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Infection and Immunity, January 2003, p. 584-587, Vol. 71, No. 1
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.1.584-587.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Biodefense-Driven Murine Model of Pneumonic Melioidosis
J. A. Jeddeloh,* D. L. Fritz, D. M. Waag, J. M. Hartings, and G. P. Andrews
Bacteriology Division, U.S. Army Medical Research Institute of Infectious Diseases, Fort Detrick, Maryland 21702-5011
Received 5 August 2002/
Returned for modification 10 September 2002/
Accepted 2 October 2002

ABSTRACT
A whole-body mouse model of pneumonic melioidosis was established
for future evaluation of biodefense vaccine candidates. The
aerosol 50% lethal doses of
Burkholderia pseudomallei strain
1026b for BALB/c and C57BL/6 mice and the times to death, dissemination
in organs, and tissue loads after exposure of the mice to low-
and high-dose aerosols are reported. In addition,
rpsL mutant
backgrounds were attenuated in this acute model of disease.

TEXT
The etiologic agent of human and animal melioidosis is
Burkholderia pseudomallei (
5,
17). Both
B. pseudomallei and its close species
relative
Burkholderia mallei are microbes recognized as having
the potential for misuse as biological weapons (
1). Typically,
signs of pneumonia in patients with clinical melioidosis are
a secondary result of sepsis (
5). Presumably, victims suffering
from a biological attack that employed
B. pseudomallei or
B. mallei would present clinically with pneumonic disease as the
initial aspect. In order to develop a vaccine program for defending
U.S. military forces against a potential
B. mallei or
B. pseudomallei attack, whole-body aerosol laboratory models of disease were
established. With these murine melioidosis models, future vaccine
candidates can be evaluated and tested for efficacy.
B. pseudomallei strain 1026b was selected because of its ease of genetic manipulation, its ability to replace genes, and its previously characterized virulence (6, 17). The 50% lethal dose (LD50) of strain 1026b for hamsters is <10 bacteria by intraperitoneal (i.p.) administration. Deleting the multidrug efflux pump encoded by the amrR, amrA, amrB, and oprA genes yields the aminoglycoside- and macrolide-sensitive strain DD503 (12). This strain retains a streptomycin resistance phenotype by virtue of a chromosomal mutation in the rpsL gene (12). rpsL encodes a ribosomal-subunit protein which regulates translational fidelity (3). The deletion of the multidrug efflux pump combined with an rpsL mutation generates the correct genotype for facilitating streptomycin-mediated gene replacement. There is no difference in the relative levels of virulence displayed by strains 1026b and DD503 in the Syrian hamster model of acute melioidosis (12).
Recently, a mucosal model of melioidosis which demonstrated that BALB/c and C57BL/6 mice display very different disease outcomes was reported (11). The BALB/c mice succumbed to an acute infection with an altered cytokine response, unlike the more resistant C57BL/6 mice. Because we also found that C57BL/6 mice are more resistant to B. mallei infections when the organisms are administered i.p., we thought it important to explore any correlation within an inhalation disease model (7).
Bacterial strains used in this study are described in Table 1. The whole-body aerosols were generated by nebulization of Luria broth-diluted overnight cultures in a modified Henderson apparatus by a previously described methodology (4, 16). Groups of 10 6- to 8-week-old female mice were exposed to the whole-body aerosol for 10 min. The aerosol concentration in the chamber was calculated on the basis of the number of CFU recovered from an integrated exposure chamber sample and on the exposure time. The dose inhaled was then derived on the basis of the respiratory minute volume of 6- to 8-week-old mice as estimated from the average weight of the animals (4, 8, 16).
Table
2 depicts the average results of two LD
50 determinations
for respiratory exposure to
B. pseudomallei strains 1026b and
DD503, with BALB/c and C57BL/6 mice as the hosts. In each repetition,
five groups of 10 mice were exposed to five different aerosols,
each containing an increasing density of
B. pseudomallei. The
targeted delivery doses were 10, 100, 1,000, 10,000, and 100,000
CFU. Actual dose groups received approximately 5, 50, 500, 5,000,
and 50,000 CFU. BALB/c and C57BL/6 mice were sensitive to aerosols
containing wild-type strain 1026b (
13). The LD
50 was calculated
for each experiment with the Reed-Muench method (
14), and the
means (± standard deviations) are reported in Table
2.
The LD
50 of 1026b was approximately 10 ± 8 CFU when it
was inhaled by a BALB/c mouse and 27 ± 20 CFU when it
was inhaled by a C57BL/6 mouse. The virulence of DD503 was attenuated
relative to that of its parent; the LD
50 of DD503 was 1,467
± 301 CFU in BALB/c mice. At equivalent doses, 1026b
killed almost 2 days faster than DD503 (data not shown).
Studies by Liu et al. showed that the genetic background of
a mouse played a major role in determining disease outcome (
11).
C57BL/6 mice exhibited approximately 2-log-greater resistance
to intranasally delivered
B. pseudomallei than BALB/c mice (
11).
Our observations suggest that both types of mice displayed similar
sensitivities to nebulized
B. pseudomallei. We do not believe
that the discrepancy in outcomes is solely attributable to our
strain selection or to the C57BL/6 genetic background of our
mice. Early work by the Naval Research Office (
13) and our collaborators
at Defence Sciences Technology Laboratories (Porton Down, United
Kingdom) found that BALB/c animals display similar sensitivities
to different strains of nebulized
B. pseudomallei (M. S. Lever,
personal communication). We believe it more likely that the
great differences in levels of virulence observed by Liu et
al. were probably due to the method of intranasal delivery employed.
The large differences reported are similar to the differences
seen between i.p. and intravenous challenge models reported
by other groups (
2,
10,
13,
15).
Figure 1 depicts the results from a serial sacrifice experiment after delivery of a high-dose (
1,000 LD50s) aerosol to BALB/c mice. Twenty-five mice were exposed to whole-body aerosols containing B. pseudomallei 1026b or DD503. At 24-h intervals, five animals were humanely sacrificed, four had their organs harvested for tissue load determination, and the final mouse was preserved in formalin for future histology studies. The numbers of deaths by day for strain 1026b were 0 deaths on day 1, 10 deaths on day 2, and 4 deaths on day 3. Strain DD503 performed differently, yielding 0 deaths on days 1 through 3 and 5 deaths on day 4. Neither of these counts reflects the number of animals sacrificed. Interestingly, both B. pseudomallei 1026b and DD503 proliferated with similar loads and kinetics in all the tissues examined, yet DD503 exhibited reduced overall virulence (Fig. 1 and Table 2). The difference in virulence appeared to manifest itself as a difference in the time to death. In this experiment, the difference in virulence observed was not likely attributable to the twofold difference in delivered dose alone. Twofold differences in delivered doses were frequently observed during our analysis. In four separate experiments where nearly identical doses were delivered, including the two LD50 determinations, DD503 repeatedly killed more slowly and required more CFU to kill than 1026b (Table 2 and data not shown).
Given the high death rate after the high-dose challenge experiment,
we repeated the preceding experiment with modified parameters.
We lowered the inhaled dose of strain 1026b, selected a time
point on the day of exposure, and isolated blood at every time
point to analyze the rate of sepsis. The results of this challenge
experiment are depicted in Fig.
2. The lower-exposure dose allowed
the animals to survive until day 4, making our later time point
data more representative. The calculated inhaled dose of strain
1026b was 260 CFU (

50 LD
50s). The death-by-day data were as
follows: 0 animals died on days 1 to 3, and 4 of the remaining
5 animals died on day 4. The data for day 4 shown in Fig.
3 are based upon the sole survivor. Two hundred sixty CFU of DD503
delivered in parallel to two groups of animals killed only 1
of 20 animals by day 5. The experiment was terminated at day
21 by humane euthanasia; no additional deaths were observed
during the 3-week period (data not shown). The tissue proliferation
data in Fig.
2 were similar to the data for the higher-dose
challenge. Curiously, the bacteria were trafficked to the spleen
and liver very quickly (within 6 h). The hepatic and spleenic
tropisms that we observed matched those in previous reports
of clinical disease (
5). The organ most heavily infected by
both low-dose and high-dose exposures was the lungs (Fig.
1 and
2). The tissue proliferation of DD503 was not examined for
the low-dose exposure.
Wild-type
B. mallei is sensitive to both aminoglycoside and
macrolide antibiotics (
9), and its genome lacks the
amrRAB oprA-encoded
multidrug efflux pump (The Institute for Genomic Research).
We wondered if the difference in virulence exhibited by
B. pseudomallei relative to that of
B. mallei (Table
2) could be explained by
the loss of the multidrug efflux pump. To discriminate between
the multidrug efflux pump's effect upon pneumonic virulence
and that of the
rpsL mutation also present in DD503, we obtained
the allelic series generated by Moore et al. (
12) and then we
exposed mice to a low-dose aerosol of each mutant and its 1026b
isogenic parent. Strain RM102 is an aminoglycoside-sensitive
1026b mutant, by virtue of a Tn
5 insertion into the
amrB gene.
RM102 SmR is a streptomycin-resistant spontaneous mutant derived
from RM102. If loss of the multidrug efflux pump attenuates
the virulence of DD503, then we would expect RM102 to be as
attenuated as DD503. If, however, the
rpsL mutation in each
background attenuates the strain, then RM102 SmR should be similar
in virulence to DD503. The results are depicted in Fig.
3.
The multidrug efflux pump played almost no role in pneumonic disease; rather, the rpsL mutation present in DD503 must have been largely responsible for its attenuation (Fig. 3). Mutations in rpsL have attenuated another gram-negative pathogen by enforcing an increase in translational fidelity (3). The i.p. hamster model must not place the same selective criteria upon the bacteria as the aerosol mouse model because, in hamsters, the LD50 of strain DD503 is indistinguishable from that of strain 1026b (a difference in LD50s of <10 by i.p. administration) (12). The different outcome reported may be due to many differences between the experiments. Simple explanations involve differences in the types of host (i.e., hamsters versus mice) or differences in the methods of administration to the tissues (i.e., i.p. versus nebulization). Alternatively, the differences in results may be attributable to differences in the targeted tissues themselves (i.e., peritoneal cavity versus lung epithelium). Because our goal was satisfied by establishing a pneumonic disease model and by characterizing the performance of available molecular genetic tools within the model, we decided against further exploration of the attenuated virulence displayed by rpsL mutants.
The important findings of this work are both that mice are sensitive to aerosols containing B. pseudomallei and that the mouse genetic background played a small role in host protection from inhaled B. pseudomallei, contrary to the results in a previous report (11). Further, our work suggests that, in future mutational analyses and vaccine studies, it is important to consider the rpsL genotype and the use of streptomycin-mediated gene replacement in B. pseudomallei and B. mallei.

ACKNOWLEDGMENTS
We gratefully acknowledge the work of an extraordinary technical
staff including A. L. O'Quinn, M. England, A. Bassett, and M.
West. D. DeShazer, E. Wargo, M. L. Dautle, and D. E. Woods provided
critical comments. R. Moore and D. E. Woods graciously provided
both strains and insight. We thank T. P. Atikins for 2 weeks
of hot lab work. Statistical analyses for LD
50 determinations
were provided by P. Gibbs.
This work was supported by funds from the U.S. Army Medical Research and Materiel Command (projects 02-4-5x-027 and 02-4-5x-030).

FOOTNOTES
* Corresponding author. Present address: Orion Genomics, LLC, Center for Emerging Technologies, 4041 Forest Park, St. Louis, MO 63108. Phone: (314) 615-6382. Fax: (314) 615-6975. E-mail:
jjeddeloh{at}oriongenomics.com.

Editor: D. L. Burns

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Infection and Immunity, January 2003, p. 584-587, Vol. 71, No. 1
0019-9567/03/$08.00+0 DOI: 10.1128/IAI.71.1.584-587.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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