Use of green fluorescent protein to monitor survival of genetically engineered bacteria in aquatic environments

Stable genetic integration of a red fluorescent protein in a virulent Group A Streptococcus strain

There are several advantages, both in vitro and in vivo, in utilizing bacteria that express a fluorescent protein. Such a protein can be transiently incorporated into the bacteria or integrated within the bacterial genome. The most widely utilized fluorescent protein is green fluorescent protein (GFP), but limitations exist on its use. Additional fluorescent proteins have been designed that have many advantages over GFP and technologies for their incorporation into bacteria have been optimized. In the current study, we report the successful integration and expression of a stable fluorescent reporter, mCherry (red fluorescent protein, RFP), into the genome of a human pathogen, Group A Streptococcus pyogenes (GAS) isolate AP53(S-). RFP was targeted at the atg codon of the fcR pseudogene that is present in the mga regulon of AP53(S-). Transcription of critical bacterial genes was not functionally altered by the genomic integration of mCherry. Host virulence both in vitro (keratinocyte infection and cytotoxicity) and in vivo (skin infection) was maintained in AP53(S-)-RFP. Additionally, survival of mice infected with either AP53(S-) or AP53(S-)-RFP was similar, demonstrating that overall pathogenicity of the AP53(S-) strain was not altered by the expression of mCherry. These studies demonstrate the feasibility of integrating a fluorescent reporter into the bacterial genome of a naturally virulent isolate of Group A S. pyogenes for comparative experimental studies.

Using GFP-Tagged Escherichia coli to Investigate the Persistence of Fecal Bacteria in Vegetated Wetlands: An Experimental Approach

The contamination of surface water by pathogenic bacteria of human origin is an important public health issue. Wetlands can be contaminated with fecal bacteria by water originating from different sources, such as wastewater treatment plants and agriculture. Escherichia coli is a commensal of the human gut flora and the major indication of fecal contamination in surface water. Little is known about the association between fecal bacteria and submerged macrophytes and how this may influence the water quality. We questioned whether macrophytes enhance or inhibit the bacterial growth in wetlands. For this purpose, we grew four different species of macrophytes (Mentha aquatica, Baldellia ranunculoides, Sparganium emersum and Elodea canadensis, in mono- or multispecies cultures) in aquatic rhizotrons and inoculated the devices with a fluorescent strain of Escherichia coli (producing a green fluorescent protein) to simulate the fecal contamination of wetlands. Bacterial survival was monitored by measuring the fluorescence for 19 days. We found (i) that contaminated sediments did not release E. coli in the water column in lentic conditions and (ii) that monocultures of E. canadensis, M. aquatica and S. emersum reduced the E. coli concentration in the water column. This suggests that aquatic plant species may be used in constructed wetlands to clear surface freshwater from bacteria of fecal origin.

A safety type of genetically engineered bacterium that degrades chemical pesticides

Chemical pesticides are used widely and their residues are found in the environment. Pesticide pollution has become a global problem. To find an economical, effective and safety way to degrade residues of pesticides in environment, we constructed a genetically engineered bacterium (GEB) having the ability to degrade pesticides, emit green fluorescence and has a containment system by using a dual plasmid expression system. One plasmid contains the genes of enhanced green fluorescent protein (EGFP) and carboxylesterase B1 (CarE B1), which were cloned downstream of lambda P_L promoter and expressed constitutively. The gene of CarE B1 encodes an insect-detoxifying enzyme possessing the degradability to organochloride pesticides, organophosphorus pesticides, carbamates, and pyrethoid insecticides. The other is the conditional suicide plasmid for containment system, in which the lethal gene used was the nuclease gene of Serratia marcescens without the leader-coding sequence and was placed downstream of T7 promoter. The GEB has wide prospects of application on cleanup of pesticide residues with its degradability to several pesticides and containment system.

Effectiveness of Plant Beneficial Microbes: Overview of the Methodological Approaches for the Assessment of Root Colonization and Persistence

Issues concerning the use of harmful chemical fertilizers and pesticides that have large negative impacts on environmental and human health have generated increasing interest in the use of beneficial microorganisms for the development of sustainable agri-food systems. A successful microbial inoculant has to colonize the root system, establish a positive interaction and persist in the environment in competition with native microorganisms living in the soil through rhizocompetence traits. Currently, several approaches based on culture-dependent, microscopic and molecular methods have been developed to follow bioinoculants in the soil and plant surface over time. Although culture-dependent methods are commonly used to estimate the persistence of bioinoculants, it is difficult to differentiate inoculated organisms from native populations based on morphological characteristics. Therefore, these methods should be used complementary to culture-independent approaches. Microscopy-based techniques (bright-field, electron and fluorescence microscopy) allow to obtain a picture of microbial colonization outside and inside plant tissues also at high resolution, but it is not possible to always distinguish living cells from dead cells by direct observation as well as distinguish bioinoculants from indigenous microbial populations living in soils. In addition, the development of metagenomic techniques, including the use of DNA probes, PCR-based methods, next-generation sequencing, whole-genome sequencing and pangenome methods, provides a complementary approach useful to understand plant-soil-microbe interactions. However, to ensure good results in microbiological analysis, the first fundamental prerequisite is correct soil sampling and sample preparation for the different methodological approaches that will be assayed. Here, we provide an overview of the advantages and limitations of the currently used methods and new methodological approaches that could be developed to assess the presence, plant colonization and soil persistence of bioinoculants in the rhizosphere. We further discuss the possibility of integrating multidisciplinary approaches to examine the variations in microbial communities after inoculation and to track the inoculated microbial strains.

The Effects of Subcellular Nanograting Geometry on the Formation and Growth of Bacterial Biofilms

Biofilm formation by bacteria protects them against environmental stresses such as desiccation, shear forces and antimicrobial agents, making them much harder to remove and increasing their virulence and persistence in industrial water systems and biomedical equipment. One promising method of disrupting biofilm formation and growth is to employ passive surface structures to inhibit bacterial adhesion and aggregation. However, most studies thus far have mainly focused on the early stages of biofilm formation and it is unclear if the influence of surface topography in the early phase will propagate to later stages. Here, we attempt to address this with an investigation into the biofilm formation of Pseudomonas aeruginosa on 25 different nanograting geometries, with dimensions that were systematically varied from subcellular to cellular sizes. The biofilms were characterized from the exponential growth phase to the decline phase, in intervals of 24 H over 4 days, using confocal scanning laser microscopy. Comparing the maximum volume of biofilm formed on each surface over 96 H, it was found that approximately 1/3 of the nanograting geometries exhibited 72 ± 16 % lower biovolume density than a flat surface. Bacteria on these nanogratings were also observed to form 40 ± 11 % smaller microcolonies that were 17 ± 6 % less compact than that found on the control surface. The majority of these nanogratings had deep trenches (i.e. depth ≥ 70% of the cell diameter). Furthermore, P. aeruginosa cells were observed to multiply at approximately twice the rate on almost all the nanogratings compared to flat surfaces, but these cell populations also began to decline 24 H earlier than those on a flat surface. Using available literature on P. aeruginosa, a qualitative model was put forth, attributing the results to increased cell motility, decreased exopolysaccharide formation and disrupted psl adhesin/signal trails on nanogratings. These factors, together, led to the net effects of reduced attachment, increased scattering of cells and rapid decline of the biofilms on nanogratings. The insights derived from this study suggest that passive surface geometries can be designed and optimized to successfully control/inhibit biofilm formation and growth.

Colonization of the Intestinal Tract of the Polyphagous Pest Spodoptera littoralis with the GFP-Tagged Indigenous Gut Bacterium Enterococcus mundtii

The alkaline gut of Lepidopterans plays a crucial role in shaping communities of bacteria. Enterococcus mundtii has emerged as one of the predominant gut microorganisms in the gastrointestinal tract of the major agricultural pest, Spodoptera littoralis. Therefore, it was selected as a model bacterium to study its adaptation to harsh alkaline gut conditions in its host insect throughout different stages of development (larvae, pupae, adults, and eggs). To date, the mechanism of bacterial survival in insects' intestinal tract has been unknown. Therefore, we have engineered a GFP-tagged species of bacteria, E. mundtii, to track how it colonizes the intestine of S. littoralis. Three promoters of different strengths were used to control the expression of GFP in E. mundtii. The promoter ermB was the most effective, exhibiting the highest GFP fluorescence intensity, and hence was chosen as our main construct. Our data show that the engineered fluorescent bacteria survived and proliferated in the intestinal tract of the insect at all life stages for up to the second generation following ingestion.

Wide field-of-view fluorescence image deconvolution with aberration-estimation from Fourier ptychography

This paper presents a method to simultaneously acquire an aberration-corrected, wide field-of-view fluorescence image and a high-resolution coherent bright-field image using a computational microscopy method. First, the procedure applies Fourier ptychographic microscopy (FPM) to retrieve the amplitude and phase of a sample, at a resolution that significantly exceeds the cutoff spatial frequency of the microscope objective lens. At the same time, redundancy within the set of acquired FPM bright-field images offers a means to estimate microscope aberrations. Second, the procedure acquires an aberrated fluorescence image, and computationally improves its resolution through deconvolution with the estimated aberration map. An experimental demonstration successfully improves the bright-field resolution of fixed, stained and fluorescently tagged HeLa cells by a factor of 4.9, and reduces the error caused by aberrations in a fluorescence image by up to 31%, over a field of view of 6.2 mm by 9.3 mm. For optimal deconvolution, we show the fluorescence image needs to have a signal-to-noise ratio of at least ~18.

Construction of genetically engineered bacteria that degrades organophosphorus pesticide residues and can be easily detected by the fluorescence

Organophosphorus compounds (OPs) are widely used in agriculture and industry and there is increased concern about their toxicological effects in the environment. Bioremediation can offer an efficient and cost-effective option for the removal of OPs. Herein, we describe the construction of a genetically engineered microorganism (GEM) that can degrade OPs and be directly detected and monitored in the environment using an enhanced green fluorescent protein (EGFP) fusion strategy. The coding regions of EGFP, a reporter protein that can fluoresce by itself, and organophosphorus hydrolase (OPH), which has a broad substrate specificity and is able to hydrolyse a number of organophosphorus pesticides, were cloned into the expression vector pET-28b. The fusion protein of EGFP-OPH was expressed in E. coli BL21 (DE3) and the protein expression reached the highest level at 11 h after isopropyl beta-D-thiogalactopyranoside induction. The fluorescence of the GEM was detected by fluorescence spectrophotometry and microscopy, and its ability to degrade OPs was determined by OPH activity assay. Those GEM that express the fusion protein (EGFP and OPH) exhibited strong fluorescence intensity and also potent hydrolase activity, which could be used to degrade organophosphorus pesticide residues in the environment and can also be directly monitored by fluorescence.

Green fluorescent protein labeling of Listeria, Salmonella, and Escherichia coli O157:H7 for safety-related studies

Many food safety-related studies require tracking of introduced foodborne pathogens to monitor their fate in complex environments. The green fluorescent protein (GFP) gene (gfp) provides an easily detectable phenotype so has been used to label many microorganisms for ecological studies. The objectives of this study were to label major foodborne pathogens and related bacteria, including Listeria monocytogenes, Listeria innocua, Salmonella, and Escherichia coli O157:H7 strains, with GFP and characterize the labeled strains for stability of the GFP plasmid and the plasmid's effect on bacterial growth. GFP plasmids were introduced into these strains by a CaCl₂ procedure, conjugation or electroporation. Stability of the label was determined through sequential propagation of labeled strains in the absence of selective pressure, and rates of plasmid-loss were calculated. Stability of the GFP plasmid varied among the labeled species and strains, with the most stable GFP label observed in E. coli O157:H7. When grown in nonselective media for two consecutive subcultures (ca. 20 generations), the rates of plasmid loss among labeled E. coli O157:H7, Salmonella and Listeria strains ranged from 0%–30%, 15.8%–99.9% and 8.1%–93.4%, respectively. Complete loss (>99.99%) of the plasmid occurred in some labeled strains after five consecutive subcultures in the absence of selective pressure, whereas it remained stable in others. The GFP plasmid had an insignificant effect on growth of most labeled strains. E. coli O157:H7, Salmonella and Listeria strains can be effectively labeled with the GFP plasmid which can be stable in some isolates for many generations without adversely affecting growth rates.

Detection of transgene in early developmental stage by GFP monitoring enhances the efficiency of genetic transformation of pepper

In order to establish a reliable and highly efficient method for genetic transformation of pepper, a monitoring system featuring GFP (green fluorescent protein) as a report marker was applied to Agrobacterium-mediated transformation. A callus-induced transformation (CIT) system was used to transform the GFP gene. GFP expression was observed in all tissues of T(0), T(1) and T(2) peppers, constituting the first instance in which the whole pepper plant has exhibited GFP fluorescence. A total of 38 T(0) peppers were obtained from 4,200 explants. The transformation rate ranged from 0.47 to 1.83% depending on the genotype, which was higher than that obtained by CIT without the GFP monitoring system. This technique could enhance selection power by monitoring GFP expression at the early stage of callus in vitro. The detection of GFP expression in the callus led to successful identification of the shoot that contained the transgene. Thus, this technique saved lots of time and money for conducting the genetic transformation process of pepper. In addition, a co-transformation technique was applied to the target transgene, CaCS (encoding capsaicinoid synthetase of Capsicum) along with GFP. Paprika varieties were transformed by the CaCS::GFP construct, and GFP expression in callus tissues of paprika was monitored to select the right transformant.

Laser-assisted cryosurgery in ex vivo mice hepatic tissue: viability assays using green fluorescent protein

An experimental investigation is carried out to develop a novel approach to cryosurgery, where laser heating counteracts tissue freezing to better confine damage to the targeted cancerous tissue within a lethal low-temperature isothermal boundary-an approach we refer to as laser-assisted cryosurgery (LAC). The advantage of this procedure relative to conventional cryosurgery assisted with urethral warmers or cryoheaters is that laser heating provides volumetric rather than superficial heating, which leads to deeper penetration, more homogeneous tissue protection and better demarcation of the destructive freezing effect to a well-defined targeted volume. Tissue viability assays are performed using green fluorescence protein (GFP) as a viability marker and correlated with temperature history after performing LAC procedures on ex vivo mice hepatic tissue. The limit for cell denaturation at the irradiated surface predicted by GFP analysis is further confirmed using reverse transcription polymerase chain reaction (RT-PCR). In addition, the correlation between GFP fluorescence and cell viability and loss of GFP fluorescence in non-viable cells has been tested and validated by histological analysis using a standard cell viability measuring method (hematoxylin and eosin staining). Analysis of our experimental measurements show that reproducible thermal gradients (of 236 °C/cm) and predictable tissue necrosis can be reliably produced by LAC without exceeding temperature thresholds for cell denaturation (of T (surf) ≈ 48 °C) beyond preset tissue boundaries (with resolution of 0.1 °C/mm). The results have shown the feasibility of controlling temperatures at specified tissue locations to prevent hyperthermal or freezing damage.

Whole-cell fluorescent biosensors for bioavailability and biodegradation of polychlorinated biphenyls

Whole-cell microbial biosensors are one of the newest molecular tools used in environmental monitoring. Such biosensors are constructed through fusing a reporter gene such as lux, gfp or lacZ, to a responsive promoter. There have been many reports of the applications of biosensors, particularly their use in assaying pollutant toxicity and bioavailability. This paper reviews the basic concepts behind the construction of whole-cell microbial biosensors for pollutant monitoring, and describes the applications of two such biosensors for detecting the bioavailability and biodegradation of Polychlorinated Biphenyls (PCBs).

A fluorescent, genetically engineered microorganism that degrades organophosphates and commits suicide when required

One way to reduce the potential risk of genetically engineered microorganisms (GEMs) to the environment is to use a containment system that does not interfere with the performance of the GEM until activated. Such a system can be created by inserting a suicide cassette consisting of a toxin-encoding gene controlled by an inducible promoter. We constructed a GEM that can degrade organophosphorus compounds, emit green fluorescence, and commit suicide when required by putting the genes that control these different functions under different promoters. The genes for enhanced green fluorescent protein (EGFP) and organophosphorus hydrolase (OPH) were cloned downstream of the lambda PL promoter in the plasmid pBV220. These genes could be expressed freely as long as the GEM was metabolizing because the repressor sequence cIts857 had been deleted. The extracellular nuclease gene of Serratia marcescens, without its leader-coding sequence, provided the suicide mechanism. This was put under the control of the T7 promoter to form a suicide cassette activated by the presence of an environmental signal, in this case, arabinose. To improve the reliability of this containment system, the suicide cassette was duplicated within the conditional suicide plasmid. The plasmid carrying the EGFP and OPH fusion genes and that containing the suicide cassette were compatible and coexisted in the same host.

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.

Transformation of Escherichia coli K-12 with a high-copy plasmid encoding the green fluorescent protein reduces growth: implications for predictive microbiology

The green fluorescent protein (GFP) of the jellyfish Aequorea victoria has been widely used as a biomarker and has potential for use in developing predictive models for growth of pathogens on naturally contaminated food. However, constitutive production of GFP can reduce growth of transformed strains. Consequently, a high-copy plasmid with gfp under the control of a tetracycline-inducible promoter (pTGP) was constructed. The plasmid was first introduced into a tetracycline-resistant strain of Escherichia coli K-12 to propagate it for subsequent transformation of tetracycline-resistant strains of Salmonella. In contrast to transformed E. coli K-12, which only fluoresced in response to tetracycline, transformed Salmonella fluoresced maximally without tetracycline induction of gfp. Although pTGP did not function as intended in Salmonella, growth of parent and GFP E. coli K-12 was compared to test the hypothesis that induction of GFP production reduced growth. Although GFP production was not induced during growth on sterile chicken in the absence of tetracycline, maximum specific growth rate (mumax) of GFP E. coli K-12 was reduced 40 to 50% (P < 0.05) at 10, 25, and 40 degrees C compared with the parent strain. When growth of parent and GFP strains of E. coli K-12 was compared in sterile broth at 40 degrees C, mumax and maximum population density of the GFP strain were reduced (P < 0.05) to the same extent (50 to 60%) in the absence and presence of tetracycline. These results indicated that transformation reduced growth of E. coli K-12 independent of gfp induction. Thus, use of a low-copy plasmid or insertion of gfp into the chromosome may be required to construct valid strains for development of predictive models for growth of pathogens on naturally contaminated food.

Formation of nonculturable Escherichia coli in drinking water

AIMS

To examine whether incubation of Escherichia coli in nondisinfected drinking water result in development of cells that are not detectable using standard procedures but maintain a potential for metabolic activity and cell division.

METHODS AND RESULTS

Survival and detectability of four different E. coli strains were studied using drinking water microcosms and samples from contaminated drinking water wells. Recovery of E. coli was compared using different cultivation-dependent methods, fluorescence in situ hybridization (FISH) using specific oligonucleotide probes, direct viable counts (DVC), and by enumeration of gfp-tagged E. coli (green fluorescent protein, GFP). Two levels of stress responses were observed after incubation of E. coli in nondisinfected drinking water: (i) the presence of cells that were not detected using standard cultivation methods but could be cultivated after gentle resuscitation on nonselective nutrient-rich media, and (ii) the presence of cells that responded to nutrient addition but could only be detected by cultivation-independent methods (DVC, FISH and GFP). Collectively, the experiments demonstrated that incubation for 20-60 days in nondisinfected drinking water resulted in detection of only 0.7-5% of the initial E. coli population using standard cultivation methods, whereas 1-20% could be resuscitated to a culturable state, and 17-49% could be clearly detected using cultivation-independent methods.

CONCLUSIONS

Resuscitation of stressed E. coli on nonselective nutrient-rich media increased cell counts in drinking water using both traditional (CFU), and cultivation-independent methods (DVC, FISH and GFP). The cultivation-independent methods resulted in detection of 10-20 times more E. coli than the traditional methods. The results indicate that a subpopulation of substrate-responsive but apparent nonculturable E. coli may develop in drinking water during long-term starvation survival.

SIGNIFICANCE AND IMPACT OF THE STUDY

The existence of substrate-responsive but nonculturable cells should be considered when evaluating the survival potential of E. coli in nondisinfected drinking water.

Genetically engineered termite gut bacteria (Enterobacter cloacae) deliver and spread foreign genes in termite colonies

Indigenous gut bacteria of the Formosan subterranean termite (Coptotermes formosanus Shiraki, Isoptera: Rhinotermitidae) were used as shuttle systems to deliver, express and spread foreign genes in termite colonies. The gut bacterium Enterobacter cloacae was transformed with a recombinant plasmid (pEGFP) containing genes encoding ampicillin resistance and green fluorescent protein (GFP). In laboratory experiments, termite workers and soldiers from three colonies were fed with filter paper inoculated with transformed bacteria. Transformed bacteria were detected in termite guts by growing the entire gut flora under selective conditions and checking the cultures visually for fluorescence. We demonstrated that (1) transformed bacteria were ingested within a few hours and the GFP gene was expressed in the termite gut; (2) transformed bacteria established a persistent population in the termite gut for up to 11 weeks; (3) transformed bacteria were efficiently transferred throughout a laboratory colony, even when the donor (termites initially fed with transformed bacteria) to recipient (not fed) ratio was low; (4) transformed E. cloacae were transferred into soil; however, they did not accumulate over time and the GFP plasmid was not transferred to other soil bacteria. In the future, transgenic bacteria may be used to shuttle detrimental genes into termite colonies for improved pest control.

Use of Genetically Engineered Escherichia coli to Monitor Ingestion, Loss, and Transfer of Bacteria in Termites

Escherichia coli was transformed with a recombinant plasmid (pEGFP) containing the genes for ampicillin resistance and Green Fluorescent Protein (GFP). Escherichia coli expressing GFP (E. coli/GFP+) was then fed to workers of the termite Coptotermes formosanus Shiraki (Isoptera: Rhinotermitidae). The transformed bacteria in the termite guts were detected by growing the gut flora under selective conditions and then checking the cultures for fluorescence. Recombinant plasmids in the termite gut were detected by plasmid extraction with subsequent restriction enzyme digest. The presence of the GFP gene in the gut of termites fed with E. coli/GFP+ was verified by PCR amplification. Transformed E. coli were ingested rapidly when workers fed on filter paper inoculated with E. coli/GFP+. After 1 day, 42% of termite guts harbored E. coli/GFP+. Transfer of E. coli/GFP+ from donor termites (fed with E. coli/GFP+) to recipients (fed with moist filter paper) occurred within 1 day. However, without continuous inoculation, termites lost the transformed bacteria within 1 week.

Effects of temperature on detection of plasmid or chromosomally encoded gfp- and lux-labeled Pseudomonas fluorescens in soil

Pseudomonas fluorescens is a normal inhabitant of the soil rhizosphere. The use of genetically altered strains of P. fluorescens in bioremediation has led to the need for effective monitoring of such cells released into the environment. In this study, we present data on the persistence in soil of P. fluorescens harboring gfp (green fluorescent protein) or lux (bioluminescence) genes. Comparisons were made between strains marked chromosomally and strains carrying these markers on a plasmid. Overall effects of plasmid carriage on culturability were also examined. Sterile soil microcosms were inoculated with washed cells to a final concentration of ca. 10(6) CFU.g(-1) and placed at 5, 23, and 35-37 degrees C. Samples were taken periodically and examined for culturability and viability, using the substrate responsiveness assay. Our results indicated no significant loss of culturability at 5 and 23 degrees C for a period of over one year. In contrast, cells of P. fluorescens incubated at 35-37 degrees C entered the viable but nonculturable state within 7 days. All cells labeled with gfp retained fluorescence regardless of culturability, suggesting that the green fluorescent protein can be of value in monitoring the presence of cells following their release to the environment. Because fluorescence was maintained regardless of the cells' physiological state, this protein may also be an indicator of cell viability.

Development of a novel recombinant adenovirus containing gfp–zeocin fusion expression cassette for conditional replication in p53-deficient human tumor cells

Two obstacles limiting the efficacy of nearly all cancer gene therapy trails are low gene transduction efficiency and the lack of tumor specificity. Fortunately, a replication-competent, E1B-deficient adenovirus (dl1520) was developed that could overcome these limitations, because it was capable of efficiently and selectively destroying tumor cells lacking functional p53. In an attempt to appraise the efficiency and safety of this approach, a novel recombinant adenovirus, r3/Ad, containing a gfp-zeocin expression cassette was constructed in this work. The study in vitro demonstrated that r3/Ad has the ability to replicate in and lyse only the p53-deficient human tumor cells such as the human glioblastoma cells (U251) and human bladder cells (EJ) but not in the human fibroblast cells (MRC-5) with functional p53. Importantly, this gfp-zeocin fusion gene driven by the bipromoter (CMV and EM-7) could be used as an effective selective marker and reporter in prokaryotic and eukaryotic cells; and also zeocin as a selective marker could minimize contamination of the recombinant virus by the wt-Ad5. Additionally, it was found that the r3/Ad could be useful for studying the selective replication of E1B-deficient adenovirus in vivo, it could be used as a "guide" to study the ability of the recombinant adenovirus to spread and to infect distant tumor cells in any tumor bearing animal model by GFP as a reporter. This may help determine the safety of using any E1B-deficient adenovirus in cancer gene therapy.

Fitness cost of the green fluorescent protein in gastrointestinal bacteria

There are surprisingly few studies that have successfully used the green fluorescent protein (GFP) as a quantitative reporter in selection experiments screening for inducible bacterial promoters. One explanation is that GFP expression may confer a fitness cost for bacteria. To test this possibility, we monitored the doubling time in enteric bacteria expressing GFP. Four bacterial species, Escherichia coli, enterohaemorrhagic E. coli, Shigella flexneri, Salmonella typhi, and Vibrio cholerae, were examined. The level of GFP expression was varied by using a salt-inducible promoter. After accounting for the increase in doubling time resulting from elevated osmolarity, the doubling time of all bacteria was found to increase proportionally with GFP expression, and some strains were more affected than others. Cultures of the bacteria most affected by GFP exhibited a proportion of elongated cells, which suggests that GFP production could interfere with cell division in these strains. The results in this study show that GFP is costly to bacteria and suggest that overly active promoters should be difficult to obtain from a genomic promoter library. They also suggest that the chances of succeeding in using GFP as a reporter in selection experiments are increased by growing the bacteria for the fewest number of generations and by subduing the expression of GFP whenever possible, such as by using a low copy vector to clone the library.

A polyphasic approach for studying the interaction between Ralstonia solanacearum and potential control agents in the tomato phytosphere

Ralstonia solanacearum biovar 2, the causative agent of brown rot in potato, has been responsible for large crop losses in Northwest Europe during the last decade. Knowledge on the ecological behaviour of R. solanacearum and its antagonists is required to develop sound procedures for its control and eradication in infested fields.A polyphasic approach was used to study the invasion of plants by a selected R. solanacearum biovar 2 strain, denoted 1609, either or not in combination with the antagonistic strains Pseudomonas corrugata IDV1 and P. fluorescens UA5-40. Thus, this study combined plating (spread and drop plate methods), reporter gene technology (gfp mutants) and serological (imunofluorescence colony staining [IFC]) and molecular techniques (fluorescent in situ hybridization [FISH], PCR with R. solanacearum specific primers and PCR-DGGE on plant DNA extracts). The behaviour of R. solanacearum 1609 and the two control strains was studied in bulk and (tomato) rhizosphere soil and the rhizoplane and stems of tomato plants. The results showed that an interaction between the pathogen and the control strains at the root surface was likely. In particular, R. solanacearum 1609 CFU numbers were significantly reduced on tomato roots treated with P. corrugata IDV1(chr:gfp1) cells as compared to those on untreated roots. Concomitant with the presence of P. corrugata IDV1(chr:gfp1), plant invasion by the pathogen was hampered, but not abolished.PCR-DGGE analyses of the tomato rhizoplane supported the evidence for antagonistic activity against the pathogen; as only weak R. solanacearum 1609 specific bands were detected in profiles derived from mixed systems versus strong bands in profiles from systems containing only the pathogen. Using FISH, a difference in root colonization was demonstrated between the pathogen and one of the two antagonists, i.e. P. corrugata IDV1(chr:gfp1); R. solanacearum strain 1609 was clearly detected in the vascular cylinder of tomato plants, whereas strain IDV1 was absent.R. solanacearum 1609 cells were also detected in stems of plants that had developed in soils treated with this strain, even in cases in which disease symptoms were absent, indicating the occurrence of symptomless infection. In contrast, strain 1609 cells were not found in stems of several plants treated with either one of the two antagonists. The polyphasic analysis is valuable for testing antagonistic strains for approval as biocontrol agents in agricultural practice.

The Use of Modified GFP as a Reporter for Metabolic Activity in Pseudomonas putida

Many bacteria are now known to enter into a "viable but nonculturable" (VBNC) state in response to various environmental stresses. In this state, the cells are no longer culturable on routine media, but retain viability and in many cases have been shown to be capable of resuscitating to the metabolically active and culturable state. There have been no simple means of measuring the metabolic activity of cells in the VBNC state. The use of green fluorescent protein (GFP) variants with short half-lives was examined in cells intended for environmental release to examine the potential of GFP as a reporter of metabolic activity. Unlike strains with the native (stable) GFP, Pseudomonas putida strains tagged with unstable GFP rapidly lost GFP fluorescence following exposure to starvation and VBNC-inducing conditions. Our results suggest that tagging cells with the modified GFP provides a method for determining metabolic activity in these cells.

Effect of starvation and the viable-but-nonculturable state on green fluorescent protein (GFP) fluorescence in GFP-tagged Pseudomonas fluorescens A506

The green fluorescent protein (GFP) gene, gfp, of the jellyfish Aequorea victoria is being used as a reporter system for gene expression and as a marker for tracking prokaryotes and eukaryotes. Cells that have been genetically altered with the gfp gene produce a protein that fluoresces when it is excited by UV light. This unique phenotype allows gfp-tagged cells to be specifically monitored by nondestructive means. In this study we determined whether a gfp-tagged strain of Pseudomonas fluorescens continued to fluoresce under conditions under which the cells were starved, viable but nonculturable (VBNC), or dead. Epifluorescent microscopy, flow cytometry, and spectrofluorometry were used to measure fluorescence intensity in starved, VBNC, and dead or dying cells. Results obtained by using flow cytometry indicated that microcosms containing VBNC cells, which were obtained by incubation under stress conditions (starvation at 37.5 degrees C), fluoresced at an intensity that was at least 80% of the intensity of nonstressed cultures. Similarly, microcosms containing starved cells incubated at 5 and 30 degrees C had fluorescence intensities that were 90 to 110% of the intensity of nonstressed cells. VBNC cells remained fluorescent during the entire 6-month incubation period. In addition, cells starved at 5 or 30 degrees C remained fluorescent for at least 11 months. Treatment of the cells with UV light or incubation at 39 or 50 degrees C resulted in a loss of GFP from the cells. There was a strong correlation between cell death and leakage of GFP from the cells, although the extent of leakage varied depending on the treatment. Most dead cells were not GFP fluorescent, but a small proportion of the dead cells retained some GFP at a lower concentration than the concentration in live cells. Our results suggest that gfp-tagged cells remain fluorescent following starvation and entry into the VBNC state but that fluorescence is lost when the cells die, presumably because membrane integrity is lost.

Green fluorescent protein functions as a reporter for protein localization in Escherichia coli

The use of green fluorescent protein (GFP) as a reporter for protein localization in Escherichia coli was explored by creating gene fusions between malE (encoding maltose-binding protein [MBP]) and a variant of gfp optimized for fluorescence in bacteria (GFPuv). These constructs encode hybrid proteins composed of GFP fused to the carboxy-terminal end of MBP. Fluorescence was not detected when the hybrid protein was synthesized with the MBP signal sequence. In contrast, when the MBP signal sequence was deleted, fluorescence was observed. Cell fractionation studies showed that the fluorescent MBP-GFP hybrid protein was localized in the cytoplasm, whereas the nonfluorescent version was localized to the periplasmic space. Smaller MBP-GFP hybrid proteins, however, exhibited abnormal fractionation. Expression of the gene fusions in different sec mutants, as well as signal sequence processing assays, confirmed that the periplasmically localized hybrid proteins were exported by the sec-dependent pathway. The distinction between fluorescent and nonfluorescent colonies was exploited as a scorable phenotype to isolate malE signal sequence mutations. While expression of hybrid proteins comprised of full-length MBP did not result in overproduction lethality characteristic of some exported beta-galactosidase hybrid proteins, synthesis of shorter, exported hybrid proteins was toxic to the cells. Purification of MBP-GFP hybrid protein from the different cellular compartments indicated that GFP is improperly folded when localized outside of the cytoplasm. These results suggest that GFP could serve as a useful reporter for genetic analysis of bacterial protein export and of protein folding.

A microscopic method to visualize Escherichia coli interaction with beef muscle

The genetic determinant for enhanced green fluorescent protein (EGFP) was introduced into Escherichia coli JM109 (ATCC 53323) and E. coli O157:H7 (ATCC 43895) on plasmid EGFP. The expression of EGFP did not change the growth kinetics or surface properties tested (hydrophobicity and electrophoretic mobility). Microscope slides were modified to allow for optimal viewing of thick meat samples with an inverted microscope. Two fluorescent dyes, nile red and Cy3 were used to stain for lipid and protein portions of beef muscle, respectively. Laser scanning confocal microscopy was used to observe interaction of the EGFP-expressing E. coli strains and the fluorescently stained muscle components without changing the spatial and temporal environment of the organisms.

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.

Survival, physiology, and lysis of Lactococcus lactis in the digestive tract

The survival and the physiology of lactococcal cells in the different compartments of the digestive tracts of rats were studied in order to know better the fate of ingested lactic acid bacteria after oral administration. For this purpose, we used strains marked with reporter genes, the luxA-luxB gene of Vibrio harveyi and the gfp gene of Aequora victoria, that allowed us to differentiate the inoculated bacteria from food and the other intestinal bacteria. Luciferase was chosen to measure the metabolic activity of Lactococcus lactis in the digestive tract because it requires NADH, which is available only in metabolically active cells. The green fluorescent protein was used to assess the bacterial lysis independently of death. We report not only that specific factors affect the cell viability and integrity in some digestive tract compartments but also that the way bacteria are administrated has a dramatic impact. Lactococci which transit with the diet are quite resistant to gastric acidity (90 to 98% survival). In contrast, only 10 to 30% of bacteria survive in the duodenum. Viable cells are metabolically active in each compartment of the digestive tract, whereas most dead cells appear to be subject to rapid lysis. This property suggests that lactococci could be used as a vector to deliver specifically into the duodenum the proteins produced in the cytoplasm. This type of delivery vector would be particularly appropriate for targeting digestive enzymes such as lipase to treat pancreatic deficiencies.

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.

A novel system for efficient gene expression and monitoring of bacteria in aquatic environments

In a previous study, we reported the identification of Escherichia coli genes with increased expression in an aquatic environment. Here, we describe the use of one of these genes, gapC, as an expression system in freshwater habitats. We have identified the transcriptional start site of gapC and analysed the synthesis of the GapC protein during incubation in aquatic medium. The promoter of gapC was used to construct fusions to the reporter genes lacZ and gfp. Analysis of these fusions indicates the potential of gapC as a valuable tool for the detection of E. coli in freshwater habitats, as well as for expressing other genes in aquatic environments.

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.

Green fluorescent protein as a novel species-specific marker in enteric dual-species biofilms

Green fluorescent protein (GFP) was used as a tool to examine the interactions between pairs of bacterial species and their effects on subsequent biofilm development over 24 h. A plasmid encoding GFP from Aequorea victoria was transformed into strains of Enterobacter agglomerans and Escherichia coli ATCC 11229. The development of dual-species biofilms, containing one fluorescent and one non-fluorescent partner, was examined using viable counts. UV illumination of plates enabled both species to be identified in a mixture. The spatial distribution of each species was examined by UV microscopy, simultaneously staining the non-fluorescent strain with propidium iodide. GFP fluorescence was measured to quantify the adhesion of the strains to other cells or cell constituents or the invasion into pre-existing biofilms. Co-operation between Ent. agglomerans/GFP and Klebsiella pneumoniae G1 resulted in a 54 and a 23% increase in biofilm formation, respectively, compared with single-species biofilms. E. coli/GFP and Serratia marcescens 87b stably co-existed in biofilms but did not affect the growth of each other. The other bacterial partnerships examined were competitive, with the end result that one species dominated the biofilm. The methods described provide a convenient technique for the examination of mixed-species biofilm communities where the unique interactions between species determine the true properties of the resultant biofilms.

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.

Use of green fluorescent protein to assess urease gene expression by uropathogenic Proteus mirabilis during experimental ascending urinary tract infection

Proteus mirabilis, a cause of complicated urinary tract infection, expresses urease when exposed to urea. While it is recognized that the positive transcriptional activator UreR induces gene expression, the levels of expression of the enzyme during experimental infection are not known. To investigate in vivo expression of P. mirabilis urease, the gene encoding green fluorescent protein (GFP) was used to construct reporter fusions. Translational fusions of urease accessory gene ureD, which is preceded by a urea-inducible promoter, were made with gfp (modified to express S65T/V68L/S72A [B. P. Cormack et al. Gene 173:33-38, 1996]). Constructs were confirmed by sequencing of the fusion junctions. UreD-GFP fusion protein was induced by urea in both Escherichia coli DH5alpha and P. mirabilis HI4320. By using Western blotting with antiserum raised against GFP, expression level was shown to correlate with urea concentration (tested from 0 to 500 mM), with highest induction at 200 to 500 mM urea. Fluorescent E. coli and P. mirabilis bacteria were observed by fluorescence microscopy following urea induction, and the fluorescence intensity of GFP in cell lysates was measured by spectrophotofluorimetry. P. mirabilis HI4320 carrying the UreD-GFP fusion plasmid was transurethrally inoculated into the bladders of CBA mice. One week postchallenge, fluorescent bacteria were detected in thin sections of both bladder and kidney samples; the fluorescence intensity of bacteria in bladder tissue was higher than that in the kidney. Kidneys were primarily infected with single-cell-form fluorescent bacteria, while aggregated bacterial clusters were observed in the bladder. Elongated swarmer cells were only rarely observed. These observations demonstrate that urease is expressed in vivo and that using GFP as a reporter protein is a viable approach to investigate in vivo expression of P. mirabilis virulence genes in experimental urinary tract infection.