Susceptibility testing of Stenotrophomonas maltophilia to carbapenems

Development and evaluation of test methods for the detection and enumeration of opportunistic waterborne pathogens from the hospital environment

BACKGROUND

Many Gram-negative bacteria other than Pseudomonas aeruginosa have been implicated in waterborne outbreaks, but standardized laboratory detection methods for these organisms have not been established.

AIM

This study aimed to establish laboratory testing methodologies for six waterborne pathogens: Acinetobacter spp., Burkholderia spp., Cupriavidus spp., Delftia acidovorans, Elizabethkingia spp. and Stenotrophomonas maltophilia.

METHODS

Water samples were spiked by UK Health Security Agency laboratories and sent to the Glasgow Royal Infirmary laboratory for analysis. Water samples were spiked with either a pure culture of target organism or the target organism in water containing normal background flora, to ensure that the methodology could identify organisms from a mixed culture. Volumes of 100 mL were filtered under negative pressure on to culture media and incubated at 30 °C and 37 °C. The incubation time was 7 days, with plates read on days 2, 5 and 7. Further identification of colonies was undertaken using matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS).

FINDINGS

Optimal recovery of organisms was obtained by culturing water samples on tryptic soy agar, chocolate bacitracin agar and pseudomonas selective agar. The optimal temperature for isolation was 30 °C. The optimal incubation time was 5 days, and MALDI-TOF MS identified all test species reliably.

CONCLUSION

The methodology described was able to detect the six tested waterborne pathogens reliably, and can be utilized by laboratories involved in testing water samples during outbreak investigations.

Rapid Detection of Imipenem Resistance in Gram-Negative Bacteria Using Tabletop Scanning Electron Microscopy: A Preliminary Evaluation

Background: Enabling faster Antimicrobial Susceptibility Testing (AST) is critical, especially to detect antibiotic resistance, to provide rapid and appropriate therapy and to improve clinical outcomes. Although several standard and automated culture-based methods are available and widely used, these techniques take between 18 and 24 h to provide robust results. Faster techniques are needed to reduce the delay between test and results.

Methods: Here we present a high throughput AST method using a new generation of tabletop scanning electron microscope, to evaluate bacterial ultra-structural modifications associated with susceptibilities to imipenem as a proof of concept. A total of 71 reference and clinical strains of Gram-negative bacteria were used to evaluate susceptibility toward imipenem after 30, 60, and 90 min of incubation. The length, width and electron density of bacteria were measured and compared between imipenem susceptible and resistant strains.

Results: We correlated the presence of these morphological changes to the bacterial susceptibility and their absence to the bacterial resistance (e.g., Pseudomonas aeruginosa length without [2.24 ± 0.61 μm] and with [2.50 ± 0.68 μm] imipenem after 30 min [p = 3.032E-15]; Escherichia coli width without [0.92 ± 0.07 μm] and with [1.28 ± 0.19 μm] imipenem after 60 min [p = 1.242E-103]). We validated our method by a blind test on a series of 58 clinical isolates where all strains were correctly classified as susceptible or resistant toward imipenem.

Conclusion: This method could be a potential tool for rapidly identifying carbapenem-resistance in Enterobacterales in clinical microbiology laboratories in <2 h, allowing the empirical treatment of patients to be rapidly adjusted.

Azithromycin Exerts Bactericidal Activity and Enhances Innate Immune Mediated Killing of MDR Achromobacter xylosoxidans

Azithromycin (AZM), the most commonly prescribed antibiotic in the United States, is thought to have no activity against multidrug-resistant Gram-negative pathogens such as Achromobacter xylosoxidans (AX) per standard minimum inhibitory concentration testing in cation-adjusted Mueller Hinton Broth. Here we provide the first report of AZM bactericidal activity against carbapenem-resistant isolates of AX, with a multifold decrease in minimum inhibitory concentration across 12 clinical isolates when examined under physiologic testing conditions that better recapitulate the in vivo human environment. This pharmaceutical activity, evident in eukaryotic tissue culture media, is associated with enhanced AZM intracellular penetration and synergistic killing with human whole blood, serum, and neutrophils. Additionally, AZM monotherapy inhibited preformed AX biofilm growth in a dose-dependent manner together with a reduction in viable bacteria. In an illustrative case, AZM in combination with piperacillin-tazobactam exerted clear therapeutic effects in a patient with carbapenem-resistant AX mediastinitis, sternal osteomyelitis, and aortic graft infection. Our study reinforces how current antimicrobial testing practices fail to recapitulate the host environment or host-pathogen interactions and may misleadingly declare complete resistance to useful agents, adversely affecting patient outcomes. We conclude that AZM merits further exploration in the treatment of drug-resistant AX infections. Novel approaches to antimicrobial susceptibility testing that better recapitulate the host environment should be considered, especially as infections caused by multidrug-resistant Gram-negative bacterial pathogens are expanding globally with high morbidity and mortality.

Double-Face Meets the Bacterial World: The Opportunistic Pathogen Stenotrophomonas maltophilia

Most studies on bacterial virulence focus on the pathogen itself. However, it is important to recall that the in-host behavior and the virulence of bacterial pathogens constitute a complex situation that depends on both the microorganisms and the infected host. While healthy people (the community) is infected by classical pathogenic microorganisms, able to cope with the anti-infection defenses of the host, in the case of people with basal diseases, debilitated or immunodepressed, the range of pathogens able to cause infection is wider and includes the so-named opportunistic pathogens, which lack the inherent ability to cause disease in healthy hosts and rarely produce infections in the community. Some of the most relevant opportunistic pathogens, as Stenotrophomonas maltophilia, have an environmental origin and, in occasions, present interesting biotechnological properties. Consequently, it is important knowing whether S. maltophilia isolates recovered from infections constitute a specific phylogenetic branch that has evolved toward acquiring a virulent phenotype as it happens in the case of classical pathogens or rather, any member of this bacterial species is capable of producing infection and its pathogenic behavior is mainly a consequence of the host situation. To address this question, we analyzed a set of environmental and clinical S. maltophilia strains. Our results indicate that this opportunistic pathogen presents a large core genome and that the distribution of genes in general, and of known virulence determinants in particular, is similar among environmental and clinical isolates. The majority of genes not belonging to the S. maltophilia core genome are present in just one or two of the analyzed strains. This indicates that, more than speciation into different lineages (virulent and environmental), the evolution of S. maltophilia is based in the strain-specific acquisition of genes, likely involved in the adaptation of this bacterial species to different microniches. In addition, both environmental and clinical isolates present low susceptibility to several antimicrobials. Altogether our results support that S. maltophilia does not present a specific evolutionary branch toward virulence and most likely infection is mainly the consequence of the impaired anti-infective response of the infected patients.

Antimicrobial therapy for Stenotrophomonas maltophilia infections

Stenotrophomonas maltophilia has emerged as an important nosocomial pathogen capable of causing respiratory, bloodstream, and urinary infections. The treatment of nosocomial infections by S. maltophilia is difficult, as this pathogen shows high levels of intrinsic or acquired resistance to different antimicrobial agents, drastically reducing the antibiotic options available for treatment. Intrinsic resistance may be due to reduced outer membrane permeability or to the multidrug efflux pumps. However, specific mechanisms of resistance such as aminoglycoside-modifying enzymes or the heterogeneous production of metallo-beta-lactamase have contributed to the multidrug-resistant phenotype displayed by this pathogen. Moreover, the lack of standardized susceptibility tests and their interpretative criteria hinder the choice of an adequate antibiotic treatment. Recommendations for the treatment of infections by S. maltophilia are based on in vitro studies, certain nonrandomized clinical trials, and anecdotal experience. Trimethoprim-sulfamethoxazole remains the drug of choice, although in vitro studies indicate that ticarcillin-clavulanic acid, minocycline, some of the new fluoroquinolones, and tigecycline may be useful agents. This review describes the main resistance mechanisms, the in vitro susceptibility profile, and treatment options for S. maltophilia infections.

Non-fermentative Gram-negative bacteria

Over the past decade, non-fermenting Gram-negative bacteria have emerged as important opportunistic pathogens in the increasing population of patients who are immunocompromised by their disease or medical treatment. These bacteria are assisted by their ubiquitous distribution in the environment and have a propensity for multiple, intrinsic or acquired drug resistance. The infections that they cause now pose significant problems in terms of treatment and infection control, whilst the commonly observed rapid emergence of bacterial resistance to new antimicrobial compounds raises concerns regarding the clinical lifespan of these agents. Studies are urgently required to assess whether combination therapy can improve the long-term utility of new drugs in the treatment of patients infected with non-fermenters.

Therapeutic advances of new fluoroquinolones

Fluoroquinolone antimicrobials have been available for over 10 years. Recent modifications to nuclear side-chains have enhanced both the antimicrobial and pharmacokinetic profiles of this class. Rapidly increasing antimicrobial resistance among community and hospital bacterial pathogens has diminished therapeutic options. Infections caused by such pathogens, including drug-resistant Streptococcus pneumoniae and multi-resistant Enterobacteriaceae are now treatable by few classes of antibacterials, one of these being the fluoroquinolones. Ciprofloxacin was one of the first effective agents available in both iv. and oral formulations for the treatment of Gram-negative infection, resistant to other antibiotics. More recent developments, such as sparfloxacin and grepafloxacin, are more effective in vitro against Gram-positive pathogens, although their safety profile may be less promising. Fluoroquinolones not yet in widespread clinical use, including trovafloxacin, clinafloxacin and moxifloxacin, hold considerable promise as community 'respiratory antimicrobials' and the results of clinical trials are awaited with anticipation. In this review, the three generations of fluoroquinolone development are examined and the relative antimicrobial, pharmacokinetic, clinical and safety profiles of available and developmental quinolones are compared.

Clonal diversity and metallo-beta-lactamase production in clinical isolates of Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is a nosocomial pathogen with an intrinsic broad-spectrum resistance to beta-lactam compounds and other antibacterial agents. It produces two chromosomal beta-lactamases: a clavulanic acid-sensitive class A (L2) and a tetrameric carbapenemase (L1 or BlaS). We screened 40 S. maltophilia multidrug-resistant clinical isolates recovered between 1995 and 1998 in the Varese Hospital (Italy) for the presence of the metallo-beta-lactamase. The isolates were investigated by phenotypic profiling (enzymatic activity and antibiotic resistance pattern) and molecular methods such as PCR and pulsed-field gel electrophoresis (PFGE) to reveal intraspecies diversity. For the tested S. maltophilia strains, we showed that the beta-lactamase production could be induced by the presence of imipenem (50 microg/ml) in the culture media. Addition of 1 mM dipicolinic acid completely inhibited the hydrolysis of imipenem but decreased that nitrocefin in a strain-dependent manner. Full activity of crude extract towards imipenem could be restored by addition of 1 mM ZnCl2. Finally, the gene encoding the carbapenem-hydrolyzing beta-lactamase from S. maltophilia ULA-511 and 39/95, a clinical strain, were isolated and sequenced. These two strains have a different profile of multidrug resistance. The two metallo-beta-lactamases were found to be isologous. The difference of sensitivity of these two strains was associated to the level of production of the metallo-beta-lactamase.

Occurrence of variants with temperature-dependent susceptibility (TDS) to antibiotics among Stenotrophomonas maltophilia clinical strains

Susceptibility to 20 antibiotics was tested in 104 Stenotrophomonas maltophilia strains at 37 and 30 degrees C by means of a dilution micromethod to verify the phenomenon of temperature-dependent susceptibility (TDS). Trimethoprim-sulfamethoxazole, pefloxacin and ofloxacin were the most active preparations at 37 degrees C (93, 90, and 86% of susceptible strains, respectively), whilst trimethoprim-sulfamethoxazole, cefoperazone-sulbactam and pefloxacin performed best at 30 degrees C (94, 94, and 76% of susceptible strains, respectively). Variants 37TDS (minimum inhibitory concentration, MIC, of tested antibiotics at least 4-times lower at 37 than at 30 degrees C) occurred in 60%. Variants 30TDS (at least 4-times lower value of MIC at 30 than at 37 degrees C) were found in 7.7%. Both variants in susceptibility to tested antibiotics appeared in 23.1%, whilst neither of them was observed in 9.6%. The 37TDS phenomenon was recorded most of all with gentamicin (51% of strains), amikacin (47), colistin (44) and tetracycline (34). The 30TDS phenomenon was found particularly with cefoperazone-sulbactam (16.0% of strains) and colistin (10.0%). The above phenomena may be due to changes in membrane permeability, temperature-dependent ribosomal changes, and insufficient adaptation to higher temperatures of some strains of the originally environmental species S. maltophilia.

Fatty acid analysis of Stenotrophomonas maltophilia clinical strains showing different susceptibility to antibiotics at 30 and 37 degrees C

Isolates of Stenotrophomonas maltophilia species display the feature "temperature-dependent susceptibility" (TDS) to antibiotics. Both 30TDS strains (at least 4 times lower value of minimum inhibitory concentration (MIC) of an antibiotic at 30 than at 37 degrees C) and 37TDS strains (at least 4 times lower value of MIC at 37 than at 30 degrees C) were described. Changes in the distribution of saturated and unsaturated fatty acids (FA) at 30 and 37 degrees C were considered as one of possible causes of the TDS phenomenon. Gas chromatography was used to determine the distribution of individual FA in five 37TDS strains of S. maltophilia (Group I); in five strains with MIC values unaffected by the cultivation temperature (Group II) and in six 30TDS (four strains) or 30/37TDS (two strains) isolates (Group III). At identical temperatures, no statistically significant differences in the distribution of major FA (iso-15:0, anteiso-15:0, 16:0 and 16:1) were registered between individual groups. Statistically significant (p < 0.05) differences between groups were found in minor FA only (iso-16:0, iso-17:0 and iso-17:1). Distribution changes of cellular FA at 30 and 37 degrees C can be considered to play only a minor role in the formation of the TDS phenomenon.

Antimicrobial susceptibilities of unique Stenotrophomonas maltophilia clinical strains

Susceptibility to 41 antimicrobials was studied with 99 Stenotrophomonas maltophilia strains, and different pulsed-field gel electrophoresis profiles were identified among 130 prospectively collected isolates. Moxalactam, doxycycline, minocycline, and clinafloxacin displayed the highest activity (> or = 98% susceptibility). Ticarcillin resistance (75%) was reverted by clavulanate in 25% of strains. Trimethoprim-sulfamethoxazole resistance was 26.2% (> or = 4 [trimethoprim]/76 [sulfamethoxazole] microg/ml) and dropped to 11.1% when an 8/152-microg/ml breakpoint was applied based on its bimodal MIC distribution. Resistance was lower when unique strains were considered, because clonal organisms contribute to resistance.