Method of preparing bacterial ghosts from gram-positive bacteria by hydrochloric acid treatment

Abstract

The present invention relates to bacterial ghosts, and more particularly, to a method of preparing bacterial ghosts from gram-positive bacteria by hydrochloric acid treatment. Specifically, according to the present invention, when gram-positive bacteria were cultured after being treated with a minimum inhibitory concentration (MIC) of hydrochloric acid capable of inhibiting colony formation of gram-positive bacteria, bacterial ghosts were effectively formed. Since the formed bacterial ghosts have no intracellular proteins or DNA while preserving cell wall integrity, the risk of side effects such as secondary infection caused by bacterial growth when the bacterial ghosts are administered to humans is low. Therefore, the bacterial ghosts prepared from gram-positive bacteria according to the method of the present invention may be effectively used as a vaccine or a foreign antigen carrier for preventing or treating gram-positive bacterial infection.

Claims

1. A method of preparing bacterial ghosts of Listeria monocytogenes, the method comprising: providing Listeria monocytogenes as pathogenic bacteria for modification in order to provide a vaccine against a disease caused by L. monocytogenes; subjecting L. monocytogenes to a treatment with hydrochloric acid for a period of 15 minutes at a concentration of 6.25 mg/ml at a temperature of 37° C. to form bacterial ghosts, which preserve envelopes of the L. monocytogenes and antigenic determinants present on the envelopes, whereby there is no DNA remaining in the bacterial ghosts and no live L. monocytogenes remaining in the culture; and collecting the bacterial ghosts of L. monocytogenes having antigenic determinants present on the envelopes, such that the bacterial ghosts provide the vaccine against the disease caused by L. monocytogenes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail embodiments thereof with reference to the accompanying drawings, in which:

(2) FIG. 1 shows the results of a culture experiment conducted to determine the minimum inhibitory concentration (MIC) of hydrochloric acid in a BHI liquid medium for preparation of bacterial ghosts using Listeria monocytogenes;

(3) FIG. 2 shows the result of confirming the viability of bacteria when the bacteria were treated with various concentrations of hydrochloric acid or sodium hydroxide. The viability was determined by the presence or absence of colony formation on a BHI agar medium;

(4) FIG. 3 shows the time course of formation of bacterial ghosts from Listeria monocytogenes after treatment with the MIC of hydrochloric acid or sodium hydroxide;

(5) FIG. 4 shows the results of SDS-PAGE showing the total amount of proteins remaining in the bacterial ghosts of Listeria monocytogenes over time after treatment with the MIC of hydrochloric acid or sodium hydroxide;

(6) FIG. 5 shows the results (images A and B) of agarose gel electrophoresis for confirming the presence of genomic DNA remaining in the bacterial ghosts of Listeria monocytogenes after treatment with the MIC of hydrochloric acid or sodium hydroxide;

(7) FIG. 6 shows the result of real-time PCR analysis for confirming the presence of genomic DNA remaining in the bacterial ghosts of Listeria monocytogenes after treatment with the MIC of hydrochloric acid or sodium hydroxide; and

(8) FIG. 7 shows the results (images A, B and C) of scanning electron microscope (SEM) analysis for observing the surface morphology of the bacterial ghosts of Listeria monocytogenes after treatment with the MIC of hydrochloric acid or sodium hydroxide.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) Hereinafter, the present invention is described in more detail.

(10) As described above, in the conventional method of preparing bacterial ghosts by chemical treatment, bacterial ghosts are prepared from gram-positive bacteria by treatment with basic sodium hydroxide. However, there has been no research on a method of preparing bacterial ghosts from gram-positive bacteria by treating with acids.

(11) According to the present invention, when gram-positive bacteria were cultured after being treated with a minimum inhibitory concentration (MIC) of hydrochloric acid capable of inhibiting colony formation of gram-positive bacteria, bacterial ghosts were effectively formed. Since the formed bacterial ghosts have no intracellular proteins or DNA while preserving cell wall integrity, the risk of side effects such as secondary infection caused by bacterial growth when the bacterial ghosts are administered to humans is low. Therefore, the bacterial ghosts prepared from gram-positive bacteria according to the method of the present invention may be effectively used as a vaccine or a foreign antigen carrier for preventing or treating gram-positive bacterial infection.

(12) Accordingly, the present invention provides a method of preparing bacterial ghosts from gram-positive bacteria, including

(13) a step of inoculating a medium with gram-positive bacteria, followed by culturing;

(14) a step of obtaining the gram-positive bacteria from the culture medium cultured in the step of inoculating;

(15) a step of treating the gram-positive bacteria obtained in the step of obtaining with hydrochloric acid to form bacterial ghosts; and

(16) a step of obtaining the bacterial ghosts formed in the step of treating.

(17) In the step of inoculating according to the present invention, gram-positive bacteria are preferably any one or more selected from the group consisting of Listeria, Staphylococcus aureus, Streptococcus pneumoniae, Bacillus anthracis, Corynebacterium diphtherias and Clostridium tetani, without being limited thereto, and may be any gram-positive bacteria, which are known as pathogenic bacteria in the art and may be used as an inactivated vaccine or a foreign antigen carrier.

(18) Specifically, Listeria is more preferably Listeria monocytogenes, Listeria denitrificans (L. denitrificans), Listeria grayi (L. grayi) or Listeria murrayi (L. murrayi). More specifically, Listeria is most preferably Listeria monocytogenes, without being limited thereto, because Listeria monocytogenes is known to be pathogenic among Listeria.

(19) In the step of inoculating according to the present invention, the culture of gram-positive bacteria is preferably performed at 30 to 40° C., more preferably at 37° C. In addition, the culture of gram-positive bacteria is preferably performed for 65 to 75 hours, more preferably for 72 hours. During culturing of the gram-positive bacteria, the cell walls of cells in the exponential growth phase of bacterial growth are susceptible to hydrochloric acid. In contrast, cells in the stationary phase after the exponential growth phase have elastic cell walls, and the degree of damage to envelopes by the MIC of hydrochloric acid is not large. Therefore, lysis tunnel structures may be effectively formed.

(20) In the step of treating, the treatment is preferably performed with the minimum inhibitory concentration (MIC) of hydrochloric acid, without being limited thereto. Specifically, the minimum inhibitory concentration of hydrochloric acid is preferably 6 to 7 mg/ml, more preferably 6.25 mg/ml. In the method of preparing bacterial ghosts from gram-positive bacteria according to the present invention, it is important to treat with hydrochloric acid at a minimum inhibitory concentration. When hydrochloric acid is treated at less than the MIC of hydrochloric acid, bacteria may survive because of incomplete lysis of gram-positive bacteria. Thus, the prepared bacterial ghosts may not be used as an inactivated vaccine or a foreign antigen carrier. In addition, when hydrochloric acid is treated at more than the MIC of hydrochloric acid, the degree of damage to the envelope structure of gram-positive bacteria is increased, and complete tunnel structures may not be formed in the cell membrane. As a result, a perfect type of bacterial ghost may not be produced, and an effect as an inactivated vaccine or a foreign antigen may be reduced.

(21) In the step of treating, the treatment is preferably performed with hydrochloric acid for 10 to 60 minutes, more preferably for 15 to 60 minutes, without being limited thereto. According to the method of preparing bacterial ghosts from gram-positive bacteria of the present invention, at about 15 minutes after the MIC of hydrochloric acid has been applied, the lysis rate of gram-positive bacteria may be 100%. However, when bacterial ghosts are used as a vaccine, the hydrochloric acid treatment is preferably performed for 60 minutes in consideration of safety.

(22) In the step of treating, the treatment is preferably performed with hydrochloric acid at 30 to 40° C., more preferably at 37° C., without being limited thereto. When the treatment is performed at a temperature of 30° C. or lower or 40° C. or higher, the MIC of hydrochloric acid may be varied, and the production efficiency of the bacterial ghosts of the present invention may be varied.

(23) In a specific embodiment of the present invention, the MIC of hydrochloric acid for preparing bacterial ghosts from Listeria monocytogenes was determined. When Listeria monocytogenes was treated with hydrochloric acid at a concentration of 6.25 mg/ml, Listeria growth was effectively inhibited. Thus, the concentration was determined as the MIC of hydrochloric acid. In addition, it was confirmed that the degree of growth of live cells was effectively inhibited at the MIC (FIGS. 1A and 2). The MIC of sodium hydroxide used as a control was 6.25 mg/ml. In the case of ammonium sulfate or calcium chloride, which is known to be used to prepare bacterial ghosts from gram-negative bacteria, an inhibitory effect on bacterial growth was not observed even when bacteria were treated with ammonium sulfate or calcium chloride at a concentration of 500 mg/ml (FIGS. 1B to 1D).

(24) In addition, in preparation of bacterial ghosts from Listeria, the optimum conditions for treating with the MIC of hydrochloric acid were determined. As a result, when Listeria were treated with the MIC of hydrochloric acid for about 15 minutes, bacterial ghosts were effectively formed (FIG. 3).

(25) In addition, it was confirmed that bacterial ghosts prepared from Listeria according to the method of the present invention may be effectively used. Lysed tunnels were effectively formed in the cell walls of the bacterial ghosts, no protein and genomic DNA remained in the cytoplasm, and only cell wall proteins for retaining the structure of the bacterial ghosts were present (FIGS. 4 and 5). On the other hand, in the case of bacterial ghosts prepared by treating with sodium hydroxide under the same conditions, trace amounts of genomic DNA remained in the cytoplasm (FIG. 6). Based on these results, it was confirmed that the method using hydrochloric acid treatment was more effective.

(26) In addition, the surface of bacterial ghosts prepared from Listeria by treatment with hydrochloric acid or sodium hydroxide was analyzed. As a result, lysis tunnel structures were formed in bacterial ghosts treated with hydrochloric acid, and a slightly lysed envelope shape was observed (FIG. 7).

(27) Accordingly, according to the present invention, when gram-positive bacteria were cultured after being treated with a minimum inhibitory concentration (MIC) of hydrochloric acid capable of inhibiting colony formation of gram-positive bacteria, bacterial ghosts were effectively formed. Since the formed bacterial ghosts have no intracellular proteins or DNA while preserving cell wall integrity, the risk of side effects such as secondary infection caused by bacterial growth when the bacterial ghosts are administered to humans is low. Therefore, the bacterial ghosts prepared from gram-positive bacteria according to the method of the present invention may be effectively used as a vaccine or a foreign antigen carrier for preventing or treating gram-positive bacterial infection.

(28) In addition, the present invention provides bacterial ghosts prepared from gram-positive bacteria according to the method of preparing bacterial ghosts from gram-positive bacteria of the present invention.

(29) In addition, the present invention provides a use of the bacterial ghosts of gram-positive bacteria, which are prepared according to the method of preparing bacterial ghosts from gram-positive bacteria of the present invention.

(30) In addition, the present invention provides a vaccine composition for preventing and treating gram-positive bacterial infection, including the bacterial ghosts prepared from gram-positive bacteria as an active ingredient.

(31) In addition, the present invention provides a method of preventing or treating gram-positive bacterial infection, including a step of administering the bacterial ghosts prepared from gram-positive bacteria to an individual in need of treatment.

(32) In addition, the present invention provides a use of a vaccine composition for preventing and treating gram-positive bacterial infection, the vaccine composition including the bacterial ghosts prepared from gram-positive bacteria as an active ingredient.

(33) In the present invention, gram-positive bacteria are preferably any one or more selected from the group consisting of Listeria, Staphylococcus aureus, Streptococcus pneumoniae, Bacillus anthracis, Corynebacterium diphtherias and Clostridium tetani, without being limited thereto, and may be any gram-positive bacteria, which are known as pathogenic bacteria in the art and may be used as an inactivated vaccine or a foreign antigen carrier.

(34) Specifically, Listeria is more preferably Listeria monocytogenes, Listeria denitrificans (L. denitrificans), Listeria grayi (L. grayi) or Listeria murrayi (L. murrayi). More specifically, Listeria is most preferably Listeria monocytogenes, without being limited thereto, because Listeria monocytogenes is known to be pathogenic among Listeria.

(35) The vaccine composition of the present invention preferably contains the bacterial ghosts of gram-positive bacteria of the present invention as an inactivated vaccine or a foreign antigen carrier, without being limited thereto.

(36) According to the present invention, when gram-positive bacteria were cultured after being treated with a minimum inhibitory concentration (MIC) of hydrochloric acid capable of inhibiting colony formation of gram-positive bacteria, bacterial ghosts were effectively formed. Since the formed bacterial ghosts have no intracellular proteins or DNA while preserving cell wall integrity, the risk of side effects such as secondary infection caused by bacterial growth when the bacterial ghosts are administered to humans is low. Therefore, the bacterial ghosts prepared from gram-positive bacteria according to the method of the present invention may be effectively used as a vaccine or a foreign antigen carrier for preventing or treating gram-positive bacterial infection.

(37) Hereinafter, the present invention will be described in more detail with reference to examples. It will be apparent to those skilled in the art that these embodiments are for illustrative purposes only and that the scope of the present invention is not construed as being limited by these examples.

EXAMPLE 1

Preparation of Bacterial Ghosts Using Listeria Monocytogenes

(38) <1-1> Identification of Minimum Inhibitory Concentration (MIC) for Preparation of Bacterial Ghosts

(39) In the present invention, the MIC of hydrochloric acid required for preparing bacterial ghosts from Listeria monocytogenes bacteria was identified.

(40) Specifically, Listeria monocytogenes, a gram-positive bacterium, was cultured in a brain-heart infusion (BHI) medium at 7° C. and 200 rpm under shaking conditions overnight, and then the degree of bacterial growth was determined by measuring optical density at 600 nm using a Biochrom Libra S22 spectrophotometer. Next, hydrochloric acid was stepwise diluted with a fresh BHI medium to the concentrations shown in Table 1, and the cultured Listeria monocytogenes bacteria were inoculated at a concentration of 10.sup.6 CFU/ml, followed by incubation at 37° C. for 18 hours. After incubation, Listeria monocytogenes bacteria growth depending on the concentrations of hydrochloric acid was determined by measuring optical density at 600 nm. As positive control groups, ammonium sulfate, calcium chloride and sodium hydroxide reported in conventional methods of preparing bacterial ghosts were added to a BHI medium at concentrations shown in Table 1, respectively. Using a BHI medium not containing hydrochloric acid as a non-treated control group, Listeria monocytogenes bacteria growth was measured in the same manner as described above.

(41) TABLE-US-00001 TABLE 1 Various concentrations of media used to identify MIC applied to prepare bacterial ghosts from Listeria monocytogenes bacteria Treatment concentrations of chemical agents (mg/ml) Test Hydrochloric Sodium Ammonium Calcium tube acid hydroxide sulfate chloride No. HCl NaOH (NH.sub.4).sub.2SO.sub.4 CaCl.sub.2 0 — — — — (Non-treated control groups) 1 50 50 500 500 2 25 25 250 250 3 12.5 12.5 125 125 4 6.25 6.25 62.5 62.5 5 3.125 3.125 31.25 31.25 6 1.5625 1.5625 15.625 15.625 7 0.78125 0.78125 7.8125 7.8125 8 0.390625 0.390625 3.90625 3.90625 9 0.1953125 0.1953125 1.953125 1.953125

(42) As a result, as shown in FIG. 1, when hydrochloric acid or sodium hydroxide was added to a BHI liquid medium at a concentration of 6.25 mg/ml, the pH of the medium was 3.16 or 10.75, respectively, and each medium effectively suppressed Listeria growth. Thus, the MICs of hydrochloric acid and sodium hydroxide for Listeria were determined to be 6.25 mg/ml (FIGS. 1A and 1B). On the other hand, in the case of ammonium sulfate and calcium chloride, previously used to prepare bacterial ghosts by effectively inhibiting the growth of Escherichia coli, a gram-negative bacterium, a growth inhibitory effect on Listeria, a gram-positive bacterium, was observed at a concentration of 500 mg/ml (FIGS. 1C and 1D).

(43) <1-2> Determination of Cell Growth Capacity of bacterial Ghosts Prepared from Listeria

(44) After the MICs of hydrochloric acid and sodium hydroxide for Listeria, a gram-positive bacterium, were determined, the degree of cell growth of bacterial ghosts prepared in a medium containing an MIC of hydrochloric acid or sodium hydroxide was then determined by a standard plating method.

(45) Specifically, the Listeria cultured in Example <1-1> was plated onto BHI agar media at concentrations indicated in Table 2 below, incubated at 37° C. for 18 hours to induce colony formation, and then observed.

(46) TABLE-US-00002 TABLE 2 Concentrations of bacteria, with which an agar medium was inoculated, to determine cell growth capacity of bacterial ghosts Hydrochloric acid-treated Sodium hydroxide-treated groups groups Test Hydrochloric acid Dilution Sodium hydroxide Dilution tube concentrations factors concentrations factors No. (mg/ml) of bacteria (mg/ml) of bacteria 0 — 10.sup.−3 — .sup. 10.sup.−3 4 6.25  10.sup.0  6.25  10.sup.0 5 3.125 10.sup.−1 3.125 10.sup.0

(47) As a result, as shown in FIG. 2, in the case of hydrochloric acid treatment, when a non-treated control group was diluted to a dilution factor of 1/1000 and plated on a medium, a large number of colonies were formed on the medium. In addition, when a group treated with 3.125 mg/ml of hydrochloric acid was diluted to a dilution factor of 1/10 and plated on a medium, tens of colonies were also formed. In contrast, when a group treated with 6.25 mg/ml of hydrochloric acid, i.e., the MIC of hydrochloric acid, was plated on a medium without dilution, no viable colony was formed (FIG. 2A). In addition, in the case of sodium hydroxide treatment, when a non-treated control group was diluted to a dilution factor of 1/1000 and plated on a medium, numerous colonies were formed in the medium. When a group treated with 3.125 mg/ml of sodium hydroxide was plated without dilution, thousands of colonies were formed. In contrast, when a group treated with 6.25 mg/ml of sodium hydroxide, i.e., the MIC of sodium hydroxide, was plated without dilution, no viable colony was formed (FIG. 2B).

(48) <1-3> Determining Optimal Time for Preparing Bacterial Ghosts from Listeria

(49) After the MICs of hydrochloric acid and sodium hydroxide for preparing bacterial ghosts from Listeria, a gram-positive bacterium, were determined, the minimum time to prepare bacterial ghosts was then determined.

(50) Specifically, a BHI liquid medium was inoculated with Listeria, and incubated for 72 hours. After incubation, centrifugation was performed at 10,000 g for 10 minutes to obtain the cultured Listeria. The cultured Listeria was washed with phosphate-buffered saline (PBS, pH 7.0), and prepared at a concentration of 10.sup.6 CFU/ml. Thereafter, hydrochloric acid or sodium hydroxide at a concentration of 12.5 mg/ml was mixed with 2 ml of the prepared Listeria so that a final concentration became 6.25 mg/ml, followed by incubation at 37° C. Listeria was obtained at 15, 30, 45, 60 and 90 minutes after the beginning of culture and the lysis rate was measured. In addition, the colony forming unit (CFU) of Listeria was determined by plating in the same manner as described in Example <1-2>. After completion of culture, Listeria was harvested, washed twice with PBS, and subjected to centrifugation at 10,000 g for 15 minutes to finally obtain the bacterial ghosts of Listeria monocytogenes. Also, in the case of sodium hydroxide treatment, bacteria were harvested at 5, 10, 15, 30, 45 and 60 minutes after the beginning of culture, and whether or not the bacterial ghosts of Listeria were prepared was determined.

(51) As a result, as shown in FIG. 3, in the case of groups treated with the MIC of hydrochloric acid, bacterial ghosts were effectively formed within 15 minutes after hydrochloric acid treatment, whereas, when bacterial ghosts prepared from 15 minutes after hydrochloric acid treatment were plated on BHI agar media, colonies were not formed (FIG. 3A). In addition, in the case of groups treated with the MIC of sodium hydroxide, preparation of bacterial ghosts was completed in about 10 minutes after sodium hydroxide treatment, whereas, when bacterial ghosts prepared from 10 minutes after sodium hydroxide treatment were plated on BHI agar media, colonies were not formed (FIG. 3B).

EXAMPLE 2

Identification of Characteristics of Listeria Monocytogenes Bacterial Ghosts

(52) <2-1> Determination of Amount of Proteins Remaining in Bacterial Ghosts

(53) To identify the characteristics of the bacterial ghosts of Listeria prepared using hydrochloric acid or sodium hydroxide, the amount of proteins remaining in the bacterial ghosts was determined.

(54) Specifically, using the manner described in Example <1-3>, Listeria was harvested at 15, 30, 45 and 60 minutes after hydrochloric acid or sodium hydroxide was added, and a denaturation buffer (Laemmli, 1970, Nature 227:680-685) was added thereto, followed by heating for 3 to 5 minutes to prepare denatured samples. The prepared samples were loaded in 12% SDS-PAGE gels, and an SDS-PAGE electrophoresis analysis was performed at a current of 40 mA for 4 hours. After electrophoresis, the gels were stained with a staining solution (containing methanol, acetic acid and water in a volume ratio of 5:1:5) containing Coomassie Brilliant Blue R-250, and the total amount of proteins in bacterial ghosts was determined. SDS-PAGE electrophoresis was performed on Escherichia coli, as a non-treated control group, in the same manner to determine the total amount of proteins.

(55) As a result, as shown in FIG. 4, in a group treated with hydrochloric acid, high molecular weight proteins mainly remained in bacterial ghosts, whereas, in a group treated with sodium hydroxide, low molecular weight proteins mainly remained. By comparison of these protein bands with protein bands seen in Escherichia coli, as a non-treated control group, it was confirmed that the amount of proteins remaining in the treated groups was small (FIG. 4). From these results, it was confirmed that bacterial ghosts prepared in the present invention lacked proteins present in the cytoplasm and only proteins present in the cell walls required to maintain the structure of the bacterial ghosts remained.

(56) <2-2> Determination of Amount of DNA Remaining in Bacterial Ghosts

(57) After confirming the loss of proteins in the bacterial ghosts of Listeria monocytogenes prepared in the present invention, the amount of DNA remaining in the bacterial ghosts was then determined.

(58) Specifically, according to the method of Example <1-3>, Listeria was harvested 60 minutes after hydrochloric acid or sodium hydroxide was added, and genomic DNA was extracted using a commercial extraction kit (iNtRON Biotechnology Inc., Korea) according to a protocol provided by the manufacturer. Then, the extracted extracts were loaded in 1% agarose gels containing 0.5 g/ml ethidium bromide (EtBr) and electrophoresis was performed.

(59) In addition, to quantitatively analyze the amount of genomic DNA using real-time PCR, 1 μl of the extracted extracts (1:100 dilution), 1 μl of each forward and reverse primer having nucleotide sequences shown in Table 3 below, 2×SYBR Green II qPCR master mix (Agilent Technology, Inc., USA) and 7 μl distilled water were mixed, and real-time PCR was performed in a Stratagene Mx3000P real-time PCR analyzer under the conditions shown in Table 4 below. In the real-time PCR, genomic DNA was amplified and the amount thereof was quantitatively determined by fluorescence analysis.

(60) Listeria monocytogenes bacteria not treated with hydrochloric acid or sodium hydroxide were used as a non-treated control group, Listeria treated with a TE buffer solution was used as a solvent control group, and Listeria treated with ammonium sulfate was used as a positive control group. Then, real-time PCR analysis was performed using the same method.

(61) TABLE-US-00003 TABLE 3 Nucleotide sequences of primers for quantitatively analyzing genomic DNA of Listeria monocytogenes bacterial ghosts of the present invention Primer name Sequences Forward primer 5′-GGAATTCCACGTGTAGCGGTGAAAT-3′ Reverse primer 5′-GACTACCAGGGTATCTAATCCTGTTTG-3′

(62) The forward and reverse primers were designed to specifically amplify a certain region of 16S rRNA of Listeria monocytogenes bacteria.

(63) TABLE-US-00004 TABLE 4 Real-time PCR analysis conditions for quantitatively analyzing genomic DNA of Listeria monocytogenes bacterial ghosts of the present invention Temperature Time Repeat (cycle) 95° C. 10 min. 1 cycle 95° C. 10 sec. 40 cycles 55° C. 10 sec. 72° C. 30 sec.

(64) As a result, as shown in FIGS. 5 and 6, there was no DNA remaining in bacterial ghosts treated with hydrochloric acid or sodium hydroxide for 60 minutes (FIG. 5). However, when quantitative analysis was performed using real-time PCR, trace amounts of DNA were amplified in the case of sodium hydroxide treatment. This indicates that trace amounts of DNA derived from Listeria monocytogenes were present in the bacterial ghosts of a group treated with sodium hydroxide. On the other hand, in the case of hydrochloric acid treatment, DNA amplification was not observed at all and the Ct value was lower than that of a TE buffer solution (FIG. 6). From these results, it was confirmed that bacterial ghosts prepared by treatment with hydrochloric acid or sodium hydroxide according to the present invention did not contain genomic DNA and cytoplasmic proteins and were structures that only retained cell walls. In particular, since the amount of genomic DNA in bacterial ghosts prepared by treatment with hydrochloric acid was significantly less than that in bacterial ghosts prepared by treatment with sodium hydroxide, the bacterial ghosts prepared by treatment with hydrochloric acid are expected to be safer when used as an inactivated vaccine or a foreign antigen carrier.

(65) <2-3> Surface Morphology Analysis of Bacterial Ghosts

(66) To confirm whether bacterial ghosts prepared according to the present invention may be effectively used as an inactivated vaccine or a foreign antigen carrier, surface morphologies of the bacterial ghosts were analyzed using a scanning electron microscope (SEM).

(67) Specifically, according to the method of Example <1-3>, the bacterial ghosts of Listeria were obtained 60 minutes after adding hydrochloric acid or sodium hydroxide, suspended in PBS containing 2.5% glutaraldehyde, and fixed at 4° C. for 2 hours, followed by washing with the same solution. Next, bacterial ghosts were transferred into a 1% osmium tetroxide (OsO.sub.4) solution, fixed at 4° C. for 1.5 hours, and dehydrated with serially diluted ethanol solutions. The dehydrated bacterial ghosts were dried using liquefied carbon dioxide, coated with gold using a Polaron high-resolution sputter coater to prepare samples, and then the surfaces of the bacterial ghosts were observed using a Hitachi S-4800 FESEM scanning electron microscope. Live Listeria bacteria were used as a non-treated control group and the surfaces of the bacteria were observed using SEM by the same method.

(68) As a result, as shown in FIG. 7, when compared with a non-treated control group (image C), in the bacterial ghosts of a hydrochloric acid-treated group, lysis tunnel structures with a definite size were formed in the cell walls (indicated by the arrows) and the morphologies of envelopes had somewhat lysed structures (image). On the other hand, in the bacterial ghosts of a sodium hydroxide-treated group, the size of lysis tunnel structures was slightly small and the morphology of envelopes retained a perfect shape compared with the hydrochloric acid-treated group (image).

(69) Accordingly, the present invention provides bacterial ghosts prepared from gram-positive bacteria by treatment with hydrochloric acid and a method of preparing the same.

(70) According to the present invention, when gram-positive bacteria are cultured after being treated with a minimum inhibitory concentration (MIC) of hydrochloric acid capable of inhibiting colony formation of gram-positive bacteria, bacterial ghosts can be effectively formed. In addition, since the formed bacterial ghosts have no intracellular proteins or DNA while preserving cell wall integrity, the risk of side effects such as secondary infection caused by bacterial growth when the bacterial ghosts are administered to humans is low. Therefore, the bacterial ghosts prepared from gram-positive bacteria according to the method of the present invention can be effectively used as a vaccine or a foreign antigen carrier for preventing or treating gram-positive bacterial infection.