Stenotrophomonas maltophilia outbreak in a university hospital: epidemiological investigation and literature review of an emerging healthcare-associated infection

ABSTRACT Stenotrophomonas maltophilia was considered to be a low-virulence organism. But it has emerged as a prominent opportunistic pathogen in patients with certain risk factors. This study aimed to describe an outbreak experienced in our hospital with all dynamics while evaluating previous S. maltophilia outbreak reports. S. maltophilia isolates were obtained from a university hospital in Türkiye in a seven-months period. Antimicrobial resistance, type of infections, predisposing factors of infected patients, antibiotic therapy, outcome of infections, and outbreak source were investigated. Also, S. maltophilia outbreaks in the literature were reviewed. In the 12 months prior to the outbreak, prevalence rate of clinical samples including S. maltophilia was 7/1,000 patient per day, opposed to 113/1,000 patient per day during the outbreak. Although a large number of cases were observed in a short seven-month period, a source of contamination could not be detected. Stable mortality rates (or remaining close to the average) during outbreaks can be attributed to the careful attention paid by laboratory and clinic physicians during procedures. S. maltophilia has potential to spread outbreaks and infect patients in operating rooms and intensive care units during invasive procedures.


INTRODUCTION
Stenotrophomonas maltophilia is a glucose nonfermenting Gram-negative bacteria found widely distributed in natural and artificial settings 1 .S. maltophilia, traditionally considered to be a low-virulence organism, has emerged as a prominent opportunistic pathogen that causes serious human infections, especially in severely debilitated patients 2 .Bloodstream infections, bone and joint infections, urinary tract infections, endocarditis, pneumonia, and meningitis are among the infections caused by S. maltophilia 1,3,4 .Many fomites and medical equipment in clinical settings may serve as promising reservoirs of S. maltophilia infection due to their ubiquitous nature and capability to form biofilm on any type of moist surface 4 .The well-known risk factors for S. maltophilia infections include: longer hospital stays requiring invasive procedures, admission to an intensive care unit, organ transplantation, mechanical ventilation, indwelling catheters, prior exposure to antibiotics or immunosuppressant therapy, cystic fibrosis, underlying malignancy, and HIV infection 5 .Aminoglycosides and routinely used carbapenems cannot be used against S. maltophilia due to its intrinsic resistance 2 .For its favorable susceptibility and positive clinical results in treated individuals, trimethoprimsulfamethoxazole (TMP-SMX) is a first-line therapy for S. maltophilia infections 6 .This report describes the results of the investigation on the S. maltophilia outbreak and aims to evaluate outbreak and patient characteristics.

MATERIALS AND METHODS
This ambidirectional observational study describes the investigation of S. maltophilia isolates obtained from a university hospital in Turkiye in a seven-months period (August 2021 to February 2022).We observed a high isolation rate from various clinical specimens in the different wards and ICUs of a university hospital.The introduction of outbreak screening and control measures in 2021 allowed us to assess the outbreak dynamics.The study investigated TMP-SMX resistance, whether isolates were infective or colonizers, the type of infections, the predisposing factors of infected patients, antibiotic therapy, and the outcome of infections, prognostic factors associated with mortality, and outbreak source.To expose the clinical characteristics of S. maltophilia infections in clinical settings, we searched the literature on S. maltophilia outbreaks and summarized the data along with the previous reports.For the literature search, the PubMed and Google Scholar databases were explored.The terms "Stenotrophomonas maltophilia outbreak", "Stenotrophomonas maltophilia epidemic", and "Stenotrophomonas maltophilia bacteremia" were searched.The reference lists of the articles under investigation have been perused for any overlooked articles and pseudooutbreaks were excluded.

Ethical approval
The study was carried out in compliance with the Declaration of Helsinki and approved by the Gaziantep University Ethics Committee for Clinical Trials and by the Gaziantep University Clinical Research Ethics Committee with the consent Nº 2021/161.

Bacterial isolates
A total of 113 consecutive nonduplicate S. maltophilia isolates were obtained from different specimens.Only one isolate was obtained from each patient.For blood culture analysis, the BACTEC FX system (Becton Dickinson, USA) was used.Bacterial identification and antibiotic susceptibility testing were done using the automatic VITEK2 (bioMérieux, France) system.Clinical breakpoints set by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) were used to assess drug susceptibility 7 .

Patient characteristics
A medical chart review was performed for data on patients' clinical backgrounds, admission wards, time (days) from admission to the occurrence of S. maltophilia infection, underlying diseases, primary focus of infection, results of blood culture, history of antibiotics use (within 30 days before the onset of S. maltophilia infection), and prognosis.The source of the infection was clinically determined by referencing the results of microbiological examinations.When some bacteria other than S. maltophilia were detected in cultures, it was regarded as a polymicrobial infection.The 30-day and 90-day mortalities were defined as the periods from the onset of S. maltophilia infection to patient death.Infection or colonization was distinguished according to clinical diagnoses given in final reports.All S. maltophilia strains isolated from sterile sites such as blood and cerebrospinal fluid, as well as those isolated from the skin, mucus membranes, wounds, and endotracheal tubes in the presence of clinical signs and symptoms were considered infections and included in the study.Selected cases were discussed with the physician to reveal the clinical relevance of isolates.The definitive diagnosis of infection was clinically established.Colonization was defined as the presence of S. maltophilia on skin, mucous membranes, in wounds, or excretions/secretions without causing adverse clinical signs or symptoms.

Epidemiological and environmental investigations and control measures
Admission and weekly screening by throat swabs were introduced in September 2021 after recognizing the S. maltophilia outbreak in ICUs.Admission screening showed that outbreak strains were acquired in the ICU, a questionnaire was applied to cover risk factors such as bed space, mechanical ventilation, surgery, and bronchoscopy.A total of 76 samples were obtained from different places in the hospital environment such as floor, bed rails, bedside table, ventilator, and intravenous pump.
Intervention techniques were addressed between the infection control unit, the physicians, and the staff throughout the outbreak.To lower the risk of nosocomial transmission, the infection control unit delivered training that emphasized proper hand hygiene.The frequency of water system disinfection was increased and microbiologic testing of pipes were performed.Since most patients were under mechanical ventilation, respiratory therapy was thoroughly examined, focusing on the equipment cleaning, disinfection, suctioning, and treatment delivery.In patient rooms, disposable aprons were implemented.The authorization for ICU access was modified and restricted.A general sanitation program using sodium hypochlorite solution was applied to all fomites in patient rooms.

RESULTS
A total of 113 isolates were obtained from nonrepetitive patients.However, 42 patients were not included in the study due to colonization or contamination when clinical data were considered.Ages of the 71 individuals that were included in the study ranged from 1 to 97, median age was 53.Of the 71 patients, 65% (n = 46) were hospitalized in the ICU, and 35% (n = 25) were under inpatient care in other wards.The distribution of ICU patients consisted of Anesthesia ICU (n = 12; 17%), Neurology ICU (n = 11; 15%), Internal Medicine ICU (n = 10; 14%), Surgical ICU (n = 7; 10%), and Pediatric ICU (n = 6; 8%).Table 1 shows the clinical and demographic characteristics of the 71 patients included in the study.
Among the specimens analyzed, blood cultures constituted the most prevalent category, accounting for 50 (70%) of the samples, followed by tracheal aspirate cultures (n = 9; 13%), sputum cultures (n = 7; 10%), cerebrospinal fluid (CSF) cultures (n = 4; 6%), and urine cultures (n = 1; 1%).Cases presenting blood culture growth were considered primary bacteremia, since a different primary focus was not detected.Bacteremia occurred in four patients after a cardiovascular interventional procedure.Patients with primary bacteremia who required mechanical ventilation in the ICU made up the entire group of patients who passed away (n = 7; 9.9%).Ventriculoperitoneal shunt was present in all four patients with growth in CSF.Additionally, seven of the patients who had S. maltophilia growth in respiratory tract samples also showed polymicrobial growth.Serratia marcescens (n = 3), Pseudomonas aeruginosa (n = 2), Acinetobacter baumannii (n = 2), and Klebsiella pneumoniae (n = 1) were accompanying microorganisms.Only two isolates were not susceptible to TMP-SMX (97%).Furthermore, susceptibility results for levofloxacin in the 71 strains included were found to be 4.2% susceptible, 16.9% intermediate susceptible, and 78.9% resistant, whilst those for ceftazidime were found to be 61.9%susceptible, 33.9% intermediate susceptible, and 4.2% resistant.
In the 12 months prior to the outbreak, the prevalence rate of clinical samples including S. maltophilia was 7/1,000 patient per days, opposed to 113/1000 patient per days during the outbreak.From March 2022 to April 2022, we performed a post-outbreak observation and the microbiological control of water was within the safety standards.After the interventions, the rates of S. maltophilia cases in the hospital decreased from 113/1000 patients to 2/682 patients receiving inpatient care.When analyzing the case distribution from 2021 to 2022 (Figure 1), we concluded that the onset of environmental sampling and control measures had created a decrease in the number of cases.although these effects were temporary.

DISCUSSION
The opportunistic pathogen S. maltophilia has emerged as a significant global threat and its infections have become more frequently reported 2 .Immunocompromised patients with underlying illnesses or those undergoing invasive procedures are typically affected, since S. maltophilia can be found on the surfaces of devices, supplies, and equipment in healthcare facilities 28 .Infections frequently progress as a result of the use of broad-spectrum antibiotics, prolonged hospital admissions, ICU stays, mechanical ventilation, indwelling urinary catheters, and the use of equipment that gets direct contact with the respiratory tract 5 .Respiratory tract infections (pneumonia and acute COPD exacerbations), bacteremia, biliary sepsis, infections of the bones and joints, of the urinary tract, and of the soft tissues, endophthalmitis, eye infections (keratitis, scleritis, and dacryocystitis), endocarditis, and meningitis are among the infections associated with S. maltophilia 1,3,9,11,12,17,27 .In our investigation, S. maltophilia was primarily isolated from blood samples (70.4%), followed by samples from the respiratory system (22.5%).Based on our literature search, S. maltophilia was predominantly isolated from blood samples (41.6%) and respiratory system samples (34.8%) in a total of 20 outbreak reports, involving 204 patients.
Despite the fact that malignancy was the most frequent underlying cause in our study, the risk factors were confirmed by a large percentage of patients requiring mechanical ventilation and being hospitalized in intensive care.Additionally, the history of antibiotic use in the hospital was at a very high level with 76%.However, considering that the average hospital length of stay before S. maltophilia infection was 13.2 days, such outcome was expected.Unfortunately, prolonged hospital stays may require increased use of antibiotics.By reviewing the outbreak reports, we found that hemodialysis was a significant factor in many of the published outbreaks and pseudo-outbreaks.Dialysis is an invasive method that often involves the insertion of needles and catheters.These procedures can ultimately introduce pathogens into the bloodstream if not carried out properly 8 .Biofilms frequently discharge bacteria, chemicals, and endotoxins that can pass through the dialysis membrane and cause an infection 29 .Likewise, bone marrow transplantation seemed to be a facilitating factor for S. maltophilia infection.An alteration in the gastrointestinal mucosal barriers may lead to S. maltophilia bacteremia in patients.A significant deterioration of the host immune system during the preengraftment stage, such as severe protracted neutropenia, as well as the selective action of antibiotics, may raise the risk of bacteremia linked with altered mucosal barriers 14 .An outbreak of S. maltophilia meningitis caused by central nervous system interventional procedures has previously been reported in the literature 27 .Similarly, in our study, S. maltophilia growth was observed in the CSF samples of four patients with ventriculoperitoneal shunts, indicating that S. maltophila is a pathogen that should be considered in procedures involving the central nervous system.We also noticed that outbreaks and pseudo-outbreaks of bronchoscopyrelated S. maltophilia infections were frequently reported.Bronchoscope examinations are performed on a significant number of patients every day, which emphasizes the need for updating current recommendations, promoting proper hygiene practices, and disclosing new concerns to ensure patient safety 30 .
Malignancy, failure to remove central lines, and ineffective antibiotic treatment were risk factors associated with mortality among patients with S. maltophilia bacteremia 31 .ICU stay, malignancy, renal disease, and inappropriate antibiotic therapy were risk factors for mortality of patients with nosocomial S. maltophilia pneumonia 32 .In our study, the whole group of deceased patients had been diagnosed with primary bacteremia and were on ventilatory support in the ICU.Previously, researchers reported that attributable mortality of S. maltophilia ranged 12%-37.5% 33 .Crude mortality rates were found range from 14% to 69% in patients with S. maltophilia bacteremia 4 .For the 71 patients included in our study, the overall mortality rate was 9.9%.Our literature research indicated that among a total of 204 cases in the 20 outbreak reports, the mortality rate was found to be 13.7%, despite the fact that it ranged from 0% to 100% in those reports.We believe that the fact that clinics and laboratories physicians pay closer attention to the protocols during outbreaks explains mortality not rising and remaining near the average.Also, in our study, the TMP-SMX resistance rate was only 3%.A recent metaanalysis of prevalence studies demonstrated that the TMP-SMX resistance rate was 9% worldwide, and that Asia was the most predominant location with a 19.2%rate 34 .However, in the literature, there are reports that define a higher prevalence of TMP-SMX resistance up to 32.8% 35 .Numerous nosocomial sources, including the hands of medical staff, ventilator circuits, dialysis machines, shower heads, sink traps, and water faucets have yielded S. maltophilia isolates 36 .In a number of countries, hospital water systems and contaminated medical equipment hold accountable for most cases of nosocomial S. maltophilia infections 37 .Several virulence factors produced by S. maltophilia aid in the spread of infection 38 .Those traits have made S. maltophilia a significant pathogen attributed to healthcare-associated infections 28 .Hospital outbreaks have been linked to S. maltophilia strains that can attach to and form biofilms on medical equipment such as prosthetic equipment, blood, and urine catheters.Antibiotics and immune system defenses are overwhelmed by biofilms 39 .Biofilm formation is facilitated by microbial contamination, presence of organic nutrients, dead ends, low fluxes, and periods of no flow, therefore it is crucial to stop a biofilm from forming at first 29 .Biofilms on faucets, shower heads, and surrounding pipes can be detected via procedures, such as6-monthly pre-flush water samples 9 .In light of the data from previous studies, water systems were the primary area of focus for our investigation into the origins of the outbreak.However, despite intensive environmental sampling and a large number of cases in a short period of seven months, a source of contamination such as tap water, etc., that would cause infection could not be detected.When we examined the epidemic reports on the literature, we found that in 60% of the cases, S. maltophilia was isolated from areas with water and water contact, including taps and devices taken together.The organoleptic quality of mains water is improved by carbon filters installed at drinking water dispensers due to the removal of chlorine and other ions.The 6 mm diameter PVC tubes allowed S. maltophilia to form a biofilm since chlorine dioxide-a disinfectant-absent after the carbon filter 9 .Interventions include design modifications, such as avoiding dead legs, blind ends, flow straighteners, and unneeded thermostatic mixing valves, as well as removing underutilized outlets and flexible tubes 9 .It is also advised that any carbon filters or narrow-diameter tubing attached to water additions in larger healthcare settings be periodically examined for the growth of nonfermenting microorganisms 9 .In most cases, the outbreak seems to have diminished after better general sterilization protocols were applied 11,14,16,18 .In our study, we observed that, although the number of cases had decreased temporarily as a result of the implementation of environmental sampling and control measures, the outbreak had not halted.It was suggested that the piping system was contaminated by microbial biofilm, due to the persistence of S. maltophilia even after the implementation of standard measures to manage the outbreak 8 .Therefore, the researchers concluded that the repair and replacement of deteriorating water system components was beneficial for managing S. maltophilia outbreaks 9,11,16 .Also, handwashing measures of the hospital staff have been identified in several studies as a critical component in stopping the outbreak 18,19 .
Hospitals worldwide perform surveillance on infections due to S. maltophilia 4 .Although outbreaks of this species have been reported to date in almost every continent, the cases concentrated especially in the Mediterranean, South Asia, and South America raise the suspicion that hot and humid weather may be related to S. maltophilia infections.Increased cell growth rates and concentrations that can come into touch with vulnerable people and potentially raise infection risk are predicted to follow an increase in the global temperature 4 .Likewise, climate change may have an effect on the spread of the S. maltophilia infection by causing the spread of waterborne infectious diseases and undermining the value of sanitation in providing clean water supplies 40 .Although, S. maltophilia is a typical water bacterium, we were unable to find any additional evidence in the literature to support this assumption.

CONCLUSIONS
The primary limitation of our investigation was the lack of genetic characterization of the strains due to economic reasons, in order to demonstrate that the same clone probably infected all patients.However, we acknowledge that S. maltophilia outbreaks are a very difficult ongoing problem and that some genetic modifications are reasonable to expect.Evaluation of outbreak reports will facilitate the identification of particular risk factors for a specific pathogen.S. maltophilia has to be given more consideration as it has the potential to spread outbreaks and infect patients in operating rooms and intensive care units during invasive procedures.Also, it is critical to proceed with the greatest caution when treating burn and bone marrow transplant patients, and patients undergoing hemodialysis and bronchoscopy.The staff members should be regularly observed and trained, and appropriate disinfection protocols should be strictly followed.

Table 1 -
Clinical and demographic characteristics of patients with S. maltophilia infection.

Table 2 -
Clinical characteristics and manifestations of patients with Stenotrophomonas maltophilia infection based on the literature review.
*Median age was described; a Preterm infants or newborns.