Abstract:
An antimicrobial composition for eucaryotic microorganisms and method of suppressing the growth of eucaryotic microorganisms requires a procaryotic DNA extract. The antimicrobial composition is effective against numerous types of eucaryotic microorganisms and can be safely added to food and cosmetic products.

Description:
FIELD OF THE INVENTION 
     This invention relates to the protection from infection by eucaryotic microorganisms. 
     BACKGROUND OF THE INVENTION 
     Chemically synthesized antimicrobial agents which suppress the growth of eucaryotic microorganisms are well known but many of these agents raise concerns about their safety. It is also known that some antibiotics produced by microorganisms suppress the growth of eucaryotic microorganisms. However, these agents cannot be applied to foods, cosmetics and industrial chemicals due to regulatory controls. Extracts of various spices or garlic have been recognized to have antimicrobial activity but they have peculiar odors, even at low doses, and therefore, their application has been limited. 
     SUMMARY OF THE INVENTION 
     This invention is directed to safe antimicrobial agents originating from natural sources which are able to suppress the growth of eucaryotic microorganisms and can be applied to foods, cosmetics, pharmaceuticals and industrial chemicals. 
     As a result of a search for natural sources which are able to suppress the growth of yeasts and molds, it was found that DNA obtained from procaryotic micro-organisms suppressed the growth of eucaryotic micro-organisms significantly. It is believed that there is a difference in the functional roles between DNA extracted from procaryotic microorganisms and DNA of eucaryotic microorganisms because of differences of methylation and frequency of CG dinucleotide sequences in both microorganisms and that the difference may relate to one of the self-protective functions in procaryotic microorganisms against eucaryotic microorganisms. 
     The present invention is directed to antimicrobial agents which are effective against eucaryotic microorganisms, in which a major component of the antimicrobial agent is DNA extracted from procaryotic microorganisms and a method of suppressing the growth of eucaryotic microorganisms by the addition of the inventive antimicrobial agent. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, the procaryotic micro-organisms include all Gram-positive and Gram-negative bacteria. 
     The Gram-positive bacteria include those of the Bacillus genus such as B. subtilis, B. cereus., B. megaterium, B. mesentericus, B. licheniformis, B. sphaericus, B. alvei, B. natto and B. circulans; lactic acid bacteria such as Lactobacillus plantarum, L. acidophilus, L. brevis, L. casei, L. delbrueckii, L. fermentum and L. helveticus and Lactococcus lactis, Leuconostoc mesenteroides, L. dextranicum, Pediococcus pentosaceus, P. acidilactici, Streptococcus faecium and Str. faecalis; and Cocci such as Micrococcus luteus, M. flavus, M. roseus, Staphylococcus aureus and S. epidermidis. 
     The Gram-negative bacteria include Aeromonas hydrophila, Alcaligenes faecalis, Enterobacter cloacae, K-lebsiella peumoniae, Escherichia coli, Proteus vulgaris, Pseudomonas aeruginosa, Ps. fluorescens, Ps. aureofaciens, Salmonella Typhimurium, Sal. Enteritidis, Serratia marcescens and Vibrio. 
     When a foodstuff is to be prevented from spoiling or being mixed with a mycotoxin, DNA of a lactic acid bacteria or B. natto, a species of B. substilis used for natto food, or DNA of Escherichia coli can be used in the invention. 
     DNA of procaryotic microorganisms can be used in the invention to protect materials other than food, plants and living things from eucaryotic microorganisms. 
     The invention can be used to prevent spoilage of food products and fruit by molds or enzymes produced by the eucaryotes. Eucaryotic microorganisms against which the present invention is effective include those which produce mycotoxins, such as Aflatoxin, Ochratoxin, Sterigmatocystin, Penicillium toxin and Fusarium toxin. Diseases by eumycetes that can be treated by the invention include infections by, for example, trichophyton and candida, plant pathology and fish pathology. Molds on houses, furniture, in the kitchen, bathroom and on floors can be treated by the present invention. 
     Procaryotic DNA degraded by restriction enzymes also can be used and the size of the DNA is preferably at least 1,000 bases in order to show the growth suppressing effects. It is also favorable for the DNA to contain a large number of CG dinucleotide sequences in the molecule. A CG dinucleotide sequence means that C and G are successive bases in the nucleotide sequence of the procaryotic DNA. 
     It is preferable that the procaryotic DNA include CG dinucleotide sequences occurring at a frequency of at least two times, more preferably at least 17 times, as in the DNA of the eucaryotic microorganisms. 
     As eucaryotic microorganisms that are treated by the present invention, yeasts such as Saccharomyces cerevisiae and Candida albicans and filamentous fungi such as Aspergillus niger and Aspergillus fumigatus are examples. 
    
    
     EXAMPLE 1 
     As an example of extracting DNA from procaryotic microorganisms, Bacillus natto, IFO 3336, is cultivated in a 500 ml-volume Erlenmeyer flask containing 200 ml L-Broth medium (0.1% glucose, 0.5% yeast extract, 1.0% peptone and 0.5% NaCl, pH 7.2) at 30° C. for 30 hours. The cells are harvested by centrifugation (8,000 rpm, 10 minutes) and washed twice with a 0.85% saline solution. The cell suspension in 40 ml of distilled water is incubated with 20 mg of lysozyme at 37° C. for 20 minutes and then heated to 65° C. to break down the cells. Cold ethanol (-200C) is gradually added to the suspension and about 500 mg of insoluble DNA is obtained by means of winding the DNA onto a glass rod. The obtained DNA is washed with 70%, 80% and 90% ethanol solutions, respectively, and dried in vacuo. Thus, highly purified DNA is prepared as a solution containing 2 mg DNA per ml of distilled water. 
     When Saccharomyces cerevisiae is cultivated in an SD medium in the presence of 1 mg/ml DNA for yeast culture (2.0% glucose and 0.67% amino acid free-yeast nitrogen bases) in the presence of the DNA obtained as described above at 30° C., for 24 hours, growth of the yeast is completely suppressed. However, the growth of Saccharomyces cerevisiae is not suppressed in the medium in the absence of the procaryotic DNA. 
     EXAMPLE 2 
     A. Preparation of the DNA 
     Procaryotic microorganisms, Bacillus natto (IFO 3336) and Escherichia coli k-12 (IFO 14410), were incubated in a 500 ml-volume Erlenmeyer flask containing 200 ml of L-Broth culture medium (0.1% glucose, 0.5% yeast extract, 1.0% peptone and 0.5% NaCl, pH 7.2) at 30° C., respectively. After 30 hours, the cells of each bacterium were harvested and washed with a 0.085% saline solution twice and then the cells were suspended in 40 ml of distilled water. After the addition of 20 mg of lysozyme to the suspension, the temperature was increased to 65° C. to degrade the cells. 500 mg of DNA insolubized by the gradual addition of -20° C. ethanol was obtained by means of winding onto a glass rod. 
     B. Tests for Growth Suppression of Yeasts 
     DNA obtained from each bacterial strain described above was washed by an ethanol solution of which the concentration was increased from 70 to 80 followed by 80 to 90% and dried in vacuo. Thus, the highly purified DNA was obtained as a solution containing 2 mg DNA per ml of distilled water. 
     A yeast, Saccharomyces cerevisiae FT-1 (Journal of Fermentation and Bioengineering, 70, 275, 1990) which was obtained from K. Kimura, was cultivated in 5 ml of the SD medium (2.0% glucose and 0.67% yeast nitrogen-base, amino acid-free) at 30° C. for 24 hours. The DNA obtained from each bacterial strain was added to four flasks containing 100 ml of the SD medium. The final concentration of the DNA in each flask was 0, 0.2, 0.4, 0.6 and 0.8 mg/ml, respectively. To these media containing different concentration of the DNA, 0.1 ml of the yeast strain cultivated as described above was inoculated. Each SD medium containing the DNA was cultivated at 30° C. for 24 hours on a shaker and the yeast cells grown in the medium were harvested and washed with 0.85% saline. The cells were suspended in 10-folds, 100-folds and 1,000-folds volume of a 0.85% saline solution, respectively, and 0.1 ml each of the cell suspension was spread on a YDP agar plate medium (2% glucose, 2% peptone, 1% yeast extract and 1.5% agar, pH 5.0). After cultivation for one to two days at 30° C., the number of colonies grown on the plate medium was counted. The results are shown in Tables 1 and 2. 
     
                       TABLE 1______________________________________DNA from Bacillus nattoConcentra-tion of DNA   0        0.2      0.4    0.6    0.8______________________________________Number of   1.2 × 10.sup.8            8.0 × 10.sup.6                     1.4 × 10.sup.4                            2.0 × 10.sup.3                                   2.0 × 10.sup.3yeast cells______________________________________ 
    
     
                       TABLE 2______________________________________DNA from E. coli K-12Concentra-tion of DNA   0        0.2      0.4    0.6    0.8______________________________________Number of   2.3 × 10.sup.8            1.0 × 10.sup.6                     7.8 × 10.sup.4                            1.1 × 10.sup.3                                   1.0 × 10.sup.3yeast cells______________________________________ 
    
     These results showed that the addition of higher than 0.4 mg/ml DNA of B. natto or E. coli k-12 strongly suppressed growth of the yeast. 
     EXAMPLE 3 
     A. Preparation of DNA 
     Two ml of the solution of E. coli DNA (10 mg) prepared as described in Example 2 were put into 4 test tubes, respectively, and then 3 times in volume of cold ethanol (-20° C.) was added. After 24 hours kept at 20° C., DNA was precipitated by centrifugation at 1,000 rpm for 10 minutes and the supernatant was discarded. The DNA precipitated was dried in vacuo. 
     Two ml of the reaction mixture for a restriction enzyme, EcoRI, (all enzymes used here were made by Takara Shuzo Co., Ltd., in Japan) containing 50 mM of Tris-HCl at a pH of 7.5, 10 mM of MgCl 2 , 1 mM dithiothreitol and 100 mM NaCl were poured into a test tube to dissolve the DNA. Two 2 ml quantities of the reaction mixture for Hind III containing 10 mM of Tris-HCl pH 7.5, 10 mM of MgCl 2 , 1 mM of dithiothreitol and 50 mM of NaCl were poured into two test tubes to dissolve the DNA. Subsequently, 10,000 U of each restriction enzyme was added to each test tube and incubated at 37° C. for two hours. 
     To one of the test tubes which were reacted with Hind III, two times the volume of cold ethanol (-20° C.) was added again, and the mixture was kept at -20° C. for a day. After centrifugation at 10,000 for 10 minutes, the DNA was precipitated and dried in vacuo. The reaction mixture for DNase (100 mM of sodium acetate, pH 5.0 and 0.5 mM of MgSO 4 ) was added to dissolve DNA and then the solution was incubated with 1,000 U of DNase I at 15° C. for two hours. The molecular size of the DNA degraded by these restriction enzymes was approximately 2 to 3,000 bases on the average. 
     B. Tests for Growth Suppression of Yeast 
     A yeast, Saccharomyces cerevisiae FT-1 (J. of Fermantation and Bioengineering 70, 275, 1990) was cultivated in 5 ml of the SD medium (2.0% glucose and 0.67% yeast nitrogen-base amino acid free) at 30° C. for 24 hours. The DNA solution prepared as described above was added to 100 ml of the SD medium in which the concentration of DNA was 0.4 mg/ml and the precultured yeast was inoculated at the size of 1/1000. 
     The yeast cell number was determined by optical density measured spectrophotometrically at 610 nm using a HITACHI Spectrophotometer, U-1100, after the yeast was cultivated for 24 hours in a shaker. Table 3 shows the growth suppression of the yeast by the DNA degraded using each enzyme. 
     
                       TABLE 3______________________________________            Cell Growth measured by O.D.______________________________________No DNA Added       1.45DNA (No restriction enzyme added)              0.11DNA (Degraded by EcoRI)              0.62DNA (Degraded by HindIII)              0.58DNA (Degraded by HindIII and DNase)              1.21______________________________________ 
    
     These results indicated that the DNA degraded by restriction enzymes exhibited a weaker growth-suppressing activity. However, the activity of the DNA degraded by both EcoRI and DNase was higher than that without the DNA. 
     The data of Table 3-1 is obtained in the same manner as that of Table 3, showing that the antimicrobial effect can be obtained by CG dinucleotide sequences. 
     
                       TABLE 3-1______________________________________               Cell growth______________________________________Chemically synthesized oligonucleotide                 0.06containing CG motifChemically synthesized oligonucleotide                 0.93containing no CG motif______________________________________ 
    
     EXAMPLE 4 
     Tests for Growth Suppression of Filamentous Fungus 
     After addition of the DNA obtained from E. Coli and that from B. natto into 5 ml of the SD medium (2.0% glucose, 0.67% yeast and nitrogen-base amino acid free, pH 5.2), in which the final concentration of the DNA was 0.2 mg/ml, respectively, a filamentous fungus, Aspergillus niger (IFO 4034) precultured in the SD agar medium was inoculated with an inoculate loop into the SD medium containing the DNA, and cultivated at 30° C. for 12, 24, 36 and 48 hours, respectively, in a shaker. 
     The mycelia of A. niger were obtained after filtration of the cultured broth through a filter-paper (Advantec No. 5) and the wet weight (gram) of the mycelia which did not pass through the paper was determined with an Electric balance, TOP FX-320. 
     Table 4 shows the growth suppression of each DNA. 
     
                       TABLE 4______________________________________Culture period (hrs)           12     24        36  48______________________________________No DNA added (mg/ml)           0.2    2.3       3.7 4.8E. coli DNA     0.01   0.1       0.2 0.3B. natto DNA    0.01   0.1       0.1 0.2______________________________________ 
    
     These results indicated that each DNA sample suppressed the growth of the fungi for 48 hours cultivation in contrast with that of no DNA added. 
     EXAMPLE 5 
     Preservative Tests for a Sauce for Grilled Meats 
     Two percent in volume of water was added to a commercially available sauce for grilled meats made by Ebara-Shokuhin Co., Ltd., in Japan (the concentration of NaCl was 7.3% after addition of water), and 0.02% or 0.04% of the DNA prepared from B. natto was further added to the sauce. In order to test the preservative effects of the DNA, the sauce was kept at 30° C. for the times indicated. The result is shown in Table 5. 
     
                       TABLE 5______________________________________Days in Preservation      0     5      9    10   15   25   30   45______________________________________No DNA added      --    --     ++0.02% DNA  --    --     --   --   --   +0.04% DNA  --    --     --   --   --   --   --   +______________________________________ --: no change  +, ++: increased gas volume 
    
     The addition of 0.02 to 0.04% DNA was recognized to protect the sauce from yeast. +shows a slightly increased volume. ++shows a visibly increased volume. 
     EXAMPLE 6 
     A commercially available noodle soup (made by Ninben Co., Ltd. in Japan) was diluted three folds in volume with water (the concentration of NaCl was 2.8% after the dilution). The DNA of E. coli prepared as described in Example 2 was added to the soup up at the level of 0.01 or 0.04%. The result is shown in Table 6. 
     
                       TABLE 6______________________________________Days in Preservation      0     12     24  36   48  60   72  84   96______________________________________No DNA added      --    --     --  --   ++0.01% DNA  --    --     --  --   --  --   +0.04% DNA  --    --     --  --   --  --   --  --   +______________________________________ --no change, + turbid, ++ turbid and increased volume 
    
     The DNA was shown to be effective on preservation of the noodle soup. 
     EXAMPLE 7 
     Tests for the Growth Inhibition of Pathogenic Eucaryotic Microorganisms 
     The minimum inhibitory concentrations (MIC) of B. natto DNA and E. coli DNA to human pathogenic eucaryotic microorganisms were determined as shown below in Table 7. 
     
                       TABLE 7______________________________________          MIC (%)          B. natto DNA                    E. coli DNA______________________________________Candida albicans (TIMM0239)            0.02        0.02Trichoderma spp. 0.02        0.02Aspergillus niger (IAM2004)            0.03        0.03Cladosporium spp.            0.03        0.03Fusarium spp.    0.04        0.04Mucor spp.       0.02        0.02Penicillium spp. 0.02        0.02______________________________________ Medium used: Potato dextrose medium Culture conditions: 25° C., 1 to 3 weeks (filamentous fungi) 48 hours (yeasts) 
    
     The DNA was shown to inhibit the growth of typical human pathogenic eucaryotic microorganisms at concentrations of 0.02 to 0.04%. 
     In the same manner as Example 7, tests for growth inhibition of Aflotoxin-producing eucaryotic microorganisms were conducted and results are shown in Table 7-1. 
     
                       TABLE 7-1______________________________________         MIC (%)         B. natto DNA                  E. coli DNA______________________________________Aspergillus flavus*           0.03       0.03Aspergillus narasiticus*           0.03       0.03Aspergillus achraceus*           0.03       0.03Penicillium viridicatum*           0.02       0.02Aspergillus versicolor           0.03       0.03______________________________________ 
    
     EXAMPLE 8 
     The inhibitory effect of the DNA on the growth of plant pathogenic and fish pathogenic eucaryotic microorganisms was examined as shown in Table 8. 
     
                       TABLE 8______________________________________          MIC (%)          B. natto DNA                   E. coli DNA______________________________________Fusarium oxysporum f. sp.            0.03       0.03Pythium ultimun  0.02       0.02Rhizoctonia solani            0.02       0.02Plasmodiophora brassiciae            0.02       0.02Phytophthora spp.            0.03       0.03Helicobasidium mompa            0.02       0.02Saproregnia spp. 0.03       0.03______________________________________ Medium used: Potato dextrose medium Culture conditions: 25° C., 7 days (fungi) 48 hours (yeasts) 
    
     The DNA inhibited the growth of typical plant pathogenic eucaryotic microorganisms at the levels of 0.02 to 0.03%. 
     EXAMPLE 9 
     Application for Cosmetics (Preservation Tests for Milky-Lotion Type Hair Tonics) 
     A milky-lotion type hair tonic was prepared using the components described below. It was manufactured under the open system since microbial infection from the air was expected. 
     
         ______________________________________(A)    Purified jojoba oil                     10.8%  Benzyl nicotinate   2  Lanolin             2  Isopropyl-myristate                      2.5  Polyoxyethylene cetylalcohol                      1.8  Sorbitan monostearate                      0.8(B)    Triethanol amine    1%  Glycerine           4  Deionized water    75  Flavours           Proper Quantity______________________________________ 
    
     The oil phase (A) and aqueous phase (B) were heated to 85° C., respectively. Then, (A) and (B) were mixed and agitated to emulsification at the same temperature. A hair tonic was prepared after the mixture was cooled down to normal temperature. 
     The hair tonic prepared was poured into a bottle with 0.03% of the DNA. The bottle was sealed and kept at 37° C. for preservation tests. 0.1% Ethyl p-hydroxy benzoate was added to the hair tonic as a control instead of the DNA. The result is shown in Table 9. 
     
                       TABLE 9______________________________________Days     0          7     14      21  28______________________________________Control  --         --    +DNA      --         --    --      --  +______________________________________ +: Fungi appeared 
    
     This result indicated that addition of 0.03% of the DNA was effective in the protection of a milky-lotion type hair tonic from fungal infection. 
     Although examples of the present invention have been presented for the purpose of explaining the present invention, the present invention is by no way limited thereto and is deemed to cover all modifications and substitutions that would be obvious to one of ordinary skill in the art.