Flow cytometric detection of specific gene expression in prokaryotic cells using in situ RT-PCR

Viable but non-culturable (VBNC) bacteria: Gene expression in planktonic and biofilm cells

Viable but non-culturable (VBNC) bacteria are common in nutrient poor and/or stressed environments as planktonic cells and biofilms. This article discusses approaches to researching VBNC bacteria to obtain knowledge that is lacking on their gene expression while in the VBNC state, and when they enter into and then recover from this state, when provided with the necessary nutrients and environmental conditions to support growth and cell division. Two-dimensional gel electrophoresis of proteins, global gene expression, reverse-transcription polymerase chain reaction (PCR) analysis and sequencing by synthesis coupled with data on cell numbers, viability and species present are central to understanding the VBNC state.

Scanning electron microscope imaging of bacteria based on DNA sequence

AIMS

To develop a scanning electron microscopic approach using in situ hybridization (SEM-ISH) for gaining both genetic and morphological information about target bacteria.

METHODS AND RESULTS

Target cells were hybridized with DNA-targeted polynucleotide probes, and a tyramide signal amplification system was used to increase the sensitivity. The protocol of SEM-ISH enabled to detect low copy number target DNA sequences in individual cells.

CONCLUSIONS

SEM-ISH allowed the in situ detection of bacteria carrying a specific gene.

SIGNIFICANCE AND IMPACT OF THE STUDY

Combining morphological study with SEM and ISH techniques appears to be a valuable tool to understand the spatial distribution of target cells in complex microbial communities on various materials.

Recognition of individual genes in diverse microorganisms by cycling primed in situ amplification

Cycling primed in situ amplification-fluorescent in situ hybridization (CPRINS-FISH) was developed to recognize individual genes in a single bacterial cell. In CPRINS, the amplicon was long single-stranded DNA and thus retained within the permeabilized microbial cells. FISH with a multiply labeled fluorescent probe set enabled significant reduction in nonspecific background while maintaining high fluorescence signals of target bacteria. The ampicillin resistance gene in Escherichia coli, chloramphenicol acetyltransferase gene in different gram-negative strains, and RNA polymerase sigma factor (rpoD) gene in Aeromonas spp. could be detected under identical permeabilization conditions. After concentration of environmental freshwater samples onto polycarbonate filters and subsequent coating of filters in gelatin, no decrease in bacterial cell numbers was observed with extensive permeabilization. The detection rates of bacterioplankton in river and pond water samples by CPRINS-FISH with a universal 16S rRNA gene primer and probe set ranged from 65 to 76% of total cell counts (mean, 71%). The concentrations of cells detected by CPRINS-FISH targeting of the rpoD genes of Aeromonas sobria and A. hydrophila in the water samples varied between 2.1 x 10(3) and 9.0 x 10(3) cells ml(-1) and between undetectable and 5.1 x 10(2) cells ml(-1), respectively. These results demonstrate that CPRINS-FISH provides a high sensitivity for microscopic detection of bacteria carrying a specific gene in natural aquatic samples.

Enhancing the efficiency of a PCR using gold nanoparticles

We found that the PCR could be dramatically enhanced by Au nanoparticles. With the addition of 0.7 nM of 13 nm Au nanoparticles into the PCR reagent, the PCR efficiency was increased. Especially when maintaining the same or higher amplification yields, the reaction time could be shortened, and the heating/cooling rates could be increased. The excellent heat transfer property of the nanoparticles should be the major factor in improving the PCR efficiency. Different PCR systems, DNA polymerases, DNA sizes and complex samples were compared in this study. Our results demonstrated that Au nanoparticles increase the sensitivity of PCR detection 5- to 10-fold in a slower PCR system (i.e. conventional PCR) and at least 10⁴-fold in a quicker PCR system (i.e. real-time PCR). After the PCR time was shortened by half, the 100 copies/µl DNA were detectable in real-time PCR with gold colloid added, however, at least 10⁶ copies/µl of DNA were needed to reach a detectable signal level using the PCR reagent without gold colloid. This innovation could improve the PCR efficiency using non-expensive polymerases, and general PCR reagent. It is a new viewpoint in PCR, that nanoparticles can be used to enhance PCR efficiency and shorten reaction times.

Microbial communities in oil-contaminated seawater

Although diverse bacteria capable of degrading petroleum hydrocarbons have been isolated and characterized, the vast majority of hydrocarbon-degrading bacteria, including anaerobes, could remain undiscovered, as a large fraction of bacteria inhabiting marine environments are uncultivable. Using culture-independent rRNA approaches, changes in the structure of microbial communities have been analyzed in marine environments contaminated by a real oil spill and in micro- or mesocosms that mimic such environments. Alcanivorax and Cycloclasticus of the gamma-Proteobacteria were identified as two key organisms with major roles in the degradation of petroleum hydrocarbons. Alcanivorax is responsible for alkane biodegradation, whereas Cycloclasticus degrades various aromatic hydrocarbons. This information will be useful to develop in situ bioremediation strategies for the clean-up of marine oil spills.

Single-cell microbiology: tools, technologies, and applications

The field of microbiology has traditionally been concerned with and focused on studies at the population level. Information on how cells respond to their environment, interact with each other, or undergo complex processes such as cellular differentiation or gene expression has been obtained mostly by inference from population-level data. Individual microorganisms, even those in supposedly "clonal" populations, may differ widely from each other in terms of their genetic composition, physiology, biochemistry, or behavior. This genetic and phenotypic heterogeneity has important practical consequences for a number of human interests, including antibiotic or biocide resistance, the productivity and stability of industrial fermentations, the efficacy of food preservatives, and the potential of pathogens to cause disease. New appreciation of the importance of cellular heterogeneity, coupled with recent advances in technology, has driven the development of new tools and techniques for the study of individual microbial cells. Because observations made at the single-cell level are not subject to the "averaging" effects characteristic of bulk-phase, population-level methods, they offer the unique capacity to observe discrete microbiological phenomena unavailable using traditional approaches. As a result, scientists have been able to characterize microorganisms, their activities, and their interactions at unprecedented levels of detail.

Bacterial gene expression detected in human faeces by reverse transcription-PCR

A method to isolate and specifically detect bacterial messenger RNA (mRNA) in human faeces is presented. The surface layer protein gene slpA of Lactobacillus acidophilus ATCC 4356(T) was chosen as a model system because it is transcribed at a high level to a relatively stable mRNA (Boot et al., 1996, J. Bacteriol. 178, 5388-5394). A simulation of the recovery of bacterial cells in the faecal ecosystem was achieved by spiking faecal homogenates with different levels of L. acidophilus cells and total RNA was isolated using a method based on Macaloid clay. The slpA transcript could be detected by reverse transcription-PCR (RT-PCR) when the L. acidophilus cells comprised more than 0.14% (approximately 2 x 10(7) cells g(-1) faeces) of the complex faecal community.

Detection of bacteria carrying the stx2 gene by in situ loop-mediated isothermal amplification

A new in situ DNA amplification technique for microscopic detection of bacteria carrying a specific gene is described. Loop-mediated isothermal amplification (LAMP) was used to detect stxA(2) in Escherichia coli O157:H7 cells. The mild permeabilization conditions and low isothermal temperature used in the in situ LAMP method caused less cell damage than in situ PCR. It allowed use of fluorescent antibody labeling in the bacterial mixture after the DNA amplification for identification of E. coli O157:H7 cells with an stxA(2) gene. Higher-contrast images were obtained with this method than with in situ PCR.

Flow cytometric detection of beta-D-glucuronidase gene in wild-type bacterial cells using in-situ PCR

An in situ PCR-based flow cytometry method useful for monitoring the presence or absence of the beta-D-glucuronidase gene in Escherichia coli has been developed. A single-step fixation and permeabilization procedure, which maintained cell integrity at the elevated temperatures used during thermal cycling in the presence of PCR reagents, was demonstrated. We have chosen a shorter DNA sequence of length 147 bp for the PCR. Cells subjected to in situ PCR using fluorescein-12-dUTP as a label, showed the presence of uid both in epifluorescence microscopic examination and flow cytometric analysis. Multi-parametric analysis of flow cytometric profiles revealed that the efficiency of labeling was found to be high. The potential of in situ PCR for the detection of uid in intact coliform cells was then successfully tested with a fecal coliform isolated from the coastal waters of Singapore.

Monitoring of Ralstonia eutropha KT1 in groundwater in an experimental bioaugmentation field by in situ PCR

Ralstonia eutropha KT1, which degrades trichloroethylene, was injected into the aquifer after activation with toluene, and then the number of bacteria was monitored by in situ PCR targeting the phenol hydroxylase gene and by fluorescent in situ hybridization (FISH) targeting 16S rRNA. Before injection of the bacterial suspension, the total concentration of bacteria in the groundwater was approximately 3 x 10(5) cells/ml and the amount of Ralstonia and bacteria carrying the phenol hydroxylase gene as a percentage of total bacterial cells was less than 0.1%. The concentration of bacteria carrying the phenol hydroxylase gene detected by in situ PCR was approximately 3 x 10(7) cells/ml 1 h after injection, and the concentration of Ralstonia detected by FISH was similar. The number of bacteria detected by in situ PCR was similar to that detected by FISH 4 days after the start of the extraction of groundwater. On and after day 7, however, the number of bacterial cells detected by FISH was less than that detected by in situ PCR.

RNA recovery and detection of mRNA by RT-PCR from preserved prokaryotic samples

The effectiveness of maintaining prokaryotic RNA in Synechococcus and Pseudomonas cells, fixed in 96% ethanol, 4% paraformaldehyde, or suspended in RNAlater, and held in cold storage for 3 months was compared. Fluorometric determination of the RNA extracted from Synechococcus and Pseudomonas cells indicated that the cell storage treatments tested were equally effective at maintaining their total RNA content. There was not any detectable decrease in the quantity of RNA isolated from the preserved samples during storage. Intact mRNA transcripts of the RuBisCO (rbcL) and nir genes were detected by reverse transcriptase-polymerase chain reaction (RT-PCR) from preserved bacterial cells throughout 3 months of storage. In contrast, RT-PCR failed to amplify the mRNA of the rbcL and nitrite reductase genes in unfixed and/or unpreserved bacterial samples, suggesting that bacterial mRNA can be well maintained during a prolonged storage when cells are preserved properly. In addition, RNAlater is a useful reagent for the storage and maintenance of high quality RNA in unfrozen samples.