Antimicrobial susceptibility and combination testing of invasive Stenotrophomonas maltophilia isolates

Clonal spread of trimethoprim-sulfamethoxazole-resistant Stenotrophomonas maltophilia isolates in a tertiary hospital

Aim

The aims of this study were to: (i) determine antibiotic susceptibility of clinical Stenotrophomonas maltophilia isolates, (ii) investigate the presence of different classes of integrons and sul genes responsible for sulphonamide resistance, (iii) assess the molecular epidemiology of the isolates by determining their clonal relatedness, and (iv) investigate the potential sources of infection by collecting environmental samples when necessary.

Methods

99 S. maltophilia isolates from clinical specimens of hospitalized patients were screened by PCR for sul1, sul2, sul3 genes, and integron-associated integrase genes: intI1, intI2, and intI3. PFGE was used to determine the clonal relatedness of the isolates.

Results

Susceptibility rates for trimethoprim-sulfamethoxazole, levofloxacin, and ceftazidime were 90.9%, 91.9%, and 53.5% respectively. All trimethoprim-sulfamethoxazole-resistant isolates were positive for intI1 and sul1. PFGE analysis revealed that 24 of the isolates were clonally related, clustering in seven different clones. Five of the nine trimethoprim-sulfamethoxazole-resistant isolates were clonally related. The first isolate in this clone was from a wound sample of a patient in the infectious diseases clinic, and the other four were isolated from the bronchoalveolar lavage samples of patients in the thoracic surgery unit. The patient with the first isolate neither underwent bronchoscopy nor stayed in the thoracic surgery unit. Although clustering was observed in bronchoalveolar lavage samples, no S. maltophilia growth was detected in environmental samples.

Conclusion

The findings demonstrated that the sul1 gene carried by class 1 integrons plays an important role in trimethoprim-sulfamethoxazole resistance in S. maltophilia isolates. PFGE analysis revealed a high degree of genetic diversity. However, detection of clonally related isolates suggests the acquisition from a common source and/or cross-transmission of this microorganism between the patients.

Global prevalence and distribution of antibiotic resistance among clinical isolates of Stenotrophomonas maltophilia: A systematic review and meta-analysis

OBJECTIVES

Stenotrophomonas maltophilia (S. maltophilia), an opportunistic pathogen, causes infection in patients undergoing immunosuppressive therapy, mechanical ventilation, or catheters and in long-term hospitalized patients. Due to its extensive resistance to various antibiotics and chemotherapeutic agents, S. maltophilia is challenging to treat. Using case reports, case series, and prevalence studies, the current study provides a systematic review and meta-analysis of antibiotic resistance profiles across clinical isolates of S. maltophilia.

METHODS

A systematic literature search was performed for original research articles published in Medline, Web of Science, and Embase databases from 2000 to 2022. Statistical analysis was performed using STATA 14 software to report antibiotic resistance of S. maltophilia clinical isolates worldwide.

RESULTS

223 studies (39 case reports/case series and 184 prevalence studies) were collected for analysis. A meta-analysis of prevalence studies demonstrated that the most antibiotic resistance worldwide was to levofloxacin, trimethoprim-sulfamethoxazole (TMP/SMX), and minocycline (14.4%, 9.2%, and 1.4%, respectively). Resistance to TMP/SMX (36.84%), levofloxacin (19.29%), and minocycline (1.75%) were the most prevalent antibiotic resistance types found in evaluated case reports/case series studies. The highest resistance rate to TMP/SMX was reported in Asia (19.29%), Europe (10.52%), and America (7.01%), respectively.

CONCLUSION

Considering the high resistance to TMP/SMX, more attention should be paid to patients' drug regimens to prevent the emergence of multidrug-resistant S. maltophilia isolates.

The prevalence of colistin resistance in clinical Stenotrophomonas maltophilia isolates worldwide: a systematic review and meta-analysis

While trimethoprim-sulfamethoxazole (TMP-SMX) is the first-line therapy of Stenotrophomonas maltophilia infections, colistin is one of the therapeutic options in cases of allergy or resistance to TMP-SMX. However, understanding the global status of resistance to colistin amongst S. maltophilia isolates could be helpful for appropriate antibiotic prescription. This study aimed to conduct a systematic review and meta-analysis to examine the prevalence of colistin resistance in clinical S. maltophilia isolates worldwide. According to eligibility criteria, a total of 61 studies were included in the analysis. The pooled prevalence for colistin resistance was 42% (95% CI: 35-49%), ranging from 0.1 to 97%. Subgroups analysis indicated that, the pooled prevalence of colistin resistance was 44% (95% CI: 29-60%) in 15 studies during 2000-2010, and it was estimated to be 41% (95% CI: 33-50%) in 46 articles from 2011 to 2021. It was 46% (95% CI: 35-58%) in the studies that used broth microdilution method, and 39% (95% CI: 30-49%) in the studies with other used methods. The resistance rate in Asian countries was 45% (95% CI: 31-60%), in European countries was 45% (95% CI: 34-56%) and in the countries of North and South America was 33% (95% CI: 20-46%). Our review showed notable resistance to colistin in clinical S. maltophilia isolates. Given the estimated resistance rates, alternative antibiotics could be preferred to treat serious infections due to S. maltophilia.

Global prevalence and antibiotic resistance in clinical isolates of Stenotrophomonas maltophilia: a systematic review and meta-analysis

Introduction

Stenotrophomonas maltophilia is a little-known environmental opportunistic bacterium that can cause broad-spectrum infections. Despite the importance of this bacterium as an emerging drug-resistant opportunistic pathogen, a comprehensive analysis of its prevalence and resistance to antibiotics has not yet been conducted.

Methods

A systematic search was performed using four electronic databases (MEDLINE via PubMed, Embase, Scopus, and Web of Science) up to October 2019. Out of 6,770 records, 179 were documented in the current meta-analysis according to our inclusion and exclusion criteria, and 95 studies were enrolled in the meta-analysis.

Results

Present analysis revealed that the global pooled prevalence of S. maltophilia was 5.3 % [95% CI, 4.1-6.7%], with a higher prevalence in the Western Pacific Region [10.5%; 95% CI, 5.7-18.6%] and a lower prevalence in the American regions [4.3%; 95% CI, 3.2-5.7%]. Based on our meta-analysis, the highest antibiotic resistance rate was against cefuroxime [99.1%; 95% CI, 97.3-99.7%], while the lowest resistance was correlated with minocycline [4·8%; 95% CI, 2.6-8.8%].

Discussion

The results of this study indicated that the prevalence of S. maltophilia infections has been increasing over time. A comparison of the antibiotic resistance of S. maltophilia before and after 2010 suggested there was an increasing trend in the resistance to some antibiotics, such as tigecycline and ticarcillin-clavulanic acid. However, trimethoprim-sulfamethoxazole is still considered an effective antibiotic for treating S. maltophilia infections.

Network analysis reveals significant joint effects of microplastics and tetracycline on the gut than the gill microbiome of marine medaka

Microplastics could accumulate and enrich antibiotics in the aquatic environment. Despite this, the joint effects of microplastics and antibiotics on aquatic organisms are not clear. Here, we investigated the changes of microbial interactions in both gill and gut of marine medaka exposed to polystyrene microbeads (PS) and/or tetracycline for 30 days by using co-occurrence network analysis based on 16S rRNA gene amplicon sequences. We found that the single and combined effects of PS and tetracycline were more profound on the gut than on the gill microbiome. SourceTracker analysis showed that the relative contributions from the gill microbiome to the gut microbiome increased under combined exposure. Moreover, the combined exposure reduced the complexity and stability of the gut microbial network more than those induced by any single exposure, suggesting the synergistic effects of PS and tetracycline on the gut microbiome. The PS and tetracycline combined exposure also caused a shift in the keystone taxa of the gut microbial network. However, no similar pattern was found for gill microbial networks. Furthermore, single and combined exposure to PS and/or tetracycline altered the associations between the gut network taxa and indicator liver metabolites. Altogether, these findings enhanced our understanding of the hazards of the co-occurring environmental microplastics and antibiotics to the fish commensal microbiome.

Antimicrobial Treatment Strategies for Stenotrophomonas maltophilia: A Focus on Novel Therapies

Stenotrophomonas maltophilia is an urgent global threat due to its increasing incidence and intrinsic antibiotic resistance. Antibiotic development has focused on carbapenem-resistant Enterobacteriaceae, Pseudomonas, and Acinetobacter, with approved antibiotics in recent years having limited activity for Stenotrophomonas. Accordingly, novel treatment strategies for Stenotrophomonas are desperately needed. We conducted a systemic literature review and offer recommendations based on current evidence for a treatment strategy of Stenotrophomonas infection.

Clinical outcomes of Stenotrophomonas maltophilia infection treated with trimethoprim/sulfamethoxazole, minocycline, or fluoroquinolone monotherapy

OBJECTIVES

The historical treatment of choice for Stenotrophomonas maltophilia infection is trimethoprim/sulfamethoxazole and this is primarily based on preclinical studies. The objective of this study was to examine the clinical outcomes of patients receiving monotherapy with different agents.

METHODS

This was a retrospective study of adult patients receiving monotherapy for S. maltophilia infection with trimethoprim/sulfamethoxazole (TMP/SMX), a fluoroquinolone, or minocycline from 2010 to 2016. The primary outcome was clinical failure, a composite of recurrence, alteration of therapy due to adverse reaction or concern for clinical failure, or 30-day in-hospital mortality. The secondary outcome was 30-day in-hospital mortality. To account for treatment selection bias, multivariate regression and propensity score weighting were conducted.

RESULTS

284 patients were included (217 received TMP/SMX, 28 received a fluoroquinolone, and 39 received minocycline). The TMP/SMX and minocycline groups appeared to include similar patients whereas the fluoroquinolone group appeared to represent a slightly less severely ill population. Clinical failure was similar between groups (36%, 29%, and 31% in the TMP/SMX, fluoroquinolone, and minocycline groups, respectively, P=0.69) as was 30-day mortality (15%, 7%, and 5% in the TMP/SMX, fluoroquinolone, and minocycline groups, respectively, P=0.16). After controlling for confounding factors, receipt of minocycline (adjusted odds ratio [OR]=0.2 [0.1-0.7]) but not a fluoroquinolone (adjusted OR=0.3 [0.1 to 2.1]) was associated with lower mortality compared with TMP/SMX. This association persisted after propensity score weighting.

CONCLUSIONS

Outcomes were similar or better with alternatives to TMP/SMX monotherapy, which indicates this may not be the treatment of choice for infections caused by S. maltophilia.

Epidemiological Trends and Resistance Associated with Stenotrophomonas maltophilia Bacteremia: A 10-Year Retrospective Cohort Study in a Tertiary-Care Hospital in Hungary

Stenotrophomonas maltophilia has been recognized as an emerging nosocomial pathogen in invasive infections of immunocompromised, severely debilitated patients with significant underlying illnesses. The first-choice drug in these infections is sulfamethoxazole-trimethoprim (SMX/TMP), and resistance to this antimicrobial is a daunting challenge for clinicians. The aim of this study was to assess the prevalence of S. maltophilia bacteremia and SMX/TMP-resistance levels at a tertiary-care university hospital. A total of 175 episodes of S. maltophilia bacteremia were identified (2008-2012: n = 82, 2013-2017: n = 93; p = 0.061), 52% of affected patients were 60 years of age, and had recent surgery, severe injuries or underlying conditions (malignant hematologic diseases and solid tumors) in their history. Sixteen percent of isolates were resistant to SMX/TMP (2008-2012: n = 13.8%, 2013-2017: n = 17.2%; p = 0.076), and out of the resistant strains, 32.7% were also resistant to levofloxacin and colistin. Our findings on the SMX/TMP-resistance were similar to global literature data.

In vitro investigation of synergy among fosfomycin and parenteral antimicrobials against carbapenemase-producing Enterobacteriaceae

Intravenous fosfomycin is undergoing clinical development in the United States for treatment of complicated urinary tract infections (cUTIs) and may be prescribed as a component of dual antibiotic regimens against carbapenemase-producing Enterobacteriaceae (CPE). Fosfomycin, aztreonam, cefepime, ceftazidime, ceftazidime/avibactam, ceftolozane/tazobactam, meropenem, piperacillin/tazobactam, and tobramycin minimum inhibitory concentrations (MICs) were determined by gradient diffusion strip (GDS) against CPE isolates (N = 49). The GDS cross method was used to assess antibiotic interactions between fosfomycin and the aforementioned parenteral antibiotics. The resultant fractional inhibitory concentration index was used to classify interactions. Fosfomycin-containing combinations were evaluated only if nonsusceptible to the second agent. The fosfomycin MIC₅₀ was ≥1024 mg/L by GDS. Synergy or additivity was detected in 80 (22%) fosfomycin-containing combinations. Antagonism was not observed. Ceftolozane/tazobactam most frequently displayed synergy [8 (16.3%) isolates]. When CPE are isolated, clinical laboratories should consider performing GDS synergy tests to identify favorable antibiotic interactions.

In vitro combination of tigecycline with other antibiotics in Stenotrophomonas maltophilia isolates

Background/aim

The aim of this study was to determine the usefulness of tigecycline in combination treatment of Stenotrophomonas maltophilia infections by evaluating the in vitro synergistic effects of tigecycline with various antibiotics using the E-test method.

Materials and methods

Synergy testing by E-test was performed with various antibiotic combinations in 10 S. maltophilia isolates identified as a cause of infection. The antibiotics used in the study included tigecycline (TGC), cefoperazone-sulbactam (CPS), ceftazidime (TZ), levofloxacin (LEV), and trimethoprim-sulfamethoxazole (cotrimoxazole) (TS). Four different combinations (TGC-CPS, TGC-TZ, TGC-LEV, TGC-TS) were studied with the E-test synergy method.

Results

S. maltophilia isolates were found to have the highest level of susceptibility to trimethoprim-sulfamethoxazole, tigecycline, and levofloxacin. The fractional inhibitory concentration (FIC) index was calculated as FIC = MICAB/MICA + MICBA/MICB. The FIC index values were calculated and classified as synergistic (FIC < 0.5), additive (FIC = 0.5–1), indifferent (FIC = 1–4), and antagonistic (FIC > 4). According to FIC index values, synergy was found with the highest rate with TGC-CPS and TGC-LEV combinations (20%). Antagonistic activity was not found in any combination.

Conclusion

When trimethoprim-sulfamethoxazole cannot be used because of resistance or allergy, tigecycline alone or in combination may be included as an alternative option. Although in vitro results are promising, clinical data are required.

Treatment Strategies for Infected Wounds

The treatment of skin wounds is a key research domain owing to the important functional and aesthetic role of this tissue. When the skin is impaired, bacteria can soon infiltrate into underlying tissues which can lead to life-threatening infections. Consequently, effective treatments are necessary to deal with such pathological conditions. Recently, wound dressings loaded with antimicrobial agents have emerged as viable options to reduce wound bacterial colonization and infection, in order to improve the healing process. In this paper, we present an overview of the most prominent antibiotic-embedded wound dressings, as well as the limitations of their use. A promising, but still an underrated group of potential antibacterial agents that can be integrated into wound dressings are natural products, especially essential oils. Some of the most commonly used essential oils against multidrug-resistant microorganisms, such as tea tree, St. John's Wort, lavender and oregano, together with their incorporation into wound dressings are presented. In addition, another natural product that exhibits encouraging antibacterial activity is honey. We highlight recent results of several studies carried out by researchers from different regions of the world on wound dressings impregnated with honey, with a special emphasis on Manuka honey. Finally, we highlight recent advances in using nanoparticles as platforms to increase the effect of pharmaceutical formulations aimed at wound healing. Silver, gold, and zinc nanoparticles alone or functionalized with diverse antimicrobial compounds have been integrated into wound dressings and demonstrated therapeutic effects on wounds.

Assessing the in vitro activity of ceftazidime/avibactam and aztreonam among carbapenemase-producing Enterobacteriaceae: Defining the zone of hope

Ceftazidime/avibactam plus aztreonam (CZA+ATM) is an emerging option to combat carbapenemase-producing Enterobacteriaceae (CPE) expressing resistance via multiple β-lactamases within Ambler classes A, B, C, and D. The benefit of this combination is apparent when the pathogen-specific resistance genotype is characterized. However, rapid molecular diagnostic systems may be unavailable to allow this precision medicine-based approach. Using synergy tests with antibiotic gradient diffusion strips (GDSs), we aimed to prove that the defined phenotypic profile of CPE is reliably predicted by the genotype to confirm the utility of this method as a phenotypic profiling tool for use in the clinical setting. Synergy assessments for CPE (n=10) that co-produce serine- and metallo-β-lactamases were performed by crossing CZA and ATM antibiotic GDSs (Liofilchem^® and Etest^®). The minimum inhibitory concentration (MIC):MIC ratio method was also conducted for five CPE. All CPE were resistant to CZA and ATM when tested alone. Using classical fractional inhibitory concentration definitions, synergy (9/10) and additivity (1/10) was detected by at least one method for all isolates. As predicted by cross-coverage of genotypically defined serine- and metallo-β-lactamases, for all isolates CZA+ATM produced a phenotypic profile distinguished by sizeable zones of inhibited growth which we term the 'zone of hope'. In conclusion, simple procedures utilizing antibiotic GDSs were concordant with the known genotypic profile of the CPE selected for study. This approach appears to be a valuable tool for guiding therapy in the absence of molecular diagnostic systems. Furthermore, this study confirms potent in vitro activity of CZA+ATM against CPE expressing multiple β-lactamases.

Therapies for multidrug resistant and extensively drug-resistant non-fermenting gram-negative bacteria causing nosocomial infections: a perilous journey toward 'molecularly targeted' therapy

INTRODUCTION

Non-fermenting Gram-negative bacilli are at the center of the antimicrobial resistance epidemic. Acinetobacter baumannii and Pseudomonas aeruginosa are both designated with a threat level to human health of 'serious' by the Centers for Disease Control and Prevention. Two other major non-fermenting Gram-negative bacilli, Stenotrophomonas maltophilia and Burkholderia cepacia complex, while not as prevalent, have devastating effects on vulnerable populations, such as those with cystic fibrosis, as well as immunosuppressed or hospitalized patients. Areas covered: In this review, we summarize the clinical impact, presentations, and mechanisms of resistance of these four major groups of non-fermenting Gram-negative bacilli. We also describe available and promising novel therapeutic options and strategies, particularly combination antibiotic strategies, with a focus on multidrug resistant variants. Expert commentary: We finally advocate for a therapeutic approach that incorporates in vitro antibiotic susceptibility testing with molecular and genotypic characterization of mechanisms of resistance, as well as pharmacokinetics and pharmacodynamics (PK/PD) parameters. The goal is to begin to formulate a precision medicine approach to antimicrobial therapy: a clinical-decision making model that integrates bacterial phenotype, genotype and patient's PK/PD to arrive at rationally-optimized combination antibiotic chemotherapy regimens tailored to individual clinical scenarios.

Surveillance of antimicrobial susceptibility patterns among Stenotrophomonas maltophilia isolated in China during the 10-year period of 2005-2014

The antimicrobial resistance pattern was studied among 300 Stenotrophomonas maltophilia isolates collected from 300 nonduplicated inpatients. The average age of the patients was 73.5 ± 14.6 years old. Isolates nonsusceptible to levofloxacin, chloramphenicol, minocycline or trimethoprim/sulfamethoxazole (SXT) were more prevalent in the 99 patients aged over 81 years than in the other patients (p < 0.05). Multidrug resistance was found to be significantly associated with isolates recovered from the blood (72.7%, p = 0.000) and elderly patients (31.3%, p = 0.003). A trend of increased resistance to antimicrobials and higher and higher frequencies of multidrug-resistant isolates were observed through the 10-year period, wherein the percentage of isolates resistant to SXT was significantly changed from 29.7% in 2005-2009 to 47.1% in 2010-2014 (p = 0.02). Clinicians are recommended to pay special attention to the possibility of multidrug-resistant S. maltophilia infection in elderly inpatients.

Evaluation of trimethoprim-sulfamethoxazole based combination therapy against Stenotrophomonas maltophilia: in vitro effects and clinical efficacy in cancer patients

OBJECTIVES

The aim of this study was to evaluate the in vitro effects and clinical efficacies of trimethoprim-sulfamethoxazole (SXT) combined with other antimicrobial agents against Stenotrophomonas maltophilia.

METHODS

In vitro analysis was conducted on 89 S. maltophilia strains isolated from blood and the respiratory tract between June 2012 and October 2014. Levofloxacin (LVX), ticarcillin-clavulanic acid (TIM), and minocycline (MIN) were selected for an examination of their effects when individually combined with SXT by the checkerboard method. In addition, 29 S. maltophilia bacteremia cases were reviewed and the clinical efficacies of SXT-based combination therapies were analyzed.

RESULTS

SXT+LVX showed synergy in 21, no interactions in 61, and antagonism in 7. SXT+TIM showed synergy in 71, and no interactions in 18. SXT+MIN showed synergy in 10, and no interactions in 79. The review of clinical data indicated that a combination of SXT+fluoroquinolone was not associated with improved prognosis compared with monotherapy.

CONCLUSIONS

The in vitro data indicated that SXT+TIM had beneficial microbiological effects and was not antagonistic. Our in vitro and clinical data analyses do not support the routine use of SXT+fluoroquinolone combination therapy for S. maltophilia infection.

Increase in the Prevalence of Resistance Determinants to Trimethoprim/Sulfamethoxazole in Clinical Stenotrophomonas maltophilia Isolates in China

Aims

This study was carried to reveal the genetic mechanisms of trimethoprim/sulfamethoxazole (SXT) resistance.

Methods

Among 300 clinical Stenotrophomonas maltophilia isolates from China, resistance determinants such as sul and dfrA genes, integrons and transposase were examined using PCR, DNA sequencing and thermal asymmetric interlaced PCR (TAIL-PCR). Data were analyzed using SPSS 20.0.

Results

Of the 300 isolates, 116 (38.7%) were resistant to SXT. An alarming trend of increased resistance to SXT were found over the 10-year period. The positive rates of sul and class 1 integrase (intI1) increased gradually with the development of SXT resistance over the 10-year period. Multiple logistic regression analyses indicated that the genes of qacEΔ1-sul1 (81% vs 46.2%, p = 0.000), sul2 (50.9% vs 9.8%, p = 0.000), intI1 (83.6% vs 65.8%, p = 0.000), dfrA12 (25% vs 3.3%, p = 0.000), dfrA17 (15.5% vs 3.8%, p = 0.000) and dfrA27 (4.3% vs 1.6%, p = 0.01) were more prevalent in SXT-resistant isolates than SXT-susceptible isolates except dfrA1(p = 0.83) and dfrA5(p = 0.18). Sequencing data revealed 12 types of resistance gene cassettes (aar-3-dfrA27, dfrA12–aadA2, dfrA17–aadA5, cmlA1, aacA4, aadA5, arr-3-aacA4, aadA1, aadB–aadA4, aacA4–catB8–aadA1, aadB–aac(6′)-II–bla_CARB-8 and aac(6′)-II–bla_CARB-8) located in the class 1 integron in 163 isolates (87% SXT-resistant vs 33.7% SXT-susceptible isolates, p = 0.000). A novel finding was the aar-3-dfrA27 (KC748137) gene cassette. The gene of sul2 linked to transposase in 50 SXT- resistant and 7 SXT- susceptible isolates was detected by TAIL-PCR.

Conclusions

The findings demonstrated a higher prevalence of sul, dfrA, intI1 and resistance gene cassettes in class 1 integron in SXT-resistant clinical S. maltophilia isolates in China. The sul1 and dfrA genes located in integrons and the sul2 linked to transposase may imply wide and rapid dissemination of resistance gene in bacteria.

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.

Mechanisms of antimicrobial resistance in Gram-negative bacilli

The burden of multidrug resistance in Gram-negative bacilli (GNB) now represents a daily issue for the management of antimicrobial therapy in intensive care unit (ICU) patients. In Enterobacteriaceae, the dramatic increase in the rates of resistance to third-generation cephalosporins mainly results from the spread of plasmid-borne extended-spectrum beta-lactamase (ESBL), especially those belonging to the CTX-M family. The efficacy of beta-lactam/beta-lactamase inhibitor associations for severe infections due to ESBL-producing Enterobacteriaceae has not been adequately evaluated in critically ill patients, and carbapenems still stands as the first-line choice in this situation. However, carbapenemase-producing strains have emerged worldwide over the past decade. VIM- and NDM-type metallo-beta-lactamases, OXA-48 and KPC appear as the most successful enzymes and may threaten the efficacy of carbapenems in the near future. ESBL- and carbapenemase-encoding plasmids frequently bear resistance determinants for other antimicrobial classes, including aminoglycosides (aminoglycoside-modifying enzymes or 16S rRNA methylases) and fluoroquinolones (Qnr, AAC(6′)-Ib-cr or efflux pumps), a key feature that fosters the spread of multidrug resistance in Enterobacteriaceae. In non-fermenting GNB such as Pseudomonas aeruginosa, Acinetobacter baumannii and Stenotrophomonas maltophilia, multidrug resistance may emerge following the sole occurrence of sequential chromosomal mutations, which may lead to the overproduction of intrinsic beta-lactamases, hyper-expression of efflux pumps, target modifications and permeability alterations. P. aeruginosa and A. baumannii also have the ability to acquire mobile genetic elements encoding resistance determinants, including carbapenemases. Available options for the treatment of ICU-acquired infections due to carbapenem-resistant GNB are currently scarce, and recent reports emphasizing the spread of colistin resistance in environments with high volume of polymyxins use elicit major concern.

Stenotrophomonas maltophilia bloodstream infection in patients with hematologic malignancies: a retrospective study and in vitro activities of antimicrobial combinations

Background

Stenotrophomonas maltophilia causes serious infections in immunocompromised hosts. Here, we analyzed the clinical characteristics of S. maltophilia bloodstream infection (BSI) in patients with hematologic malignancies and evaluated in vitro synergistic effects of antimicrobial combinations.

Methods

We retrospectively reviewed all consecutive episodes of S. maltophilia BSIs in adult hematologic patients from June 2009 to May 2014, with in vitro susceptibility and synergy tests using high-throughput bioluminescence assay performed for available clinical isolates.

Results

Among 11,004 admissions during 5-year period, 31 cases were identified as S. maltophilia BSIs. The incidence rate of S. maltophilia BSI was 0.134 cases/1,000 patient-days. Overall and attributable mortality of S. maltophilia BSI was 64.5% and 38.7%, respectively. Severe neutropenia (adjusted hazard ratio [HR] 5.24, p =0.013), shock at the onset of BSI (adjusted HR 6.05, p <0.001), and pneumonia (adjusted HR 3.15, p =0.017) were independent risk factors for mortality. In vitro susceptibilities to ceftazidime, levofloxacin, ticarcillin-clavulanic acid (TIM) and trimethoprim-sulfamethoxazole (SXT) were 11.1%, 44.0%, 40.7%, and 88.9%, respectively. MIC₅₀/MIC₉₀ for moxifloxacin and tigecycline were 1/4 mg/L and 4/8 mg/L. The 50% and 90% fractional inhibitory concentrations (FIC⁵⁰/FIC⁹⁰) of clinical isolates against a combination of SXT and TIM were 0.500/0.750. For SXT plus levofloxacin or moxifloxacin, FIC⁵⁰/FIC⁹⁰ were 0.625/1.000 and 0.625/0.625, respectively.

Conclusion

S. maltophilia BSIs show high mortality, which is related to severe neutropenia, shock, and S. maltophilia pneumonia. Based upon drug susceptibility testing, the primary treatment of choice for S. maltophilia BSIs should be SXT in hematologic patients, rather than quinolones, with combination therapies including SXT serving as a feasible treatment option.

Infections Caused by Stenotrophomonas maltophilia in Recipients of Hematopoietic Stem Cell Transplantation

Stenotrophomonas maltophilia (S. maltophilia) is a globally emerging Gram-negative bacillus that is widely spread in environment and hospital equipment. Recently, the incidence of infections caused by this organism has increased, particularly in patients with hematological malignancy and in recipients of hematopoietic stem cell transplantation (HSCT) having neutropenia, mucositis, diarrhea, central venous catheters or graft versus host disease and receiving intensive cytotoxic chemotherapy, immunosuppressive therapy, or broad-spectrum antibiotics. The spectrum of infections in HSCT recipients includes pneumonia, urinary tract and surgical site infection, peritonitis, bacteremia, septic shock, and infection of indwelling medical devices. The organism exhibits intrinsic resistance to many classes of antibiotics including carbapenems, aminoglycosides, most of the third-generation cephalosporins, and other β-lactams. Despite the increasingly reported drug resistance, trimethoprim-sulfamethoxazole is still the drug of choice. However, the organism is still susceptible to ticarcillin-clavulanic acid, tigecycline, fluoroquinolones, polymyxin-B, and rifampicin. Genetic factors play a significant role not only in evolution of drug resistance but also in virulence of the organism. The outcome of patients having S. maltophilia infections can be improved by: using various combinations of novel therapeutic agents and aerosolized aminoglycosides or colistin, prompt administration of in vitro active antibiotics, removal of possible sources of infection such as infected indwelling intravascular catheters, and application of strict infection control measures.

Activity of colistin in combination with tigecycline or rifampicin against multidrug-resistant Stenotrophomonas maltophilia

The antimicrobial treatment of Stenotrophomonas maltophilia infections is complicated by intrinsic multidrug resistance and a lack of reliable susceptibility data. We assessed the activity of colistin (COL), rifampicin (RIF) and tigecycline (TGC) alone and in combination using a range of in vitro susceptibility testing methodologies and a simple invertebrate model of S. maltophilia infection (Galleria mellonella). Synergy [fractional inhibitory concentration indices (FICIs) ≤0.5] between COL and either RIF or TGC was observed against 92 % and 88 % of 25 S. maltophilia isolates, respectively, despite resistance to one or another of the single agents alone. In time-kill assays, COL combined with either RIF or TGC was superior to single agents, but only the COL/RIF regimen was reliably bactericidal. The in vitro findings correlated with treatment outcomes in G. mellonella, with heightened survival observed for larvae treated with COL/RIF or COL/TGC compared with COL, RIF or TGC alone. COL combined with RIF was the most effective combination overall in both in vitro and in vivo (p < 0.05) assays. Given the difficulty in selecting appropriate therapy for S. maltophilia infections, regimens consisting of COL combined with RIF or TGC could be considered for clinical use.