Objectives: Although Pseudomonas species infections have decreased since the introduction of anti-pseudomonal antibiotics, these infections still cause mortality in immunocompromised patients. This study investigated the antibiotic resistance and mortality risk factors of Pseudomonas species isolated from various materials in solid-organ transplant recipients.
Materials and Methods: Infections due to Pseudomonas species between 2020 and 2024 were retrospectively analyzed in solid-organ transplant recipients aged >18 years. Recurrent infections in the same patient were excluded. We analyzed resistance profiles to commonly used antibiotics, demographic and laboratory data, and risk factors affecting 30-day hospital mortality using the SPSS version 25 statistical program.
Results: Pseudomonas species infection was detected in 80 solid-organ transplant recipients: 70% were male patients, 78% had living donors, and 57% were kidney transplant recipients. Pseudomonas aeruginosa was the most frequently detected species (95%). The most common agent was detected in body fluid samples. Thirty-day hospital mortality was 18.8%. Mortality was significantly higher in patients with resistance to amikacin, ciprofloxacin, cefepime, and colistin, and in those with invasive procedures and mechanical ventilation, and in patients with sepsis and in intensive care. Resistance rates were higher between 2020 and 2023, when the COVID-19 pandemic was active.
Conclusions: Although changes occurred over the years, antibiotic resistance increased during the COVID-19 pandemic. This increased resistance created difficulties in empirical and pathogen-targeted treatments. Amikacin and colistin are recommended for empirical treatment of Pseudomonas species infections due to their low resistance rates. In addition, unnecessary invasive procedures should be avoided, discontinuing them when the indication is no longer there. Adhering to infection prevention can reduce risk factors, as sepsis, intensive care unit stay, and invasive procedures affect mortality.

Key words : Bacterial infections, Sensitivity to antibiotics, Transplantation

Introduction
Pseudomonas species are gram-negative bacilli that are commonly found in nature. Pseudomonas aeruginosa is one of the most frequently encountered pathogens causing serious infections. The toxins, pili, flagellum, and biofilm-forming ability of P aeruginosa increase its virulence, giving this species the capacity to cause serious infections. Pseudomonas infections are a significant cause of health care-associated and hospital-acquired infections in patients with HIV, patients with cystic fibrosis, febrile neutropenic patients, and those receiving immunosuppressive therapy for various reasons.^1-3 However, in immunosuppressed patients and solid-organ transplant recipients (SOTR), P aeruginosa is problematic to monitor due to its potential to cause severe infections in wounds and burns, including bacteremia, urinary tract infections, pneumonia, and the development of multidrug resistance over the years.^4-6 Risk factors for Pseudomonas species infections include transplant, skin burns, neutropenia, structural lung diseases, having received antimicrobial treatment within the last 3 months, catheterization, invasive procedures, prolonged hospital stays, and being on a ventilator for more than 3 days.^7,8 Although Pseudomonas species infections have decreased since the introduction of anti-pseudomonal antibiotics, these infections still cause mortality in immunocompromised patients.⁹ As a result of multidrug resistance, fully drug resistance, and difficult-to-treat Pseudomonas strains, patients can experience inadequate clinical responses.¹⁰ Here, we investigated the antibiotic resistance and mortality risk factors of Pseudomonas species isolated from various materials in SOTR.

Materials and Methods
We retrospectively analyzed infections due to Pseudomonas species shown in SOTRs aged >18 year between 2020 and 2024. Various materials from SOTRs, such as blood, wounds, urine, and body fluids, were cultured in blood culture medium and eosin-methylene blue medium (Figure 1, Figure 2), and disk diffusion was performed in Müller-Hinton medium for antibiograms (Figure 3). Recurrent infections in the same patient were excluded. Polymicrobial growth, that is, growth considered as contamination in SOTR, were also not included in the study. We analyzed resistance profiles to commonly used antibiotics, demographic and laboratory data, and risk factors affecting 30-day hospital mortality. We used SPSS version 25.0 (SPSS Inc) for statistical analyses. We examined conformity of the variables to the normal distribution with histogram charts and the Shapiro-Wilk test. We presented descriptive data as mean ± SD and as median (minimum to maximum) values. We used the Kruskal Wallis test to compare nonnormally distributed (nonparametric) variables between more than 2 groups. We used Bonferroni multiple comparisons to investigate reasons for significant differences between groups. We presented results for categorical variables as frequency and percentage values and used the χ2 exact test to analyze results. P < .⁰⁵ was considered statistically significant. The protocols conformed to the ethical guidelines of the 1975 Helsinki Declaration and the Declaration of Istanbul on Organ Trafficking and Transplant Tourism.

Results
Between 2020 and 2024, infections caused by Pseudomonas spp. were detected in 80 patients; 56 patients (70%) were male. Average age was 41.6 ± 15.6 years. Of the transplants performed in these patients, 63 were from living donors (78.8%) and 17 were from deceased donors (21.3%). Of the Pseudomonas species strains that grew, 76 (95%) were Pseudomonas aeruginosa and the remaining 4 (5%) were other types. The number of Pseudomonas species strains detected varied between 14 and 19 over the years. The most common comorbidities in patients were chronic renal failure (57.5%), hypertension (40%), and diabetes mellitus (25%). Pseudomonas bacteria were most frequently detected in body fluid samples (28.7%), urine cultures (23.8%), respiratory samples (22.6%), wound and tissue samples (12.5%), and blood cultures (12.5%). Median C-reactive protein value was 78 mg/dL (2-411 mg/dL). Twenty-seven patients (33.8%) were followed up in the intensive care unit. Nine patients (11.3%) had signs of sepsis. Thirty-day hospital mortality was observed in 15 patients (18.8%) (Table 1). When the factors affecting 30-day mortality were investigated, no difference was found between male versus female patients and the transplanted organ (P > .⁰⁵). However, mortality was significantly higher in transplants from living donors (P = .⁰¹⁸). Furthermore, in Pseudomonas infections, mortality was significantly higher in the presence of multidrug resistance (P = .⁰³³), amikacin resistance (P = .⁰²⁸), ciprofloxacin resistance (P = .⁰⁴⁴), cefepime resistance (P = .⁰²¹), and colistin resistance (P = .⁰⁰⁶) (Table 2). Furthermore, mortality was higher in patients who underwent invasive procedures such as central venous catheterization (P = .⁰⁰¹), urinary catheterization (P = .⁰⁰¹), mechanical ventilation (P = .⁰⁰¹), and total parenteral nutrition (P = .⁰⁰¹). Having received antibiotics in the last 3 months did not significantly affect mortality (P = .¹⁰⁵). Mortality was higher in SOTRs who were monitored in the intensive care unit (P = .⁰¹) and those with concomitant sepsis (P = .⁰¹). When the antibiotic resistance of Pseudomonas bacteria was investigated over the years, multidrug resistance, particularly resistance to meropenem and colistin, was highest in 2021. Resistance to amikacin, levofloxacin, cefepime, ceftazidime, ciprofloxacin, and piperacillin-tazobactam was detected at a higher rate in 2020 compared with other years. A general decrease in antimicrobial resistance was observed in 2023. However, in 2024, resistance to meropenem, cefepime, ceftazidime, and piperacillin-tazobactam increased again. Complete drug resistance was detected at a rate of 6% in 2020 and 7% in 2021 and was not observed in subsequent years (Figure 4).

Discussion
Pseudomonas species bacteria are common microorganisms found in nature and hospital settings, and their treatment has become problematic in recent years because of increasing resistance. Recently, the term “difficult-to-treat resistance” has begun to be used, especially for Pseudomonas bacteria resistant to carbapenems, β-lactam-β-lactamase inhibitor combinations, and fluoroquinolones.¹¹ In patients who are particularly vulnerable to infections because of immunosuppressive treatments such as solid-organ transplant and multiple invasive procedures, bacteremia can lead to high mortality rates, ranging from 20% to 40%.^12-14 In our study, which covered the period from March 11, 2020, the start date of the COVID-19 pandemic, to the following 4 years, we observed between 14 and 19 Pseudomonas species infections. During the pandemic, the increased use of antibiotics worldwide, coupled with a significant rise in hospitalizations, led to a rapid increase in antibiotic-resistant infections.¹⁵ As shown in our study, P aeruginosa is a leading cause of health care-associated and hospital-acquired skin and soft tissue infections, surgical site infections, pneumonia, bloodstream infections, and urinary tract infections and is responsible for 10% to 20% of hospital-acquired cases.^16,17 Among hospital-acquired infections, P aeruginosa is known to cause serious morbidity and mortality. In a retrospective study of 136 patients with P aeruginosa bacteremia, the median age of the patients was 55 years, 78.7% were hospital-acquired infections, and the 30-day mortality rate was 39% (53 of 136 patients).¹⁸ Although our study had a smaller number of patients, our median age was 53 years, similar to the literature, the 30-day mortality rate was 18.8%, and our sepsis rate was lower at 11.3%. Antibiotic resistance studies have shown that 50% of P aeruginosa isolates are multidrug resistant.¹⁹ After colonization, P aeruginosa accumulates genetic mutations, both natural and acquired, conferring antibiotic resistance and improved survival in the host environment. Modification of antibiotic targets can foster numerous intrinsic and acquired resistance mechanisms, including reduced membrane permeability, expression of efflux systems, biofilm formation, and quorum sensing.^20,21 We found the overall average MDR rate to be 22.5%. However, although this rate is low, an examination of the annual distributions in our study showed that the MDR rate reached 40% in 2021. This situation is thought to be related to the intensive antibiotic use during the COVID-19 pandemic. Antibiotic resistance makes it difficult to provide effective treatment and increases mortality. Therapies targeting bacterial virulence are an effective approach to alleviating growing bacterial resistance to traditional antibiotics that aim to either kill bacteria or inhibit their growth.¹⁶ Combination therapies can lead to an increased frequency of side effects. In our study, we found that mortality was increased in the presence of complete drug resistance, amikacin resistance, ciprofloxacin resistance, cefepime resistance, and colistin resistance. Immunosuppressive therapies, previous antibiotic use, corticosteroids, surgery and invasive procedures, diabetes, and mechanical ventilation can all contribute to the development of multidrug resistance.^22-25 In our study, mortality was also frequently observed in the presence of central venous catheterization, urinary catheterization, mechanical ventilation, and total parenteral nutrition. In conclusion, although rates have changed over the years, increased antibiotic resistance was observed during the pandemic period. This increased resistance created difficulties in empirical and pathogen-targeted treatments. Amikacin and colistin are recommended for the empirical treatment of Pseudomonas species infections due to their low resistance rates. Furthermore, avoiding unnecessary invasive procedures, discontinuing them when the indication is no longer there, and adhering to infection prevention will reduce risk factors, as sepsis, intensive care unit stay, and all invasive procedures can affect mortality.