg). Quality control was ensured using P. aeruginosa (ATCC 27853). Plates were incubated aerobically at 37°C for 16–20 h, and results were interpreted accordingly.
2.5 Statistical Analysis
Descriptive statistical analyses were performed using SPSS software, version 21.0 (IBM Corp., Armonk, NY). In accordance with the guidelines and the Statistical Analyses and Methods in the Published Literature (SAMPL) guidelines, we prioritized the accurate and transparent presentation of descriptive statistics with clinical relevance. Categorical variables (e.g., infection source, resistance pattern) are reported as absolute frequencies and percentages. Continuous variables (e.g., patient age) are presented as mean ± standard deviation (SD). No inferential statistical tests (e.g., χ2 or t-tests) were conducted, as the study was not designed or powered for subgroup comparison. Effect sizes and confidence intervals were not calculated due to the descriptive nature of the study. All analyses were prespecified, with no post hoc hypothesis testing. While subgroup patterns are qualitatively discussed in Sections 3 and 4, no formal statistical comparisons were made.
2.6 Ethics Approval
This study was approved by the Ethics Committee of Guilan University of Medical Sciences (IR.GUMS.REC.1399.277). Since it was a retrospective analysis of routinely collected clinical data, individual patient consent was waived in accordance with national ethical guidelines.
3 Results
3.1 Prevalence of P. aeruginosa Infections
During the study period, 535 nonduplicate samples were collected from 193 patients, and P. aeruginosa was a significant contributor, constituting 36.1% (193/535) of bacterial isolates. The annual prevalence of P. aeruginosa infections exhibited variations, ranging from 19.6% to 44.4%. The highest prevalence was observed in 2018 (44.4%), followed by a decline in 2019 (19.6%) and a subsequent rise in 2020 (45.8%).
3.2 Patient Demographics and Clinical Characteristics
The mean age of the patients in this study was 39.2 ± 19.8 years, with a broad age range from 1 to 86 years. Among the isolates, 80.3% (155/193) were obtained from male patients and 19.7% (38/193) from female patients. Table 1 summarizes the demographic data of the study population.
| Variable | Value |
|---|---|
| Median age (years) | 39.2 ± 19.8 |
| Gender (male/female) | 80.3%/19.7% |
| Type of burns | Thermal (85%), electrical (13%), chemical (2%) |
| ICU admissions | 62.7% |
| TBSA categories (%) | 11%–25% (27%), 26%–50% (40.7%), > 50% (30%) |
3.3 Clinical Sources and Distribution
SSTIs emerged as the predominant clinical source of P. aeruginosa, accounting for 75.1% of cases, followed by bloodstream infections (BSIs) at 15%. Notably, the majority of P. aeruginosa isolates, 62.7%, were traced back to patients in the intensive care unit (ICU), underscoring the nosocomial nature of these infections. Further, burn patients with a total body surface area (TBSA) of 26%–50% exhibited the highest frequency of P. aeruginosa isolation at 40.7%.
3.4 Outcome and Mortality
Among the 193 cases of P. aeruginosa infection, 36.3% of patients experienced fatal outcomes, while 63.7% were discharged.
3.5 Antibiotic Resistance Patterns
The antibiotic susceptibility testing revealed concerning resistance patterns among P. aeruginosa isolates. High resistance levels were observed against key antibiotics commonly used in clinical practice. Ceftazidime, a third-generation cephalosporin, exhibited the highest resistance at 80%, followed closely by ciprofloxacin (77.2%), imipenem (76.6%), and meropenem (76.1%). Conversely, aminoglycosides demonstrated relatively higher susceptibility, with amikacin and gentamicin showing 78.2% and 45% susceptibility, respectively (Table 2).
| Class | Antibiotics | Resistant | Intermediate | Sensitive | Total | |||
|---|---|---|---|---|---|---|---|---|
| No. | % | No. | % | No. | % | No. | ||
| Cephalosporins III | Ceftazidime | 156 | 80 | 4 | 2 | 33 | 17.1 | 193 |
| Carbapenem | Meropenem | 147 | 76.1 | 12 | 6.2 | 34 | 17.6 | 193 |
| Carbapenem | Imipenem | 148 | 76.6 | 8 | 4.1 | 37 | 19.1 | 193 |
| Aminoglycosides | Amikacin | 32 | 16.5 | 10 | 5.1 | 151 | 78.2 | 193 |
| Aminoglycosides | Gentamicin | 72 | 37.3 | 34 | 17.6 | 87 | 45 | 193 |
| Aminoglycosides | Tobramycin | 103 | 53.3 | 32 | 16.5 | 58 | 30 | 193 |
| Quinolones | Ciprofloxacin | 149 | 77.2 | 7 | 3.6 | 37 | 19.1 | 193 |
4 Discussion
The emergence and evolution of antibiotic resistance in P. aeruginosa present a formidable challenge in the context of burn wound infections. In this 3-year surveillance study conducted in a referral burn center in Northern Iran, we observed a notable prevalence of P. aeruginosa (36.1%) among bacterial isolates from burn patients. This finding is consistent with reports from various burn centers globally, highlighting the significance of P. aeruginosa in burn wound infections [14-17]. The increasing resistance of P. aeruginosa to commonly used antibiotics, particularly cephalosporins, carbapenems, and fluoroquinolones, raises concerns about the limited therapeutic options available. Our study revealed high resistance rates against ceftazidime (80%), ciprofloxacin (77.2%), imipenem (76.6%), and meropenem (76.1%). Notably, the observed resistance to carbapenems is alarming, considering their crucial role in treating MDR P. aeruginosa infections [8, 9]. However, it is essential to note that CLSI updated the breakpoints for aminoglycosides in 2023, significantly impacting the interpretation of susceptibility results. According to the latest guidelines, gentamicin is no longer recommended for P. aeruginosa; amikacin should only be considered for urinary tract infections. The evolving breakpoints emphasize the need for updated testing and cautious interpretation of aminoglycoside efficacy. Several studies from 2006 to 2015 identified imipenem as the most effective antibiotic among the groups tested on P. aeruginosa isolates, while some studies in recent years have reported the high resistance of these strains to carbapenems [3, 8, 18-22]. The changing resistance patterns observed in this study, with a significant increase in carbapenem resistance over the 3 years, emphasize the dynamic nature of antibiotic resistance. The factors contributing to this evolution are likely multifaceted, involving selective pressure from antibiotic usage, nosocomial transmission, and the acquisition of resistance genes. The rise of Pseudomonas strains with DTR necessitates urgent attention to antibiotic stewardship practices in burn care settings. In contrast to the observed resistance, aminoglycosides, specifically amikacin, exhibited relatively higher susceptibility (78.2%). This finding suggests that aminoglycosides might be more effective treatment options for P. aeruginosa infections in our setting. This result aligns with the evolving nature of antibiotic susceptibility patterns and underscores the importance of ongoing surveillance to guide empirical treatment strategies. In confirmation of these findings, the results of some past studies (2012–2013) indicated the high resistance of P. aeruginosa isolates to aminoglycosides and ceftazidime, while recent studies have shown a decrease in their resistance to aminoglycosides [20, 23-26]. However, further research and multicenter studies with higher sample sizes are needed to generalize the results. The observed high prevalence of P. aeruginosa in the ICU and its association with SSTIs underline the nosocomial nature of these infections. Burn patients with a TBSA of 26%–50% were more prone to P. aeruginosa infections, emphasizing the need for targeted preventive measures in this high-risk population. The clinical outcomes revealed a concerning mortality rate of 36.3% among P. aeruginosa-infected patients, highlighting the severity and impact of these infections. The association between resistance patterns and clinical outcomes warrants further investigation to guide appropriate therapeutic interventions.
While this study was designed primarily as a descriptive epidemiological surveillance, we observed notable differences in resistance rates across clinical subgroups. For example, P. aeruginosa isolates were more frequent among ICU patients (62.7%) and burn patients with TBSA involvement of 26%–50% (40.7%). These patterns suggest that certain high-risk populations may be more prone to MDR infections. Although formal statistical comparisons were not performed due to retrospective limitations, our findings align with previous studies that reported increased resistance burden in ICU settings and among patients with more extensive burns [17, 16]. Future studies with prospective data collection and larger sample sizes should include subgroup analyses to explore these trends further and guide targeted antimicrobial strategies.
5 Conclusion
This study provides a 3-year surveillance of P. aeruginosa infections in burn patients, highlighting a high prevalence and concerning resistance levels, particularly among carbapenem-resistant strains. The presence of Pseudomonas strains with DTR underscores the critical need for enhanced antibiotic stewardship in burn care settings. Although amikacin demonstrated relatively higher susceptibility, recent CLSI guidelines limit its recommended use, urging a reassessment of aminoglycoside efficacy in such infections. The findings suggest that while effective antibiotic options are becoming increasingly scarce, focused infection control practices and regional resistance monitoring may help manage the spread and impact of resistant Pseudomonas strains in burn centers.
5.1 Limitations
This study has several limitations. First and most notably, our antimicrobial susceptibility testing relied on the 2021 CLSI breakpoints, which were the most current at the time of data collection. Since then, the 2023 CLSI update has introduced significant changes to the interpretive criteria, particularly for aminoglycosides—including the recommendation against the use of gentamicin for P. aeruginosa and restriction of amikacin use to urinary tract isolates only. As a result, our reported susceptibility rates for these antibiotics may not reflect current clinical guidelines, potentially limiting the translational applicability of our findings to real-world clinical decision-making.
Moreover, the reliance on earlier breakpoints may also hinder reproducibility, as future studies applying the newer CLSI standards could yield different susceptibility profiles from the same isolates. A retrospective reevaluation of our susceptibility data based on updated breakpoints was not feasible due to the lack of raw inhibition zone measurements. To address this, we recommend that future studies collect and report zone diameters in addition to susceptibility categories to enable retrospective reanalysis as guidelines evolve.
Additionally, this was a single-center study in Northern Iran, which may limit the generalizability of the results to other regions or national trends. The absence of centralized surveillance data from other Iranian burn centers further constrains broader comparisons.
5.2 Suggestions for Future Studies
Future research should focus on conducting multicenter studies across multiple burn centers to capture a broader range of regional variations in P. aeruginosa resistance, thereby enhancing the generalizability of findings. Prospective studies incorporating real-time CLSI guidelines will also ensure that resistance data align with the latest standards, maintaining clinical relevance. Expanding antibiotic testing to include agents like cefepime could offer insights into potential alternative treatments. Additionally, investigating the association between resistance profiles, such as DTR, and patient outcomes would help refine treatment strategies and improve management practices in high-risk burn patient populations.
Author Contributions
Mahsa Sadeghi: writing – original draft, investigation. Mohammadreza Mobayen: conceptualization, supervision. Tofigh Yaghubi Kalurazi: methodology, visualization. Zahra Mehrdad: investigation, data curation, resources. Mohammadamin Khajavi Gaskarei: investigation, formal analysis, software. Hadi Sedigh Ebrahim-Saraie: supervision, resources, project administration, writing – review and editing, validation. Meysam Hasannejad-Bibalan: visualization, project administration, resources, supervision, methodology. All authors contributed to the study conception, design, data collection, analysis, and manuscript drafting and revising. All authors have read and approved the final version of the manuscript.
Acknowledgments
The authors express gratitude to the Burn and Regenerative Medicine Research Center of Guilan University of Medical Sciences for their technical support.
Ethics Statement
This study was approved by the Ethics Committee of Guilan University of Medical Sciences (IR.GUMS.REC.1399.277).
Consent
Since it was a retrospective analysis of routinely collected clinical data, individual patient consent was waived in accordance with national ethical guidelines.
Conflicts of Interest
The authors declare no conflicts of interest.
Transparency Statement
The lead authors Hadi Sedigh Ebrahim-Saraie and Meysam Hasannejad-Bibalan affirm that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.
Open Research
Data Availability Statement
The data supporting the findings of this study are available from the corresponding author upon reasonable request. Dr. Meysam Hasannejad-Bibalan had full access to all of the data in this study and takes complete responsibility for the integrity of the data and the accuracy of the data analysis.
