SHORT COMMUNICATION
An opportunistic pathogen isolated from the gut of
an obese human causes obesity in germfree mice
Na Fei1 and Liping Zhao1,2
1State Key Laboratory of Microbial Metabolism and School of Life Sciences and Biotechnology, Shanghai Jiao
Tong University, Shanghai, China and 2Shanghai Centre for Systems Biomedicine, Shanghai Jiao Tong
University, Shanghai, China
Lipopolysaccharide endotoxin is the only known bacterial product which, when subcutaneously
infused into mice in its purified form, can induce obesity and insulin resistance via an inflammation-
mediated pathway. Here we show that one endotoxin-producing bacterium isolated from a morbidly
obese human’s gut induced obesity and insulin resistance in germfree mice. The endotoxin-
producing Enterobacter decreased in relative abundance from 35% of the volunteer’s gut bacteria to
non-detectable, during which time the volunteer lost 51.4 kg of 174.8 kg initial weight and recovered
from hyperglycemia and hypertension after 23 weeks on a diet of whole grains, traditional Chinese
medicinal foods and prebiotics. A decreased abundance of endotoxin biosynthetic genes in the
gut of the volunteer was correlated with a decreased circulating endotoxin load and alleviated
inflammation. Mono-association of germfree C57BL/6J mice with strain Enterobacter cloacae B29
isolated from the volunteer’s gut induced fully developed obesity and insulin resistance on a high-
fat diet but not on normal chow diet, whereas the germfree control mice on a high-fat diet did not
exhibit the same disease phenotypes. The Enterobacter-induced obese mice showed increased
serum endotoxin load and aggravated inflammatory conditions. The obesity-inducing capacity of
this human-derived endotoxin producer in gnotobiotic mice suggests that it may causatively
contribute to the development of obesity in its human host.
The ISME Journal advance online publication, 13 December 2012; doi:10.1038/ismej.2012.153
Subject Category: microbe-microbe and microbe-host interactions
Keywords: gut microbiota; germfree mice; endotoxin-producing bacterium; obesity; insulin resistance;
high-fat diet
The role of the gut microbiota in the pathogenesis of
obesity has emerged into an important research area
(Backhed et al., 2004). Gram-negative opportunistic
pathogens in the gut may be pivotal in obesity
(Schumann et al., 1990; Zhang et al., 2010, 2012).
Lipopolysaccharide (LPS) endotoxin purified from
Escherichia coli induced obese and insulin-resistant
phenotypes when subcutaneously infused into mice
at a concentration comparable to what can be found
in a mouse model of high-fat diet (HFD)-induced
obesity (Cani et al., 2007). Endotoxin-induced
inflammation seems to be essential for the develop-
ment of obese and insulin-resistant phenotypes in
the mouse model involving LPS infusion, as CD14-
knockout mice did not develop these phenotypes
after endotoxin infusion (Cani et al., 2007).
Epidemiological studies show increased population
of endotoxin producers and elevated endotoxin load
in various obese cohorts (Lepper et al., 2007; Ruiz
et al., 2007; Moreno-Navarrete et al., 2011), but
experimental evidence of endotoxin producers hav-
ing a causative role in human obesity is lacking.
During our clinical studies, we found that Enter-
obacter, a genus of opportunistic, endotoxin-
producing pathogens (Sanders and Sanders, 1997),
made up 35% of the gut bacteria in a morbidly obese
volunteer (weight 174.8 kg, body mass index
58.8 kgm� 2) suffering from diabetes, hypertension
and other serious metabolic deteriorations (Table 1).
The volunteer lost 30.1 kg after 9 weeks, and 51.4 kg
after 23 weeks, on a diet composed of whole
grains, traditional Chinese medicinal foods and
prebiotics (WTP diet, Supplementary Information;
Supplementary Figure 1), with continued ameliora-
tion of hyperinsulinemia, hyperglycemia and hyper-
tension until most metabolic parameters improved
to normal ranges (Table 1). After 9 weeks on the
WTP diet, this Enterobacter population in the
volunteer’s gut reduced to 1.8%, and became
undetectable by the end of the 23-week trial, as
Correspondence: L Zhao, State Key Laboratory of Systems
Biomedicine, Shanghai Centre for Systems Biomedicine, Shang-
hai Jiao Tong University, Room 3-517, Biology Building, 800
Dongchuan Road, Minhang Campus, Shanghai 200240, China.
E-mail: lpzhao3517@gmail.com or lpzhao@sjtu.edu.cn
Received 1 August 2012; revised 24 October 2012; accepted 28
October 2012
The ISME Journal (2012), 1–5
& 2012 International Society for Microbial Ecology All rights reserved 1751-7362/12
www.nature.com/ismej
shown in the clone library analysis (Table 1;
Supplementary Figures 2 and 3). The serum–
endotoxin load, measured as LPS-binding protein
(Schumann et al., 1990), dropped markedly during
weight loss, along with substantial improvement of
inflammation, decreased level of interleukin-6 and
increased adiponectin (Table 1). Metagenomic
sequencing of the volunteer’s fecal samples at 0, 9
and 23 weeks on the WTP diet confirmed that
during weight loss, the Enterobacteriaceae family
was the most significantly reduced population
(Supplementary Figure 4). The abundance of 25
KEGG Orthologies involved in the LPS biosynthetic
pathway diminished considerably, together indicat-
ing a significant reduction of the endotoxin-produ-
cing capacity of the volunteer’s gut microbiota after
the intervention (Supplementary Figures 5–7). In
light of previous reports of the pivotal role that
endotoxins have in metabolic diseases in mice (Cani
et al., 2007), we hypothesized that this endotoxin-
producing Enterobacter population may have a
causative role in the metabolic deteriorations of its
human host. To confirm the causative role it may
have in obesity development, we confirm Koch’s
postulate in an experimental host with an isolated
strain of this Enterobacter population (Evans, 1976).
We then obtained one clinical isolate (B29) from
the volunteer’s fecal sample via a ‘sequence-guided
isolation’ scheme (Rappe´ et al., 2002; Supple-
mentary Figure 8), and identified it as Enterobacter
cloacae through biochemical tests and 16S riboso-
mal RNA gene sequencing (Supplementary Table 1).
We performed whole-genome sequencing on B29,
and phylogenetic analysis using CVTree (Qi et al.,
2004) and identified its nearest neighbor as E.
cloacae subsp. cloacae ATCC 13047 (Supplemen-
tary Information). A limulus amebocyte lysate test
showed that B29 LPS has strong endotoxin activity
(Supplementary Figure 9), and the draft genome
sequence revealed LPS biosynthesis genes similar to
those in the metagenome from the day 0 fecal
sample (Supplementary Figure 10).
Previous studies show that germfree mice are
resistant to HFD-induced obesity (Backhed et al.,
2007; Ding et al., 2010; Rabot et al., 2010). To test
whether B29 can overcome this resistance to obesity
by colonizing the gut of germfree mice (Supple-
mentary Figure 11), we inoculated 1010 cells of B29
every day for the first week into 6- to 10-week-old
germfree C57BL/6J mice (n¼ 7 per group) under
either normal chow diet (NCD) or HFD. We observed
a slight body weight reduction among the mice
during the inoculation period (Supplementary
Figures 11–14). One mouse in each group died
during inoculation because of the translocation of
B29 into various organs (Sanders and Sanders, 1997;
Supplementary Table 2). After the first week, the
HFD-fed gnotobiotic mice inoculated with B29
(HFDþB29) showed a steady weight gain until
eventually reaching an obese state comparable to
that of the HFD-fed conventional mice (n¼ 8 per
group; Figures1a–c; Supplementary Figures 14–17).
The excessive fat accumulation in the HFDþB29
gnotobiotic mice was associated with an altered
lipometabolism including a leptin-resistant pheno-
type, reduced expression of fasting-induced adipose
factor in the ileum, and increased expression of
acetyl-CoA carboxylase 1, fatty acid synthase and
peroxisome proliferator-activated receptor-gamma
genes in the liver (Supplementary Figures 18–19;
Backhed et al., 2004, 2007). The HFDþB29 gnoto-
biotic mice developed the most significant insulin-
resistant phenotype as shown in the oral glucose
tolerance test and 2h post load insulin levels at the
end of the trial (Figures 1d and e). This group also
had the greatest increases in liver and spleen
weights and the greatest decrease in cecum weight
(Supplementary Table 3). The NCD-fed mice inocu-
lated with either B29 (NCDþB29) or Luria–Bertani
(LB) medium (NCDþLB) both remained lean
throughout the trial (Figures 1a–c). The HFD-fed
germfree mice inoculated with LB (HFDþLB)
experienced significant weight gain over the first 9
weeks but eventually became no different, based on
Table 1 Changes of endotoxin load, inflammation indicators,
metabolic phenotypes and the gut microbiota during weight loss
of a morbidly obese volunteer
Measurements Day 0 9
Weeks
23
Weeks
Reference
range
Body weight (kg) 174.8 144.8 123.5 —
BMI (kgm�2) 58.78 48.66 41.50 18–23
SBP (mmHg) 150 120 120 p140
DBP (mmHg) 110 80 75 p90
Triglycerides (mmol l�1) 2.68 1.72 1.18 0–1.7
Total cholesterol
(mmol l�1)
5.53 4.64 4.78 3.00–5.17
HDL cholesterol
(mmol l�1)
0.89 0.70 0.82 40.91
LDL cholesterol
(mmol l�1)
3.42 3.15 3.42 0–4.16
Fasting plasma glucose
(mmol l�1)
8.95 4.76 5.40 3.90–6.10
Fasting plasma insulin
(mIUml�1)
58.7 25.8 23.0 6–27
HbA1c (%) 7.58 5.44 4.52 3.8–5.8
AST (U l�1) 122 51 31 10–47
ALT (U l�1) 97 50 33 0–41
GGT (U l�1) 168 49 59 0–56
LBP (mgml� 1) 7.03 2.29 4.78 —
C-reactive protein (mg l�1) 14.1 9.4 9.51 0–10
IL-6 (pgml� 1) 6.71 4.46 2.76 —
Adiponectin (mgml�1) 2.00 2.09 4.27 —
Enterobactera 34.98% 1.77% 0%
Enterobacteriaceaeb 13.23% 0.45% 0.32%
Abbreviations: AST, aspartate aminotransferase; ALT, alanine
aminotransferase; BMI, body mass index; DBP, diastolic blood
pressure; GGT, gamm-glutamyl transferase; HbA1c, glycated
hemoglobin; HDL, high-density lipoprotein cholesterol; IL-6,
interleukin-6; LBP, lipopolysaccharide-binding protein; LDL,
low-density lipoprotein cholesterol; SBP, systolic blood pressure.
aBased on the near full-length 16S ribosomal RNA gene sequence
clone libraries of the fecal microbiota.
bBased on the metagenomic sequencing analysis of the fecal
microbiota.
Human pathobiont causes obesity in germfree mice
N Fei and L Zhao
2
The ISME Journal
the obesity parameters tested, from the NCD-fed
groups by the end of the 16-week trial, except for
a moderately increased epididymal fat pad and a
low level of insulin resistance (Supplementary
Figure 14; Figures 1b and d). Our repeat of
the animal test with HFD-fed gnotobiotic mice
mono-associated with B29 confirmed that a single
endotoxin producer such as B29 can function in the
capacity of the whole microbiota for inducing obese
and insulin-resistant phenotypes (Supplementary
Figure 20). Inoculating 6- to 10-week-old germfree
mice (n¼ 4–6 per group) with a strain of
Figure 1 Gnotobiotic mice mono-associated with E. cloacae B29 become obese and insulin resistant with increased endotoxin load and
provoked systemic inflammation under HFD feeding (data collected at the end of 16 weeks after inoculation). (a) Body weight; (b) mass of
epididymal, mesenteric, subcutaneous inguinal and retroperitoneal fat pad; (c) abdominal photographs; (d) oral glucose tolerance test
(OGTT) and the areas under the curve (AUC) for the plasma glucose; (e) serum 2h post load insulin; (f) enzyme-linked immunosorbent assay
(ELISA) analysis of serum LPS-binding protein (LBP); (g) serum amyloid A (SAA); and (h) adiponectin corrected for bodyweight. The two-
way analysis of variance (ANOVA) revealed a significant effect of the diet (Po0.01), a significant effect of B29 (Po0.01) and a significant
diet�B29 interaction effect (Po0.01) on body weight, mass of epididymal, mesenteric, subcutaneous inguinal and retroperitoneal fat pad,
serum LBP; a significant effect of the diet (Po0.01) and a significant effect of B29 (Po0.01) on OGTT; a significant effect of B29 (Po0.05) on
serum 2h post load insulin. Data are shown as means±s.e.m. (n¼ 6). NS, no significant difference; *Po0.05; **Po0.01. Color code for
animal groups: NCDþLB, blue slash; NCDþB29, blue; HFDþLB, red slash; HFDþB29, red. LB, Luria–Bertani medium.
Human pathobiont causes obesity in germfree mice
N Fei and L Zhao
3
The ISME Journal
Bifidobacterium animals via alternation of NCD and
HFD feeding did not induce the same obese
phenotype (Supplementary Figure 21), suggesting
that obesity cannot be induced by introducing any
bacteria in the germfree mice under HFD feeding.
A slightly increased endotoxin load can induce a
low-grade, chronic inflammation as a driving force
for insulin resistance and altered lipometabolism in
mice (Hotamisligil et al., 1996; Cani et al., 2007). The
serum LPS-binding protein was significantly higher
in the HFDþB29 gnotobiotic mice than in the
NCDþB29 gnotobiotic mice (Figure 1f), despite the
fact that B29 reached a significantly greater popula-
tion size in the gut of the NCD-fed gnotobiotic mice
(Supplementary Figure 13). As B29 was the only LPS
producer in the gnotobiotic-mouse gut (Supple-
mentary Figure 22), the increased serum–endotoxin
load in the HFDþB29 gnotobiotic mice could only
come from B29. As the gene expression levels of
the two tight junction proteins occludin and ZO-1
(Cani et al., 2008) in the ileum were not signifi-
cantly different among the groups (Supplementary
Figure 23), the high amount of endotoxin transloca-
tion from the gut to the serum in the HFDþB29
gnotobiotic mice may be facilitated by chylomicrons
induced by long-chain fatty acids in the HFD (Cani
et al., 2007; Ghoshal et al., 2009), rather than by
impaired gut barrier function (Cani et al., 2007;
Zhang et al., 2010, 2012). In accordance with the
increased endotoxin load, the HFDþB29 gnotobiotic
mice had the greatest increase in serum amyloid A
protein levels and the greatest decrease in adipo-
nectin secretion, suggesting that these mice had the
greatest increase in systemic inflammation (Figures
1g and h). The expression of the tumor necrosis
factor-alpha, interleukin-1b, interleukin-6, I kappa B
kinase epsilon and Toll-like receptor 4 pro-inflam-
matory genes increased significantly in the liver and
epididymal fat pad but not in the ileum of the HFDþ
B29 gnotobiotic mice (Supplementary Figure 24),
indicating local inflammation induced in the former
two tissues but not in the gut, in contrast to a
previous report (Ding et al., 2010). The HFDþLB
germfree mice had moderately higher levels of serum
serum amyloid A and liver tumor necrosis factor-
alpha expression than the NCD-fed groups, suggest-
ing that the HFD induced some host inflammation
(Tripathy et al., 2003), which is, however, much
lower than that induced by B29. Taken together, our
results suggest that endotoxin-induced inflammation
may have a pivotal role in obesity induced by
E. cloacae B29, supporting the existence of a
putative chain of causation from endotoxin produ-
cers in the gut to the obesity end points.
Germfree mice have been extensively used for
obesity studies. For example, Gordon et al. showed
that co-inoculation of germfree mice with the plant
polysaccharide-fermenting Bacteroides thetaiotao-
micron and the methane-producing Methanobrevi-
bacter smithii significantly increased the epididymal
fat pad but not the total bodyweight (Samuel and
Gordon, 2006). As a step forward, our study has
followed a procedure modified from Koch’s Postu-
lates (Evans, 1976) and, for the first time, established
a gnotobiotic-mouse obesity model combining HFD
with a human-originated endotoxin producer. This
work suggests that the overgrowth of an endotoxin-
producing gut bacterium is a contributing factor to,
rather than a consequence of, the metabolic dete-
riorations in its human host. In fact, this strain B29 is
probably not the only contributor to human obesity
in vivo, and its relative contribution needs to be
assessed. Nevertheless, by following the protocol
established in this study, we hope to identify more
such obesity-inducing bacteria from various human
populations, gain a better understanding of the
molecular mechanisms of their interactions with
other members of the gut microbiota, diet and host
for obesity, and develop new strategies for reducing
the devastating epidemic of metabolic diseases.
Acknowledgements
We appreciate Professor R. Losick, L Neuhauser, M Obin and
M Pop for critical reading of the manuscript and kind
suggestions. We are also grateful to the following individuals
for their kind assistance during the study: S Xiao, J Shen, X
Pang, M Zhang, XJ Zhang, Y Zhao, LWang, J Wang, Y Zhang,
G Wu, G Wang, H Ou, J Qi, JJ Wang, X Zhang, R Wang, M
Song, J Xu, H Tang, T Liu, Q Zhang, N Zhao, C Zhang, Y Fan,
S Liu, YZ Fan, T Wang, Z Hu, R Xi, XY Zhang, C Liu, H Wu,
X Guo, X Li, G Ning, S Yang and G Zhao.
This work was supported by Project 30730005 of the
National Nature Science Foundation of China (NSFC), 863
Projects 2008AA02Z315 and 2009AA02Z310, Key Projects
2007DFC30450 and 075407001 of International Coopera-
tion Program Grants and Project in the National Science
and Technology Pillar Program 2006BAI11B08.
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