Molecular detection and quantification of Enterococci

Abstract

A primer pair and probe for the large subunit ribosomal RNA gene of enterococci for use in a real-time nucleic acid sequence based amplification (NASBA) assay.

Description

FIELD OF INVENTION

This invention relates to rapid detection of an organism, specifically, this is a method for detecting and quantifying Enterococci (a human fecal indicator organism) from sea water.

BACKGROUND OF THE INVENTION

The coastal ocean is an important economic and recreational resource that is constantly influenced by human activities. In 2003, there were more than 18,000 days of beach closings throughout the US due to high concentrations of fecal bacteria. This was an increase of more than 51% over the previous year. Health related management of recreational coastal sites is currently undertaken by monitoring fecal coliform and enterococci by membrane filtration. Elevated concentrations of enterococci in marine waters have been shown to have a strong correlation with illness (especially gastrointestinal disease) in exposed individuals therefore making enterococci the indicator organism of choice for saline waters. The problem with this type of standard indicator monitoring is that there is a lag of at least 24-48 hours between when the sample is collected and when the data become available. Changes in the water quality and potential exposures during this delay lead to management decisions and public notifications that are often times inaccurate. To improve our management abilities, a primer pair and probe have been adapted for the large subunit ribosomal RNA gene of enterococci for use in a real-time nucleic acid sequence based amplification (NASBA) assay. This region is highly conserved within all reported species of the Enterococcus genus. Using this assay, the equivalent of less than one enterococci colony forming unit (CFU) was detected from a spiked sea water sample (100 ml). Further, there is a negative linear relationship (R²=0.9484) between CFUs obtained by membrane filtration and time to positivity (TTP) readings obtained with the NASBA assay. Therefore, quantitative estimates of enterococci are possible over at least four orders of magnitude and all positive samples amplified within forty-three minutes. By coupling this real-time NASBA assay to the inventors' existing field RNA extraction procedure and portable NASBA detection device, this technology will provide a simple, rapid (<1 hr), convenient testing format for coastal sites and greatly improve the health risk assessment of these regions.

Unlike polymerase chain reaction (PCR) based methods, NASBA is able to amplify RNA in a DNA background, and thus, only viable targets will be detected. This is important for indicator monitoring because only recent pollution events will be detected and false positive amplifications of naked DNA present in the sample will be eliminated. Indicator bacteria are chosen because of their ability to survive longer than the pathogenic organisms in question and therefore only viable bacteria should be considered as part of a risk analysis.

SUMMARY OF INVENTION

This is a method for the detection and quantification of Enterococci. Enterococci is the USEPA approved indicator organism used to monitor for fecal pollution in saline waters. Current methods rely on membrane filtration and even though the direct enumeration of these microorganisms using membrane filtration and plating has been available for years, there continue to be outbreaks associated with fecal contamination. The problem is that there is a lag of at least 24-48 hours between when the sample is collected and when the data become available. Changes in the water quality and potential exposures during this delay lead to management decisions and public notifications that are often times moot. To improve management abilities, there is a need for rapid detection and quantification of appropriate bacterial indicators in coastal waters and sediments to ensure the safety of these resources for their multiple users. The present NASBA based method addresses this issue by quantitatively amplifying the target RNA and providing results in less than one hour.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:

The FIGURE shows sequences for primers and probe for NASBA amplification of Enterococci.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

NASBA primers and probes were designed based on homologous regions of the large subunit ribosomal RNA gene (The FIGURE). This region is highly conserved within all reported species of the Enterococcus genus (Haugland et al., 2005). Previous researchers (Frahm and Obst, 2003) used this region as the priming site for the development of a Taqman PCR assay for enterococci detection. The primers have been modified to best fit the NASBA amplification requirements and molecular beacon design criteria as well as minimize the cross hybridization to each other. This primer set has been tested against Enterococcus faecalis and E. faecium, and E. avium, as well as several other genera of Gram positive and Gram negative bacteria to determine the specificity of the assay and ensure that no cross reactivity occurs to non-target organisms. Data thus far indicate that the primer set is specific to the genus enterococci.

The assay sensitivity has been evaluated using serially diluted RNA isolated from enterococci cells. It is also possible to include cell extract samples from individual species as well as mixtures of all of the enterococci species available. Because ribosomal RNA tends to have a complex secondary structure, the NASBA assay was initiated by a 65° C. denaturing step for 3 minutes prior to the 41° C. amplification step (60 to 90 minutes). This initial denaturation is required for some, but not all NASBA assays and its requirement for efficient amplification of enterococci still needs to be evaluated.

Amplification inhibitors are often co-purified with natural environmental samples. These inhibitors have been encountered in coastal water filtrates (Casper et al., 2005b). However, this inhibition tends to occur in sample volumes greater than 200 ml. For enterococci samples, the sample volume will always be less than 100 ml (as used by the DOH). Also, an internal control (IC-NASBA protocol) is included that will be used to normalize the amplification results (Patterson et al., 2005). This approach will increase the precision of quantification, and reduce the amount of false negative results. Because the internal control molecule contains the same priming site as the target, the amplification reaction becomes competitive. Serial titrations of internal control RNA determine the optimal concentration to allow for consistent quantification of enterococci within the expected environmental concentrations (Patterson et al, 2005).

REFERENCES

The following citations are incorporated herein by reference:

• Haugland R A, Siefring S C, Wymer L J, Brenner K P, Dufour A P. 2005. Comparison of Enterococcus measurements in freshwater at two recreational beaches by quantitative polymerase chain reaction and membrane filter culture analysis. Water Res. 39(4):559-68.

Frahm E, Obst U. 2003. Application of the fluorogenic probe technique (TaqMan PCR) to the detection of Enterococcus spp. And Escherichia coli in water samples. J Microb Meth 52: 123-131.

It will be seen that the advantages set forth above, and those made apparent from the foregoing description, are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matters contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. Now that the invention has been described,

Claims

1. A method for quantifying Enterococci in a sample of saline water, said method comprising: lysing at least one test microbe in the sample;extracting a sample nucleic acid from the at least one microbe;performing nucleic acid sequence based amplification, wherein said amplifying step comprises contacting said sample nucleic acid with a pair of primers to produce an amplification product, wherein said pair of primers comprises a first primer and a second primer, wherein said first primer comprises SEQ ID NO:1, and said second primer comprises SEQ ID NO: 2; andquantifying the amplification product, wherein the amount of amplification product is indicative of the amount of Enterococci in a sample of saline water.