The observed hospital is a 900-bed tertiary hospital for adults and children with different clinics and eight intensive care units. In the hospital, consultations regarding infectious diseases are carried out by infectious disease and clinical microbiology specialists, and the infection control team nurses monitor the growth and infection control measures with active surveillance conducted throughout the hospital. During these consultations and during follow-ups, an increase in S. maltophilia growth in the clinical samples was noted. Therefore, growth was reviewed and detected in various cultures of 81 children and adult patients from different clinics between 15.09.2021 and 19.11.2021. Because this increase was significant when compared to all instances S. maltophilia growth in 2019 and 2020–2021, it was decided that an analysis be conducted.
First, patients with S. maltophilia growth between 15.09.2021 and 19.11.2021 in various clinical samples were examined. It was determined that blood culture growth was predominant among all types of growth. The case description was determined as ‘clinical and intensive care patients with at least one blood culture were positive for S. maltophilia between 15.09.2021–19.11.2021’. The outbreak analysis and case-control study were performed considering risk factors for bacteraemia, such as catheter type, presence of IV solutions, drugs, blood and blood products. The control group included ‘patients with similar characteristics (age, clinic, etc.) but with no S. maltophilia growth in their blood culture over the same period’. In the comparison of S. maltophilia growth between the case and control groups, when the effect size was determined to be 20%, the minimum number of patients required to be included in the groups was determined to be 50 for 5% significance and 80% power.
The patients’ data were obtained from the hospital data processing system. The analysis included characteristics of samples with growth, growth dates, general characteristics of patients, prognosis, underlying diseases, clinics where the patients were hospitalized, whether the patients used antibiotics, surgery and haemodialysis, intubation history, whether interventional procedures were performed, bronchoscopy, enteral nutrition, history of total parenteral nutrition, blood product collection and history of potassium replacement use. Relevant procedure dates were also sourced from the patients
Epidemiological surveillance
Samples from possible foci (distilled water, sink water from patients’ rooms, heparinized flush solutions, treatment solutions [saline, dextrose, ringer lactate, etc.], alcohol, povidone-iodine, chlorhexidine and any syringes used on the patient) and unopened syringes within the same batch were cultured.
Arterial catheter: Blood gas sampling
Arterial blood gas tests were performed on patients with arterial catheters. Arterial catheters consisting of a standard arterial flush system were placed directly in the systemic arteries. The flush solution contained 2 U/mL heparin with 0.9% sodium chloride. Flush solution and arterial line connected with vascular three-way stopcock. First, the three-way stop cock was disinfected with 2% chlorhexidine, and then the nurse aspirated 2 mL of blood with sterile syringes. This blood sample was thrown into a medical waste container. Next, 1 mL of blood was aspirated with heparinised syringes using a three-way stopcock.
Microbiologic identification and molecular analysis
Blood cultures were monitored using the BACTEC™ 9240 (Becton Dickinson, Sparks, MD, USA) fully automated blood culture device for 5 days or until bacterial growth was identified. Gram staining was prepared from the samples showing growth signals. The samples were inoculated on 5% sheep blood agar and eosin methylene blue (EMB) agar and incubated at 37°C for 24–48 hours. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) (Bruker Daltonik, Bremen, Germany) was used to identify microorganisms reproduced in the media. The antibiotic susceptibility of these causative microorganisms was studied with the Phoenix™ 100 fully automated system and evaluated in accordance with the criteria of the European Committee on Antimicrobial Susceptibility Testing (EUCAST).
The solutions, drugs and equipment thought to be a source for the outbreak (blood products, blood collection and component separation bags, liquid containing 0.9% NaCl, 5% dextrose, ready alcohol used in the fields, 2% chlorhexidine, hand antiseptic and alcohol prepared in the warehouse [70% diluted alcohol, tap water], the outer surface of multidose heparin vials taken from intensive care areas in which cases were common and heparin inside of the vials, and commercial kits consisting of syringes (containing liquid lithium and heparin used to collect arterial blood gas) were cultured. The samples were inoculated on 5% sheep blood agar and thioglycolate liquid and incubated at 37°C for 24–48 hours. MALDI-TOF was used to identify microorganisms with growth in the media.
On detecting the blood gas collection kit as a possible source of the outbreak, the outbreak analysis was repeated with this risk factor. After detecting the significant OR value in the second analysis, the blood culture samples and sterile blood gas collection kit samples were randomly selected and sent to a reference laboratory (National Antimicrobial Resistance Surveillance Laboratory) to investigate genetic linkage and clonal relationships. Bacterial identification of the 20 submitted strains (14 patients’ blood culture samples and 6 blood gas collection kits) was first confirmed by MALDI-TOF MS (Bruker Daltonics, Bremen, Germany). The source/epidemiological analysis of the 20 strains isolated from patients and medical equipment was performed with the PFGE method using the CHEF-DR III system (Bio-RadLaboratories-Belgium). In this test, bacterial cells were embedded in 1% low-melting-point agarose plugs (SeaKem® Gold Agarose, Lonza Rockland, USA) and lysed with lysozyme and proteinase K. The digestion of chromosomal DNA was carried out using SpeI (Thermo Scientific-Fermantas Corporation, Vilnius, Lithuania). DNA fragments were separated on 1% pulsed-field certified agarose (Lonza Rockland, USA) using a CHEF-DR III system (Bio-Rad Lab, Nazareth, Belgium) at 6 V/cm2 for 18 h at 14°C and the pulse time was changed from 5 s to 70 s. Band profiles were analysed with BioNumerics version 7.5 software (Applied Maths, Sint-Martens-Latem, Belgium), and a similarity analysis was conducted using a Dice coefficient with a tolerance of 1.5% and an optimization of 1%. A dendrogram was constructed using the unweighted-pair group method with arithmetic mean (UPGMA). The clonal relationship among isolates was interpreted and evaluated according to the 1997 criteria developed by Tenover et al. [4]. Isolates with identical patterns were regarded as genotypically indistinguishable. Those with 1–3 band differences were considered closely related, those with 4–6 band differences were possibly related, and those with 7 or more band differences were considered unrelated or different.
Patient follow-up
We conducted follow-ups for patients with potential colonisations or infections with contaminated arterial gas kits between September and December 2021. All inpatients who used heparinized syringes for blood gas measurement were monitored and followed to the last discharged patient. Furthermore, after the use of arterial blood gas collection kits was stopped, cultures with S. maltophilia growth were closely monitored for two months.
Statistical methods
The results are presented as frequencies and percentages. Binary logistic regression was performed, and the crude odds ratios (OR) and their 95% confidence intervals (CIs) were used to test confounding variables associated with risk factors and reported. p < 0.05 was considered as the significance level. Statistical analyses were performed using IBM SPSS ver. 28.0 (IBM Corp. Released 2021. IBM SPSS Statistics for Windows, Version 28.0. Armonk, NY: IBM Corp.).