3. Please cite this article in press as: Campbell, A.K., et al., Bacterial metabolic ‘toxins’: A new mechanism for lactose and food intolerance, and
irritable bowel syndrome. Toxicology (2010), doi:10.1016/j.tox.2010.09.001
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1. Lactose and food intolerance
Lactose and food intolerance, and irritable bowel syndrome,
cause a wide range of gut and systemic symptoms. These include:
gas, gut pain, bloating, borborygmi, diarrhoea or constipation,
headache, severe fatigue, cognitive dysfunction, muscle and joint
pain, heart palpitations, various allergies such as eczema and
urticaria, increased micturition, and infertility (Matthews and
Campbell, 2000; Campbell and Matthews, 2005a,b; Matthews et al.,
2005; Waud et al., 2008; Campbell et al., 2005, 2009). We propose
that these symptoms are caused by hydrogen and methane gas
and toxins, produced by bacteria in the large intestine when they
metabolise carbohydrates not digested fully in the small intestine
(Campbell et al., 2005, 2009). These carbohydrates include lactose,
sucrose, fructose, and starch from rice, potatoes, pasta, and flour. In
order to digest carbohydrates such as lactose, sucrose and starch,
these must first be degraded to monosaccharides, which can then
be absorbed. There are thus two major causes of lack of full diges-
tion of carbohydrates in the small intestine – deficiency in the
enzyme degrading the carbohydrate to monosaccharides, or defi-
ciency in the transporter enabling monosaccharides to be absorbed
into the blood.
All mammals, apart from white Northern Europeans and a few
races such as the Bedouins, start to lose the enzyme lactase-
phlorizin hydrolase (lactase for short) after weaning. Thus some
two thirds of the world’s adult population, about 4000 million
people, cannot digest lactose properly. Everyone can digest some
lactose, unless they have the very rare disorder of congenital lac-
tase deficiency. But those with a low lactase have threshold, which
if crossed results in gut and systemic symptoms. Lactase is unique in
having two active sites within the same polypeptide chain, and thus
has two enzyme commission numbers (EC 3.2.1 62 and 108). One
site cleaves lactose into its two constituent sugars – galactose and
glucose (Fig. 1), while the other, discovered because it cleaves the
diabetogenic compound phlorizin from apple bark, cleaves cere-
brosides, providing essential sphingosine for tissues such as the
brain. This is why we have to retain some to the enzyme after wean-
ing. Dairying is only 6000–8000 years old (Campbell and Matthews,
2005b; Campbell et al., 2005, 2009). So, in evolutionary terms, since
lactose is only found in significant quantities in mammalian milk,
there would otherwise be no need to retain lactase after weaning.
Thus, in order to digest cerebrosides, we all need some lactase, even
if our diet contains no lactose. After cleavage of lactose by lactase,
the galactose and glucose are absorbed through the sodium depen-
dent glucose transporter, SGLUT1. This is distinct from GLUT5,
responsible for absorbing fructose in fruits or formed from cleav-
age of sucrose (Fig. 1) by sucrase. Moreover, glucose and galactose
uptake through SGLUT1 is inhibited by certain non-metabolisable
tri- and tetra-saccharides, such as raffinose and stachyose (Fig. 1).
These are found in many root vegetables, pulses and beans, includ-
ing soya, and are the reason why large consumption of these during
a meal causes gas.
2. The bacterial metabolic toxin hypothesis
Carbohydrates not digested and absorbed in the small intestine
reach the large intestine, where there are over one hundred times
as many bacterial cells than cells in the rest of the body, approx-
imately 1 kg in weight, representing over 1000 species (Qin et al.,
2010). There is little oxygen here. Thus, in order to make ATP via
Fig. 1. The sugars.
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Fig. 2. Hypoxic bacterial metabolism and the production of metabolic toxins. Methylglyoxal is generated by glycolysis from dihydroxyacetone phosphate {Campbell et al.,
2007a,b; Kalapos, 1999}.
glycolysis, the bacteria have to get rid of the hydrogen in carbohy-
drates, not as H2O in aerobic metabolism, but rather as hydrogen
gas, and through a range of metabolites – alcohols, diols, aldehy-
des, ketones and acids (Fig. 2). Archaebacteria can convert the H2
to methane.
Our ‘bacterial toxin’ hypothesis proposes that these carbohy-
drate metabolites affect the balance of microflora in the large
intestine, through effects on gene expression and growth, and also
affect neurones, skeletal, smooth and cardiac myocytes, mast cells
and cells of the immune system, as well as several other cell types,
to cause the systemic symptoms we have identified in lactose and
food intolerance, and irritable bowel syndrome. The latter is the
most common referral to gastroenterologists. People suffering car-
bohydrate intolerance are often mistakenly diagnosed as intolerant
or allergic (coeliac disease) to foods such as flour containing gluten.
Most of the proteins in food responsible for the immune reaction in
coeliac disease are the prolamins. These are storage proteins, rich in
proline and glutamine, and are found in cereal grains. But each grain
type has a different, but related, prolamin; e.g. wheat has gliadin,
barley has hordein, rye has secalin, and corn has zein. A minor
protein, avenin is found in oats. The test for coeliac disease is the
presence of anti-transglutaminase antibodies, found in the major-
ity of cases. However, a patient may be intolerant to wheat without
producing such antibodies. We propose that gluten reduces access
to the starch of the degradative enzymes released from the pan-
creas into the small intestine. Thus some of the starch ends up in
the large intestine, where the bacteria generate the metabolic tox-
ins. Our bacterial ‘toxin’ hypothesis also explains the illness that
afflicted Charles Darwin for 50 years. We propose it has roles in
several other major diseases such as type 2 diabetes, which is at
epidemic levels in the Asian population.
A particularly potent toxin is methylglyoxal (CH3.CO.CHO),
produced primarily from the glycolytic intermediate dihydroxy-
acetone phosphate (Fig. 2). This has been detected at around 1 M
in human plasma. Methylglyoxal was originally discovered as an
inhibitor of bacterial cell growth (Freedberg et al., 1971; Ackerman
et al., 1974), and has been shown to have several toxic effects
on eukaryotic cells (Kalapos, 1999), including inhibition of insulin
secretion and action (Cook et al., 1998; Jia et al., 2006; Riboulet-
Chavey et al., 2006; Guo et al., 2009).
A key question now is: what is the molecular mechanism by
which methylglyoxal, and the other putative ‘bacterial toxins’,
affect both bacterial and eukaryotic cells, to explain the systemic
symptoms in lactose and food intolerance, and IBS?
3. Evidence to support the hypothesis
There is considerable evidence that metabolism of sugars
such as glucose, galactose, fructose and lactose by bacteria and
yeast, generates methylglyoxal and other metabolites, and that
this affects growth and gene expression (Freedberg et al., 1971;
Ackerman et al., 1974; Murata et al., 1989; Inoue and Kimura,
1995; Ferguson et al., 1998; Kalapos, 1999; Syu, 2001; Booth et al.,
2003; Ward and McLeish, 2004). It has also been shown that these
metabolites are found in human plasma and can affect the physiol-
ogy of eukaryotic cells (Kalapos, 1999; Sheader et al., 2001; Nemet
5. Please cite this article in press as: Campbell, A.K., et al., Bacterial metabolic ‘toxins’: A new mechanism for lactose and food intolerance, and
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et al., 2004; Riboulet-Chavey et al., 2006; Guo et al., 2009; Ogawa
et al., 2010).
Thus there are four key sets of observations that provide key
evidence to support the bacterial toxin hypothesis:
1. Anaerobic metabolism of sugars by many microbes produces
methylglyoxal and other metabolites (Fig. 2), consistent with
the fact that these bacteria contain the enzymes necessary for
the synthesis and degradation of these metabolites.
2. These metabolites do affect bacterial cell growth and gene
expression.
3. These metabolites are found in human plasma.
4. They have been shown to affect eukaryotic cells in vitro.
Here we provide a molecular mechanism to show how methyl-
glyoxal and other carbohydrate metabolites can affect both growth
and physiology of bacterial and eukaryotic cells.
3.1. The symptoms
Our recommended procedure for diagnosing lactose intoler-
ance is first to carry out a DNA test, based on a polymorphism
in the intron of a helicase upstream from lactase on chromosome
2 (Waud et al., 2008), followed by a 50 g oral lactose challenge,
measuring breath H2 and CH4 for 6 h, recording both gut and sys-
temic symptoms for 24–48 h (Table 1). Few patients exhibit all
Table 1
The symptoms of lactose and food intolerance, and irritable bowel syndrome.
Symptom
Gut
Abdominal pain
Gut distension
Flatulence
Diarrhoea
Ileus
Constipation
Nausea and vomiting
Systemic
Headache
Light headedness and concentration loss
Cognitive dysfunction – memory loss and reasoning deficiency
Tiredness
Joint pain and/or swelling and stiffness
Muscle pain
Allergy – eczema, pruritis, rhinitis, sinusitis, asthma
Arrhythmia tachycardia
Sore throat and mouth ulcers
Increased frequency of micturition
Depression
Any one patient will only exhibit some of these symptoms.
the symptoms. However, after a positive diagnosis, most improve
markedly within 1–2 weeks on a lactose free diet, being careful to
avoid foods and drinks where lactose is ‘hidden’ because of poor
labelling.
Fig. 3. Effect of glucose and methylglyoxal on cytosolic free Ca2+
in E. coli. JM109 E. coli cells expressing aequorin, as previously described (refs), were incubated in medium
A (25 mM HEPES, 125 mM NaCl, 1 mM MgCl2 pH 7.5). After 60 s Ca2+
(1 mM final) was added. After a further 120 s (a) glucose (30 M to 10 mM final) or (b) methylglyoxal
(3 mM final) was added. Light emission was converted to free Ca2+
as previously described (refs). Results represent the mean ± SEM of three observations.
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The hypothesis predicts that sugars and toxic bacterial metabo-
lites will induce signalling events, and effects on growth, in bacteria.
We have used Ca2+ signalling as a model in E. coli followed by testing
the effect of one toxin, methylglyoxal, on eukaryotic cells (Nemet
et al., 2004; Ogawa et al., 2010).
3.2. Effect of glucose and methylglyoxal on free Ca2+ and growth
in E. coli
In order to test the effects of glucose and methylglyoxal on free
Ca2+ in E. coli, genetically engineered aequorin was expressed in
live cells, in order to monitor changes in cytosolic free Ca2+. Both
glucose (10 M to 10 mM) and methylglyoxal (0.1–10 mM) induced
rapid transients in cytosolic free Ca2+ (Fig. 3a and b), the maximum
free Ca2+ concentration reaching 8 M and 30 M respectively.
We have shown that rises in cytosolic free Ca2+ result in changes
in the expression of at least 90 genes, and a reduction in gen-
eration time, through a rise in intracellular ATP (Naseem et al.,
2009). Our results also show clearly that E. coli has closely regu-
lated mechanisms for Ca2+ influx and efflux. The best candidate for
influx is the non-proteinaceous complex of polyhydroxybutyrate
and polyphosphate (Norris, 2005; Reusch et al., 1995). By using
knock-outs from the Keio collection (Baba et al., 2006), we have
also shown that there is a defect in Ca2+ efflux in the knock-out of
the F1ATPase, atpD, and that, as a consequence, ATP is required
for maximum Ca2+ efflux (Naseem et al., 2008, 2009).
Methylglyoxal inhibited growth of wild type E. coli cells (Fig. 4a).
Methylglyoxal at 10 mM virtually stopped growth completely. But
at 1 mM, there was a clear lag phase, when growth was completely
stopped for several hours. This is consistent with our previous
results (Campbell et al., 2007a,b; Naseem et al., 2007, 2008, 2009).
By measuring methylglyoxal using HPLC, and by using knock-outs
from the Keio collection, we showed that the lag phase was depen-
dent on the time the cells took to degrade the methylglyoxal,
primarily via the enzyme GloA. Thus, the lag phase at lower con-
centrations was markedly increased in the knock-outs gloA and
gloB (Fig. 4b), conditions that led to a large reduction in methyl-
glyoxal degradation, compared to wild type cells, as confirmed by
HPLC.
Other bacterial carbohydrate metabolites induce Ca2+ sig-
nals and affect cell growth (Campbell et al., 2007a,b), but are
less potent than methylglyoxal. The potency of these metabo-
lites in inducing Ca2+ signals is: methylglyoxal > acetylmethyl
carbinol > diacetyl > butane 2,3 diol > propan 1,2 diol.
3.3. Effect of methylglyoxal on eukaryotic cells
In order to show whether the symptoms of lactose and food
intolerance might be explained by bacterial metabolic toxins, the
effects of methylglyoxal on cell growth and K+ channel activity in
MCF-7 and MG63 cells respectively, the contraction of the guinea
pig ileum, and the perfused isolated guinea pig heart were investi-
gated (Fig. 5).
Methylglyoxal, 1 and 3 mM, inhibited cell growth of MCF-7 cells
(Fig. 5a). It was not clear however whether this effect was through
inhibition of cell proliferation or through induction of apoptosis.
Similar effects were also seen with MG63 cells (data not shown).
Using single channel patch clamp, the activity of the maxi-
K channel (Ca2+-activated voltage-dependent) in MG63 cells was
investigated in cell-attached patches (Fig. 5b). The maxi-K channel
activity was very low at a patch potential of +60 mV in the con-
trol bath solution. This channel typically has a unitary conductance
Fig. 4. Effect of methylglyoxal on growth of E. coli. E. coli cells obtained from the Keio collection (ref); (a) BW2133 wild type; (b) JW1643 gloA, the knock-out of glyoxalase
A, the first enzyme in the degradation pathway of methylglyoxal were incubated in growth medium LB at 37 ◦
C as previously described (Jones et al., 2002; Campbell et al.,
2007a,b; Naseem et al., 2009) in the presence of methylglyoxal (1 M to 10 mM). Growth was measured by a rise in absorbance at 600 nm. Results represent the mean of three
independent determinations, and show the increase in lag in growth as the methylglyoxal concentration was increased. They also show that at any particular concentration
of methylglyoxal the lag in growth was longer in the gloA knock-out cells compared with wild type.
7. Please cite this article in press as: Campbell, A.K., et al., Bacterial metabolic ‘toxins’: A new mechanism for lactose and food intolerance, and
irritable bowel syndrome. Toxicology (2010), doi:10.1016/j.tox.2010.09.001
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Fig. 5. Effects of methylglyoxal on eukaryotic cells. (a) Inhibition of growth in MCF-7 cells. MCF-7 cells were grown in DMEM at 37 ◦
C. Methylglyoxal (0–3 mM) was added
and the effect on cell number was estimated from an MTS assay. Methylglyoxal at 1 and 3 mM caused a significant decrease in cell proliferation. (b) Opening of K+
channels.
MG63 cells were patch clamped using a standard patch electrode filled with 140 mM KCl. Maxi-K channel activity was evident at +60 mV. Maxi-K channels typically have a
unitary conductance between 200 and 250 pS in symmetrical 140 mM KCl. Addition of methylglyoxal; 2 mM, caused a small increase in maxi-K activity shown by a slight
increase in the open probability of the channel, NPo from 0.25 to 0.33. Addition of a further 20 mM methylglyoxal caused a large increase in maxi-K activity, coupled with
an increase in Po. It took some 70 s before this increase was observed. (c) Effect on guinea pig ileum. A standard organ preparation and isometric transducer was used to
investigate the contractility of guinea pig ileum. The buffer was Krebs Ringer pH 7.4, temp. 37 ◦
C. Methylglyoxal was added at 10 mM, and the effect on isometric contraction
measured. (d) Effect on guinea pig heart. A standard Langendorff guinea pig heart preparation was used, perfused with Krebs Ringer buffer pH 7.4, temp. 37 ◦
C. Methylglyoxal
(10 mM) was added to the perfusate and the effect on heart left ventricular pressure measured. Methylglyoxal caused a large negative inotropic effect. This was followed by
a small positive inotropic effect.
of 200–250 pS in symmetrical KCl (140 mM) solutions. Bath appli-
cation of 2 mM methylglyoxal caused a small increase in maxi-K
channel activity (Fig. 5b Trace B), compared with the control (Fig. 5b
Trace A), with an increase in the open probability of the patch from
0.25 to 0.33. Addition of a further 20 mM methylglyoxal, resulted
in a large increase in maxi-K channel activity (Fig. 5b, Trace C), cou-
pled with a marked increase in NPo (0.33–3.0). This increase was
observed after a delay of 70 s.
Using a standard organ bath preparation, with Krebs Ringer
(pH 7.4, and 37 ◦C), and measuring isometric contraction, methyl-
glyoxal (10 mM) was shown to induce contraction of guinea pig
ileum (Fig. 5c), the tension induced being 50% of that first induced
by carbachol (5 M). Additionally, and in contrast, in a standard
Langendorff preparation methylglyoxal (10 mM) induced a large
negative inotropic effect on the perfused guinea pig heart (Fig. 5d),
followed by a small positive inotropic effect. This was observed as
an initial decrease in left ventricular systolic pressure (LVSP), fol-
lowed by a subsequent rise back to base line. These effects on the
heart can be compared with the effects on heart beat and arrhyth-
mia induced in the heart of the water flea, Daphnia pulex, that
we have established as a whole animal model to investigate our
‘bacterial toxin’ hypothesis (Campbell et al., 2004).
3.4. Covalent modification of insulin induced by methylglyoxal
A further prediction of the hypothesis is that methylglyoxal
reacts with amino and other groups, non-enzymatically, to cause
covalent modification of proteins, hormones and neurotransmit-
ters. Using HPLC, it was shown that modification of insulin can be
induced by methylglyoxal (Fig. 6), as well as 5HT and adrenaline
(data not shown). The biological activity of the modified insulin
now needs to be investigated (Jia et al., 2006; Riboulet-Chavey
et al., 2006; Vander Jagt, 2008; Guo et al., 2009). This cova-
lent modification is analogous to the established glycosylation of
Fig. 6. Covalent modification of insulin by methylglyoxal. The figure shows the
HPLC chromatogram of the effect of methylglyoxal treatment on insulin using gra-
dient analysis on a reverse phase silica based column. Human insulin (10 mg ml−1
)
was incubated with 10 mM methylglyoxal in 50 mM sodium phosphate buffer pH
8.2 for 24 h at RT. The solution was then dialysed overnight against 2× 500 ml
50 mM sodium phosphate buffer pH 8.2. The sample was then run on an HPLC col-
umn as follows: solvent delivery module: Varian prostar 210; detector: prostar
325 UV–visible: 220 nm; column: supelcosil LC-3DP, 5 m, 25 cm × 4.6 mm C18
HPLC column; mobile phase: A = 10:90 isopropanol:0.1% trifluoroacetic acid, pH
2.1, B = 90:10 isopropanol:0.1% trifluoroacetic acid, pH 2.1; injection volume: 20 l;
temp: 40 ◦
C; flow rate: 1 ml min−1
; gradient: at 0 min 5% B, at 15–20 min 60% B, at
22–32 min 5% B. The peak of normal insulin versus modified insulin was detected
by absorbance at 210 nm.
8. Please cite this article in press as: Campbell, A.K., et al., Bacterial metabolic ‘toxins’: A new mechanism for lactose and food intolerance, and
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Table 2
Systemic lactose and food intolerance versus Darwin’s 50 year illness.
Symptoms of lactose intolerance LI people who have
this symptom (%)
Darwin’s description Occurrence of
Darwin’s symptoms
Gut (pain, bloating, diarrhoea and/or
constipation)
100% Stomach ache Common
Flatulence 100% Flatulence/belching Common
Headache 86% Severe headache Common
Nausea and vomiting 82% Sickness for days Very common
Light head and concentration loss 78% Swimming head and concentration
problem
Common
Muscle and joint pain 71% Rheumatic pain Often
Severe fatigue 63% Chronic fatigue and exhaustion Very common
Allergy (eczema, hay fever, sinusitis) 40% Skin rash and boils Often
Mouth ulcers 30% Mouth sores Common
Heart palpitations 24% Chest palpitations Common
Depression Common Depression Frequent
haemoglobin to form HbA1C, currently used in the diagnosis of
diabetes.
3.5. Darwin’s illness
“I have had a bad spell, vomiting every day for eleven days, &
some days after every meal.”
Thus Charles Darwin wrote to his friend James Hooker in
1863, describing the illness that was to afflict him for some 50
years (Campbell and Matthews, 2005a; Colp, 2008). Explanations
for Darwin’s illness include Chagas disease, arsenic poisoning,
bereavement syndrome, and more recently the vague cyclical vom-
iting syndrome, and Crohn’s disease (Orrego and Quintana, 2007;
Sheehan et al., 2008; Greene and Greene, 2009; Hayman, 2009). Yet,
none of these explain all of his symptoms. In fact, the only disor-
der that Darwin’s symptoms fit exactly is that we have revealed in
patients with lactose and food intolerance, and irritable bowel syn-
drome (Table 2) (Campbell and Matthews, 2005a,b). Darwin saw
over twenty doctors, including his father. None were able to solve
his problem. His illness was the main reason he left London to live
in the Kent village of Downe. The only time he got better was when
he took little or no milk. He did not suffer his main illness while
on the Beagle for five years. He did suffer sea-sickness, but this is a
red herring. Later in life he visited the famous Dr Gully in Malvern
for his water therapy cure. Darwin got better on a diet with little
milk. And there is a clear family history. His paternal grandmother,
Mary Darwin, Erasmus’ first wife, and Charles’ mother, Susannah,
both died young with some of his symptoms, as did his daughter
Annie at the age of ten. His brother, Erasmus, never worked, and
several of Charles’ children also exhibited several of his symptoms,
often being dismissed as hypochondriacs. Darwin’s story bears a
remarkable similarity to many of our patients, who have often
suffered similar symptoms for years without anyone finding the
correct diagnosis of lactose or food intolerance (Campbell et al.,
2005, 2009).
4. Conclusions
Irritable bowel syndrome is the most common condition seen
by gastroenterologists, involving up to 50% of referrals, affecting
tens of thousands of people in the UK alone. Successful treatment
of this condition is poor, resulting in loss of quality of life, some-
times for years, loss of efficiency at work, and massive cost to the
NHS through doctor time and drug therapy. The key to understand-
ing this condition, and then managing it, is the mechanism that
causes the wide range of gut and systemic symptoms. We have
shown that lactose and food intolerance is a major cause of irrita-
ble bowel syndrome (Campbell and Matthews, 2005a,b; Campbell
et al., 2005; Waud et al., 2008). What is poorly recognised is that
these conditions result in many systemic symptoms, in addition
to the well-known gut problems (Table 1). Furthermore, it is not
generally recognised that lactose sensitivity is a major cause of IBS.
Surprisingly, the NICE recommendations currently ignore both of
these, and fail to understand the molecular and cellular mecha-
nisms involved.
Application of our studies on lactose intolerance, supported by
our bacterial toxin hypothesis, has already improved the quality of
life of over a thousand patients in South Wales alone. The results
reported here show that one of the most potent of the putative
toxins, methylglyoxal, not only can act as a negative quorum sensor
in bacteria, but also affects signalling mechanisms in eukaryotic
cells that could explain effects on neurones, muscle, cells of the
immune system, and other cells that are the cause of the wide range
of gut and systemic symptoms. In a competitive situation such as
the large intestine, where there are over 1000 species of bacteria
(Qin et al., 2010), those that can inhibit the growth of others would
have a clear Darwinian-Wallace selective advantage. The question
now arises whether there is an intestinal analogy to Helicobacter
pylori, identified as the prime cause of gastric ulcers (Gustafson
and Welling, 2010; Sheu et al., 2010).
The concentration of lactose in cow’s milk is approximately 47 g
per litre, equivalent to 130 mM. Thus the lactose present in one
250 ml glass of milk (ca 12 g) has the maximum capacity to gen-
erate over 500 mmol of methylglyoxal. With a plasma volume of
some 3 l, this would produce a level in the blood of over 170 mM. In
fact, measurement of fasting plasma methylglyoxal has shown that
it is normally in the range 0.1–100 M. However, these calculations
show that the level that bacteria in the gut and cells lining the gut
is likely to be in the 1–100 mM range after ingestion of lactose, well
within the range we have shown affects bacterial cell growth, and
eukaryotic physiology (see Figs. 3–5). A study now needs to be con-
ducted measuring the effect of lactose ingestion on gut and plasma
methylglyoxal concentrations. Thus the production of methylgly-
oxal and other carbohydrates by gut bacteria is likely to be much
higher that by endogenous eukaryotic cells.
Calcium is a well established as a universal intracellular sig-
nal in eukaryotic cells (Berridge et al., 2003; Campbell, 1983), and
our results show clearly that it does also have a significant role
in bacteria (Jones et al., 2002; Campbell et al., 2007a,b; Naseem
et al., 2007, 2008, 2009). The effects of methylglyoxal on eukaryotic
cells also clearly involve ionic signalling mechanisms. The question
now is whether agents can be discovered that manipulate calcium
and other intracellular signals in gut bacteria and cells around
the body in order to alleviate the symptoms of irritable bowel
syndrome. We have also recently shown that lactose sensitivity
may cause problems in some patients with inflammatory bowel
disease.
9. Please cite this article in press as: Campbell, A.K., et al., Bacterial metabolic ‘toxins’: A new mechanism for lactose and food intolerance, and
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A major target for the putative metabolic toxins in the gut and
brain is the 5 hydroxytryptamine (5HT) receptor, of which there
are at least five types. This is supported by the fact that the drug
tegaserod, a 5HT receptor 4 agonist, is used to treat IBS, whereas
naratriptan, a 5HT receptor 1 agonist, is used to treat migraine.
Methylglyoxal and other putative metabolic toxins react with pro-
teins and other molecules, such as insulin, 5HT and adrenaline,
resulting in covalent modification, which affects their biological
activity. Thus the data presented here provide a key mechanism
to explain how methylglyoxal and other carbohydrate metabolites
can affect bacterial cell growth and gene expression, as well as
affecting the physiology of cells in the body, by interacting with
calcium and ionic signals.
The question now is: what is the role of bacterial metabolic tox-
ins in diseases such as diabetes (Nemet et al., 2004; Ogawa et al.,
2010), cancer, multiple sclerosis, rheumatoid arthritis, autism,
attention deficit hyperactivity disorder (ADHD), as well as infer-
tility and hearing loss, where no molecular mechanism has, until
now, been able to explain the apparent link between milk, diet and
the condition. Interestingly, the idea that toxins are cause of many
diseases was proposed over one hundred years ago, by one of the
founders of immunology – Elie Metchnikoff (1845–1916), working
at Institute Pasteur in Paris. Metchnikoff won the Nobel Prize in
1908 for his discovery of macrophages. But his real focus was the
role of gut bacteria in disease. He wrote in his book ‘The Nature of
Man’, which was a Darwinian approach to the human body; ‘The
large intestine must be regarded as one of the organs possessed by
man and yet harmful to his health and his life. The large intestine
is the reservoir of the waste of the digestive processes, and this
waste stagnates long enough to putrify. The products of putrefac-
tion are harmful. . . Bacterial putrefaction is the cause of all disease’
(Metchnikoff, 1908). He published several papers investigating the
effects of putative bacterial toxins such as cresol on the health and
survival. Our work shows it is time to revive his vision.
Conflicts of interest
There are none.
Acknowledgements
We thank the Waterloo Foundation, the Welsh School of Phar-
macy for financial support. We also thank National BioSource
Resource project (NIG, Japan), for supplying the E. coli knock-out
Keio collection.
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