Potential of Stenotrophomonas for Bioremediation of Recalcitrant Pollutants
Authors: Vipin Chandra Kalia

Introduction

Microbes can do highly diverse things ranging from producing biofuels, biopolymers to carry out bioremediation. Recently, the microbes which are gaining importance as the most effective for biotechnological applications belong to Bacillus, Citrobacter and Stenotrophomonas. Microbial diversity in soils contaminated with polycyclic aromatic hydrocarbons (PAHs), Effluent Treatment Plants (ETPs) treating pesticide and pharmaceutical industrial waste waters, includes: Alcaligenes, Bacillus, Citrobacter, Pseudomonas, Rhodococcus and Stenotrophomonas. Most versatile organisms belonging toStenotrophomonas are able to treat aromatic compounds either individually or in combination with Bacillus, Pseudomonas, Flavimonas, Morganella spp.

Diversity of Stenotrophomonas

Initially these organisms were grouped under Pseudomonas, and Xanthomonas and only more recently have been named asStenotrophomonas. Presently, 8 recognized Stenotrophomonas spp. exist: S. maltophilia, S. nitritireducens,S. acidominiphilia, S. rhizophilia, S. koreensis, S. terrae, S. humi, and S. chelatiphaga. S. dokdonensis has been transferred to Pseudoxanthomonas as P. dokdonensis.

Ecological and metabolic (genetic and functional) diversity of S. maltophilia implies high taxonomic heterogeneity. S. maltophilia shows 4 phylogenetic lineages. S. maltophilia posses 40 genomic islands (3-70 Kb in size), encoding for around 50 proteins. Large islands encode for integrases, etc., which are instrumental in horizontal gene transfer (HGT) from Xanthomonas compestris and X. axonopodis. High diversity among clinical isolates of S. maltophilia is due to polymorphic mutations. Restriction Endonucleases (REs) - DraI and XbaI digestion revealed unique DNA profile in Stenotrophomonas spp. S. maltophilia was distinguished from Xanthomonas through restriction mapping and 16S rRNA gene sequencing.

As Biodegraders

• Excessive use of nitrogenous fertilizers leads to emission of green house gases such as NO and N2O. S. maltophilia can carry out aerobic denitrification, which reduces nitrate to nitrite.

• S. maltophilia degrades xenobiotics: pesticides, insecticides, dyes, herbicides - atrazine (2-chloro-4-ethylamino-1,3,5 triazine) and 2,4-dichlorophenoxyacetic acid (2,4-D), hexachlorocyclohexane (HCH), detoxify high molecular weight PAHs, cyclotrimethylene trinitroamine derivatives (RDX), oil products. S. maltophilia Ac degrades different substrates, including polyol-L-glucitol, nonylphenol polyethoxylates – the synthetic nonionic surfactants used for textile, leather and paper industries.

• Nylon, Feather degradation and alfatoxins

• Complete degradation of catechol, 4-methylcatechol and hydroquinone within 48 h.

Heavy metal toxicity

S. maltophilia shows resistance to multiple antimicrobial agents, can tolerate a wide range of heavy metals: Cd, Pb, Co, Zn, Hg, Ag, can reduce selenite (50 mM) to selenium (Seo) and tellurite (25 mM) to tellurium (Teo). S. chelatiphaga sp. nov. has an ability to degrade EDTA, can oxidize arsenite As(III) to arsenate As(V). S. maltophilia can be exploited for bioremediation of contaminated ground water, detoxify selenate and arsenate contaminated sites.

Biofilm in bioremediation

Biofilms represent microbial communities, gene expression varies with respect to free floating or planktonic members: antibiotic resistance, aggregation, biofilm formation and expression of virulence factors. Stenotrophomonas produces biofilms even in 100 ng/L [CrVI], can transform Cr(VI) to Cr(III) at 300 mg Cr(VI)/L. S. maltophilia embedded in biofilms could biodegrade dodecylbenzene sulfonates 2 times faster than the free floaters. Polycyclic alkanes are recalcitrant to microbial degradation, however it was found to induce bacterial cell aggregation, which augmented degradation of xenobiotics.


The versatile Stenotrophomonas

It acts as plant growth enhancers, biopesticides and antagonistic against pathogens: (i) S. maltophilia R13 has the potential to act as PGPR bioinoculant as it produces 18 free amino acids, keratinolytic activity and IAA production, (ii) Amenable to exploitation for biocontrol of plant pathogens and act against the human pathogenic fungus Candida albicans. It removes Au(III) from contaminated wastewater through its absorption capacity, can grow in limited nutrient – siderophore and. Its refractile toxic bodies enhance competitiveness

Biotechnological importance: (i) Agriculture - to compete in rhizosphere, (ii) Medicine - by producing antibiotics, and (iii) Industry – enzymes (proteases) - may “revolutionize” detergents.

A cautionary note

Stenotrophomonas are opportunistic pathogen next to Pseudomonas aeruginosa. S. maltophilia causes bacteraemia and pneumonia, endocarditis, respiratory tract infections. S. maltophilia can be serious pathogen in cancer patients. S. maltophilia can also produce macrocyclic lactam antibiotics: (i) alteramid A, and (ii) maltophilin.

Most “dangerous” properties:

· Capacity to resist drugs (as it possess a multidrug efflux pump SmeDEF), heavy metals

· Produces extracellular enzymes – lipases, fibrolysin, proteases and biofilm formation - characteristics of emerging pathogens.

Difficult to develop an optimal therapy against S maltophilia.

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About Author / Additional Info:
Researcher at Microbial Biotechnology and Genomics at CSIR-IGIB, Delhi.