Advances in Microbiology, 2012, 2, 382-387 Published Online September 2012 (
Bacillus pumilus: Possible Model for the Bioweapon
Bacillus anthracis
Shannon B. Murphy1, Merranda D. Holmes1, Stephen M. Wright2*
1East Tennessee State University, James H. Quillin College of Medicine, Johnson City, USA
2Department of Biology, Middle Tennessee State University, Murfreesboro, USA
Email: *
Received July 5, 2012; revised August 13, 2012; accepted August 20, 2012
The misuse of Bacillus anthra cis as a bioweapon continues to be a serious concern. Medical personnel and researchers
are served well if appropriate non-pathogenic anthrax simulants can be used as countermeasures in preparative planning.
While there are several accepted simulants of B. anthracis, the addition of another model organism would be beneficial.
This investigation was undertaken to evaluate the suitability of B. pumilus as a simulant for B. anth racis. All organisms
were grown on AK Agar #2 to foster sporulation. Optimum conditions for spore formation were determined for B.
pumilus as well as for currently used anthrax surrogates B. atrophaeus and B. thuringiensis. Spore dimensions were
determined by scanning electron microscopy. Comparative antibody binding studies using commercially available
anti-Bacillus antisera were completed with the simulants as well as with a negative control organism, Clostridium
sporogenes. We report that B. pumilus sporulated readily (2.9 × 1010 viable spores per plate), had appropriate spore size
(1.24 μm × 0.59 μm) and reactivity with anti-Bacillus antibodies. The characteristics of B. pumilus determined in this
study suggest this organism represents a novel, suitable model for B. anthracis.
Keywords: Bacillus pumilus; Bacillus anthracis; Anthrax Simulant; Bioweapon
1. Introduction
The intentional use of a biological agent resulting in
morbidity or mortality represents a real and on-going
threat [1]. Ba cillus anthracis is considered a leading can-
didate as a biological weapon [2]. One of the characteris-
tics of B. anthracis that makes it an attractive bioweapon
is its ability to form spores in harsh environments and
subsequently germinate when conditions become favor-
able. Survivability of dormant spores is remarkable.
Spores of B. anthracis recovered from dirt retained le-
thality in guinea pigs after 60 years [3].
Exposure to aerosolized B. anthracis spores could re-
sult in inhalational anthrax, a disease that approaches
nearly universal mortality if left untreated [4]. The far-
reaching implication of bioterrorism involving anthrax in
the US was demonstrated by delivery of B. anthracis
spores through the mail, resulting in over 1000 individu-
als being considered at risk of exposure [5].
The seriousness of anthrax dictates that investigations
into new technologies for detection of anthrax spores
continue. Due to the virulent nature of B. anthracis, strict
adherence to safety protocols must be followed. Inactiva-
tion of spores from B. anthracis may result in alterations
in antigenic structures or changes in targeted nucleic acid
sequences that may affect detection [6]. To permit more
thorough investigation of possible detection methodolo-
gies, it is often desirable to use spores from other Bacil-
lus spp. that can serve as models for anthrax spores. The
Bacillus cereus group, a genetically closely related group
of Bacilli, is comprised of B. anthracis, B. cereus, B.
mycoides, B. thuringiensis and B. weihenstephanesis [7].
From this group, B. cereus and B. thuringiensis have
been used as surrogates for B. anthracis [8,9]. Other Ba-
cilli have also been used as simulants including B. atro-
phaeus and B. subtilis [10-12]. Ba cillus p umilus has been
phylogenetically placed within the branch for B. subtilis,
an accepted simulant, based on the sequencing of 16S
rDNA [13]. Several criteria have been suggested when
considering an appropriate organism to model B. an-
thracis including virulence, genetic and morphologic
similarity to B. anthracis, and how the simulant responds
to challenges from chemicals or the environment [14].
Bacillus anthracis has been well-characterized and spore
dimensions have been reported [11].
This investigation was undertaken to determine suit-
ability of B. pumilus as a simulant for B. anthracis. Ba-
cillus pumilus was chosen for study for several reasons,
including lack of virulence, ease of growth and sporula-
*Corresponding author.
opyright © 2012 SciRes. AiM
tion as well as its genetic relatedness to other model or-
ganisms. Since spore sizes for B. pumilus have not been
reported, this study is the first to provide spore dimen-
sions. Bacillus pumilus is an environmental organism and
has been investigated primarily for commercial applica-
tion of various enzymes it produces [15,16]. Compari-
sons were made with other currently used simulants, B.
atrophaeus and B. thuringiensis. Bacillus pumilus sporulates
readily, is non-pathogenic, has comparable spore size with
B. anthracis and reacts serologically with other simulants
suggesting it merits inclusion as another model organism
for B. anthracis.
2. Materials and Methods
2.1. Organisms and Spore Preparation
Organisms used in these studies included Bacillus atro-
phaeus (ATCC #9372), B. thuringiensis (ATCC #10792)
and B. pumilus (ATCC #700814). Clostridium sporo-
genes (ATCC #3584) was used as a negative control for
antibody binding studies. All organisms were inoculated
into 5 ml T-soy broth and incubated overnight at 35˚C.
Clostridium sporogen es was kept under anaerobic condi-
tions for all manipulations. One hundred μl of each broth
was spread onto nutrient-deficient AK Agar #2 plates
(Becton, Dickinson and Company, Sparks, MD), sealed
with parafilm and incubated at 30˚C. For B. atrophaeus,
B. pumilus and Cl. sporogenes, plates were evaluated
daily for sporulation through 10 d post-inoculation. Sam-
ples from B. thuringiensis were evaluated daily through
20 d post-inoculation. Optimum sporulation was consid-
ered to have occurred when 90% - 95% of organisms had
formed spores. Determination of sporulation was made
by Schaeffer-Fulton spore stain and light microscopy.
After appropriate incubation, 10 ml of cold phosphate
buffered saline was added to each plate for 5 min to loosen
spores. Spores were scraped off plates using a sterile loop.
The spore suspension was pelleted by centrifugation in a
15 ml centrifuge tube. The pellet was resuspended in 1 ml
of sterile deionized water (dH2O) and transferred to a 1.5
ml microfuge tube. Spores were washed at least five times
with dH2O. The washed spores were resuspended in 1 ml
of dH2O and heated in a heat block to 65˚C for 30 min to
destroy any vegetative cells. After a final wash, spores
were resuspended in 1 ml of dH2O and stored at 4˚C.
Spore counts were determined in two ways. Viable
counts were determined by spreading dilutions on T-soy
agar plates and counting colonies after overnight incuba-
tion. Total direct counts were done in a hemocytometer
using a phase-contrast microscope. All counts were done
at least three times and averages ± SE were determined.
2.2. Electron Microscopy
In preparation for electron microscopy, an aliquot of each
spore stock was washed three additional times in dH2O
and diluted in dH2O resulting in 10, 100 or 500 spores
μl–1. One μl of each dilution was placed on an aluminum
specimen stage and allowed to dry completely under a
laminar flow hood. The spores were coated with an 8 nm
layer of gold (Hummer 6.2 Sputter Coater, Ladd Re-
search, Williston, VT) prior to scanning in a Hitachi
SEM 3400 (Hitachi High-Technologies, Tokyo). An ac-
celerated voltage between 15 and 20 kV was applied. A
minimum of 100 spores from each organism were evalu-
ated. Image J software was used to measure spores [17].
2.3. Microarray Preparation and Detection
In order to evaluate multiple antibodies binding with
spores from different organisms, spore microarrays were
developed. In this process, spores from each Bacillus spp.
and Cl. sporogenes were applied to slides as an array.
Appropriate spore dilutions of each organism were pre-
pared in spotting solution (ArrayIt, Sunnyvale, CA) such
that 100 spores were applied in triplicate as a microarray
to an epoxy-coated slide. Microarrays were prepared
with a SMP8 pin guided by a SpotBot microarray robot
(ArrayIt). The microarray was allowed to air dry at room
temperature for at least two h. The slide was heated at
95˚C for 25 min to ensure fixation of the spore samples
to the slide. The slide was placed in a UV crosslinker
(Spectronics Corporation, Westbury, NY) for two cycles
(6500 μJ·cm2 – 1) to deactivate any remaining epoxy func-
tional groups. The slide was washed with 0.1% sodium
dodecyl sulfate (SDS) for 5 min followed by washing
with dH2O for 3 min. ArrayIt blocker was applied to the
edge of the slide and a coverslip was placed over it
yielding 1 µl blocker mm2 – 1 coverslip. The slide was
incubated at room temperature for one h, after which the
coverslip was removed for additional SDS and dH2O
washes and drying cycles performed in the same manner
as described above.
The primary monoclonal anti-B. anthracis was ob-
tained from Virostat (Portland, ME). All other primary
anti-Bacillus antibodies were purchased from Tetracore
(Rockville, MD). Secondary fluorescently labeled anti-
bodies were acquired from Jackson ImmunoResearch
Laboratories (West Grove, PA). Primary antibody was
diluted 1:50 in a 1:1 mixture of dH2O and blocker and
added to the array and covered with a coverslip as 1 μl
antibody dilution mm2 – 1 coverslip. The slide was incu-
bated at 35˚C for 30 min. Following incubation, the slide
was washed with 0.1% SDS and dH2O as before. The
secondary fluorescently labeled antibody was diluted
1:100 and applied, incubated and washed in identical
fashion as the primary antibody. The dry slide was ana-
lyzed for fluorescence using a confocal laser scanner
(Genetix Ltd., Queensway, Hampshire, UK).
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Copyright © 2012 S AiM
3. Results percent viability among the Bacilli was similar, ranging
from 59 - 64. Clostridium sporogenes spores exhibited a
lower viability at 36%.
3.1. Sporulation
AK Agar #2 proved to be an ideal medium to promote
sporulation for most organisms used in these studies.
Bacillus pumilus yielded the highest number of spores,
for both viable plate count (2.92 ± 0.19 × 1010) and total
direct count (4.58 ± 0.42 × 1010) (Table 1). Optimum
sporulation occurred on d 6 or 8 post-inoculation for B.
atrophaeus, B. pumilus and Cl. sporogenes. Bacillus
thuringiensis required 16 d post-inoculation to demon-
strate 90% sporulation. The number of spores produced
by B. thuringiensis was the lowest for all organisms (vi-
able: 6.98 ± 0.13 × 108, direct: 1.19 ± 0.11 × 109). The
3.2. SEM Spore Comparisons
Electron micrographs of each of the Bacilli are seen in
Figure 1. Spores of each Bacillus were measured using
Image J. Length and width dimensions are summarized
in Table 2. Table 2 also lists spore dimensions reported
by others. The values for each of the Bacilli investigated
in this study represent averages ± SE of a minimum of
100 spore measurements. Bacillus atrophaeus spores
were 1.314 ± 0.018 μm in length and 0.752 ± 0.013μm in
width. Bacillus pumilus spores were 1.242 ± 0.022 μm
Table 1. Time for optimum sporulation and enumeration of spores.
Organism Optimum Day Plate Count Total Count % Viability
B. atrophaeus 8 8.66 ± 0.29 × 109 1.37 ± 0.14 × 1010 63
B. pumilus 8 2.92 ± 0.19 × 1010 4.58 ± 0.42 × 1010 64
B. thuringiensis 16 6.98 ± 0.13 × 108 1.19 ± 0.11 × 109 59
Cl. sporogenes 6 1.02 ± 0.14 × 109 2.81 ± 0.14 × 109 36
Bacillus atrophaeus Bacillus pumilus
Bacillus thuringiensis
Figure 1. Scanning electron micrograph of Bacillus spp. spores. The image at the top left is B. atrophaeus. Spores from B.
pumilus are at the top right and B. thuringiensis is at the bottom.
Table 2. Comparison of Bacillus spore dimensions.
Source B. anthracis B. atrophaeus B. pumilus B. thuringiens is
Buhr [22] 1.21 × 0.68a
Carrera [11] <1.26 × 0.81 - 0.86
1.49 - 1.67 × 0.81 - 0.86 1.22 × 0.65 1.61 × 0.80
Fazzini [23] 1.63 × 0.97
Plomp [21] 1.68 × 0.65 2.17 × 0.94
This study 1.31 × 0.75 1.24 × 0.59 1.58 × 0.75
aDimensions are given as length × width, in μm.
in length by 0.594 ± 0.016 μm in width. Bacillus thur-
ingiensis spores were 1.580 ± 0.024 μm in length and
0.754 ± 0.022 μm in width.
3.3. Serologic Reactivity
When evaluating a potential anthrax simulant organism,
it is important to consider how the organism lends itself
to detection. Binding with antibody through immunoas-
says is a common approach for detection. Both poly-
clonal and monoclonal commercially available antibodies
were used to evaluate binding among the organisms in
this study. All polyclonal antisera bound with the Bacilli
under investigation (Table 3). No binding occurred be-
tween any anti-Bacillus antisera and the negative control
Cl. sporogenes. The monoclonal anti-B. anthracis was
highly specific and failed to bind with any Ba cilli used in
this study. Similarly, monoclonal anti-B. thuringiensis
was specific and bound only with B. thuringiensis. Ba-
cillus pumilus displayed reactivity patterns in almost
identical fashion as B. atrophaeus and even bound with
monoclonal anti-B. atrophaeus.
4. Discussion
It is likely that the potential for misuse of B. an thrac is as
a bioweapon will continue. This threat requires vigilance
on the part of medical and research personnel. Due to
biosafety and containment issues, it is highly desirable to
use organisms that can serve as model simulants for B.
anthracis as new detection technologies are developed.
Bacillus pumilus shows promise as a new anthrax surro-
AK Agar #2 was an ideal medium to promote sporula-
tion for B. atrophaeus, B. pumilus and Cl. sporogenes.
Bacillus pumilus produced the highest number of spores
of all organisms in this study. However, AK Agar was
not as effective at promoting spore formation for B.
thuringiensis. With this medium, B. thuringiensis yielded
the lowest number of spores and required 16 days to
achieve 90% sporulation. Successful sporulation of B.
thuringiensis has been reported using nutrient broth yeast
extract agar [9], T-soy agar supplemented with 5% sheep
blood [6] and sporulation medium S [18].
The percent viability exhibited by the Bacilli cultured
on T-soy agar following spore formation on AK Agar #2
was similar with an average just over 60%. In our hands,
Cl. sporogenes was nearly half that at 36%. Yang re-
ported [19] that the viability of Cl. sporogenes cultured
on T-soy was 30%.
Some variability is evident among reports of spore
sizes (Table 2). An explanation for this variation may be
the amount of water remaining in the spore during
preparation [20,21]. Additionally, the method used to de-
termine spore dimensions would impact measurements.
Spore dimensions reported by Buhr [22] were based on
phase microscopy. Scanning electron microscopy has
been used [23, this study] and Carrera [11] used trans-
mission electron microscopy. The measurements made
by Plomp [21], resulting in the largest reported spore
sizes, were based on atomic force microscopy. Spore
dimensions for B. atrophaeus in this study (1.31 × 0.75
μm) are in agreement with other reports [11,22] (1.22 ×
0.65 μm and 1.21 × 0.68 μm, respectively). Our meas-
urements for B. thuringiensis were also similar with di-
mensions reported by Carrera [11] (1.58 × 0.75 μm vs
1.61 × 0.80 μm). To our knowledge, this is the first re-
port of spore dimensions for B. pumilus. One criterion for
an appropriate simulant for B. anthracis is spore size [14].
Carrera reported [11] that spores of B. anthracis fall into
two size categories, a larger size of 1.49 - 1.67 μm × 0.81
- 0.86 μm and a smaller spore size of less than 1.26 μm ×
0.81 - 0.86 μm. We determined that B. pumilus spores
represent the smaller sized category with dimensions of
1.24 × 0.59 μm.
There are risks associated with working with B. an-
thracis and inactivation of spores by heat, chemicals or
radiation may alter antigenicity [6]. Many detection as-
says are immunologically centered. An inherent problem
with antibody based detection is the high level of cross-
reactivity that occurs among Bacilli, particularly when
using polyclonal antisera [24-26]. Such cross-reactivity
was evident in the current study (Table 3). Based on our
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Table 3. Anti-Bacillus antibody reactivity.
Antibody B. atrophaeus B. pumilus B. thuringiensis Cl. sporogenes
Polyclonal anti-B. atrophaeus +++a +++ ++
Monoclonal anti-B. atrophaeus +++ ++ + —
Polyclonal anti-B. thuringiensis ++ ++ +++ —
Monoclonal anti-B. thuringiensis — — ++ —
Polyclonal anti-B. anthracis + + ++ —
Monoclonal anti-B. anthracis — — — —
a+++ = strong binding, ++ = moderate binding, + = weak binding, — = no binding, as determined by fluorescence.
results of antibody binding, there are several noteworthy
observations. First, Cl. sporogenes appears to be an ap-
propriate negative control spore forming organism, help-
ing avoid false positive results. Secondly, immunoassays
used for development of anthrax spore detection must
employ specific monoclonal antibodies similar to those
used in the current study for B. thuringiensis and perhaps
B. anthracis. Finally, the reactivity patterns shown by B.
pumilus suggest it could serve as an appropriate model
organism, binding with antibody to B. atrophaeus in
nearly identical manner.
Bacillus pumilus also may represent a useful anthrax
simulant since it is rarely implicated as a cause of disease.
There is one report of B. pumilus being responsible for
cutaneous infection in humans [27]. In that report, the
authors noted that the lesions were similar to lesions that
occur during cutaneous anthrax infection. However, it
was also stated that infection of humans by B. pumilus
was “exceptional”. Bacillus pumilus seems to represent
an appropriate organism to model B. anthracis. It sporu-
lates to high titer readily, produces spores of similar size
as B. anthracis and may be considered a non-pathogenic
5. Acknowledgements
This work was supported, in part, by the Department of
Homeland Security through the Southeast Region Re-
search Initiative by contract number 4000086311.
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