Reading Guide
Food Microbiology-THE BUGS!
The second part of this material focuses on the organisms that can cause food
poisoning. In this section we will focus on the specifics about each organism, what type
of symptoms they cause, the food that they are most likely associated with, and specific
mechanisms of pathogenesis.
Before talking about the organisms let’s look at the features present in the
digestive tract to prevent infection. Once food is eaten, the stomach coats the bolus in
gastric juice as it is mechanically digested by the layers of the stomach contracting and
almost “wringing” the food. The pH of the gastric juice is around 1.0 so any organisms
present in food are easily destroyed by this very low pH. Individuals who don’t make
enough gastric juice or are taking acid reducing drugs (as a treatment for ulcers) will not
have a good layer of protection from food poisoning.
So if the gastric juice can protect from food poisoning, why does it happen at all?
While there is no easy answer to this question there are some factors that play a role in
increasing the chance of food poisoning. For instance, we know that in order to cause
food poisoning (food borne infection) you need to eat bacteria, lots of bacteria. When so
many cells enter the stomach there is a greater chance that not all of them will be
destroyed. In order for bacteria to cause disease (foodborne infection) they need to gain
entry into the body and then find a location (small or large intestine) to bind and grow.
Pay attention to how the organism attaches, does it use pili, adhesions, flagella, or even a
capsule? Also when organisms grow in the intestines they may produce toxins that cause
changes to the epithelial tissue or even destroy cells. Notice if there are special toxins
made by an organism and how the toxin works.
Let’s begin a quick look at the bugs. The first category to cover are the organisms
that cause food borne intoxication. We covered these in class so I won’t expand on them
here. Remember that these organisms cause food poisoning by creating toxins in food,
not by ingestion of the organisms directly.
Organisms that cause foodborne infection are bugs that you ingest in food and
then the organisms grow inside and may produce more toxins and lead to the common
symptoms of food poisoning.
Campylobacter jejuni is an organism found naturally occurring on poultry and in
raw (unpasteurized) milk. It is a gram negative curved rod, microaerophile, with one or
more polar flagella. The flagella are used for attachment to the surface of epithelial cells.
This organism produces a toxin known as a cytotoxin which leads to epithelial cell death.
Symptoms caused by this organism are bloody diarrhea with mucus which can last for a
week.
Salmonella is a genus of organisms that consists of over 2400 serovars. All of
these are gram negative rods, facultative anaerobes, with peritrichous flagella. The
nomenclature for this group has identified two species of Salmonella, Salmonella
enterica and Salmonella bongori. Of these two species they are further divided into
serotypes, S. enterica is divided into 6 groups. Salmonella are found to be part of the
normal flora of poultry and are present on reptiles (turtles, lizards, and snakes). Infection
of this organism is due to contamination of fecal material from these animals and often
requires ingestion of a large number of organisms. Salmonella can bind to epithelial cells
using an adhesion and activate a Type III secretion system which facilitates endocytosis
of the bacterial cell into the epithelial cell. The organism (Salmonella enterica Typhi) can
also exhibit phase variation of flagella genes and survive inside the macrophage if
ingested! Wow!
Salmonella Typhi is an organism transmitted from human to human and the cause
of Typhoid Fever. It is very rare in the US, most cases are the result of people traveling to
countries where sanitation practices are not good. This organism has a much longer
incubation of 2 weeks and the symptoms are different-note that you have a high fever,
chills, and a headache for a week, then diarrhea.
Shigella is an organism that is a gram negative non-motile rod and a facultative
anaerobe. It is also an organism that is transmitted from other humans with transmission
occurring via the five F’s (see lecture notes). It is found worldwide and in the US there
are about 21,500 cases each year. The symptoms of shigellosis are dysentery….what does
this mean??(see p595) You will see the passage of small volume bloody stools about 3-4
days after a person ingests this organism. It is not destroyed by stomach acid and only
takes 10 cells to cause disease….YIKES! The organism is taken up by M cells in the
digestive tract and then are ingested by macrophages. They multiply in the macrophage,
lyse the macrophage, and enter the epithelial cells by inducing endocytosis. Once inside
the cells they begin to build “actin” tails and become motile! The organism uses the Type
III secretion system to modify the arrangement of the actin fibers in the cell and generates
this “actin” tail. Shigella dysenteriae is a species that is more virulent because it
produces the Shiga toxin. This toxin is a cytotoxin since it inhibits protein synthesis.
There are also strains of E. coli which produce the Shiga toxin.
Vibrio cholerae is a curved gram negative rod able to tolerate strong alkaline and
high salt conditions. It is killed by stomach acid so you must eats LOTS of them to get
sick, mostly through ingestion of contaminated shellfish or undercooked fish. They
attach to the small intestine using pili and other adhesions. They produce the cholera
toxin, an A-B toxin which causes the levels of cAMP in the cell to increase. This increase
in cAMP levels promotes an active pumping of other ions and electrolytes out of the cell
and into the intestinal lumen. Where ions (especially sodium go) water follows…so it is
easy to see how this leads to a huge loss in fluids. Patients can lose up to 3-5 gallons of
fluids per day. The symptoms of this type of food poisoning are rice-watery stools with
often a sudden onset of explosive watery diarrhea. Symptoms will resolve without
antibiotic treatment but most patients do need fluid supplements.
The non-cholera Vibrios are two species of Vibrio that are also associated with
food poisoning. Vibrio parahemolyticus is a halophile, motile by single polar flagella.
Ingestion of this bug will lead to vomiting and diarrhea about 12 hours after ingestion and
last about 2-5 days. Food most often associated are fish and undercooked or raw shellfish.
Vibrio vulnificus is the most virulent of the non-cholera vibrios, found to cause food
poisoning with ingestion of fish and undercooked shellfish. It has also been found to
cause tissue wounds (cellulitis) and can cause septicemiain patients who are
immunosuppressed or have liver disease.
The textbook highlights the general characteristics of the different strains of E.
coli on pages 599-600. The strain ETEC is responsible for Traveler’s diarrhea and
produces toxins that are similar in function to the cholera toxin. The STEC is the E. coli
that was famous here in Seattle….the Jack-in-the-Box E. coli. This organism produces
two toxins that are similar to the Shiga toxin. The STEC release the toxin in the large
intestine and do not invade the epithelial cells (like Shigella).
Listeria monocytogenes is a motile, faculatative anerobe, gram postive rod that
can grow at 4oC. It is found in natural waters and vegetation as well as intestines of
humans and animals. It can be found to contaminated unpasteurized cheeses, lunchmeat,
hot dogs, or coleslaw. When ingested symptoms appear like the flu with a headache and
stiff neck as signs of meningitis. GI symptoms are not always present. The organism
enters the body through the intestinal epithelium, macrophages uptake it and it can
continue to grow and migrate through the body in these cells. If the organism penetrates
the meninges, meningitis can occur which is fatal in 20% of cases.
Clostridium perfringens is the last bug on my list….and another obligate
anaerobe, endospore former, and gram positive rod. It is most often the cause of food
poisoning from foods that are rich in proteins and made in large pots/volumes, like stews
and soups. The endospore germinates after cooking and the cells grow as the food is left
at cooler temperature. Once ingested (the bugs) it can produce an enterotoxin that leads to
watery diarrhea. Symptoms appear 7-15 hours after ingestion and resolves in about 24
hours. Short lived at least! =)