Update on infections caused by Stenotrophomonas maltophilia with particular attention to resistance mechanisms and therapeutic options

Stenotrophomonas maltophilia is a Gram-negative, biofilm-forming bacterium. Although generally regarded as an organism of low virulence, S. maltophilia is an emerging multi-drug resistant opportunistic pathogen in hospital and community settings, especially among immunocompromised hosts. Risk factors associated with S. maltophilia infection include underlying malignancy, cystic fibrosis, corticosteroid or immunosuppressant therapy, the presence of an indwelling central venous catheter and exposure to broad spectrum antibiotics. In this review, we provide a synthesis of information on current global trends in S. maltophilia pathogenicity as well as updated information on the molecular mechanisms contributing to its resistance to an array of antimicrobial agents. The prevalence of S. maltophilia infection in the general population increased from 0.8–1.4% during 1997–2003 to 1.3–1.68% during 2007–2012. The most important molecular mechanisms contributing to its resistance to antibiotics include β-lactamase production, the expression of Qnr genes, and the presence of class 1 integrons and efflux pumps. Trimethoprim/sulfamethoxazole (TMP/SMX) is the antimicrobial drug of choice. Although a few studies have reported increased resistance to TMP/SMX, the majority of studies worldwide show that S. maltophilia continues to be highly susceptible. Drugs with historically good susceptibility results include ceftazidime, ticarcillin-clavulanate, and fluoroquinolones; however, a number of studies show an alarming trend in resistance to those agents. Tetracyclines such as tigecycline, minocycline, and doxycycline are also effective agents and consistently display good activity against S. maltophilia in various geographic regions and across different time periods. Combination therapies, novel agents, and aerosolized forms of antimicrobial drugs are currently being tested for their ability to treat infections caused by this multi-drug resistant organism.

Stenotrophomonas maltophilia is a Gram-negative, biofilm-forming bacterium. Although generally regarded as an organism of low virulence, S. maltophilia is an emerging multi-drug resistant opportunistic pathogen in hospital and community settings, especially among immunocompromised hosts. Risk factors associated with S. maltophilia infection include underlying malignancy, cystic fibrosis, corticosteroid or immunosuppressant therapy, the presence of an indwelling central venous catheter and exposure to broad spectrum antibiotics. In this review, we provide a synthesis of information on current global trends in S. maltophilia pathogenicity as well as updated information on the molecular mechanisms contributing to its resistance to an array of antimicrobial agents. The prevalence of S. maltophilia infection in the general population increased from 0.8-1. 4% during 1997-2003 to 1.3-1.68% during 2007-2012. The most important molecular mechanisms contributing to its resistance to antibiotics include β-lactamase production, the expression of Qnr genes, and the presence of class 1 integrons and efflux pumps. Trimethoprim/sulfamethoxazole (TMP/SMX) is the antimicrobial drug of choice. Although a few studies have reported increased resistance to TMP/SMX, the majority of studies worldwide show that S. maltophilia continues to be highly susceptible. Drugs with historically good susceptibility results include ceftazidime, ticarcillin-clavulanate, and fluoroquinolones; however, a number of studies show an alarming trend in resistance to those agents. Tetracyclines such as tigecycline, minocycline, and doxycycline are also effective agents and consistently display good activity against S. maltophilia in various geographic regions and across different time periods. Combination therapies, novel agents, and aerosolized forms of antimicrobial drugs are currently being tested for their ability to treat infections caused by this multi-drug resistant organism. Keywords: Stenotrophomonas maltophilia, prevalence, susceptibility, surveillance, treatment Stenotrophomonas maltophilia is a Gram-negative, aerobic, glucose non-fermenting, motile bacillus. S. maltophilia was first isolated from pleural effusion in 1943 and initially named Bacterium bookeri. The organism was reclassified as a member of the genus Pseudomonas in 1961, Xanthomonas in 1983, and then Stenotrophomonas in 1993(Al-Anazi and Al-Jasser, 2014. It survives on almost any humid surface and has been isolated from a wide variety of aquatic sources, such as suction tubing, nebulizers, endoscopes, hemodialysis dialysate samples, plant rhizosphere, faucets, sink drains, and shower heads (Brooke, 2012). S. maltophilia is characterized by its ability to form biofilms on various abiotic and biotic surfaces, including lung cells (de Oliveira-Garcia et al., 2003;Pompilio et al., 2010), and by its resistance to a broad array of antimicrobial agents. The World Health Organization recently classified S. maltophilia as one of the leading multidrug resistant organisms (MDROs) in hospital settings (Brooke, 2014).
S. maltophilia is generally regarded as an organism of low virulence and therefore an opportunistic pathogen, especially in immunocompromised hosts. The risk factors associated with acquiring S. maltophilia infections are well-known and include underlying malignancy (especially hematologic malignancy), organ transplantation, human immunodeficiency virus (HIV) infection, cystic fibrosis, prolonged hospitalization, intensive care unit (ICU) admission, mechanical ventilation, indwelling catheters (vascular, urinary, biliary), corticosteroid or immunosuppressant therapy, and recent antibiotics treatment (Al-Anazi and Al-Jasser, 2014). These risk factors reflect specific features of S. maltophilia, such as its ability to survive on almost any humid surface, its propensity to form biofilm and colonize humid surfaces, and its employment of several mechanisms that confer resistance to a number of antimicrobial agents.
S. maltophilia causes a wide range of infections including respiratory tract infections (RTI), blood stream infections (BSI) and, less commonly, skin and soft tissue infections (SSTI), bone and joint infections, biliary tract infections, urinary tract infections, endophthalmitis, endocarditis, and meningitis (Falagas et al., 2009a;Looney et al., 2009). The correlations between S. maltophilia infection and structural abnormalities with or without obstruction or procedural manipulation are well documented. Biliary tract infections caused by obstruction due to hepatobiliary neoplasms (Papadakis et al., 1995;Chang et al., 2014) or post-operative anastomotic strictures of the gastrointestinal tract (Perez et al., 2014) have been reported in patients with biliary S. maltophilia sepsis. Pleural infections caused by post-surgical/tube thoracostomy or fistula (broncho-/esophageal-/bilio-) , postneurosurgical meningitis (Sood et al., 2013;Lai et al., 2014b), complicated urinary tract infections (Vartivarian et al., 1996), and obstructive lung cancer (Fujita et al., 1996;Vartivarian et al., 2000) have all been reported to create a milieu for S. maltophilia infection. In addition, although commonly perceived as nosocomial pathogens, community-acquired infections appear to be on the rise (Falagas et al., 2009a;Chang et al., 2014).

Prevalence
There were few data before 1970 regarding the prevalence or clinical characteristics of S. maltophilia (previously Pseudomonas maltophilia or Xanthomonas maltophilia) because of its rarity and relative clinical insignificance. It was in the 1980s when S. maltophilia became more frequently reported as an emerging nosocomial pathogen (Jang et al., 1992;Victor et al., 1994), especially in patients with post-chemotherapy neutropenia (Kerr et al., 1990;Labarca et al., 2000) and in those with indwelling central venous catheters (CVC) (Victor et al., 1994;Lai et al., 2006;Chen et al., 2014). Beginning in the late 1990s worldwide surveillance programs and multi-center studies began to provide more comprehensive information about the pathogenicity of S. maltophilia. Of the global surveillance programs, the SENTRY Antimicrobial Surveillance Program initiated in 1997 and the Study for Monitoring Antimicrobial Resistance Trends (SMART) initiated in 2002 are the most well-known (Jean et al., 2015). A number of nationwide and antimicrobial agent-targeted projects were also launched during the late 1990s, including the Canadian Ward Surveillance Study (CANWARD), the Surveillance and Control of Pathogens of Epidemiologic Importance (SCOPE) study, the British Society for Antimicrobial Chemotherapy (BSAC) Resistance Surveillance Project, the Taiwan Surveillance of Antimicrobial Resistance (TSAR) study, and the Tigecycline Evaluation Surveillance Trial (TEST).
Despite the massive scale of these surveillance studies, there are still limited integrated data on the prevalence and susceptibility patterns of S. maltophilia. The heterogeneity among the studies stems from the diverse patient demographics, geographic differences, and the ratio of the isolates collected from different sources, making inter-literature comparison difficult. To add to the complexity, there are no worldwide guidelines on susceptibility testing methodology and breakpoints for S. maltophilia (Nicodemo et al., 2004;Hombach et al., 2012), which results in different or absence of susceptibility breakpoints for some antibiotics. The lack of universal references for evaluating resistance of S. maltophilia to antimicrobial agents leads to confusion and complications when interpreting clinical data. Table 1 shows the prevalence rates of infection due to S. maltophilia, categorized by sources of infection, reported by worldwide and nationwide surveillance projects as well as multicenter studies. Specific patient groups such as the critically ill in intensive care units (ICUs) and the pediatric population are presented separately in Table 1. By comparing data gathered by large surveillance studies over time we can estimate longitudinal change in prevalence of S. maltophilia infection in the general population. The frequency of occurrence among isolates from all sources ranged from 0.8 to 1.4% in five SENTRY studies during 1997∼2003 (Fluit et al., 2001a;Gales et al., 2001a;Sader et al., 2004;Sader and Jones, 2005;Fedler et al., 2006b). During 2007-2012, the CANWARD surveillance study (Zhanel et al., 2011(Zhanel et al., , 2013Walkty et al., 2014) and the SENTRY antimicrobial surveillance program (Farrell et al., 2010b;Sader et al., 2013) reported prevalence rates ranging from 1.3 to 1.68%. These data indicate that there is an increasing trend in infections due to S. maltophilia in the general population.    It has been observed in the general population (Gales et al., 2001a) and in ICUs (Fluit et al., 2001b) alike that S. maltophilia is most frequently associated with respiratory tract infections (RTIs), followed by bloodstream infections (BSIs), and, rarely, skin and soft tissue infections (SSTIs) and urinary tract infections (UTI) (Gales et al., 2001a). The prevalence of RTIs due to S. maltophilia is generally higher than that of other infections caused by that pathogen, but varies widely among countries and continents, ranging from 1.6 to 6.3% during the period 1997-2012 (Sader et al., 1998(Sader et al., , 2014aJones et al., 2000;Fluit et al., 2001a;Gales et al., 2001aGales et al., , 2002Mathai et al., 2001;Hoban et al., 2003;Jones, 2010;Zhanel et al., 2010;Farrell et al., 2014). The United States has the most consecutive records regarding RTI isolates collected by the SENTRY program. Based on data from four SENTRY studies (Gales et al., 2001a;Hoban et al., 2003;Jones, 2010;Sader et al., 2014a), the prevalence rates increased from 3.3-3. 5% during 1997-2004 to 4.4% during 2009-2012. During that 15-year period, S. maltophilia went from being the eighth to the sixth most common cause of RTI. In a large study on 2968 RTI isolates collected from 59 medical centers in the USA and 15 centers in European countries in 2012, 6.3% of the pathogens were S. maltophilia (Farrell et al., 2014). These observations suggest an increasing frequency of occurrence of respiratory tract infections due to S. maltophilia.
Among NFGNB, S. maltophilia has been reported to be the third most commonly isolated pathogen after Pseudomonas aeruginosa and Acinetobacter baumannii. In a large survey conducted as a part of the SENTRY program, 221,084 GNB isolates were collected worldwide, including 25,305 (11.5%) NFGNB isolates, of which Acinetobacter spp. and P. aeruginosa accounted for the vast majority (87.7%). The remaining 3509 isolates were deemed unusual NFGNB species. Of them, S. maltophilia was the most frequently isolated (n = 2076, 59.16%) (Sader and Jones, 2005). A similar finding was reported in a prospective multi-center study involving nine teaching hospitals in France, in which S. maltophilia was the most commonly isolated NFGNB among all unusual NFGNB species (39%) (Fihman et al., 2012). Other surveillance studies, namely SCOPE , SENTRY Gales et al., 2006), and SMART  showed a steady increase in isolation of S. maltophilia among all NFGNB pathogens during the period 1995-2010 (6.7% in 1995-1996, 8.0% in 1997-2001, and 9.1% in 2001-2010). These findings show that S. maltophilia is not an insignificant pathogen among disease-causing GNB and NFGNB species.
There is limited information on the worldwide prevalence of S. maltophilia infections in the general pediatric population. SENTRY studies conducted during 1998-2003 (Fedler et al., 2006b) and in 2004 (Fedler et al., 2006a) showed that the prevalence of infections due to S. maltophilia was 1.2% among children ≤ 7 years and 1.4% among children ≤ 18 years old. The rates are similar to those in the adult population. A comparison of two single-center studies in China and the USA revealed markedly different incidence rates of ventilatorassociated pneumonia due to S. maltophilia among pediatric patients in ICUs. Ning et al. reported a rate of 20.3% among patients aged 2 months to 16 years in a pediatric ICU in China (Ning et al., 2013) whereas Arthur et al. found that the rate of infection due to S. maltophilia among infants aged 0-6 months in a cardiac ICU in the USA was only 0.8% (Arthur et al., 2015).
Several recent studies have shown that S. maltophilia is also an emerging opportunistic pathogen in community settings (Falagas et al., 2009a;Chang et al., 2014). Results of a worldwide SENTRY study in 1997 (Diekema et al., 1999) and the British Society for Antimicrobial Chemotherapy Resistance surveillance project conducted during (Livermore et al., 2008 showed that 33.3 and 32%, respectively, of S. maltophilia isolates were collected within 48 h after admission (defined as community-acquired in these studies) from patients with bloodstream infections. The results from two recent SMART studies revealed that 14.3-17.2% of isolates from patients with community-acquired IAI (also defined by a 48-h time frame within admission) during 2003-2010 were S. maltophilia Liu et al., 2012). Another recent study on the prevalence of community-acquired S. maltophilia BSI in Taiwan, which specifically divided the patients into three categories based on whether they had community-acquired (excluding patients hospitalized within 90 days before admission, cared in a nursing facility, etc.), healthcare-associated or hospitalacquired infections, reported that 17.6% of all communityacquired bloodstream infections were due to S. maltophilia . A similar study in France revealed that 23.7% of all community-acquired BSIs were due to S. maltophilia (Fihman et al., 2012). These studies show that communityacquired S. maltophilia infections are far less rare than previously thought.

Risk Factors of Mortalty
A number of risk factors for death due to S. maltophilia infections have been reported. Paez et al. (Paez and Costa, 2008) reviewed the literature from 1985 to 2008 and found that BSI and pneumonia, shock, thrombocytopenia, and Acute Physiological Assessment and Chronic Health Evaluation (APACHE) score >15 are independent risk factors associated with outcome. In addition, underlying hematological malignancy and admission to ICU are independent risk factors for cancer patients. The impact of appropriate antimicrobial treatment and removal of CVC on mortality were concluded to require further clinical studies (Paez and Costa, 2008). The conclusion of the review corresponds to the aforementioned studies. Falagas et al. analyzed 15 articles for attributable mortality of S. maltophilia infections. Only four studies provided relevant data regarding inappropriate antibiotic treatment, and three out of the four studies found significantly higher mortality when compared with initial appropriate therapy (Falagas et al., 2009b).

Molecular Mechanisms in Antimicrobial Resistance
S. maltophilia has several molecular mechanisms contributing to its extensive antimicrobial resistance. The mechanisms are summarized in Table 3. Detailed descriptions of the major mechanisms are elaborated as follows.

β-Lactamases
S. maltophilia has two chromosomal-mediated inducible βlactamases, namely L1 and L2. L1 is a molecular class B Zn 2+ -dependent metallo-β-lactamase and L2 is a molecular class A clavulanic acid-sensitive cephalosporinase. The L1 and L2 β-lactamases are simultaneously regulated by AmpR, a transcriptional regulator in the L2 upstream region (Okazaki and Avison, 2008). The ampR-L2 module is homologous to the ampR-ampC systems, which are widely distributed in some members of the family Enterobacteriaceae and in P. aeruginosa (Lodge et al., 1990). The regulation of chromosomal ampR-ampC systems has been well studied in Citrobacter freundii, where the AmpC β-lactamase induction is linked to peptidoglycan recycling and involves several regulatory genes, such as as ampR, ampG, and ampD (Lindberg et al., 1985). A similar induction mechanism was proposed for the ampR-ampC and the ampR-L2 modules (Okazaki and Avison, 2008). But unlike P. aeruginosa, the permease system in S. maltophilia requires an intact ampN-ampG operon for the induction of β-lactamase . Two ampD homologs, ampD I and ampD II , were found in S. maltophilia, but only ampD I appears to be relevant to the regulation of β-lactamase (Yang et al., 2009). Penicillin-binding proteins (PBPs) participate in peptidoglycan biosynthesis and the inactivation of PBP4 in P. aeruginosa has been shown to confer AmpC overexpression and β-lactam resistance (Moya et al., 2009). The inactivation of a putative PBP1a gene, mrcA, recently was found to cause basal-level L1/L2 β-lactamase hyperproduction in S. maltophilia KJ. The inactivation of mrcA only affects basal L1/L2 production β-lactamase, which is ampR-and ampN-ampG-dependent, and does not augment their induction (Lin et al., 2011). The universality of disruption of ampD I or mrcA in β-lactamase-hyperproducing S. maltophilia mutants and clinical isolates has been proved by the existence of wild-type ampD I and mrcA genes. The result implicates mutation of at least one additional gene in this phenotype (Talfan et al., 2013).

SmeABC
The overexpression of smeABC genes confers resistance to aminoglycosides, β-lactams, and fluoroquinolones. SmeC was identified to function independently of SmeAB, while deletions in smeC but not smeB compromised the antimicrobial resistance (Li et al., 2002).

SmeDEF
SmeDEF is a complex formed by an efflux pump located on the inner membrane (SmeE), an outer membrane protein (SmeF) and a periplasmic membrane fusion protein (SmeD). It is involved in resistance to quinolones, tetracyclines, macrolides, chloramphenicol and novobiocin (Alonso and Martinez, 2000). Expression of the smeDEF operon is regulated by the SmeT repressor (Hernandez et al., 2009). A recent study showed that the SmeDEF efflux pump is associated with plant root colonization by S. maltophilia, and that deletion of the smeE gene impairs this function (García-León et al., 2014a).

SmeVWX
The SmeVWX pump, encoded by a five-gene operon (smeU1, smeV, smeW, smeU2, and smeX), was identified and characterized in a multidrug-resistant mutant of S. maltophilia KJ. Overexpression of the SmeVWX pump resulted in increased resistance to chloramphenicol, quinolones, and tetracyclines but increased aminoglycoside susceptibility (Chen et al., 2011).

SmeYZ and SmeJK
The smeZ, smeJ, and smeK genes were identified in S. maltophilia K M5, a selected mutant derivative. SmeZ contributes to elevated aminoglycoside MICs. SmeJ and SmeK jointly elevate tetracycline, minocycline, and ciprofloxacin MICs and confer resistance to levofloxacin (Gould et al., 2013). In addition to drug extrusion, the SmeIJK pump has been reported to play a physiologic role in the maintenance of cell membrane integrity . A recent study further elucidated the physiologic significance of the SmeYZ pump and demonstrated its correlation with virulence-related functions, including swimming, flagella formation, oxidative stress susceptibility, biofilm formation, and protease secretion (Lin et al., 2015).

SmeOP
A pcm-tolCsm operon was recently verified in S. maltophilia KJ2. The tolCsm gene is involved in the resistance of several antimicrobial agents, including aminoglycosides, macrolides, β-lactams, chloramphenicol, nalidixic acid, doxycycline and TMP/SMX. The deletion of pcm was shown to result in decreased expression of tolCsm, which compromised the pathogen's resistance to amikacin and gentamicin (Huang et al., 2013b). A very recent study characterized a five-gene cluster efflux pump (tolCSm-pcm-smeRo-smeO-smeP) in S. maltophilia. The study showed that SmeOP requires TolCSm for efflux pump function and suggested that TolCSm is the cognate outer membrane protein (OMP) for the SmeOP pump. The SmeOP-TolCSm efflux pump was shown to be associated with resistance to nalidixic acid, doxycycline, amikacin, gentamicin, erythromycin, and leucomycin .

ABC Family: SmrA and MacABCsm
SmrA, the first ABC-type efflux pump identified in S. maltophilia, has been shown to confer resistance to fluoroquinolones and tetracycline (Al-Hamad et al., 2009). The MacABCsm efflux pump in S. maltophilia was recently shown to confer intrinsic resistance to antimicrobials (aminoglycosides, macrolides, and polymyxins) and to play an important role in regulating oxidative and envelope stress tolerance and biofilm formation (Lin et al., 2014b).

MFS Family: EmrCABsm
Only one MFS-type efflux pump, EmrCABsm, has been characterized so far. It is involved in the extrusion of hydrophobic compounds, including the antibiotics nalidixic acid and erythromycin (Huang et al., 2013a).

Fusaric Acid Tripartite Efflux Pump
A novel tripartite fusaric acid efflux pump was found in S. maltophilia, namely FuaABC, which may constitute a new subfamily of the tripartite efflux pump. The fuaABC operon was demonstrated to be induced by fusaric acid and to contribute to fusaric acid resistance when overexpressed (Hu et al., 2012).

Trimethoprim/Sulfamethoxazole (TMP/SMX) Resistance Mechanisms
The sul1 gene carried by class 1 integrons and the sul2 gene, which is linked to insertion sequence common region (ISCR) elements, are known to be responsible for resistance to TMP/SMX in S. maltophilia (Barbolla et al., 2004;Toleman et al., 2007;Chung et al., 2015). The dfrA gene cassettes, which are located in class 1 integrons and encode for the dihydrofolate reductase enzyme, have also been reported to confer high-level resistance to TMP/SMX . Moreover, SmeDEF, TolCsm, and SmeYZ efflux pumps have recently been reported to be associated with TMP/SMX resistance (Huang et al., 2013b;Lin et al., 2015;Sánchez and Martínez, 2015).

Quinolones Resistance Mechanisms
Two mechanisms are associated with resistance of S. maltophilia to quinolones: efflux pumps and a chromosomally encoded qnr gene (Smqnr) that protects both gyrase and topoisomerase IV from quinolones (Sanchez et al., 2009). Unlike other bacteria, clinical isolates of quinolone-resistant S. maltophilia do not present mutations in topoisomerases (Valdezate et al., 2005). To date, the characterized genetic determinants involving resistance to quinolones are smeDEF, smeIJK, smeABC, smeVWX, and Smqnr genes, among which smeDEF and Smqnr are the best-described. Smqnr belongs to the qnr family. It confers low-level resistance and contributes to intrinsic resistance to quinolones in S. maltophilia (Sánchez et al., 2008;Shimizu et al., 2008;Sánchez and Martínez, 2010). García-León et al. further elucidated the interplay between intrinsic and acquired resistance to quinolones in S. maltophilia. Their study demonstrated that the capacity to develop mutation-driven antibiotic resistance is highly dependent on the intrinsic resistome. Their findings indicate that the most prevalent cause of acquired quinolone resistance in S. maltophilia is the overproduction of multidrug efflux pumps, among which SmeDEF efflux pump plays the most important role (García-León et al., 2014b). In addition, a more recent report by García-León et al. confirmed that overexpression of smeVWX in clinical isolates of S. maltophilia is associated with high-level quinolone resistance (García-León et al., 2015).

Antimicrobial Treatment Studies
Trimethoprim/sulfamethoxazole (TMP/SMX) remains the most effective antimicrobial agent against S. maltophilia, with an overall susceptibility rate higher than 90% (Falagas et al., 2008). A recent study investigated the efficacy of sulfametrole/trimethoprim, an alternative sulphonamide/trimethoprim combination available in several Europe countries against non-fermenters (40 S. maltophilia included) and found that the activity of the alternative combination was similar to that of TMP/SMX (Livermore et al., 2014). Other common options include ceftazidime, ticarcillin-clavulanate, fluoroquinolones, and tetracyclines such as tigecycline, minocycline, and doxycyclines. As previously mentioned, resistance rates to ceftazidime and ticarcillinclavulanate are high and rising and are, therefore, unreliable choices. Fluoroquinolones are now popular alternatives because of their less prominent side effects compared to TMP/SMX and their greater potency compared to β-lactams.

Fluoroquinolones
Fluoroquinolones (FQs) are commonly used to treat infections due to S. maltophilia (Nicodemo and Paez, 2007). However, their overuse worldwide has resulted in higher resistance rates in many kinds of pathogenic bacteria, including S. maltophilia Pien et al., 2015).
To evaluate the effectiveness of FQs in this era of high FQ resistance, a retrospective study published in 2014 compared the outcomes of patients with S. maltophilia infections treated with TMP/SMX and those of patients treated with FQs monotherapy (Wang et al., 2014b). A total of 38 adults received TMP/SMX and 63 adults received FQs (levofloxacin n = 48 and ciprofloxacin n = 15). The overall microbiological cure rate was 63% (65% in the TMP/SMX group and 62% in the FQ group), and the overall clinical success rate was 55% (61% in the TMP/SXT group and 52% in the FQ group). The antibiotic regimens were equally effective in both groups. Another retrospective study compared the effectiveness of TMP/SMX (n = 51) with that of levofloxacin (n = 35) in treating S. maltophilia bacteremia and revealed no significant differences in treatment outcome between the two groups, including 30-day mortality, length of hospital day, and antibiotic withdrawal (Cho et al., 2014). However, the rate of adverse events was significantly lower in the levofloxacin group (0%) than in the TMP/SXT group (23.5%, p = 0.001).
Several new quinolones have been developed and some of them have recently been approved for clinical application, including nemonoxacin  and delafloxacin (Bassetti et al., 2015). Oral nemonoxacin, a novel nonfluorinated quinolone antibiotic, has been shown to have good activity against Gram-positive bacilli, such as methicillin-resistant Staphylococcus aureus . However, in vitro susceptibility assays on 32 clinical isolates of S. maltophilia revealed high MIC 90 (32 mg/L) and MIC 50 (8 mg/L) values (Lai et al., 2014a). Regarding delafloxacin, a phase II study published in 2009 that compared two doses of delafloxacin to tigecycline in adults with complicated skin and skin structure infections found that only one patient was infected with S. maltophilia and was treated successfully with delafloxacin (O'Riordan et al., 2015). More in vivo studies are needed to better understand the effectiveness of these new quinolones in treating S. maltophilia infections.

Tetracyclines
Tetracyclines such as tigecycline, minocycline, and doxycycline are some of the most active antimicrobial agents against S. maltophilia other than TMP/SMX, even in the cystic fibrosis population (Cantón et al., 2003;San Gabriel et al., 2004;Gülmez et al., 2010;Milne and Gould, 2012;Castanheira et al., 2014). The results agree with our aforementioned observation that these antibiotics consistently exhibited good activity against S. maltophilia in global surveillance studies (Table 2).
Tigecycline, a derivative of minocycline, has broad-spectrum antimicrobial activity (Stein and Babinchak, 2013) and is an alternative agent against S. maltophilia infections. A recent study from Brazil showed that the MIC 50 and MIC 90 values of tigecyline for S. maltophilia isolates, including isolates resistant to levofloxacin and/or TMP/SMX harboring sul-1, sul-2, and qnrMR, were 1 and 4 µg/ml, respectively (Rizek et al., 2015). However, tigecycline has great bio-distribution after intravenous injection, which leads to lower serum drug levels. So, there are concerns about its efficacy for the treatment of bacteremia due to S. maltophilia (Stein and Babinchak, 2013). In a recent study, a high dose of tigecycline was effective at treating S. maltophilia bacteremia (Wu and Shao, 2014), although in another study, high-dose tigecycline treatment was associated with significant adverse effects (Falagas et al., 2014). In addition, a 3-year clinical therapeutic study that compared the effectiveness of TMP/SMX and tigecycline in treating nosocomial S. maltophilia infections revealed no significant differences in mortality or clinical response rates between the two regimens. Clinical improvement rates on the 14th day were 69.2% in the TMP/SMX group and 68.4% in the tigecycline group (P = 0.954) and mortality rates on the 30th day were 30.8% in the TMP/SMX group and 21.1% in the tigecycline group (P = 0.517) (Tekce et al., 2012). Therefore, tigecycline might be an alternative for patients who are unable to tolerate TMP/SMX. In addition to monotherapy, a combination regimen with tigecycline might be a better option for severe infections, especially for nosocomial infections (Samonis et al., 2012).
In a recent large collection of resistant organisms from the SENTRY program during 2007-2011 (1706 S. maltophilia included), minocycline was shown to be significantly more active than other tetracyclines against S. maltophilia. The rate of susceptibility of S. maltophilia to minocycline exceeded 97% across all geographic regions, and the potency was 2-fold higher than doxycycline (MIC 50/90 : 0.5/2 µg/mL) (Castanheira et al., 2014). A study evaluated 53 multidrug resistant isolates of S. maltophilia, including 48 that were resistant to levofloxacin and/or TMP/SMX, and found that minocycline exhibited excellent activity against S. maltophilia. However, the clinical experience is still anecdotal (Rizek et al., 2015). A patient with pneumonia was reportedly successfully treated with minocycline (Irifune et al., 1994), and the combination of minocycline with TMP and ticarcillin/clavulanate has been suggested to be effective (Vartivarian et al., 1994).

Polymyxins and Fosfomycin
Polymyxins and fosfomycin are being reconsidered as alternatives or "last-resort" options because of the increasing emergence of multidrug-resistant organisms. Unfortunately, interpreting the susceptibility rates of S. maltophilia to polymyxins is problematic because of the discordance between different testing methods (Nicodemo et al., 2004;Gülmez et al., 2010;Moskowitz et al., 2010;Betts et al., 2014). A SENTRY surveillance program study conducted during 2001-2004 assessed the antimicrobial activity of polymyxin B among 54731 isolates of GNB and 1256 isolates of S. maltophilia and found that 72.4% of the S. maltophilia isolates were susceptible to polymyxin B (MIC 50 and MIC 90 values, 1 and 8 mg/L, respectively) (Gales et al., 2006). Colistin (polymyxin E) appears to have a considerable in vitro activity against S. maltophilia (83-88%) (Falagas and Kasiakou, 2005). In addition, synergy with rifampin, TMP/SMX, and doxycycline has been shown (Giamarellos-Bourboulis et al., 2002;Gülmez et al., 2010). Two studies by the SENTRY program in the year 2011 (globally) and during 2009-2012 (USA and Europe) that collected 362 and 494 isolates of S. maltophilia, respectively, reported very different rates of susceptibility to colistin (98.5%, Sader et al., 2013 and38.7-49.7%, Sader et al., 2014a). In a recent study that collected 641 isolates of S. maltophilia in a university hospital in Argentina , Rodríguez et al. showed that colistin resistance increased from 8% in 1996 to 45% in 2013 and found that the increase correlated with a marked increase (11.4-fold) in colistin consumption during the study period. Fosfomycin has been shown in several reports to be an inappropriate treatment option because of its poor activity and high MICs against S. maltophilia (Macleod et al., 2009;Khan et al., 2014;Rizek et al., 2015).

Combination Therapy
Owing to the impressive array of antimicrobial resistance mechanisms of S. maltophilia, various combinations of antimicrobial agents have been surveyed in order to overcome resistance or to attain synergism ( Table 5). Combinations of two or three agents with good susceptibility results such as TMP/SMX, ceftazidime, ticarcillin/clavulanate, and aminoglycosides have demonstrated synergistic effects to different degrees in prior studies. In more recent studies, combinations of TMP/SMX or β-lactam/β-lactam inhibitors with new or old antibiotics such as tigecycline, fluoroquinolones, televancin (Hornsey et al., 2012), rifampin (Betts et al., 2014), and aerosolized colistin have been investigated; they demonstrated various extents of synergism and the ability to maintain effectivity in biofilm. TMP/SMX plus ceftazidime plus levofloxacin has been shown to be effective in treatment of meningitis (Correia et al., 2014) and intrabiliary infusion of colistin plus parenteral fosfomycin with tigecycline was reported to be effective at treating complicated biliary tract infection (Perez et al., 2014). Several combinations of novel agents are currently under investigation, including a β-lactam and dual β-lactamase inhibitor combination (Page et al., 2011) and MD3 (a novel synthetic inhibitor of peptidases) plus colistin (Personne et al., 2014). It is important to mention that in vitro synergy attained by combination should be further correlated with clinical outcomes.

Nebulized Antimicrobial Agents
Nebulization of antimicrobials results in high concentrations in the respiratory tract and is associated with low toxicity because this method of delivery results in limited systemic absorption ( Table 5). These characteristics are especially important for patients with cystic fibrosis, who are prone to frequent infection and colonization of multidrug resistant pathogens including S. maltophilia. Wood et al. reported a case of recurrent ventilatorassociated pneumonia successfully treated with aerosolized colistin and doxycycline (Wood et al., 2010). King et al. surveyed the in vitro pharmacodynamics of aerosolized levofloxacin and suggested that the high concentrations of levofloxacin achieved in the lung by aerosol delivery may be useful for the treatment of patients with cystic fibrosis (King et al., 2010). When S.  Nord et al., 1974Nord et al., 1979 TMP/SMX + carbenicillin Felegie et al., 1979Felegie et al., 1980 Gentamicin + carbenicillin + rifampin TMP/SMX + carbenicillin + rifampin Yu et al., 1980Yu et al., 1983 Gentamicin + carbenicillin + rifampicin TMP/SMX + carbenicillin + rifampicin Berenbaum et al., 1983Berenbaum et al., 1988 Ciprofloxacin + ceftazidime Chow et al., 1988Chow et al., 1995 Ticarcillin/clavulanate + TMP/SMX Ticarcillin/clavulanate + ciprofloxacin Poulos et al., 1995Poulos et al., 1998 Levofloxacin + various beta-lactams Visalli et al., 1998Visalli et al., 2001   maltophilia isolates were grown as a biofilm, the top 3 most effective antibiotic combinations included high-dose levofloxacin or colistin delivered at doses achievable by aerosolization plus a β-lactam or TMP/SMX ( Table 5) (Wu et al., 2013). The potentials of other antibiotics to be nebulized to achieve high drug levels in airway have been investigated in order to overcome the high MICs that cannot be conquered when the agents are administered systemically. A device (Podhaler device) that delivers new inhalational tobramycin (tobramycin inhalation powder, TIP) and attains high drug levels to the lung may be able to exceed current high MICs of tobramycin in S. maltophilia (Ratjen et al., 2015). Waters suggested a potential role of inhaled aztreonam lysine in the treatment of S. maltophilia pulmonary infection because of its resistance to the L1 β-lactamase produced by S. maltophilia and the ability to achieve high drug levels in respiratory secretions (approximately 1000-fold higher than the corresponding plasma concentration) (Waters, 2012). The antibacterial activity of a novel inhaled combination of fosfomycin and tobramycin (FTI) was investigated in patients with bronchiectasis. However, FTI demonstrated relatively poor activity against S. maltophilia (Macleod et al., 2009).

Conclusions
Worldwide, multi-institutional studies confirm that S. maltophilia is an emerging multi-drug resistant opportunistic pathogen in hospital and community settings, especially among immunocompromised hosts. TMP/SMX remains the most effective antimicrobial agent in the general population. Drugs with historically good susceptibility results include ceftazidime, ticarcillin-clavulanate, and fluoroquinolones; however, a number of studies show an alarming trend in resistance to those agents. Tetracyclines such as tigecycline, minocycline, and doxycycline are also effective agents and consistently display good activity against S. maltophilia in various geographic regions and across different time periods. Combination therapies, novel agents, and aerosolized forms of antimicrobial drugs are currently being tested for their ability to treat infections caused by this multi-drug resistant organism. In addition, recent advances in molecular methods have identified various new mechanisms contributing to drug resistance, which hopefully will lead to future breakthroughs in the treatment of infections due to S. maltophilia.