A transposon for green fluorescent protein transcriptional fusions: application for bacterial transport experiments

Visualization study on aniline-degrading bacteria AN-1 transport in the aquifer with the low-permeability lens

The geological conditions of the contaminated sites will affect the migration of microorganisms in the underground environment. In order to study the effect of low-permeability lens on bacterial transport, green fluorescent protein labeling combined with light transmission method was used to reveal the bacterial transport in the heterogeneous aquifer. The experiment has the advantages of real-time monitoring and no disturbance. The results showed that the bacteria gave priority to bypass the lens to flow away. The lens had a significant effect on hindering the bacterial transport due to adsorption and straining. The larger permeability coefficient ratio between the bulk media and the low-permeability lens was, the more obvious the obstruction was. AN-1 cannot enter the lens until the ratio decreased to the order of 10². With the increase of the flow velocity, the bacterial plume changed a lot. The higher flow velocity reduced the adsorption and retention of AN-1 to the media, resulting in some microorganisms remaining in the pores washed down. When the flow came to 2.0 m·d^-1, AN-1 cannot adhere to the media due to the excessive fluid shear stress.

Microbial remediation of nitro-aromatic compounds: an overview

Nitro-aromatic compounds are produced by incomplete combustion of fossil fuel or nitration reactions and are used as chemical feedstock for synthesis of explosives, pesticides, herbicides, dyes, pharmaceuticals, etc. The indiscriminate use of nitro-aromatics in the past due to wide applications has resulted in inexorable environmental pollution. Hence, nitro-aromatics are recognized as recalcitrant and given Hazardous Rating-3. Although several conventional pump and treat clean up methods are currently in use for the removal of nitro-aromatics, none has proved to be sustainable. Recently, remediation by biological systems has attracted worldwide attention to decontaminate nitro-aromatics polluted sources. The incredible versatility inherited in microbes has rendered these compounds as a part of the biogeochemical cycle. Several microbes catalyze mineralization and/or non-specific transformation of nitro-aromatics either by aerobic or anaerobic processes. Aerobic degradation of nitro-aromatics applies mainly to mono-, dinitro-derivatives and to some extent to poly-nitro-aromatics through oxygenation by: (i) monooxygenase, (ii) dioxygenase catalyzed reactions, (iii) Meisenheimer complex formation, and (iv) partial reduction of aromatic ring. Under anaerobic conditions, nitro-aromatics are reduced to amino-aromatics to facilitate complete mineralization. The nitro-aromatic explosives from contaminated sediments are effectively degraded at field scale using in situ bioremediation strategies, while ex situ techniques using whole cell/enzyme(s) immobilized on a suitable matrix/support are gaining acceptance for decontamination of nitrophenolic pesticides from soils at high chemical loading rates. Presently, the qualitative and quantitative performance of biological approaches of remediation is undergoing improvement due to: (i) knowledge of catabolic pathways of degradation, (ii) optimization of various parameters for accelerated degradation, and (iii) design of microbe(s) through molecular biology tools, capable of detoxifying nitro-aromatic pollutants. Among them, degradative plasmids have provided a major handle in construction of recombinant strains. Although recombinants designed for high performance seem to provide a ray of hope, their true assessment under field conditions is required to address ecological considerations for sustainable bioremediation.

Bioformulation of Burkholderia sp. MSSP with a multispecies consortium for growth promotion of Cajanus cajan

The present work was undertaken to formulate an effective bioformulation using Burkholderia sp. strain MSSP, a known plant-growth-promoting rhizobacterium. MSSP was tagged with the reporter gene of green fluorescent protein (gfp) to monitor its population in cost-effective solid carriers, including sugarcane-bagasse, sawdust, cocoa peat, rice husk, wheat bran, charcoal, and rock phosphate, and paneer-whey as liquid carrier. Physical and chemical properties of different low-cost carrier materials were studied. The viability of the green fluorescent tagged variant of MSSP was estimated in different sterile carrier materials. Whey and wheat bran proved to be efficient carrier materials for the bioformulation. Sawdust, rock phosphate, rice husk, and cocoa peat were average, while charcoal and sugarcane-bagasse proved to be inferior carriers. The viability of strain MSSP was also assessed in wheat bran and whey-based consortium, having three other bacterial strains, namely Sinorhizobium meliloti PP3, Rhizobium leguminosarum Pcc, and Bacillus sp. strain B1. Presence of other plant-growth-promoting bacteria did not have any detrimental effect on the viability of MSSP. Efficiency of the wheat-bran-based multispecies consortium was studied on the growth of pigeonpea in field conditions. A considerable increase in plant biomass, nodule number and weight, and number of pods was recorded as compared with individual trials and with the control.

Paenibacillus polymyxa invades plant roots and forms biofilms

Paenibacillus polymyxa is a plant growth-promoting rhizobacterium with a broad host range, but so far the use of this organism as a biocontrol agent has not been very efficient. In previous work we showed that this bacterium protects Arabidopsis thaliana against pathogens and abiotic stress (S. Timmusk and E. G. H. Wagner, Mol. Plant-Microbe Interact. 12:951-959, 1999; S. Timmusk, P. van West, N. A. R. Gow, and E. G. H. Wagner, p. 1-28, in Mechanism of action of the plant growth promoting bacterium Paenibacillus polymyxa, 2003). Here, we studied colonization of plant roots by a natural isolate of P. polymyxa which had been tagged with a plasmid-borne gfp gene. Fluorescence microscopy and electron scanning microscopy indicated that the bacteria colonized predominantly the root tip, where they formed biofilms. Accumulation of bacteria was observed in the intercellular spaces outside the vascular cylinder. Systemic spreading did not occur, as indicated by the absence of bacteria in aerial tissues. Studies were performed in both a gnotobiotic system and a soil system. The fact that similar observations were made in both systems suggests that colonization by this bacterium can be studied in a more defined system. Problems associated with green fluorescent protein tagging of natural isolates and deleterious effects of the plant growth-promoting bacteria are discussed.

Construction of green fluorescent protein (GFP)-marked strains of Bradyrhizobium for ecological studies

AIM

To introduce the gfp gene encoding green fluorescent protein (GFP) into bradyrhizobia for their identification in nodules, soil and carrier-based inoculants.

METHODS AND RESULTS

Bradyrhizobium sp. strains M29 and GN7, which nodulate mungbean (Vigna radiata), were conjugated with Escherichia coli S17-1 carrying plasmid EDS 15 (a suicide plasmid carrying a promoterless gfp gene fused with Tn5). The GFP-marked strain expressed the gfp gene from a Bradyrhizobium promoter and gave green fluorescence when observed under an epifluorescent microscope or u.v. transilluminater. All the GFP-marked strains were able to nodulate mungbean and fix nitrogen. The GFP-marked bradyrhizobia were recovered at a frequency of 90-100% and 16-63% from nodules formed under sterilized and unsterilized conditions, respectively. The GFP-marked bradyrhizobia were identified from soil and from charcoal-based inoculants on the basis of green fluorescence.

CONCLUSIONS

The GFP-marked Bradyrhizobium was successfully identified on the basis of green fluorescence to study its competition and survival in the soil and in charcoal-based inoculants.

SIGNIFICANCE AND IMPACT OF THE STUDY

Introduction of the gfp gene into Bradyrhizobium provides a simple, specific and cost-effective method of strain identification for ecological studies.

Comparison of a range of green fluorescent protein-tagging vectors for monitoring a microbial inoculant in soil

Two new transposon-based tagging vectors have been constructed using the gfp marker gene under control of either constitutive or inducible promoters. The two vectors, along with the established pUTminiTn5gfp were used to tag a diesel-degrading Pseudomonas strain. Tagged strains were obtained that were not affected in terms of their growth or ability to use diesel as a carbon source. The transposon tags were stably maintained in the strains without selection and provided visible fluorescence as colonies or single cells in suspension. Tagging did not impede the survival of tagged Pseudomonas aeruginosa GP41B strains in diesel-contaminated soil microcosms. The tagged strains were easily recovered from the microcosms after a 3-month period. The tagging of bacteria with gfp using either native or introduced constitutive/inducible promoters is an effective and easy way to monitor their survival in soil.

A comparison of enumeration methods for culturable Pseudomonas fluorescens cells marked with green fluorescent protein

The detection of bacteria in environmental samples using genetic markers is valuable in microbial ecology. The green fluorescent protein (GFP) reporter gene was studied under nutrient starvation conditions at 4 degrees C, 23 degrees C and 30 degrees C in Pseudomonas fluorescens R2fG1 cells tagged with a red-shifted gfp. Fluorescence intensity was not significantly different in cells maintained in a buffer for at least 48 days at all the tested temperatures. gfp-Tagged R2fG1 cells were introduced into bulk soil microcosms and soil microcosms with wheat seedlings. GFP-marked cells were enumerated immediately after inoculation into soil and again in soil and root samples after 10 days. Counts of culturable colonies were obtained from drop plates using 5-microl aliquots of serial dilutions viewed with an epifluorescent microscope. Traditional spread plates (using 100-microl aliquots) and the most-probable-number (MPN) method using a spectrofluorometer were also used to enumerate the GFP-marked Pseudomonas cells in soil, rhizosphere and rhizoplane samples. Microcolonies were visualized on root surfaces under the epifluorescent microscope after immobilizing in agar and incubation for 24 h. Counts from traditional spread plates were significantly higher (P<0.05) than the population estimates of the MPN method for all treatments at any sampling time. Counts using the drop plate method, however, were not significantly different (P<0.05) except in one treatment, and provided similar estimates in half the time of spread plates and at an estimated third of the cost.

Bacterial survival and mineralization of p-nitrophenol in soil by green fluorescent protein-marked Moraxella sp. G21 encapsulated cells

Moraxella sp. G21 cells marked with the green fluorescent protein (gfp) survived in kappa-carrageenan beads and as free cells for a month after inoculation into autoclaved soil and non-sterile soil contaminated with p-nitrophenol (PNP). Similar [U-(14)C]PNP mineralization values were produced by encapsulated Moraxella sp. G21 cells and as free cells (53 and 60% mineralization). There was no significant difference between cell survival and [U-(14)C]PNP mineralization activity in soil by the rifampicin-resistant Moraxella sp. mental strain and Moraxella sp. G21. The ability of encapsulated Moraxella sp. G21 cells to survive, retain their green fluorescence and mineralize [U-(14)C]PNP suggests that the GFP-marked strain encapsulated in kappa-carrageenan may be useful for bioremediation of toxic chemicals in soil.

Green fluorescent protein-based direct viable count to verify a viable but non-culturable state of Salmonella typhi in environmental samples

The gfp-tagging method and lux-tagging method were compared to select a better method for verifying a viable but nonculturable (VBNC) state of bacteria in the environment. An environmental isolate of Salmonella typhi was chromosomally marked with a gfp gene encoding green fluorescent protein (GFP). The hybrid transposon mini-Tn5 gfp was transconjugated from E. coli to S. typhi. Using the same method, S. typhi was chromosomally marked with luxAB genes encoding luciferase. The survival of gfp-tagged S. typhi introduced into groundwater microcosms was examined by GFP-based plate count, total cell count, and a direct viable count method. In microcosms containing lux-tagged S. typhi, luminescence-based plate count and the measurement of bioluminescence of each microcosm sample were performed. In microcosms containing lux-tagged S. typhi, viable but nonculturable cells could not be detected by using luminometry. As no distinguishable luminescence signals from the background signals were found in samples containing no culturable cells, a VBNC state of S. typhi could not be verified in lux-based systems. However, comparison between GFP-based direct viable counts and plate counts was a good method for verifying the VBNC state of S. typhi. Because GFP-based direct viable count method provided a direct and precise estimation of viable cells of introduced bacteria into natural environments, it can be used for verifying the VBNC state of bacteria in environmental samples.

Applications of the green fluorescent protein as a molecular marker in environmental microorganisms

In this review, we examine numerous applications of the green fluorescent protein (GFP) marker gene in environmental microbiology research. The GFP and its variants are reviewed and applications in plant-microbe interactions, biofilms, biodegradation, bacterial-protozoan interactions, gene transfer, and biosensors are discussed. Methods for detecting GFP-marked cells are also examined. The GFP is a useful marker in environmental microorganisms, allowing new research that will increase our understanding of microorganisms in the environment.

Viable, but non-culturable, state of a green fluorescence protein-tagged environmental isolate of Salmonella typhi in groundwater and pond water

An environmental isolate of Salmonella typhi was chromosomally marked with a gfp gene encoding green fluorescence protein (GFP) isolated from Aequorea victoria. The hybrid transposon mini Tn5 gfp was transconjugated from E. coli to S. typhi, resulting in constitutive GFP production. The survival of S. typhi GFP155 introduced into groundwater and pond water microcosms was examined by GFP-based plate counts, total cell counts, and direct viable counts. A comparison between GFP-based direct viable counts and plate counts was a good method for verifying the viable, but non-culturable (VBNC), state of S. typhi. The entry into a VBNC state of S. typhi was shown in all microcosms. S. typhi survived longer in groundwater than in pond water as both a culturable and a VBNC state.

Use of green fluorescent protein to tag and investigate gene expression in marine bacteria

Two broad-host-range vectors previously constructed for use in soil bacteria (A. G. Matthysse, S. Stretton, C. Dandie, N. C. McClure, and A. E. Goodman, FEMS Microbiol. Lett. 145:87-94, 1996) were assessed by epifluorescence microscopy for use in tagging three marine bacterial species. Expression of gfp could be visualized in Vibrio sp. strain S141 cells at uniform levels of intensity from either the lac or the npt-2 promoter, whereas expression of gfp could be visualized in Psychrobacter sp. strain SW5H cells at various levels of intensity only from the npt-2 promoter. Green fluorescent protein (GFP) fluorescence was not detected in the third species, Pseudoalteromonas sp. strain S91, when the gfp gene was expressed from either promoter. A new mini-Tn10-kan-gfp transposon was constructed to investigate further the possibilities of fluorescence tagging of marine bacteria. Insertion of mini-Tn10-kan-gfp generated random stable mutants at high frequencies with all three marine species. With this transposon, strongly and weakly expressed S91 promoters were isolated. Visualization of GFP by epifluorescence microscopy was markedly reduced when S91 (mini-Tn10-kan-gfp) cells were grown in rich medium compared to that when cells were grown in minimal medium. Mini-Tn10-kan-gfp was used to create an S91 chitinase-negative, GFP-positive mutant. Expression of the chi-gfp fusion was induced in cells exposed to N'-acetylglucosamine or attached to chitin particles. By laser scanning confocal microscopy, biofilms consisting of microcolonies of chi-negative, GFP+ S91 cells were found to be localized several microns from a natural chitin substratum. Tagging bacterial strains with GFP enables visualization of, as well as monitoring of gene expression in, living single cells in situ and in real time.

Green fluorescent protein as a visual marker in a p-nitrophenol degrading Moraxella sp

The green fluorescent protein gene (gfp) was introduced into a p-nitrophenol-metabolizing strain of Moraxella sp. by chromosomal integration. The gfp-marked transformants, designated Moraxella sp. strains G21 and G25, exhibited green fluorescence under UV light. Molecular characterization by PCR and Southern hybridization showed the presence of gfp in both transformants. Both transformants and the parent strain degraded 720 microM of p-nitrophenol with nitrite release within 4 h after inoculation in minimal medium supplemented with yeast extract. Transformants degraded up to 1440 microM p-nitrophenol and mineralized about 60% of 720 microM p-nitrophenol, both in broth and in soil, to the same extent as the parent strain. Insertion of gfp did not adversely affect the expression of p-nitrophenol-degrading genes in the transformants. Survival studies indicated that individual green fluorescent colonies of transformants can be detected up to 2 weeks after inoculation in soil. These marked strains could be of value in studies on microbial survival in the environment.

Use of green fluorescent protein to detect expression of nif genes of Azoarcus sp. BH72, a grass-associated diazotroph, on rice roots

A gfp (green fluorescent protein) cassette for transcriptional fusions has been developed to study gene expression in Azoarcus sp. BH72 in association with plant roots. The bacteria expressed nitrogenase genes (nifHDK) in the rhizosphere, on root tips, and in epidermal cells of rice seedlings. Green fluorescent protein fusions also visualized promoter activity of single cells in soil.

Green fluorescent protein-based reporter systems for genetic analysis of bacteria including monocopy applications

The green fluorescent protein (GFP) gene, gfp, was used to develop versatile reporter systems for genetic analysis in, and monitoring of bacteria. This reporter system is available on a plasmid and on a mini-transposon located in a suicide delivery plasmid for generation of chromosomal fusions. To achieve sensitivity levels necessary for use in monocopy applications and for detection of single cells, the 3'-end of gfp was replaced by that of a modified gfp gene characterized by a 45-fold stronger fluorescence signal than that exhibited by the natural GFP. This modified gfp gene was also equipped with the strong translation signals of the atpE gene. Transfer of the mini-transposon into two different Pseudomonas spp. and Alcaligenes eutrophus produced random chromosomal fusions, some 5% of which exhibited fluorescence detectable by eye. Individual GFP+ cells were readily observed by fluorescence microscopy.