Download informational handout - Western Connecticut State University

INFORMATIONAL HANDOUT:
FIRST, REVIEW THE DIGESTIVE TRACT:
Humans (and most animals) digest all their food extracellularly; that is, outside of cells. Digestive enzymes are
secreted from cells lining the inner surfaces of the digestive tract. The enzymes hydrolyze macromolecules in
food into small, soluble molecules that can be absorbed into cells. This should remind you of the need that
BACTERIA have for exoenzymes.
In diagram, note that the linings of the tract are all continuous with the
surface of the body. Anything placed within the lumen is, strictly
speaking, outside the body. This includes the secretions of all exocrine
glands (in contrast to the secretions of endocrine glands, which are
deposited in the blood). Any indigestible material placed in the mouth
will appear, in due course, at the other end.
Ingestion: Food placed in the mouth is ground into finer particles by the
teeth, moistened and lubricated by saliva (secreted by three pairs of
salivary glands). Small amounts of starch are digested by the amylase
present in saliva and the resulting food is swallowed into the esophagus
and carried by peristalsis to the stomach.
The stomach: The wall of stomach is lined with millions of gastric
glands, which together secrete 400-800 ml of gastric juice at each meal.
Cells may secrete compounds like hydrochloric acid (stomach juice has
pH near 1!), proteolytic enzymes, and hormones related to digestion.
Very little absorption occurs in the stomach. However, some water,
certain ions, and such drugs as aspirin and ethanol are absorbed from the stomach into the blood (accounting
for the quick relief of a headache after swallowing aspirin and the rapid appearance of ethanol in the blood after
drinking alcohol). As the contents of the stomach become thoroughly liquefied, they pass into the duodenum,
the first segment (about 10 inches long) of the small intestine.
LIVER AND PANCREAS: Two ducts enter the duodenum, one draining the gall bladder and hence the liver and
the other draining the exocrine portion of the pancreas.
The liver: Remember - the liver receives blood from the small intestine - blood that carries the small food
molecules that were absorbed there. In the liver, some glucose from the small intestine is removed and
converted into glycogen for storage. Other monosaccharides are removed and converted into glucose. Excess
amino acids are removed and deaminated and the amino group is converted into urea for excretion. The rest of
ingested food molecules can then enter the bloodstream to be delivered to cells for respiration (oxidation to
obtain energy). The liver also secretes bile. Between meals bile accumulates in the gall bladder. When food,
especially fatty food, enters the duodenum, a hormone stimulates the gall bladder to contract and discharge its
bile into the duodenum. Bile contains detergent-like molecules that break up fat globules, and pigments which
are hemoglobin breakdown products that have accumulated in the liver - these pigments give the characteristic
brown color to feces. Many nonnutritive molecules, such as ingested drugs, are removed by the liver and, often,
detoxified. So the liver serves as a gatekeeper between the intestines and the general circulation. It screens
blood reaching it so that its composition when it leaves will be close to normal for the body. Furthermore, this
homeostatic mechanism works both ways. When, for example, the concentration of glucose in the blood drops
between meals, the liver releases more to the blood by converting its glycogen stores to glucose or converting
certain amino acids into glucose.
The pancreas: The pancreas consists of clusters of cells that make and send glucagon and insulin into the
blood. These hormones help regulate blood sugar levels. The pancreas secretes other compounds directly into
the duodenum. Pancreatic fluid contains sodium bicarbonate to neutralize the acidity of the fluid arriving from
Biology 215, Ruth A. Gyure
Western CT State University
the stomach, amylase (to break down starch), and lipase (to break down fats). It also contains proteolytic
enzymes and enzymes to break down DNA.
The small intestine: Digestion within the small intestine produces a mixture of disaccharides, peptides, fatty
acids, and monoglycerides. The final digestion and absorption of these substances occurs in the villi. Enzymes
such as sucrase and lactase can be found here.
The large intestine (colon): The large intestine receives the liquid residue after digestion and absorption are
complete. This residue consists mostly of water as well as materials (e.g. cellulose) that were not digested. It
nourishes a large population of bacteria (the contents of the small intestine are normally sterile). Most of these
bacteria (of which one common species is E. coli) are harmless. And some are actually helpful, for example, by
synthesizing vitamin K. Bacteria flourish to such an extent that as much as 50% of the dry weight of the feces
may consist of bacterial cells. Reabsorption of water is the chief function of the large intestine. The large
amounts of water secreted into the stomach and small intestine by the various digestive glands must be
reclaimed to avoid dehydration. If the large intestine becomes irritated, it may discharge its contents before
water reabsorption is complete causing diarrhea. On the other hand, if the colon retains its contents too long,
the fecal matter becomes dried out and compressed into hard masses causing constipation.
NORMAL MICROBIOTA OF LOWER GI TRACT - who's in there???
The bacterial flora of the GI tract of animals has been studied more extensively than that of any other site. And
this does not consider all the studies on the colon bacterium Escherichia coli, which is a favorite experimental
organism for microbiologists, geneticists, and molecular biologists! The composition of the GI flora differs
between various animal species, and within individual animal species. In humans, the composition of the flora is
also influenced by age, diet and cultural conditions. In the upper GI tract of adult humans, the esophagus
contains only the bacteria swallowed with saliva and food. Because of the high acidity of the gastric juice only a
few bacteria (mainly lactobacilli) can be cultured from the normal stomach. The small intestine has a relatively
sparse gram-positive flora, consisting mainly of lactobacilli and Streptococcus faecalis The flora of the large
intestine (colon) is qualitatively similar to that found in feces. The predominant species are anaerobic
Bacteroides and Bifidobacteria. These organisms may outnumber E. coli by 10,000 to 1!
Strict anaerobes: Bacteroides (gram negative rod), Bifidobacterium (gram positive coccus),
Enterococcus (gram positive coccus), Lactobacillus (gram positive rod), Clostridium (gram positive rod)
Facultative: Escherichia, Enterobacter, Klebsiella, Citrobacter, Proteus, Serratia and more!
This group of gram-negative facultative rods is of great interest. As a group they form the 'enterics' for short.
They share many characteristics, which probably reflects their common environment, which is dark, moist,
mostly anaerobic, and full of partially digested small organic molecules. Because it is an active 'fermentation vat'
the products of fermentation also serve as food sources. What one organism makes can be used as food by
another. We divide the enterics into two groups:
The lactose fermenters = COLIFORMS
The non-lactose fermenters = ALL THE REST
MEDIA: We use the following to help us isolate and identify enterics and other inhabitants of lower GI:
MAC-CONKEY AGAR: A selective and differential medium that contains bile salts and crystal violet (to inhibit
gram positives), lactose as main carbon source to help detect coliforms, and peptone to allow some growth of
everybody. On MacConkey agar - lactose fermenting colonies appear pink-red and opaque.
EMB AGAR: Selective and differential, contains bile salts, eosin, and methylene blue - all inhibit gram positives.
Lactose fermenters appear blackish with a metallic green sheen on this medium.
SALMONELLA SHIGELLA AGAR: This selective and differential medium contains bile salts, sodium citrate, and
brilliant green - which all inhibit gram positive organisms. It also has some ferrous sulfate and color indicator so
that lactose fermenters will turn pink, and non-lactose fermenters that make hydrogen sulfide (Salmonella) will
be black. Shigella appear colorless on this medium.
TRIPLE SUGAR IRON AGAR A differential medium that contains a tiny amount glucose, large amount sucrose
and lactose, ferrous sulfate, phenol red, nutrient agar. It helps differentiate between organisms that ferment
sucrose/lactose, or just glucose. Also, it can reveal whether or not an organism produces hydrogen sulfide gas.
Biology 215, Ruth A. Gyure
Western CT State University
UREA AGAR: This differential medium contains small amounts of peptone and glucose, lots of urea, and phenol
red. Organisms that have the enzyme urease can break urea apart, releasing the ammonia. This causes pH to
go up, turning phenol red a deep fuschia pink color.
Invasion of GI tract:
World-wide gastrointestinal infections are a major cause of morbidity and mortality. In the Third World they are
responsible for many deaths, particularly in children. In "developed" countries they are a major cause of
economic loss. Ingestion of a microbe or its products may elicit a wide range of illnesses with a spectrum of
severity. Microbial intoxication does not require the ingestion of live microbes: just their biologically active toxins.
Intoxications range in severity. At one end of the spectrum is the mild, self-limiting food poisoning caused by
Staphylococcus aureus and "Chinese restaurant syndrome" caused by Bacillus cereus through to the potentially
fatal botulism caused when food is contaminated with toxin produced by Clostridium botulinum. Intoxications are
generally characterised by a short incubation period. This may be complicated by the need for some toxigenic
bacteria to grow in the bowel before releasing toxin. This is the case with Clostridium perfringens food
poisoning. Many cases of gastrointestinal infection are caused by bacteria, particularly in otherwise healthy
adults, but viruses and protozoa are also common causes of gastroenteritis.
Infection (delayed symptoms)
 Fever usually symptom
 Diarrhea
 Diarrhea + mucous = dysentery
 Cramps, nausea, vomiting
Inflammation of stomach lining and intestinal mucosa = GASTROENTERITIS
Intoxication, (immediate symptoms) ingestion of preformed toxin
 Fever usually NOT a symptom
 Diarrhea
 Diarrhea + mucous = dysentery
 Cramps, nausea, vomiting
Inflammation of stomach lining and intestinal mucosa = GASTROENTERITIS
WHAT TYPES OF ORGANISMS CAN INVADE OR DAMAGE THE LOWER GI TRACT?
See text pages 689-699
E. coli, pathogenic forms only; Shigella and Salmonella; Vibrio; Campylobacter
Yersinia; Closteridium; Bacillus; Staphylococcus
Non-bacteria: Hepatitus A, Rotaviruses, Protozoans, Worms
Campylobacter and Helicobacter: Campylobacters were relatively recently discovered to be a cause of
gastrointestinal disease, and yet they are now amongst the commonest bacterial causes of diarrhoea in
humans. Typically the diarrhoea is very watery, and blood and mucus may be present in the stools.
Campylobacters are microaerophilic bacteria that can easily become non-cultivable, although they remain viable
and infectious in this resting state. It is for this reason that campylobacters can be difficult to isolate, making
source tracing difficult in many cases. The incubation period of campylobacter enteritis is highly variable,
ranging from a matter of a few hours through to more than ten days. This variability is correlated with the density
of the inoculum of the infecting dose. Campylobacter infections may be spread by drinking contaminated water
or drinking unpasteurised milk, as well as by eating particular foods, especially undercooked chickens.
[Helicobacter pylori is a bacterium that is related to the campylobacters, and is intimately associated with cases
of gastritis and duodenal ulcers.]
Salmonella: Salmonella strains are a common cause of diarrheal disease. The species of
salmonella that commonly causes gastroenteritis is Salmonella enteritidis. This species is
divided into over 2,000 variants or 'serovars' depending upon their expression of "O" and
flagellar "H" antigens. Each serovar has an apparent species name by which they are commonly
known; for example, Salmonella typhimurium. Salmonella infections are associated with drinking
unpasteurised milk, or with consumption of various foodstuffs, particularly poultry or poultry products. Pork pies
are another potential source of salmonella infection. Salmonella typhi is a bacterium that causes an invasive,
systemic infection, that may be life-threatening known as enteric fever. This is an exclusively human pathogen.
Biology 215, Ruth A. Gyure
Western CT State University
This contrasts with Salmonella enteritidis, a bacterium that has a very wide distribution, and that can be
isolated from reptiles as well as mammals. The ability of Salmonella typhi to cause invasive infection is
considerably enhanced by production of a capsule that protects the bacterial cell from intracellular killing. The
antigen associated with the capsular material is known as the 'Vi' antigen, because its presence confers
virulence upon its producing strain. Salmonella typhi can be distinguished from other bacteria of the genus
Salmonella because it produces only acid from the fermentation of glucose, whereas others produce both acid
and gas when fermenting glucose. Salmonella typhi infections are spread by the faecal-oral route, and water
that is contaminated with human faeces is the most common source, although raw vegetables washed in
contaminated water can act as a vector for enteric fever.
Shigella: Bacilliary dysentery is caused by bacteria of the genus Shigella. Bacilliary dysentery is characterised
by the presence of blood, pus and mucus in stools, accompanied by abdominal pain and cramps. In the
developed world, Shigella sonnei is the commonest bacterium of the genus associated with dysentery. In the
Third World, Shigella dysenteriae is the commonest isolate, and the cause of the most severe form of the
disease. Because of the highly infectious nature of these bacteria, and due to the personal habits of small
children, in the United States incidents of dysentery caused by Shigella sonnei often occur in preschool
playgroups or in primary schools. Whereas most species of the genus Shigella are non-lactose fermenters,
Shigella sonnei can ferment lactose after prolonged incubation, and it is described as a late lactose fermenter.
Escherichia coli: Several types of Escherichia coli are associated with diarrheal disease. This
bacterium is a major component of the aerobic bowel flora, and we become acclimatised to
strains in our local environment. Escherichia coli diarrhoea is most often associated with
journeys, and is often referred to as 'Traveller's diarrhea'. Children under 2 years old are also
vulnerable to Escherichia coli diarrhoea, and without prompt rehydration, they may die from
their infection. These strains of Escherichia coli produce a heat-labile toxin (LT) that causes a watery diarrhea.
This toxin is similar to the toxin produced by campylobacters, Shigella, and by Vibrio cholerae, the bacterium
that causes cholera. Some E. coli strains make something called vero-toxin. Verotoxin production is associated
with the O-157 antigen. Such strains are associated with consumption of undercooked hamburgers or drinking
unpasteurised milk. These bacteria, unlike most strains of Escherichia coli cannot ferment sorbitol. A preliminary
screen can be performed using a modified MacConkey agar in which sorbitol is substituted for lactose.
Escherichia coli strains that produce verotoxin are responsible for two life-threatening conditions, heamorrhagic
colitis and haemolytic uraemic syndrome. Both conditions can kill young and otherwise fit people.
Vibrio spp.: Cholera is an exclusively human disease, and like typhoid is spread through consumption of
contaminated water. For many years this disease was only associated with a single serovar of Vibrio cholerae
now referred to as the 'classical' serotype. More recently, a second serovar has been identified that causes a
less severe form of the disease. This is the 'El Tor' serotype. The disease is caused by the activity of a toxin,
'choleragen' The characteristic appearance of the stool of a cholera victim is that of water in which rice has been
boiled the 'Rice-Water Stool'. Patients die of electrolyte imbalance caused by severe dehydration. Even so, at a
microscopic level there is no apparent and observable damage to the structures of the gut. Vibrio
parahaemolyticus is a second vibrio that is associated with diarrheal disease. This is a marine organism with a
World-wide distribution, and is particularly associated with the consumption of shellfish. Because of the great
quantities of fish in the Japanese diet, this organism is frequently isolated in Japan.
poor sanitation leads to outbreaks
Rehydration therapy for an infant
(Images courtesy U of Wisconsin and Medecin Sans Frontieres)
The organism, Vibrio cholerae
GRAM POSITIVE ORGANISMS - We already met these in our previous unit:
Clostridium spp: Toxigenic strains Clostridium perfringens cause a diarrheal disease. Spores survive initial
cooking, and often the subsequent reheating of meat stews, etc. particularly when cooked in large quantities.
Biology 215, Ruth A. Gyure
Western CT State University
Clostridium perfringens diarrhoea is most commonly seen in institutional catering, such as schools, hospitals
and university halls of residence, particularly if large quantities of stews and so on are reheated prior to serving.
Upon ingestion, the spores enter the bowel where they germinate. Vegetative bacteria grow and sporulate.
Toxin production is associated with sporulation. The incubation for clostridial food poisoning is thus longer than
for many intoxications, since toxin production relies first upon germination and growth of the bacterium in the gut
before sporulation and toxin production occurs. Very rarely, Clostridium perfringens has been associated with
a rapidly fatal diarrhoeal illness, from which patients can die in under two days. Use of any antibiotic can lead to
the disruption of the bowel flora, in particular the anaerobic bacteria. This leaves patients vulnerable to
colonisation with Clostridium difficile. This bacterium flourishes in the absence of competing organisms in the
bowel flora, and if the strain is toxigenic, this may cause diarrhoea. There may be life-threatening complications
with a high mortality rate. In general Clostridium difficile infection primarily affects the hospitalised elderly. It is a
more common cause of gastrointestinal infection in the elderly that either campylobacter or salmonella
infections. Clostridium botulinum is the cause of botulism, a flaccid paralysis that can lead to death as a result
of respiratory or cardiac failure. The disease results from the activity of a highly potent heat-labile neurotoxin. It
is most commonly associated with improperly or inadequately processed or home-canned food.
Bacillus cereus: Bacillus cereus is associated with two types of gastrointestinal
disease. Bacillus cereus intoxication is typically associated with the consumption of
fried rice the 'Chinese Restaurant Syndrome'. Spores survive the initial boiling of rice
that is then allowed to cool in bulk. During the slow cooling, spores germinate and
vegetative bacteria multiply, then sporulate again. Sporulation is also associated with
toxin production. Any uncooked boiled rice used to be kept for production of fried rice
on the second day. If the batch was contaminated with Bacillus cereus, overnight
storage in a refrigerator is sufficient to allow toxin production. The toxin is heat-stable,
and can easily withstand the brief high temperatures used to cook fried rice. Within 16 hours of eating
contaminated fried rice, patients suffer a bout of vomiting that generally lasts for less than a day.
Staphylococcus aureus: Food poisoning strains of this bacterium are probably the commonest cause of food
poisoning in the United States, but as the disease is relatively mild, and it generally clears up in less than a day,
the majority of cases probably never get reported. Strains of Staphylococcus aureus can produce a toxin that is
absorbed, and that acts upon the vomiting center in the brain. Symptoms of staphylococcal food poisoning
include vomiting that starts between 30 minutes and 6 hours following the consumption of the contaminated
food. Because of its salt tolerance staphylococcal food poisoning can be associated with salty foods such as
hams, and because it can be found on human skin, staphylococci cause food poisoning in foods that require a
lot of manipulation in their preparation, including cream cakes, and shrimp cocktails.
Listeria monocytogenes: Although it causes systemic infections, particularly in neonates and the
immunocompromised, Listeria monocytogenes is spread by the consumption of contaminated foods (often
processed meats) and dairy products. Because of the ability of this bacterium to grow at low temperatures,
concerns have been expressed about the safety of cook-chill foods, where food is cooked than stored
refrigerated at temperatures above freezing for up to a week prior to rewarming then consumption.
TEXT RESOURCE: Chapter 25
ACKNOWLEDGEMENTS: http://www.leeds.ac.uk/mbiology/ug/med/gut.html#Introduction
Biology 215, Ruth A. Gyure
Western CT State University