# Stenotrophomonas maltophilia
**Stenotrophomonas maltophilia** is an aerobic, glucose non-fermenting, Gram-negative bacillus that is ubiquitous in water environments and has emerged as a multidrug-resistant opportunistic human pathogen, particularly in immunocompromised and critically ill patients[1](https://doi.org/10.1093/cid/ciae403). Once regarded as an organism of low virulence, it is now an important cause of hospital- and community-acquired infection, and its intrinsic and acquired antibiotic resistance makes treatment difficult[2](https://journals.asm.org/doi/10.1128/cmr.00019-11).
| Key fact | Detail |
|---|---|
| Taxonomy | First isolated in 1943 as *Bacterium bookeri*; named *Pseudomonas maltophilia* (1961), *Xanthomonas maltophilia* (1983), and placed in the new genus *Stenotrophomonas* in 1993[2](https://journals.asm.org/doi/10.1128/cmr.00019-11) • [3](https://pubmed.ncbi.nlm.nih.gov/8347518/) |
| Habitat | Aqueous environments, including hospital water systems, soil, plant rhizospheres, foods, and animals[1](https://doi.org/10.1093/cid/ciae403) • [4](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article) |
| Signature resistance | Two chromosomal inducible β-lactamases, L1 (class B3 metallo-β-lactamase) and L2 (class A cephalosporinase), plus multidrug efflux pumps and low outer membrane permeability[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC9071536/) |
| Clinical weight | Crude mortality for S. maltophilia bloodstream infections ranges from 14% to 69%[4](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article) |
| First-line therapy | Trimethoprim-sulfamethoxazole (TMP-SMX); US surveillance shows susceptibility in more than 90% of isolates[6](https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciag481/8756906) |
| Species complex | Genomic analysis of 734 strains labeled *S. maltophilia* delineated 24 species-level clades; only 42.8% were confirmed as *S. maltophilia*[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10806987/) |
## Taxonomy and nomenclature
The species has one of the more checkered taxonomic histories among clinical bacteria. It was first isolated in 1943 from human pleural fluid in England and described as *Bacterium bookeri*; it was classified in the genus *Pseudomonas* in 1961, moved to *Xanthomonas* in 1983 on the basis of ribosomal RNA analysis, and finally placed in a new genus, *Stenotrophomonas*, proposed in 1993 by N. J. Palleroni, a microbiologist at New York University Medical Center, and J. F. Bradbury[2](https://journals.asm.org/doi/10.1128/cmr.00019-11) • [3](https://pubmed.ncbi.nlm.nih.gov/8347518/) • [8](https://fedorabg.bg.ac.rs/fedora/get/o%3A16661/bdef%3AContent/download). The 1993 proposal restored the genus *Xanthomonas* to its former definition and initially included a single species, *S. maltophilia*[3](https://pubmed.ncbi.nlm.nih.gov/8347518/).
The species itself has since been shown to contain multiple cryptic species, giving rise to the *Stenotrophomonas maltophilia* complex (Smc). In a phylogenomic study of 734 genomes identified as *S. maltophilia*, just 314 (41.4%) were verified to belong to that species; the study resolved 24 clades at the species level, with *S. maltophilia* accounting for the largest share (42.8%), followed by *S. sepilia* (13.6%) and *S. geniculata* (9.9%)[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10806987/). Strains were distributed across 39 countries, with the largest shares in Europe (34.2%), Asia (33.7%), and North America (24.0%)[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10806987/).
## Ecology and beneficial uses
Outside the hospital, *S. maltophilia* is a versatile environmental bacterium. It has been isolated from water sources, soil, foods, a dishwasher seal, dental biofilm, and contact lens storage cases, and it associates closely with plants[9](https://pmc.ncbi.nlm.nih.gov/articles/PMC8262804/). It possesses multiple dioxygenases that biodegrade alkanes and polycyclic aromatic hydrocarbons, and it can adsorb heavy metals, which gives it value in bioremediation[10](https://biotech.aiijournal.com/EN/Y2015/V31/I8/35). Rhizospheric strains act as plant growth-promoting bacteria, secreting hydrolytic enzymes and secondary metabolites that enhance plant growth and help control fungal diseases and nematodes[10](https://biotech.aiijournal.com/EN/Y2015/V31/I8/35).
## Antimicrobial resistance
Resistance in *S. maltophilia* is layered. The intrinsic component rests on chromosomally encoded enzymes, efflux pumps, and reduced outer membrane permeability; the acquired component arises through genetic mutation and horizontal gene transfer of resistance determinants on mobile elements[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/) • [12](https://pubmed.ncbi.nlm.nih.gov/32052662/). Together these can convert a multidrug-resistant strain into an extensively drug-resistant or pandrug-resistant one[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/).
**β-lactamases.** The two inducible chromosomal β-lactamases L1 and L2 are the defining resistance feature. L1 is a Zn²⁺-dependent metallo-β-lactamase of Ambler subclass B3 that hydrolyzes penicillins, cephalosporins, and [carbapenems](https://www.edgepedia.org/carbapenem), and is not inhibited by currently available β-lactamase inhibitors; aztreonam is an important exception that L1 does not inactivate[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC9071536/). L2 is an active-site serine β-lactamase of class A, a broad-spectrum cephalosporinase that hydrolyzes first- to third-generation cephalosporins, penicillins, and monobactams, and is inhibited by clavulanic acid and avibactam[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC9071536/) • [11](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/). Both are regulated by AmpR, a [LysR-type transcriptional regulator](https://www.edgepedia.org/lysr-type-transcriptional-regulator) whose activity is tied to peptidoglycan recycling; β-lactam exposure disrupts cell wall synthesis and triggers induction[11](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/). Mutations in regulators of these chromosomal genes are a major route to acquired resistance, and additional β-lactamase variant genes can arrive on transferable plasmids[12](https://pubmed.ncbi.nlm.nih.gov/32052662/) • [11](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/).
**Other mechanisms.** Efflux pumps of the RND family (SmeABC, SmeDEF, SmeYZ, and others), together with ABC and MFS family pumps, reduce susceptibility to quinolones, tetracyclines, TMP-SMX, β-lactams, and macrolides[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4557615/) • [9](https://pmc.ncbi.nlm.nih.gov/articles/PMC8262804/). Chromosomal *Smqnr* genes reduce fluoroquinolone effectiveness, chromosomal aminoglycoside acetyltransferases confer intrinsic aminoglycoside resistance, and class 1 integrons carrying *sul1*, *sul2*, and *dfrA* genes underlie TMP-SMX resistance[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4557615/) • [1](https://doi.org/10.1093/cid/ciae403). Biofilm formation further protects cells from antibiotics by reducing drug diffusion and increasing inactivation[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC11089167/).
## Clinical significance
*S. maltophilia* most often causes respiratory tract infections but also causes bloodstream, intraabdominal, urinary, catheter, and device-associated infections[4](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article). Risk factors include underlying malignancy, cystic fibrosis, corticosteroid or immunosuppressant therapy, indwelling central venous catheters, and exposure to broad-spectrum antibiotics[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4557615/). Among bacterial isolates, S. maltophilia accounted for 0.8–1.4% in SENTRY studies during 1997–2003, a proportion that rose to 1.3–1.68% in the following decade[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4557615/) • [20](https://ejmm.journals.ekb.eg/article_228827_d02a4454c3b8cb2e0dcce8565d4a2144.pdf). In patients with hematologic malignancies it can cause hemorrhagic pneumonia or bacteremia, and crude mortality for bloodstream infections ranges from 14% to 69%[1](https://doi.org/10.1093/cid/ciae403) • [4](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article).
A recurring management problem is distinguishing colonization from true infection, since the organism is often recovered as part of a polymicrobial culture, particularly in patients with cystic fibrosis or ventilator dependence[1](https://doi.org/10.1093/cid/ciae403). Hospital outbreaks are frequently linked to contaminated water systems, sinks, and medical devices[15](https://pubmed.ncbi.nlm.nih.gov/42603304/). In one California hospital outbreak during May 2022–September 2023, 13 intensive care unit patients developed bloodstream infections from a highly related sequence type 239 strain; a matched case-control study found increased odds of infection with exposure to iodinated contrast (OR 12.0), injectable propofol (OR 12.2), or fentanyl (OR 9.2), and investigators suspected transmission via nonsterile water from a common source[4](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article).
## Treatment
Trimethoprim-sulfamethoxazole remains the traditional first-line therapy, with US surveillance showing in vitro susceptibility in more than 90% of isolates (MIC ≤2/38 µg/mL); resistance when it occurs is commonly mediated by efflux pump overexpression and acquisition of *sul* and *dfrA* genes[6](https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciag481/8756906). Minocycline is the preferred tetracycline derivative, with susceptibility in approximately 90% of US isolates (MIC ≤1 µg/mL), and is preferred over tigecycline because CLSI breakpoints exist for it and its pharmacodynamic targets are better characterized[6](https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciag481/8756906).
The Infectious Diseases Society of America (IDSA) guidance has evolved as evidence has accumulated. The 2024 guidance suggested, for moderate to severe infections, either two of cefiderocol, minocycline, TMP-SMX, or levofloxacin, or the combination of ceftazidime-avibactam and aztreonam[1](https://doi.org/10.1093/cid/ciae403). The 2026 update designated cefiderocol monotherapy as the preferred treatment for invasive *S. maltophilia* infections, while acknowledging that this rests mainly on susceptibility data and neutropenic animal studies with very little supportive clinical data, and named aztreonam-avibactam, preferably with a second agent, as an alternative[16](https://www.idsociety.org/practice-guideline/amr-guidance/). Ceftazidime is not suggested, because L1 and L2 render it ineffective[17](https://academic.oup.com/cid/article/74/12/2089/6453394). Comparative effectiveness data remain limited to observational studies with selection bias, small samples, and heterogeneity, and no standard-of-care regimen against which to compare alternatives has been established[1](https://doi.org/10.1093/cid/ciae403). Emerging strategies under study include efflux pump inhibitors, anti-virulence approaches, and bacteriophage therapy[18](https://pubmed.ncbi.nlm.nih.gov/42577614/).
## Virulence and pathogenesis
Although not a highly virulent pathogen, *S. maltophilia* carries a broad set of virulence factors: surface structures such as lipopolysaccharide, type IV pili, flagella, and fimbriae; extracellular enzymes including proteases, esterases, and lipases; hemolysin, siderophores, and cytotoxins; and the ability to form biofilms on abiotic surfaces and host tissues[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC11089167/). Biofilm formation is central to persistent infection and reduces the efficacy of aminoglycosides, fluoroquinolones, and tetracyclines[14](https://pmc.ncbi.nlm.nih.gov/articles/PMC11089167/). Cell-cell communication proceeds through a quorum sensing system based on the diffusible signal factor cis-11-methyl-2-dodecenoic acid, governed by genes in the *rpf* (regulation of pathogenicity factors) cluster, which coordinates motility, biofilm formation, extracellular effector production, and resistance properties[19](https://pmc.ncbi.nlm.nih.gov/articles/PMC5928129/). In cystic fibrosis airways the organism colonizes the respiratory tract and elevates the risk of pulmonary exacerbations and mortality[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC10806987/).
## References
1. [Infectious Diseases Society of America 2024 Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections, Clinical Infectious Diseases](https://doi.org/10.1093/cid/ciae403)
2. [Brooke JS (2012). Stenotrophomonas maltophilia: an Emerging Global Opportunistic Pathogen, Clinical Microbiology Reviews](https://journals.asm.org/doi/10.1128/cmr.00019-11)
3. [Palleroni NJ & Bradbury JF (1993). Stenotrophomonas, a new bacterial genus for Xanthomonas maltophilia, Int J Syst Bacteriol](https://pubmed.ncbi.nlm.nih.gov/8347518/)
4. [Stenotrophomonas maltophilia Bloodstream Infection Outbreak in Acute Care Hospital, California, USA, 2022–2023, Emerging Infectious Diseases](https://wwwnc.cdc.gov/eid/article/32/3/25-0835_article)
5. [Clinical challenges treating Stenotrophomonas maltophilia infections: an update, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC9071536/)
6. [Infectious Diseases Society of America 2026 Guidance on the Treatment of Antimicrobial-Resistant Gram-Negative Infections, Clinical Infectious Diseases](https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciag481/8756906)
7. [Stenotrophomonas maltophilia complex: insights into evolutionary relationships, global distribution and pathogenicity, Frontiers in Cellular and Infection Microbiology](https://pmc.ncbi.nlm.nih.gov/articles/PMC10806987/)
8. [Taxonomy of the genus Stenotrophomonas (thesis introduction)](https://fedorabg.bg.ac.rs/fedora/get/o%3A16661/bdef%3AContent/download)
9. [Advances in the Microbiology of Stenotrophomonas maltophilia, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC8262804/)
10. [Li Yulong & Han Zhengmin (2015). The Application of Stenotrophomonas maltophilia in Environmental Remediation and Agriculture, Biotechnology Bulletin](https://biotech.aiijournal.com/EN/Y2015/V31/I8/35)
11. [Acquired resistance in Stenotrophomonas maltophilia: Mechanisms underlying the shift from multidrug to pandrug resistance, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC13003763/)
12. [Mechanisms of antimicrobial resistance in Stenotrophomonas maltophilia: a review of current knowledge, PubMed](https://pubmed.ncbi.nlm.nih.gov/32052662/)
13. [Update on infections caused by Stenotrophomonas maltophilia with particular attention to resistance mechanisms and therapeutic options, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4557615/)
14. [Stenotrophomonas maltophilia virulence: a current view, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11089167/)
15. [Stenotrophomonas maltophilia infections: Biofilm-mediated virulence, multidrug resistance, clinical impact, and emerging treatment strategies, PubMed](https://pubmed.ncbi.nlm.nih.gov/42603304/)
16. [IDSA 2026 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections](https://www.idsociety.org/practice-guideline/amr-guidance/)
17. [IDSA Guidance on Treatment of AmpC β-Lactamase-Producing Enterobacterales, CRAB, and S. maltophilia Infections, Clinical Infectious Diseases](https://academic.oup.com/cid/article/74/12/2089/6453394)
18. [Antimicrobial resistance mechanisms, treatment challenges, and emerging strategies in Stenotrophomonas maltophilia infections, PubMed](https://pubmed.ncbi.nlm.nih.gov/42577614/)
19. [Quorum Sensing Signaling and Quenching in the Multidrug-Resistant Pathogen Stenotrophomonas maltophilia, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5928129/)
20. [ejmm.journals.ekb.eg](https://ejmm.journals.ekb.eg/article_228827_d02a4454c3b8cb2e0dcce8565d4a2144.pdf)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Bacteria › Proteobacteria*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license