Stenotrophomonas maltophilia (Stenos, Greek: narrow; trophos, Greek: one who feeds; monas, Greek: a unit, monad; i.e., a unit feeding on few substrates; and malt, Old English: malt; philos, Greek: friend; i.e., a friend of malt). The type strain was isolated in 1958 from an oropharyngeal swab of a patient with oral carcinoma [1]. and named Pseudomonas maltophilia [2]. Subsequently, Swings et al, (1981) further proposed that P. maltophilia be reclassified in the genus Xanthomonas as X. maltophilia [3].

Stenotrophomonas maltophilia is a Gram-negative, aerobic, non-fermenting bacillus belonging to the family Xanthomonadaceae. Initially, it was classified under the genera Pseudomonas and Xanthomonas, but later reassigned to the genus Stenotrophomonas in the early 1990s based on genotypic and phenotypic characteristics [4,5].

Morphologically, S. maltophilia appears as straight or slightly curved rods measuring approximately 0.4–0.7 µm in width and 0.7–1.8 µm in length. Microscopically, the cells occur singly or in short chains and exhibit motility due to polar flagella [5].

Stenotrophomonas maltophilia is a ubiquitous environmental bacterium found in water, soil, animals and plant rhizospheres, and it can colonise damp surfaces in residential and healthcare settings [5]. During the past decade, it has been increasingly recognised as an important Gram-negative multidrug-resistant (MDR) pathogen [6].

As an opportunistic pathogen, S. maltophilia is frequently associated with healthcare-associated infections in weakened and immunocompromised individuals. Community-acquired infections have also been reported, although they are less common [7]. In hospital settings, S. maltophilia is clinically significant as a cause of catheter-related bacteraemia, pneumonia, soft-tissue infection, meningitis, prosthetic valve endocarditis and ocular infection, particularly among critical-care and oncology patients [8].

Although S. maltophilia is not considered highly virulent, its clinical importance is related to underlying malignancy, indwelling devices, chronic respiratory disease, immunosuppression, prior antibiotic exposure, prolonged hospitalization and ICU stay [5]. Virulence-associated properties such as adhesion, biofilm formation, hydrophobicity, motility and extracellular enzyme production may contribute to colonization and infection [9,10]. Biofilm formation on abiotic and biotic surfaces may protect the organism from host immune responses and antimicrobial penetration, facilitating persistence on ventilator circuits and central venous catheters [11,12].

maltophilia has significant intrinsic and acquired resistance to multiple antimicrobial classes, limiting treatment options [13-16]. It commonly shows resistance to expanded-spectrum penicillins, third-generation cephalosporins, carbapenems, aminoglycosides and several quinolones. Trimethoprim-sulfamethoxazole has traditionally been considered the preferred agent, but resistance and treatment limitations are increasingly recognised [5,8]. Recent guidance has highlighted trimethoprim-sulfamethoxazole, levofloxacin, minocycline and cefiderocol as important agents, while CLSI breakpoint changes have revised the interpretation of susceptibility for this organism [17,18].

The present study aims to determine the bacteriological profile and antimicrobial susceptibility patterns of Stenotrophomonas maltophilia isolated from patients admitted to intensive care units in a tertiary care hospital. The study also aims to assess the distribution of isolates across clinical specimens and ICU settings, and to evaluate the resistance patterns of S. maltophilia to commonly used antimicrobial agents to support appropriate antimicrobial therapy and infection control practices.

Considering the increasing recognition of Stenotrophomonas maltophilia as an important opportunistic nosocomial pathogen in critically ill patients, continuous surveillance of its occurrence and antimicrobial susceptibility pattern is essential. ICU patients are particularly vulnerable due to prolonged hospitalization, mechanical ventilation, invasive devices, prior exposure to broad-spectrum antibiotics, and underlying comorbid conditions. Since S. maltophilia possesses intrinsic resistance to multiple antimicrobial classes and has limited therapeutic options, local data on its distribution and susceptibility profile are required for guiding effective treatment strategies. Therefore, the present observational cross-sectional study was undertaken to evaluate the bacteriological profile and antimicrobial susceptibility patterns of S. maltophilia isolated from intensive care unit patients at a tertiary care hospital.

MATERIALS AND METHODS

Study design and setting

This observational cross-sectional laboratory-based study was conducted in the Department of Microbiology, Index Medical College Hospital & Research Centre, Malwanchal University, Indore, Madhya Pradesh, India, from 1 January 2024 to 1 December 2025. The study included clinical samples from patients admitted to Medical ICU, Surgical ICU, Respiratory ICU and Paediatric ICU.

Inclusion and exclusion criteria

All ICU patients from whom S. maltophilia was isolated from clinically relevant samples during the study period were included. To avoid duplication, only the first isolate from each patient was considered. Samples were included only when relevant clinical and microbiological data were available. Repeat isolates from the same patient, isolates from non-ICU patients, and contaminated or clinically insignificant mixed-growth specimens were excluded.

Sample collection and processing

Clinical specimens, including blood, endotracheal aspirate, sputum, bronchoalveolar lavage, urine, pus/wound swab, catheter tip and body fluids, were collected using aseptic precautions and transported promptly to the microbiology laboratory. Blood samples were inoculated into blood-culture bottles and incubated using an automated blood culture system, where available. Other specimens were inoculated on blood agar, MacConkey agar and other appropriate media and incubated aerobically at 37°C for 18-24 hours. Preliminary identification was based on colony morphology, Gram staining, oxidase test, motility and biochemical reactions. Final identification was performed by conventional biochemical tests and/or automated identification systems such as VITEK 2, depending on availability.

Antimicrobial susceptibility testing

Antimicrobial susceptibility testing was performed by the Kirby-Bauer disk diffusion method on Mueller-Hinton agar. Results were interpreted according to the applicable CLSI recommendations during the study period. The antimicrobial agents analysed for S. maltophilia were trimethoprim-sulfamethoxazole, levofloxacin and minocycline.

Statistical analysis

Data were entered in Microsoft Excel and analysed using IBM SPSS Statistics version 29. Categorical variables were expressed as frequencies and percentages. Associations between categorical variables were assessed using the chi-square test or Fisher's exact test, as appropriate. A p-value of <0.05 was considered statistically significant.

Ethical approval

The study was approved by the Institutional Ethics Committee of Malwanchal University (IEC: MU/Research/EC/Ph.D/2023/347). Patient confidentiality was maintained throughout the study.

RESULTS

A total of 22 S. maltophilia isolates were obtained from 266 ICU patients during the study period, giving an isolation rate of 8.3%. The majority of isolates were recovered from patients aged 30-60 years, 13(59.1%), followed by patients aged >60 years, 8(36.4%). Only one isolate (4.5%) was recovered from a patient aged <30 years. The mean age was 56.8 ± 15.4 years. Male patients accounted for 14 isolates (63.6%) and female patients for 8 isolates (36.4%). ICU-wise distribution showed that the highest number of isolates came from the Respiratory ICU, 10 (45.5%), followed by Surgical ICU 7(31.8%) and Medical ICU 5(22.7%). No isolate was recovered from the Paediatric ICU. Sample-wise, endotracheal aspirate was the most common specimen 10(45.5%), followed by blood, 8 (36.4%). Pus/wound swab contributed 2 isolates (9.1%), while urine and sputum/BAL contributed 1 isolate each (4.5%). In antimicrobial susceptibility testing, trimethoprim-sulfamethoxazole showed the highest susceptibility,18(81.8%), followed by minocycline 17 (77.3%) and levofloxacin 14(63.6%). Statistical analysis showed that ventilator use was significantly associated with infection/colonization with S. maltophilia (p<0.05), whereas gender was not significantly associated (p>0.05).

Table 1. Age-wise distribution of patients with S. maltophilia infection/colonization

Table 2. Gender-wise distribution of patients with S. maltophilia infection/colonization

Table 3. Distribution of S. maltophilia isolates according to ICU type

Table 4. Sample-wise distribution of confirmed S. maltophilia isolates

Figure 1.Sample-wise distribution of Stenotrophomonas maltophilia isolates.

Table 5. Antimicrobial susceptibility pattern of S. maltophilia isolates

Figure 2. Antimicrobial susceptibility pattern of Stenotrophomonas maltophilia isolates.

Table 6. Statistical analysis

DISCUSSION

Stenotrophomonas maltophilia is increasingly recognized as an important opportunistic, non-fermenting Gram-negative bacillus in hospital and intensive care settings. Although it is not considered a highly virulent organism, its ability to survive in moist hospital environments, colonize respiratory equipment, form biofilm and resist multiple antimicrobial classes makes it clinically significant among critically ill and immunocompromised patients. Brooke reported that S. maltophilia has emerged globally as an opportunistic pathogen, particularly among patients exposed to prolonged hospitalization, invasive devices and broad-spectrum antibiotics [5]. Similarly, Abbott et al. described changing disease patterns of S. maltophilia and highlighted its growing importance in nosocomial infections [16].

In the present study, S. maltophilia was isolated from 22 ICU patients. The majority of isolates were recovered from patients in the 30-60 years age group, followed by patients aged >60 years. This finding suggests that middle-aged and elderly patients admitted to ICUs may be more vulnerable to colonization or infection, possibly due to comorbidities, prolonged ICU stay, invasive procedures and previous antimicrobial exposure. Muder et al. similarly noted that S. maltophilia infections are commonly associated with hospitalized and critically ill patients, especially those with severe underlying illness [8].

Male predominance was observed in the present study, with 14 of 22 isolates recovered from male patients. However, gender was not statistically significant in our analysis. This indicates that the occurrence of S. maltophilia infection/colonization may be more strongly associated with ICU-related risk factors such as ventilator use, catheterization and antibiotic exposure rather than gender alone. Falagas et al. also emphasized that host-related and healthcare-associated risk factors play a major role in the development of S. maltophilia infections [7].

In our study, the highest number of isolates was obtained from the Respiratory ICU, followed by Surgical ICU and Medical ICU. This supports the role of S. maltophilia as an important pathogen in critically ill patients with respiratory involvement. Respiratory ICU patients commonly require mechanical ventilation, endotracheal intubation, oxygen therapy and prolonged antibiotic treatment, all of which increase the risk of colonization and infection. Di Bonaventura et al. demonstrated that S. maltophilia can adhere to respiratory epithelial cells and contribute to inflammatory lung infection [9]. De Oliveira-Garcia et al. also reported that fimbrial structures and adherence mechanisms help S. maltophilia attach to epithelial and abiotic surfaces, contributing to persistence in hospital environments [10].

Sample-wise distribution showed that endotracheal aspirate was the most common specimen, accounting for 45.5% of isolates, followed by blood culture at 36.4%. This indicates that S. maltophilia in ICU settings is mainly associated with respiratory tract infection/colonization and ventilator-associated events. Isolation from blood samples also highlights its role as a cause of bloodstream infections, especially in patients with central venous catheters and other invasive devices. Muder et al. reported that S. maltophilia is associated with pneumonia, catheter-related bacteraemia, soft-tissue infection, meningitis and endocarditis in hospitalised patients [8]. Biofilm formation on indwelling medical devices may further contribute to persistence and bloodstream invasion. Elvers et al. and De Vidipo et al. demonstrated biofilm-forming ability and epithelial interaction of S. maltophilia, which may explain its association with ventilator circuits, catheters and ICU-related infections [11,12].

Antimicrobial susceptibility testing showed that trimethoprim-sulfamethoxazole had the highest susceptibility rate, with 18 of 22 isolates susceptible. Minocycline also showed good activity, followed by levofloxacin. These findings support the continued role of trimethoprim-sulfamethoxazole as an important therapeutic option for S. maltophilia. However, resistance to trimethoprim-sulfamethoxazole was observed in 18.2% of isolates, which is clinically important because therapeutic options are limited. Hu et al. reported increasing resistance determinants against trimethoprim-sulfamethoxazole among clinical S. maltophilia isolates, supporting the need for continuous antimicrobial surveillance [14]. Sanchez et al. also emphasized that intrinsic and acquired resistance mechanisms make S. maltophilia difficult to treat [15].

Levofloxacin susceptibility was observed in 63.6% of isolates, while resistance was seen in 36.4%. Although levofloxacin may be considered as an alternative agent, the comparatively higher resistance in the present study indicates that it should be used only after susceptibility confirmation. Minocycline showed 77.3% susceptibility and may be considered a useful alternative, particularly when trimethoprim-sulfamethoxazole resistance or intolerance is present. Bakthavatchalam et al. reported that CLSI revised the minocycline susceptibility breakpoints for S. maltophilia in 2024, which can affect interpretation of susceptibility rates when compared with older breakpoints [19]. Recent IDSA guidance and PK/PD-based studies also support susceptibility-guided treatment using active agents such as trimethoprim-sulfamethoxazole, minocycline, levofloxacin and cefiderocol in appropriate clinical settings [18,20].

The resistance pattern of S. maltophilia is mainly due to intrinsic mechanisms such as multidrug efflux pumps, reduced outer-membrane permeability, aminoglycoside-modifying enzymes and beta-lactamases. The organism is intrinsically resistant to many beta-lactams, including carbapenems, which may explain its emergence in patients previously exposed to broad-spectrum antibiotics. Abbott et al. suggested that increased use of carbapenems and other broad-spectrum antibiotics may contribute to the rising isolation of S. maltophilia in hospitals [16]. CLSI has also updated breakpoints for this organism, including removal of ceftazidime breakpoints and revision of minocycline breakpoints, reflecting changing understanding of its treatment relevance [17].

A statistically significant association was found between ventilator use and infection/colonisation in the present study. This finding is important because ventilator-associated colonisation and biofilm formation are major contributors to ICU-acquired respiratory infections, particularly among critically ill patients requiring mechanical ventilation and prolonged ICU care [24]. The predominance of endotracheal aspirate isolates further supports this observation. Carbonell et al. reported that S. maltophilia is an important pathogen in critically ill ICU patients and is frequently associated with respiratory infections, invasive devices, prior broad-spectrum antibiotic exposure and prolonged ICU stay [21]. Puech et al. observed that ventilator-associated pneumonia caused by S. maltophilia is clinically important and that appropriate antimicrobial therapy was associated with better outcomes [22]. Cristina et al. described an ICU outbreak of S. maltophilia and emphasized the role of environmental surveillance and infection-control measures in preventing transmission [23]. Therefore, strict infection-control practices, proper disinfection of respiratory equipment, antimicrobial stewardship and early removal of invasive devices whenever possible are essential to reduce the burden of S. maltophilia in ICU settings [21-24].

This study shows a high prevalence of LRTIs and multidrug-resistant Gram-negative organisms. Empirical antibiotic policies should consider local resistance patterns, with emphasis on rational antibiotic use to mitigate AMR [25].

Overall, the present study highlights the emergence of S. maltophilia as an important ICU-associated pathogen, particularly in respiratory specimens and bloodstream infections. The organism showed the highest susceptibility to trimethoprim-sulfamethoxazole, followed by minocycline and levofloxacin. However, resistance even to preferred therapeutic agents emphasizes the need for routine susceptibility testing, continuous surveillance, antimicrobial stewardship and strict infection-control measures.

Limitations

This study has certain limitations. It was a single-centre study with a small number of S. maltophilia isolates. Differentiation between colonization and true infection was based on available clinical and microbiological documentation and may not be absolute in all cases. Molecular characterization of resistance mechanisms and MIC-based testing were not performed. Exact risk-factor denominators and exact p-values should be added in future analyses to strengthen statistical interpretation.

CONCLUSION

Stenotrophomonas maltophilia is an emerging opportunistic pathogen in intensive care units, especially among critically ill patients with respiratory involvement and invasive devices. Its intrinsic multidrug resistance limits therapeutic options and makes accurate identification essential. Trimethoprim-sulfamethoxazole, minocycline and levofloxacin remain important therapeutic options; however, treatment should be guided by antimicrobial susceptibility testing. Continuous surveillance of antimicrobial susceptibility patterns, strict infection-control practices and antimicrobial stewardship are necessary to guide empirical therapy and reduce ICU-associated transmission.

Declarations

Ethical approval: Approved by the Institutional Ethics Committee of Malwanchal University (IEC: MU/Research/EC/Ph.D/2023/347).

Funding: To be declared by the authors, as per journal requirement.

Conflicts of interest: To be declared by the authors, as per journal requirement.

Data availability: The data supporting the findings of this study are available from the corresponding author on reasonable request, subject to institutional policies.

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