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Bhat et al. AMB Express 2013, 3:36
http://www.amb-express.com/content/3/1/36ORIGINAL ARTICLE Open AccessBacillus subtilis natto: a non-toxic source of
poly-γ-glutamic acid that could be used as a
cryoprotectant for probiotic bacteria
Aditya R Bhat1, Victor U Irorere1, Terry Bartlett1, David Hill1, Gopal Kedia1, Mark R Morris1,
Dimitris Charalampopoulos2 and Iza Radecka1*Abstract
It is common practice to freeze dry probiotic bacteria to improve their shelf life. However, the freeze drying process
itself can be detrimental to their viability. The viability of probiotics could be maintained if they are administered
within a microbially produced biodegradable polymer - poly-γ-glutamic acid (γ-PGA) - matrix. Although the
antifreeze activity of γ-PGA is well known, it has not been used for maintaining the viability of probiotic bacteria
during freeze drying. The aim of this study was to test the effect of γ-PGA (produced by B. subtilis natto ATCC
15245) on the viability of probiotic bacteria during freeze drying and to test the toxigenic potential of B. subtilis
natto. 10% γ-PGA was found to protect Lactobacillus paracasei significantly better than 10% sucrose, whereas it
showed comparable cryoprotectant activity to sucrose when it was used to protect Bifidobacterium breve and
Bifidobacterium longum. Although γ-PGA is known to be non-toxic, it is crucial to ascertain the toxigenic potential
of its source, B. subtilis natto. Presence of six genes that are known to encode for toxins were investigated: three
component hemolysin (hbl D/A), three component non-haemolytic enterotoxin (nheB), B. cereus enterotoxin T
(bceT), enterotoxin FM (entFM), sphingomyelinase (sph) and phosphatidylcholine-specific phospholipase (piplc). From
our investigations, none of these six genes were present in B. subtilis natto. Moreover, haemolytic and lecithinase
activities were found to be absent. Our work contributes a biodegradable polymer from a non-toxic source for the
cryoprotection of probiotic bacteria, thus improving their survival during the manufacturing process.
Keywords: Probiotics; γ-PGA; Cryoprotectant; Toxicity; Bifidobacteria; LactobacillusIntroduction
Over the years, extensive research has been done to determine the efficacy of probiotic foods in controlling and
alleviating disorders/diseases (de Moreno de LeBlanc
et al. 2007, Falagas et al. 2007, Garrait et al. 2009, LaraVilloslada et al. 2007). Probiotic bacteria are helpful
in maintaining a healthy gut and have been used for
controlling several types of gastrointestinal infections
(Anukam et al. 2008, Benchimol and Mack 2004, Kligler
et al. 2007, Lara-Villoslada et al. 2007, Park et al. 2007,
Pochapin 2000, Szymanski et al. 2006). Some lactic acid
bacteria have been shown to have antitumor activity
(de LeBlanc et al. 2005). Research has also shown a
marked reduction in total serum cholesterol in human* Correspondence: i.radecka@wlv.ac.uk
1University of Wolverhampton, Wolverhampton, UK
Full list of author information is available at the end of the article
© 2013 Bhat et al.; licensee Springer. This is an
Attribution License (http://creativecommons.or
in any medium, provided the original work is pvolunteers after ingestion of Enterococcus faecium (EF)
M-74 enriched with selenium (Hlivak et al. 2005). Furthermore, strains of Lactobacillus and Bifidobacterium
have been shown to cure dental disorders (Allaker and
Douglas 2009). Because of these beneficial effects, probiotic microorganisms have been introduced into a variety of food and drink products for administration to
humans or animals. Various strains of Lactobacillus and
Bifidobacterium are used commonly as probiotic bacteria to benefit the health of the host (Benno and
Mitsuoka 1992, de LeBlanc et al. 2005, De Simone et al.
1992, Kailasapathy and Rybka 1997).
One of the most important manufacturing steps for
producing a probiotic food product is to introduce the
bacteria into the foodstuff as dry cultures. Working with
dry cultures is advantageous since they are easier to handle and have a longer shelf life than wet cultures. FreezeOpen Access article distributed under the terms of the Creative Commons
g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction
roperly cited.
Bhat et al. AMB Express 2013, 3:36 Page 2 of 9
http://www.amb-express.com/content/3/1/36drying has been used most commonly for producing
dry bacterial powders. However, the methods for preparation of freeze dried probiotic bacteria are often detrimental to the cell structure and viability (Saarela et al.
2005). Previously, it has been shown that the viability of
lactic acid bacteria reduces by 3 log CFU/g when freeze
dried (Jagannath et al. 2010). Therefore, there remains
a need to improve the viability of probiotic microorganisms as they pass through, in particular, the freeze drying process.
This research uses bacterial poly-γ-glutamic acid (γ-PGA)
for protecting probiotic bacteria during freeze drying.
γ-PGA is a non-toxic, non-immunogenic and biodegradable biopolymer that is produced by bacteria for
use in various applications. γ-PGA is edible and is
present in a traditional Japanese dish Natto which is made
by fermenting soyabean with Bacillus strains. Most
commonly, strains of Bacillus subtilis and Bacillus
licheniformis have been researched for its production
(Buescher and Margaritis 2007, Candela and Fouet 2006,
Shih and Van 2001). Although the antifreeze properties of
γ-PGA have been determined (Mitsuiki et al. 1998, Shih
et al. 2003), it has never been used to protect live probiotic
bacteria during freeze drying. This research was designed
to analyse the effect of different concentrations of γ-PGA
as a cryoprotectant for probiotic bacteria during freeze
drying. The cryoprotective effect of the polymer produced
by B. subtilis natto was investigated by assessing the viability
of several probiotic bacteria (Lactobacillus paracasei,
Bifidobacterium breve and Bifidobacterium longum) during
freeze drying.
Whilst γ-PGA is suggested to be non-toxic and nonimmunogenic (Bajaj and Singhal 2011), it is important to
ascertain the toxigenic potential of the specific bacterium used to produce the polymer for a novel food application (SCAN 2000). Consequently, B. subtilis natto was
also screened for genes encoding toxins in other members of Bacillus sp. (Matarante et al. 2004). The presence/absence of haemolytic and lecithinase activities in
this bacterium was also determined. Therefore, this research not only assesses the potential of γ-PGA as a
cryoprotectant for probiotic bacteria, but also ascertains
the toxigenic potential of its source, B. subtilis natto.
Material and methods
Poly- γ-glutamic acid (γ-PGA)
γ-PGA (MW: 257,000 Da) was produced and extracted
from B. subtilis natto ATCC 15245 and identified by
Fourier transform infrared (FTIR) spectroscopy as reported
earlier (Kedia et al. 2010).
Bacterial strains
Three probiotic bacteria (Bifidobacterium longum NCIMB
8809, Bifidobacterium breve NCIMB 8807 and Lactobacillusparacasei NCIMB 8835) were obtained from National
Collection of Industrial and Marine Bacteria (NCIMB),
Aberdeen, UK. The stock cultures were freeze-dried
and stored at −20°C. Before use, the cultures were revived
aseptically and grown anaerobically on Bifidobacteria Selective Medium (BSM) Agar for Bifidobacteria and MRS
agar for Lactobacillus at 37°C in anaerobic gas jars using
an atmosphere generation system (Oxoid Anaerogen™, UK)
and indicator strip according to manufacturer’s instructions. Both BSM and MRS broth/agar were purchased from
Sigma-Aldrich, U.K. and prepared according to the manufacturer’s protocol.
Growth media
BSM agar/broth were used for the growth and enumeration of Bifidobacteria under study (Nualkaekul et al.
2011) and MRS medium was used for Lactobacillus.
γ-PGA as a cryoprotectant
Initially, unsterilised γ-PGA was used for preliminary
cryoprotection experiment. However, due to potential
concerns on the safety of future products, γ-PGA was
sterilised by autoclaving at 110°C and 0.35 bar for
30 min before being used for cryoprotection studies.
The structural integrity of the polymer was determined
by comparing the FTIR spectra (Genesis II FTIR™, UK)
of the polymer before and after sterilization.
To prepare cells for freeze drying, all microorganisms
were cultured anaerobically at 37°C. B. breve and B.
longum were cultured in 250 ml of BSM broth for 22 h
and 16 h respectively, while L. paracasei was cultured in
250 ml of MRS broth for 48 h. After incubation, viable
counts were performed on BSM agar (Bifidobacteria) and
MRS agar (Lactobacillus) to determine the number of
viable cells prior to freeze drying. The cultures were
centrifuged and washed with PBS to obtain cell pellets and
then resuspended in 10 ml solutions of either 10% (w/v)
γ-PGA, 5% (w/v) γ-PGA or 10% (w/v) sucrose. For cells
without a cryoprotectant, 10 ml of sterile distilled water
was added. The suspensions were incubated at room
temperature for 1 h and then frozen at −80°C for 24 h. The
frozen cultures were then freeze dried (Edwards Freeze
Dryer Modulyo) at −40°C and 5 mbar for 48 h. After freeze
drying, 10 ml of PBS was added to each treatment and the
viability was determined. Cells were enumerated by the
Miles and Misra technique which involves a 10 fold dilution series in PBS followed by aseptically plating out 20 μl
of each cell suspension in triplicate on appropriate media,
which were then incubated anaerobically at 37°C.
Scanning electron microscopy (SEM) analysis
SEM analysis was performed to determine the surface
structure of freeze dried cells protected with γ-PGA. All
freeze dried samples were coated with gold using a sputter
Bhat et al. AMB Express 2013, 3:36 Page 3 of 9
http://www.amb-express.com/content/3/1/36coater (Emscope Sc 500). SEM analysis was performed
using a Zeiss EVO50 and the images analysed using the
Smart SEM software.
Toxigenic analysis
The presence of major enterotoxins and virulence factors
in B. subtilis natto was investigated and compared to B.
cereus which was used as a positive control (Matarante
et al. 2004). The genes encoding various enterotoxins and
enzymes screened were: HBL - a three component hemolysin (hbl D/A), NHE - three component non-haemolytic enterotoxin (nheB), B. cereus enterotoxin T (bceT), enterotoxin
FM (entFM), sphingomyelinase (sph), phosphatidylinositol
and phosphatidylcholine specific phospholipase (piplc).
Phosphofructokinase A (pfkA), a housekeeping gene,
was used as positive control.
DNA isolation and purification
5 ml of Tryptone Soya Broth was inoculated with a single
colony of B. subtilis natto or B. cereus and the cultures were
incubated at 30°C overnight with shaking at 150 rpm. Bacterial cells were harvested by centrifugation at 1,000 × g for
5 min and the bacterial pellets were washed with sterile
water. DNA was isolated using the GenElute Bacterial Genomic DNA kit (Sigma-Aldrich, UK) according to the protocol provided by the manufacturer. The DNA quantity was
determined using a Nanodrop2000 Spectrophotometer
(Thermo Scientific, UK).
PCR analysis
The primers for PCR analysis were purchased from
Sigma-Aldrich, UK. The details of the primers used for
screening the genes of interest are provided in Table 1.
The gene sequence of pfkA was obtained from the nucleotide collection of the National Centre for Biotechnology
Information (NCBI) and the primers for the gene were
designed using the NCBI Primer-Blast. PCR analysis was
performed on isolated and purified DNA from each
microorganism as described by (Matarante et al. 2004).
Amplification comprised of an initial denaturation at 94°C
for 3 min followed by 31 cycles of denaturation (94°C for
30 s), annealing (58°C for 45 s) and extension (72°C for
1 min 30 s). A final extension at 72°C for 5 min was also
performed. Control mixtures without template DNA were
also included in each experiment. PCR amplification was
carried out in an MJ Research PTC-200 Peltier Thermal
Cycler. Taq polymerase, deoxynucleotide triphosphates
and DNA molecular weight markers were purchased from
Roche, UK. The amplified fragments were separated by
electrophoresis on a 2% agarose gel.
Determination of haemolytic and lecithinase activities
Haemolytic and lecithinase activies were determined
using a method described previously (Matarante et al.2004). Haemolytic activity was determined at 30°C on
blood agar plates containing 5% horse blood. Lecithinase
(phosphatidylinositol-specific phospholipase C) activity
was determined at 30°C on nutrient agar supplemented
with 8% egg yolk emulsion. B. cereus was used as a positive control. 5 μl of overnight TSB cultures of both microorganisms incubated at 37°C were used to inoculate
each of the media plates in triplicate. The cell-free supernatants of each microorganism were also tested for
their haemolytic and lecithinase activities by inoculating
5 μl of each cell free supernatants into the aforementioned media (Matarante et al. 2004).Statistical analyses
All results were statistically analysed using Microsoft
Excel 2010 and GraphPad Prism 5. Two-factor Anova
and student’s T test were used to compare data. The
Bonferroni multiple comparison test was used for the
non-parametric analysis of the data to determine the difference between individual groups in a data set. P
value ≤ 0.05 was considered to be statistically significant.Results
γ-PGA as a cryoprotectant for probiotic bacteria
The effect of sterilization on γ-PGA was investigated
using FTIR to determine whether steam sterilization alters the structure of the polymer. Results from FTIR
spectroscopy (Figure 1) indicated that the FTIR spectra
for sterilised and unsterilised polymer showed all the
peaks representing the characteristic bonds in γ-PGA.
This indicates that steam sterilisation does not alter the
chemical integrity of the polymer.
10% γ-PGA, 5% γ-PGA and 10% sucrose were used to
protect probiotic bacteria during freeze drying and the
effect of cryoprotection was assessed (Figure 2). It was
observed that when no cryoprotectant was used to protect the cells, L. paracasei showed a reduction in viability of 1.34 log CFU/ml. When 10% sucrose was used to
protect L. paracasei during freeze drying, 0.91 log
CFU/ml reduction in viability was observed after freeze
drying. However, for 10% γ-PGA-protected cells, the
loss in viability was reduced to 0.51 log CFU/ml. For L.
paracasei, 10% γ-PGA was able to protect the cells
significantly (P ≤ 0.05) better than 10% sucrose. The
cryoprotectant ability of 5% γ-PGA (Figure 2) was comparable to 10% sucrose (P > 0.05). A more pronounced
reduction in viability (~2.5 log CFU/ml) was observed
when both Bifidobacterium strains were freeze dried
without any cryoprotectant (Figure 2). When the cells
were protected with 10% γ-PGA, only 1.24 - 1.26 log
CFU/ml reduction in viability was observed. The cryoprotectant ability of 10% sucrose and 10% γ-PGA for
the Bifidobacterium strains was comparable (P ≥ 0.05).
Table 1 PCR primers used and virulence factors (adapted from Matarante et al. 2004)
Target gene Primer name Primer sequence (5′-3′) Amplicon size (bp) Reference
hbl – D/A hblD-f GGAGCGGTCGTTATTGTTGT 623 (Matarante et al. 2004)
hblA-r GCCGTATCTCCATTGTTCGT
nheB nheB 1500 S CTATCAGCACTTATGGCAG 769 (Matarante et al. 2004)
nhe B 2269 A ACTCCTAGCGGTGTTCC
bceT ETF TTACATTACCAGGACGTGCTT 428 (Matarante et al. 2004)
ETR TGTTTGTGATTGTAATTCAGG
entFM EntA ATGAAAAAAGTAATTTGCAGG 1269 (Matarante et al. 2004)
EntB TTAGTATGCTTTTGTGTAACC
sph Ph1 CGTGCCGATTTAATTGGGGC 558 (Matarante et al. 2004)
Ph2 CAATGTTTTAAACATGGATGCG
piplc PC105 CGCTATCAATGGACCATGG 569 (Matarante et al. 2004)
PC106 GGACTATTCCATGCTGTACC
pfkA* pfkA-F CCATCAGCTAAACCAGCC 370 This study
pfkA-R CGCGGTGGTACGAAATTA
* pfkA primer was designed using the NCBI Primer-Blast.
Bhat et al. AMB Express 2013, 3:36 Page 4 of 9
http://www.amb-express.com/content/3/1/36SEM analysis
Freeze dried bifidobacteria with and without γ-PGA as a
cryoprotectant were analysed using SEM to understand
how the cells may be protected. As is evident from
Figure 3a and b, freeze dried B. longum cells protectedFigure 1 FT-IR Spectroscopy of (a) unsterilised γ-PGA and (b) sterilisewith γ-PGA appear to be encapsulated within a material,
suggesting cluster of cells within a γ-PGA matrix. The
thickness of γ-PGA coating could be potentially calculated with further testing using transmission electron
microscopy (TEM).d γ-PGA showing relevant peaks.
Figure 2 Effect of γ-PGA and sucrose on viability of probiotic bacteria during freeze drying. Cells were freeze dried at −40°C and 5 mbar
pressure and viability was measured before and after freeze drying on BSM agar. Experiments were conducted in triplicate (n = 3).
Bhat et al. AMB Express 2013, 3:36 Page 5 of 9
http://www.amb-express.com/content/3/1/36Screening of B. subtilis for toxin genes using PCR
The housekeeping gene pfkA was seen to be present in
both DNA extracts of B. subtilis natto and B. cereus,
confirming successful DNA extraction and PCR amplification (Figure 4). Of the six genes coding for toxinb
a Figure 3 SEM image of a) Freeze dried B. longum cells with no γ-PGA
b) Freeze dried B. longum protected with γ-PGA (EHT = 20.00 kV; Sign
EVO50, U.K. and photographs were analysed using the software provided bproduction, four (nheB, entFM, sph, piplc) were present
in the control organism B. cereus (Figure 4). However
none of these six genes were present in B. subtilis natto.
No band was seen in any of the negative control experiments lacking DNA.B.
B.
protection (EHT = 20.00 kV; Signal A = SE1; WD = 4.0 mm)
al A = SE1; WD = 4.5 mm). SEM analysis was performed using Zeiss
y Zeiss EVO50.
2000–100 bp
2000–100 bp
2000–100 bp
Figure 4 PCR patterns of B. subtilis and B. cereus for screening
of genes coding for toxins. A - hbl- D/A, B – nheB, C – bceT,
D – entFM, E – sph, F – piplc, G – pfkA, W =Water, S = B. subtilis,
C = B. cereus.
Bhat et al. AMB Express 2013, 3:36 Page 6 of 9
http://www.amb-express.com/content/3/1/36Haemolytic and lecithinase activities of B. subtilis
B. subtilis natto and B. cereus (positive control) were
tested for haemolytic (Figure 5a and b) and lecithinase
(Figure 5c & d) activities on blood agar plates and nutrient agar, supplemented with 8% egg yolk emulsion.
Large and clear halo formation was seen around the colonies of B. cereus on both agars, thus confirming that
this bacterium exhibits haemolytic and lecithinase activities (Figure 5). In contrast, no halo formation was seen
around the colonies of B. subtilis natto, indicating that
this bacterium does not exhibit haemolytic or lecithinase
activity. The cell-free supernatants of both these bacteria
showed similar results.Discussion
The antifreeze activity of γ-PGA has been assessed
previously (Mitsuiki et al. 1998, Mizuno et al. 1997,Figure 5 Physiological analysis of the haemolytic and lecithinase acti
activity showing halo around the cells, b – B. subtilis natto haemolyti
lecithinase activity with halo indicating the presence of lecithinase ac
indicating the absence of lecithinase activity.Shih et al. 2003). However, it has never been used to
maintain and protect the viability of bacteria. This
study is the first to assess the effect of γ-PGA on the
viability of probiotic bacteria during freeze drying.
On conclusion of the cryoprotectant tests, it was seen
that L. paracasei was more resilience to the freeze drying
process than both the Bifidobacterium strains under
study, since there was only 1.34 log CFU/ml reduction
in viability of unprotected L. paracasei when they were
freeze dried, whereas the unprotected Bifidobacterium
strains showed a reduction in viability of around 2.5 log
CFU/ml (Figure 2). Wang et al. (2004) found that
Streptococcus thermophilus and Lactobacillus acidophilus exhibited greater survival during freeze drying than
did B. longum and B. infantis (Wang et al. 2004). In contrast, Heidebach et al. (2010) found that probiotic
Bifidobacterium Bb12 survived better than Lactobacillus
F19 (Heidebach et al. 2010). Otero et al. (2007) studied
the effect of freeze drying on different Lactobacillus species and found considerable variation in survival between species and even strains of the same species
(Otero et al. 2007). Similar results have been found for
members of Bifidobacterium species and strains (Lian
et al. 2002). Based on the above, it could be concluded
that resistance to the freeze drying process varies between species and even strains of the same species. This
study demonstrates that L. paracasei is more resistant to
the freeze drying process than B. longum and B. breve.
The results for freeze drying with γ-PGA as a cryoprotectant showed that for L. paracasei, sterilised 10%
γ-PGA could protect the cells during freeze drying significantly better than 10% sucrose (P < 0.05). Although
5% γ-PGA was also able to protect the cells during
freeze drying as efficiently as sucrose (P > 0.05), it was
not as efficient as 10% γ-PGA. For B. longum and B.
breve, 10% sterilised γ-PGA and 10% sucrose were
equally efficient in maintaining viability during freeze
drying (P > 0.05).
It was also observed that the sterilised γ-PGA
(obtained by autoclaving an aqueous solution of γ-PGA)vities of B. subtilis natto and B. cereus: a – B. cereus haemolytic
c activity with no halo observed around the cells, c: B. cereus
tion, d – B. subtilis natto lecithinase activity test without halo,
Bhat et al. AMB Express 2013, 3:36 Page 7 of 9
http://www.amb-express.com/content/3/1/36was a better cryoprotectant than unsterilised polymer
which was initially used for preliminary cryoprotection
analysis (results not shown). It has been shown that
heating an aqueous solution of γ-PGA can reduce its
molecular weight (Goto and Kunioka 1992). Previous
studies have also demonstrated that γ-PGA with a lower
molecular weight has a higher antifreeze activity than a
high molecular weight polymer (Mitsuiki et al. 1998,
Shih et al. 2003). This may explain why sterilising
γ-PGA enhanced its cryoprotectant ability. FTIR analysis
revealed that the structural integrity of the polymer
remained intact after steam sterilization (Figure 1). This
is in agreement with the study done by Goto and
Kunioka (1992) which suggested that the activation
energy of the polymer chain scission due to steam
sterilization of γ-PGA by heating is approximately
120 kJ/M (Goto and Kunioka 1992). The relative bond
strengths of the C-C, C-N and C-O bonds are greater
than 300 kJ/M, hence, the breaking of the bonds by
heating at 110°C is unexpected.
It has been found that sucrose offers better protection during freeze drying of lactobacilli when compared
to trehalose and sorbitol (Siaterlis et al. 2009). Since γPGA could protect lactobacilli better than sucrose in
this study, it could indicate that γ-PGA is a better cryoprotectant than trehalose and sorbitol as well. Nata, a
bacterial cellulose produced by Acetobacter xylinum,
has also been used to protect different lactobacilli during freeze drying (Jagannath et al. 2010). When the
cells were immobilized using nata and freeze dried, it
was observed that viable cell number decreased from
109 – 1010 CFU/g to 107 CFU/g. In this study, there
was only a 0.51 log CFU/ml and 1.3 log CFU/ml reduction in the viability of γ-PGA-protected Lactobacilli
and Bifidobacteria respectively (Figure 2), indicating
that γ-PGA may be a better cryoprotectant compared
to nata. However, the protection offered by γ-PGA, trehalose, sorbitol and nata during freeze drying of Lactobacilli needs to be directly compared under identical
conditions.
Following the discovery and use of B. subtilis natto
in the solid state fermentation of soybean to produce
the common Japanese food natto, there has been a
surge in the industrial application of Bacillus sp. for
the production of a wide range of useful products
(Schallmey et al. 2004). However the detection of toxins
and the genes that produce them in some strains of Bacillus
(Beattie and Williams 1999, Phelps and McKillip 2002)
has questioned the use of Bacillus sp. in industrial
production of several products, especially food and
health products. There have been reports where the
production of emetic toxins by different Bacillus sp.,
including B. subtilis isolated from food, water and
food plants has been demonstrated (From et al. 2005).However, it is important to note that the authors concluded that the tendency of toxin production in strains
of Bacillus other than B. cereus isolated from food,
water and food plants is rare. The occurrence of genes
capable of producing toxins in other strains of Bacillus
sp. has already prompted the Scientific Committee on
Animal Nutrition (SCAN) to recommend that products from Bacillus sp. other than those from B. cereus
group should be accepted only if there is no detection of toxin production (SCAN 2000). For other
products of Bacillus sp. which do not include the
whole microorganism, it has been recommended that
the producing strain should be shown not to produce toxins under production conditions. Therefore,
to make γ-PGA applicable in the probiotic food industry as a cryoprotectant, it was crucial to analyse
the γ-PGA producing strain for the presence of
genes that are known to produce toxins. Moreover,
there is presently no report on the toxigenic potential of Bacillus subtilis natto, which was used for the
production of γ-PGA in our study.
Therefore, the toxigenic potential of B. subtilis natto
was assessed. The results of the toxigenic analysis in this
study showed that none of the six genes known to produce toxins (hbl D/A, nheB, bceT, entFM, sph, piplc)
were present in B. subtilis natto. Also, physiological analysis showed the absence of haemolytic and lecithinase
activities in B. subtilis natto. In contrast, the positive
control B. cereus showed the presence of four of the
genes (nheB, entFM, sph, piplc) and also exhibited both
haemolytic and lecithinase activities. These results are
similar to those obtained by (Matarante et al. 2004) who
reported the absence of the genes (hbl D/A, nheB, bceT,
entFM, sph and piplc) in all the strains of Bacillus sp.
isolated from sausages and more importantly in B.
subtilis. Our results are in accordance with another
study that investigated the cytotoxic potential of other
strains of B. subtilis, B. licheniformis, B. cereus and B.
amyloliquefaciens used in industrial production of enzyme products and discovered that none of the industrial strains demonstrated any in vitro cytotoxic activity
(Pedersen et al. 2002).
In conclusion, this study has established that γ-PGA
could be used successfully to improve the survival of
probiotic bacteria when during freeze drying. Three
probiotic bacteria (L. paracasei, B. longum, B. breve)
were successfully protected with γ-PGA when they
were freeze dried. It was also seen that Lactobacillus
was more resistant to the freeze drying process than
Bifidobacteria. While choosing an agent to protect probiotic bacteria, it is essential to choose one that protects
them during the different stages of their production and
upon incorporation into food products. Therefore, future
work will involve studying the ability of γ-PGA to
Bhat et al. AMB Express 2013, 3:36 Page 8 of 9
http://www.amb-express.com/content/3/1/36protect probiotic bacteria during storage in a foodstuff
and during passage through the gastrointestinal tract.
For γ-PGA to be used in health and food products, it
is important to analyse γ-PGA producing strains for the
presence of genes that are known to produce toxins.
This research successfully confirmed that B. subtilis
natto, the bacterium used to produce γ-PGA for the
novel application, does not contain any of the genes that
are usually responsible for toxin production. In addition,
the absence of haemolytic or lecithinase activity in B.
subtilis natto was also demonstrated.
Therefore, this study suggests that γ-PGA could be
used as a food ingredient for the delivery of probiotic
bacteria.
Competing interests
The authors declare that they have no competing interests.
Acknowledgments
Many thanks to the University of Wolverhampton, School of Applied
Sciences for their financial support and granting access to their laboratory
facilities, without which this research could not have been carried out.
Author details
1University of Wolverhampton, Wolverhampton, UK. 2University of Reading,
Reading, UK.
Received: 7 June 2013 Accepted: 3 July 2013
Published: 5 July 2013
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doi:10.1186/2191-0855-3-36
Cite this article as: Bhat et al.: Bacillus subtilis natto: a non-toxic source
of poly-γ-glutamic acid that could be used as a cryoprotectant for
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