Abstract:
Provided are a single-stranded nucleic acid aptamer specifically binding to  E. coli  and a method for detecting  E. coli  using the same. The method, kits or sensors of the present disclosure enable  E. coli  to be specifically detected among microorganisms existing in a water system, but also be applied in fields such as food sanitation or medical diagnosis.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2012-0086395, filed on Aug. 7, 2012, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    The present disclosure relates to single-stranded nucleic acid aptamers specifically binding to  Escherichia coli  ( E. coli ) and methods for detecting  E. coli  using the same. 
         [0004]    2. Description of the Related Art 
         [0005]    To identify existence of harmful microorganisms in a water system, current practice involves detection of total coliforms present in the water, indicated by an index number. With regards to drinking water, existence of coliforms is measured based on the ability of degrading lactose. However, experimental methods of measuring the existence of coliforms are very complicated to require long experimental turnaround time (over 4 days), and also has high error rate and inaccuracy since the results are calculated based on statistical table. Therefore, a highly accurate  E. coli  monitoring sensor system for real-time monitoring of microorganism contamination of a water system due to pathogenic microorganism existence is required. In particular, securing of a receptor that recognizes  E. coli  with high specificity is essential for development of a highly sensitive and stable  E. coli  monitoring system. 
         [0006]    Aptamer is a single-stranded nucleic acid with high specificity and affinity for various target substances. The related art sensor technologies for diagnosing diseases or for detecting harmful microorganisms mostly rely on immunological analysis using antibodies. Recent studies have been relying upon biosensor technology, wherein aptamers of high specificity and affinity for target substances are used in diagnosing diseases. Various chemical functional groups may be given to terminals of aptamers, and the specificity and affinity of aptamers may be maximized during a procedure of being selected in vitro. An aptamer may be mass-produced with high-purity and low-cost through chemical synthesis, and may be stored under room temperature for extended periods, as aptamers are thermally stable. In addition, aptamers may be enhanced with respect to their specificity for target substances by performing a counter selection on various materials which have a similar structure with the target or are largely present in actual samples. 
         [0007]    Therefore, the present disclosure provides a simple, quick and accurate method of detecting the presence of  E. coli  within a water system by incorporating single-stranded nucleic acid aptamers specifically binding to  E. coli.    
       SUMMARY 
       [0008]    Provided are single-stranded nucleic acid aptamers that specifically bind to  E. coli.    
         [0009]    Provided are methods for detecting  E. coli  from a sample using single-stranded nucleic acid aptamers specifically binding to  E. coli.    
         [0010]    Provided are  E. coli  detection kits and  E. coli  detection sensors that include single-stranded nucleic acid aptamers specifically binding to  E. coli.    
         [0011]    Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments. 
         [0012]    According to an aspect of the present invention, a single-stranded nucleic acid aptamer specifically binding to  E. coli  includes deoxyribonucleic acid (DNA), ribonucleic acid (RNA), peptide nucleic acid (PNA) and any combinations thereof. The terminology “aptamer” is a single-stranded nucleic molecule that has a stable 3-dimensional structure by itself and is characterized by being able to bind to a target molecule with high affinity and specificity. The aptamer may have a nucleotide sequence of SEQ ID NO: 1 to 28 or combinations thereof, for example, SEQ ID NO: 1, 2, 10, 12 or combinations thereof. 
         [0013]    The aptamer may include a detectable label attached thereto. The detectable label may be a moiety that may be detected by detection methods known in the art. For example, the detectable marker may be an optical label, an electrochemical label, a radioisotope or combinations thereof. The label may be attached to a certain base or certain structure of an aptamer, for example, a certain site of a hairpin-loop structure or a 3′ end or a 5′ end of an aptamer. 
         [0014]    The optical label may be, for example, a fluorescent substance. The fluorescent substance may be chosen from the group consisting of fluorescein, 6-FAM, rhodamine, Texas Red, tetramethylrhodamine, carboxyrhodamine, carboxyrhodamine 6G, carboxyrhodol, carboxyrhodamine 110, Cascade Blue, Cascade Yellow, Comarin, Cy2 (cyanine 2), Cy3, Cy3.5, Cy5, Cy5.5, Cy-chrome, phycoerythrin, PerCP (peridinine chlorophyl-a protein), PerCP-Cy5.5, JOE (6-carboxy-4′,5′-dichloro-2′,7′-dimethoxyfluorescein), NED, ROX (5-(and -6)-carboxy-X-rhodamine), HEX, Lucifer Yellow, Marina Blue, Oregon Green 488, Oregon Green 500, Oregon Green 514, Alexa Fluor, 7-amino-4-methylcomarin-3-acetate, BODIPY FL, BODIPY FL-Br 2, BODIPY 530/550, their conjugates and mixture thereof. For example, the fluorescent substance may be fluorescein, Cy3 or Cy5. 
         [0015]    Also, the optical label may be a pair of fluorescence resonance energy transfers (FRET) that include donor fluorophore and acceptor fluorophore separated by an appropriate distance in which the fluorescence of the donor is inhibited by the acceptor. The donor fluorophore may include FAM, TAMRA, VIC, JOE, Cy3, Cy5 and Texas Red. The acceptor fluorophore may be selected such that excitation spectrum thereof overlaps emission spectrum of the donor. Also, the acceptor may be a non-fluorescent acceptor that quenches donors of broad range. Other examples of donor-acceptor FRET pair are known in the art. 
         [0016]    The electrochemical label may include electrochemical labels known in the art. For example, the electrochemical label may be methylene blue. 
         [0017]    According to another aspect of the present invention, a method for detecting  E. coli  includes: contacting a sample with a single-stranded nucleic acid aptamer which includes DNA, RNA, PNA or combinations thereof and binds specifically to  E. coli  to form an  E. coli - aptamer  complex; measuring a signal from the  E. coli - aptamer  complex; and identifying a presence or concentration of  E. coli  within the sample based on the measured signal. 
         [0018]    In the above method, the contact between the sample and the aptamer may be performed in a reacting solution. For example, the contact may be performed in a reacting solution that includes a composition of salt that enables good binding between the aptamer and  E. coli  and a component that prevents nonspecific binding. The component preventing the nonspecific binding may include, for example, salmon sperm DNA, BSA and/or Tween-20. The reaction temperature may be, for example, about 5° C. to about 35° C., about 10° C. to about 30° C., about 15° C. to about 25° C., or about 20° C. to about 25° C. The reaction time may be, for example, over about a 1 hour, about 10 minutes to about 60 minutes, about 20 minutes to about 60 minutes, about 30 minutes to about 50 minutes, or about 40 minutes to about 50 minutes. The aptamer may have a nucleotide sequence that includes SEQ ID NO: 1 to 28 or any combinations thereof, for example, SEQ ID NO: 1, 2, 10, 12 or any combinations thereof. The aptamer may have a detectable label attached thereto. The detectable label may be an optical label, an electrochemical label, a radioisotope or combinations thereof. 
         [0019]    In the above method, the signal from  E. coli - aptamer  complex may be generated from the optical label, electrochemical label or combinations thereof. The optical label may be, for example, donor-acceptor FRET pair. The electrochemical label may be, for example, methylene blue. 
         [0020]    In the above method, the identifying of an existence or concentration of  E. coli  within the sample may be performed via comparison with a control group. The control group may be an aptamer in a status not bound with  E. coli . For example, if the label attached to the aptamer is a donor-acceptor FRET pair, the control group may be an aptamer in which a fluorescent signal of donor fluorophore is inhibited by the acceptor fluorophore. When  E. coli - aptamer  complex is formed, the fluorescent signal changes by having FRET efficiency reduced, and existence or concentration of  E. coli  may be identified from the signal change. For example, if the label attached to an aptamer is an electrochemical label, the control group may be the labeled aptamer fixed to an electrode. The electrochemical signal changes by having the electrochemical label being distant or close to the electrode, or separated from the aptamer due to structural change of the aptamer bound with  E. coli , and existence or concentration of  E. coli  may be identified from the signal change. 
         [0021]    The method may further include separating the  E. coli - aptamer  complex from a reactant that includes the sample and the aptamer. The separating may be performed after forming the  E. coli - aptamer  complex, before measuring the signal from the complex. The separating may be, for example, performed by a membrane filtration method or centrifugation method. Also, the separating may be performed by washing off the aptamer when the aptamer is bound to the electrode. The optical label attached to the aptamer may be fluorescent substances as mentioned above. For example, the fluorescent substance may be fluorescein, Cy3 or Cy5. The signal generated from the separated  E. coli - aptamer  complex may be measured by, for example, a fluorometry or radioisotope detecting method. 
         [0022]    According to another aspect of the present invention, an  E. coli  detection kit includes a single-stranded nucleic acid aptamer including DNA, RNA, PNA or combinations thereof, specifically binding to  E. coli.    
         [0023]    In the above kit, the aptamer may have a nucleotide sequence of SEQ ID NO: 1 to 28 or combinations thereof, for example, SEQ ID NO: 1, 2, 10, 12 or combinations thereof. The aptamer may include a detectable label attached thereto. The detectable label may be, for example, an optical label, a radioisotope or combinations thereof. 
         [0024]    The kit may further include substances known to be required in forming  E. coli -aptamer complex. The kit may further include components required for stimulation of specific  E. coli - aptamer  complex formation or inhibition of nonspecific  E. coli - aptamer  complex formation, for example, salmon sperm DNA, BSA and/or Tween-20. Also, the kit may further include manual written about method of identifying  E. coli  within samples with the aptamer. 
         [0025]    According to another aspect of the present invention, an  E. coli  detection sensor includes a single-stranded nucleic acid aptamer including DNA, RNA, PNA or combinations thereof, specifically binding to  E. coli.    
         [0026]    In the above sensor, the aptamer may have nucleotide sequence of SEQ ID NO: 1 to 28 or combinations thereof, for example, SEQ ID NO: 1, 2, 10, 12 or combinations thereof. The aptamer may include a detectable label attached thereto. The detectable label may be an optical label, an electrochemical label, or combinations thereof. The sensor including the optical label may use, for example, FRET effect. The sensor including the electrochemical label may be based on principle of electrochemical signal changes performed by having the electrochemical label being distant or close to the electrode, or separated from the aptamer due to structural change of the aptamer bound with target substance. 
         [0027]    The sensor may include a substrate with more than two of the aptamers fixed thereon, for example, be an array form. An array refers to a substrate with certain regions having numbers of specific molecules fixed thereon. The array may include a substrate and a fixation region that is formed on the substrate and includes aptamers capable of binding with  E. coli . The aptamer may be covalently attached within the fixation region. The aptamer may further include a plurality of compounds with functional groups that may be covalently attached. The functional group may be any type as long as it is capable of attaching the aptamer to the substrate. For example, the functional group may be aldehyde, epoxy or amine group, and the compound may be siloxane with aldehyde, epoxy or amine group at the end. The material of the substrate may be, for example, glass, silicon, polypropylene and polyethylene. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The above and other aspects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0029]      FIG. 1  is a schematic view of a Bacteria Cell-SELEX method to select a DNA aptamer that may specifically bind to  E. coli;    
           [0030]      FIG. 2  is a percentage graph of a single-stranded DNA that binds to  E. coli  among the single-stranded DNA library used in each selection round; 
           [0031]      FIG. 3A to 3D  are schematic views of an predicted secondary structure for a DNA aptamer that specifically binds to  E. coli , based on an mfold program; 
           [0032]      FIG. 4  is a coherence analysis result graph for a DNA aptamer that specifically binds to  E. coli ; and 
           [0033]      FIG. 5  is a specificity analysis result graph for a DNA aptamer that specifically binds to  E. coli.    
       
    
    
     DETAILED DESCRIPTION 
       [0034]    Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. 
       Example 1 
     Manufacture of a DNA Aptamer Specifically Binding to  E. coli    
       [0035]    1.  E. coli  Cultivation 
         [0036]    Feces-originated  E. coli  KCTC2571 was cultured at 37° C. after inoculation in nutrient broth (beef extract 37%, pepton 63%, 8 g broth/L D.W, and pH 6.8). Once the concentration of the  E. coli  reached 10 8  CFU/ml, culture media was removed by washing it off 3 times with phosphate buffered saline (PBS). The  E. coli  cells were then suspended in a binding buffer (1×PBS, 0.1 mg/ml salmon sperm DNA, 1% BSA, and 0.05% Tween-20). 
         [0000]    2. Manufacture of Random Single-Stranded DNA (ssDNA) Library 
         [0037]    A ssDNA library consisting of the following single-stranded DNA oligonucleotide was synthesized. As a result, a random ssDNA library having 10 15  different oligonucleotides was synthesized. 
         [0000]    
       
         
               
               
             
           
               
                   
                 (SEQ ID NO: 29) 
               
               
                   
                 5′- GCAATGGTACGGTACTTCC -N 45 - CAAAAGTGCACGCTACTTT   
               
               
                   
                   
               
               
                   
                   GCTAA -3′ 
               
             
          
         
       
     
         [0038]    The underlined sites in the above represent the fixed sequences where the following primer pairs are annealed, and N 45  represents random existence of adenine (A), guanine (G), thymine (T), and cytosine (C) bases in each site. 
       3. Whole-Cell SELEX 
       [0039]    The random ssDNA library having 10 15  different oligonucleotides was dissolved in a binding buffer (1×PBS, 0.1 mg/ml salmon sperm DNA, 1% BSA, 0.05% Tween-20) and heated for 5 minutes at 95° C. Temperature was then immediately reduced to 4° C. and held for 10 minutes, and the random ssDNA library was mixed with 1 mL of  E. coli  suspension (10 7  cells) for an hour at room temperature. The  E. coli /ssDNA complex was separated from the solution using a centrifuge at 13,000 rpm for 10 minutes. The separated  E. coli /ssDNA complex was suspended in PBS buffer and the procedures were repeated 3 times. Finally, the  E. coli /ssDNA complex was resuspended in sterilized distilled water. The resuspension was heated for 10 minutes at 95° C. and then held at 4° C. for 10 minutes in order to separate ssDNA from  E. coli /ssDNA complex. The separated ssDNA was collected by using the centrifuge set at 13,000 rpm for 10 minutes. 
         [0040]    The amount of the ssDNA selected from the procedure was amplified through polymerase chain reaction (PCR). The forward primer was a primer labeled with fluorescein at its 5′-end and the reverse primer was a primer labeled with biotin at its 5′-end. This was done to separate the dsDNA, the product of PCR, into a single-stranded DNA. 
         [0000]    
       
         
               
               
             
           
               
                   
                 Forward: 
               
               
                   
                 (SEQ ID NO: 30) 
               
               
                   
                 5′-fluorescein-GCAATGGTACGGTACTTCC-3′ 
               
               
                   
                   
               
               
                   
                 Reverse: 
               
               
                   
                 (SEQ ID NO: 31) 
               
               
                   
                 5′-biotin-TTAGCAAAGTAGCGTGCACTTTTG-3′ 
               
             
          
         
       
     
         [0041]    PCR was performed under the conditions of 95° C. (30 seconds), 56.3° C. (30 seconds) and 72° C. (10 seconds), with a total of 25 μl volume obtained by mixing 10 μl of ssDNA (which is approximately 100 ng), 1.25 μl of each primers with 10 μM, and 12.5 μl of PCR master mix, and the number of repeating cycle was 10. Electrophoresis was performed in 2% agarose gel to identify the proper performance of PCR. Afterwards, the PCR product was purified using a PCR purification kit (MinElute PCR Purification kit by QIAGEN). 100 μl of the purified PCR product was mixed with magnetic beads coated with 50 μl of avidin (Dynabeads MyOne™ Streptavidin, Invitrogen), allowed to react for 10 minutes at room temperature, and then washed with 1 ml of PBS buffer using a magnet. 500 μl of 200 mM sodium hydroxide (NaOH) was added to the mixture and the mixture was allowed to react for 5 minutes in order to denature dsDNA into ssDNA. Biotin-attached ssDNA was removed from the reacting solution using a magnet, and fluorescein-attached ssDNA was collected. The collected fluorescein-attached ssDNA was purified/condensed using the PCR purification kit and the concentration was analyzed. Concentrated ssDNA was used for the next round&#39;s selection procedure, wherein selection and amplification procedures were repeated multiple times as shown in  FIG. 2 . After completing a total of 10 selection procedures, it was ultimately found that 93.7% of ssDNA among the ssDNA mixed with  E. coli  were bound with  E. coli.    
         [0042]    Among SELEX procedures, a counter selection procedure was performed 3 times in total after 5th, 6th and 9th selection procedures using 4 types of bacteria ( Klebsiella pneumoniae, Citrobacter freundii, Enterobacter aerogenes, Staphylococcus epidermidis ) that are vastly found with target  E. coli  in water systems such as sewage. The methods and conditions of this counter selection procedure were equal to the prior selection procedure, however in the counter selection procedure, ssDNAs binding to the bacteria were removed, and ssDNAs not binding to the bacteria were collected, amplified and used for the next stage of the selection procedure. 
         [0043]    The finally obtained ssDNAs were subjected to PCR using the primer pairs and were cloned using a cloning kit (TOPO TA cloning kit). Plasmids were extracted from each colony, and base sequence analysis was performed, wherein a total of 28 different base sequences were obtained. The following Table 1 consists of the total 28 ssDNA base sequences selected from SELEX method. 
         [0000]    
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Random Site Base Sequence 
               
               
                   
               
             
             
               
                 E1 
                 5′-ACTTAGGTCGAGGTTAGTTTGTCTTGCTGGCGCATCCACT 
               
               
                   
                 GAGCG-3′ 
               
               
                   
                 (SEQ ID NO: 1) 
               
               
                   
               
               
                 E2 
                 5′-CCATGAGTGTTGTGAAATGTTGGGACACTAGGTGGCATAG 
               
               
                   
                 AGCCG-3′ 
               
               
                   
                 (SEQ ID NO: 2) 
               
               
                   
               
               
                 E3 
                 5′-AGGTTCGAACGTCATGATGCCCCAGTTGACATCATTCAAA 
               
               
                   
                 CGGAT-3′ 
               
               
                   
                 (SEQ ID NO: 3) 
               
               
                   
               
               
                 E4 
                 5′-TGTTCGTCGCCGCGTGGCCTGGGGGGAGGGTTGCTTGATT 
               
               
                   
                 GCCTA-3′ 
               
               
                   
                 (SEQ ID NO: 4) 
               
               
                   
               
               
                 E5 
                 5′-CCACTGCGGTTGGCGCGCCGACCTTTCTGTTAGCCTGAAA 
               
               
                   
                 CGCAG-3′ 
               
               
                   
                 (SEQ ID NO: 5) 
               
               
                   
               
               
                 E6 
                 5′-GGGTTTGGTTTGGGACGGGGACCAGATTTTCAGCTTCACT 
               
               
                   
                 CGTGG-3′ 
               
               
                   
                 (SEQ ID NO: 6) 
               
               
                   
               
               
                 E7 
                 5′-GTGATACGTAGGTAGTGGTGTGGGTCCTCTTTGCACCTTA 
               
               
                   
                 TGGTC-3′ 
               
               
                   
                 (SEQ ID NO: 7) 
               
               
                   
               
               
                 E8 
                 5′-GGGGTGCTCGGGGTCGCAGGTCGGTGCCCGCTTGTCACAG 
               
               
                   
                 ATCAGC-3′ 
               
               
                   
                 (SEQ ID NO: 8) 
               
               
                   
               
               
                 E9 
                 5′-TGGTGTAATCAGCCTAGGAGTGGGTCGTGACAGGTTGTAT 
               
               
                   
                 ATCTC-3′ 
               
               
                   
                 (SEQ ID NO: 9) 
               
               
                   
               
               
                 E10 
                 5′-GTTGCACTGTGCGGCCGAGCTGCCCCCTGGTTTGTGAATA 
               
               
                   
                 CCCTGGG-3′ 
               
               
                   
                 (SEQ ID NO: 10) 
               
               
                   
               
               
                 E11 
                 5′-GCGTGTGTGTTAGCGACGAGTTGGGGCAACGATAGTCATA 
               
               
                   
                 TGACT-3′ 
               
               
                   
                 (SEQ ID NO: 11) 
               
               
                   
               
               
                 E12 
                 5′-GCGAGGGCCAACGGTGGTTACGTCGCTACGGCGCTACTGG 
               
               
                   
                 TTGAT-3′ 
               
               
                   
                 (SEQ ID NO: 12) 
               
               
                   
               
               
                 E13 
                 5′-AAAGGAGGCATGGTCTCTGCTAAGCCATCCTACCTATACT 
               
               
                   
                 GAGGC-3′ 
               
               
                   
                 (SEQ ID NO: 13) 
               
               
                   
               
               
                 E14 
                 5′-TTGGGCATGAGTTGGTGATGCGGGTTTCGATTACGGGTTA 
               
               
                   
                 CTGCG-3′ 
               
               
                   
                 (SEQ ID NO: 14) 
               
               
                   
               
               
                 E15 
                 GCGCTCTACCGCCAGGCTACTCTTGGAGAGTCTTTTGGGCTCT 
               
               
                   
                 TG-3′ 
               
               
                   
                 (SEQ ID NO: 15) 
               
               
                   
               
               
                 E16 
                 5′-GAAAGGCTGTTTGCGCGCGAGTCTGTAAGATGTGCACTGA 
               
               
                   
                 TCGGT-3′ 
               
               
                   
                 (SEQ ID NO: 16) 
               
               
                   
               
               
                 E17 
                 5′-GAAATAGTGCTGGCTCTTCATCCGTCGGTTGTGTGAGGGG 
               
               
                   
                 TAGTG-3′ 
               
               
                   
                 (SEQ ID NO: 17) 
               
               
                   
               
               
                 E18 
                 5′-GATGAACTAATCGTGCGCCTCGGGTGGGTTTCCTGCTGCA 
               
               
                   
                 TCCAT-3′ 
               
               
                   
                 (SEQ ID NO: 18) 
               
               
                   
               
               
                 E19 
                 5′-TGAGGGCAGGCCTTAGTATTCTGCCTCTTAGAAAGGCAGG 
               
               
                   
                 TGCGG-3′ 
               
               
                   
                 (SEQ ID NO: 19) 
               
               
                   
               
               
                 E20 
                 5′-CTACCTGCTATTCACTGACTTTCGTCGCTTGTTTCACAGT 
               
               
                   
                 GGGAC-3′ 
               
               
                   
                 (SEQ ID NO: 20) 
               
               
                   
               
               
                 E21 
                 5′-CAATTTGCAATTTGGGTATGTCATTATTAGCGTTATTTGA 
               
               
                   
                 CTCAT-3′ 
               
               
                   
                 (SEQ ID NO: 21) 
               
               
                   
               
               
                 E22 
                 5′-ATTCAAGTCCATCTTACCGGTTGTGCTCCTCCTGTGCTAC 
               
               
                   
                 TTACT-3′ 
               
               
                   
                 (SEQ ID NO: 22) 
               
               
                   
               
               
                 E23 
                 5′-GGCCGTGTTCGGACGGTTTCATGCTCTATGAGGGCCCTTT 
               
               
                   
                 ACCCT-3′ 
               
               
                   
                 (SEQ ID NO: 23) 
               
               
                   
               
               
                 E24 
                 5′-CAGTTGAGGTCCCGATTTCACACGTATATCAGGACATTTA 
               
               
                   
                 TCTGA-3′ 
               
               
                   
                 (SEQ ID NO: 24) 
               
               
                   
               
               
                 E25 
                 5′-GCTGTAGATCTATGGGTGGTTGCCTGTTCTGGTTTCCATC 
               
               
                   
                 GATTT-3′ 
               
               
                   
                 (SEQ ID NO: 25) 
               
               
                   
               
               
                 E26 
                 5′-GCCTTGTTTTGCATTGTATGCACGTACGTGGGGTGCTGCG 
               
               
                   
                 TCAGAG-3′ 
               
               
                   
                 (SEQ ID NO: 26) 
               
               
                   
               
               
                 E27 
                 5′-AACGCATAGTTTCGGGAATTCTACATGAGCACGCCCATTA 
               
               
                   
                 CCCCG-3′ 
               
               
                   
                 (SEQ ID NO: 27) 
               
               
                   
               
               
                 E28 
                 5′-GCTAGTACGCGTTGTTCTGCAGATTATTTTTGCGAGTTTG 
               
               
                   
                 TTCGT-3′ 
               
               
                   
                 (SEQ ID NO: 28) 
               
               
                   
               
             
          
         
       
     
       Example 2 
     Analysis of Coherence and Specificity for  E. coli    
       [0044]    Secondary structure analysis was performed using an mfold program (http://mfold.rna.albany.edu/?q=mfold; Zuker, M. Nucleic Acids Res. 2003, 31, 3406) among 28 ssDNAs with their base sequence analyzed. As a result of the analysis, approximately 10 types of ssDNAs were expected of high coherence for  E. coli , and coherence and specificity of the ssDNAs for  E. coli  were analyzed for verification. 
         [0045]    First, for coherence analysis for  E. coli, E. coli  KCTC2571 was cultured at 37° C. after inoculation in nutrient broth (beef extract 37%, pepton 63%, 8 g broth/L in distilled water, and pH 6.8). After the concentration of  E. coli  reached 10 8  CFU/ml, culture media was removed by washing it off 3 times with PBS buffer, and the remaining  E. coli  was then suspended in a binding buffer (1×PBS, 0.1 mg/ml salmon sperm DNA, 1% BSA, 0.05% Tween-20). 100 μl of  E. coli  (10 7  cells) was mixed with 100 μl of fluorescently labeled ssDNA in various concentrations (0, 5, 12.5, 25, 50, 125, and 250 nM as final concentration) and allowed to react for 45 minutes at room temperature. After reaction, ssDNAs not bound to  E. coli  were removed by washing the product of the reaction twice with PBS buffer, followed by a fluorescence intensity analysis of  E. coli /ssDNA complex (Ex/Em=494 nm/521 nm, slit width: 10 nm, exposure time: 1 s) using a fluorescence spectrometer (LS50B, PerkinElmer Co., USA). Fluorescence intensity in each ssDNA concentration condition was plotted using a nonlinear regression method and a single-region saturation ligand binding method of a SigmaPlot program involving the equation F=B max *C/(Kd+C), wherein F represents fluorescence intensity, B max  represents maximum binding location, Kd is a dissociation constant, and C is a concentration of ssDNA. 
         [0046]    As a result of the above coherence analysis, 4 types of ssDNAs showing high affinity to  E. coli  is shown in  FIG. 4 . The coherence of the determined 4 types of ssDNAs showing high affinity to  E. coli  is as follows, wherein Kd is a dissociation constant: 
         [0000]    E1 (Kd=12.4 nM), E2 (Kd=25.2 nM), E10 (Kd=14.2 nM), and E12 (Kd=16.8 nM). 
         [0047]    Selectivity analysis for the 4 types of ssDNAs was performed using 4 other types of bacteria ( Klebsiella pneumoniae, Citrobacter freundii, Enterobacter aerogenes , and  Staphylococcus epidermidis ) and  E. coli  types different from the target  E. coli  (KCTC1682, KCTC2617, and KCTC2618). The selectivity analysis was performed by mixing 100 μl of each bacteria (i.e. 10 7  cells of each bacteria) with 100 μl of 500 nM ssDNA, allowing the mixtures to react for 45 minutes at room temperature, washing the products of the reactions with PBS buffer and comparing the final results by measuring fluorescence intensity of bacteria/ssDNA complex. As shown in  FIG. 5 , the 4 types of ssDNAs E1, E2, E10 and E12 all exhibited fluorescence intensity of less than 10% when mixed with  Klebsiella, Citrobacter, Enterobacter  and  Staphylococcus , while exhibiting fluorescence intensity ranging from approximately 0.5× to 1× that of the target  E. coli  for  E. coli  types different from the target  E. coli  (KCTC1682, KCTC2617, KCTC2618). 
         [0048]    According to the examples above, the existence and concentration of  E. coli  within a water system may be quickly and accurately detected by using the single-stranded nucleic acid aptamers specifically binding to  E. coli  of the present disclosure. 
         [0049]    While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.