Salmonellosis
Salmonellosis is the most
frequently occurring bacterial food-borne illness.
Salmonellae
are gram-negative non-spore-forming rods that ferment glucose, usually with
gas, but usually do not ferment lactose or sucrose. They grow over a wider
range of temperature, pH, and water activity. They grow well at room
temperatures, optimum is about 37oC. The pH range for growth is 4.1
to 9.0. The lowest aw for growth varies with the food but is about 0.93 to
0.95.
The likelihood of
infection by consumption of a food containing salmonellae
depends on the resistance of the consumer, the infectiveness of the particular
strain of Salmonella. and the number of organisms ingested. Salmonellae can
attain considerable numbers in foods without causing detectable alterations in
appearance, odor, or even taste. Human beings and animals are directly or
indirectly the source of the contamination of foods with salmonellae. The
organisms may come from cases of the disease or from carriers.
Most frequently isolated
serovars, such as S. typhimurium cause human gastroenteritis.
A large variety of foods
are involved in causing outbreaks of Salmonella infections. Most common are
various kinds of meats, poultry and products from them, especially if they are
held unrefrigerated for long periods. Fresh meats may carry Salmonella bacteria
that caused disease in the slaughtered animals or may be contaminated by
handlers. Meat products, such as meat pies, hash, sausages, cured meats (ham,
bacon, and tongue), sandwiches, and chili, often are allowed to stand at room
temperatures, permitting the growth of salmonellae. Milk and milk products,
including fresh milk, fermented milks, ice cream, and cheese, have caused
infections. Since eggs may carry the salmonellae, foods made with eggs and not
sufficiently cooked or pasteurized may carry live organisms, e.g., pastries filled
with cream or custard, cream cakes, etc.
The organisms also may
come from cats, dogs, swine, and cattle, but more important sources for foods
are poultry and their eggs and rodents. About one-third of all the food
products involved in Salmonella outbreaks are meat and poultry products, eggs etc.
Infected rodents, rats and mice, may contaminate unprotected foods with
their faeces and thus spread Salmonella bacteria. Flies may play an
important role in the spread of Salmonella, especially from contaminated fecal
matter to foods.
Changes in processing,
packaging, and compounding of foods and feeds in recent years
have resulted in an apparent increase in salmonellosis from these products.
Salmonellae have been introduced by the incorporation of cracked and dried eggs
in baked goods, candy, ice cream, and convenience foods such as cake and cookie
mixes.
Large-scale handling
of foods, increase the spread of trouble, precooked foods and food vending
machines add to the risk. Feeds, especially those from meat or fish
by-products, may carry Salmonellae to poultry or meat animals.
As with other infectious
diseases, individuals differ in their susceptibility to Salmonella infections,
but in general morbidity is high in any outbreak. The susceptibility of humans
varies with the species and strain of the organism and the total numbers of
bacteria ingested
Salmonellosis has
a longer incubation period - usually 12 to 36 hr. The principal symptoms of a
Salmonella gastrointestinal infection are nausea, vomiting, abdominal pain, and
diarrhea that usually appear suddenly. This may be preceded by a headache and
chills. Other evidences of the disease are watery, greenish foul-smelling
stools, prostration, muscular weakness, faintness, usually a moderate fever,
restlessness, twitching, and drowsiness. The mortality is low, being less than 1
percent. The severity and duration vary not only with the amount of food eaten
and hence the numbers of Salmonella bacteria ingested and with the individual.
Intensity may vary from
slight discomfort and diarrhea to death in 2 to 6 days. Usually, the symptoms
persist for 2 to 3 days, followed by uncomplicated recovery, but they may
linger for weeks or months. About 0.2 to 5 percent of the patients may become
carriers of the Salmonella organism.
The laboratory diagnosis
of the disease is difficult unless Salmonella can be isolated from the
suspected food and from the stools of individuals.
For the prevention of
outbreaks of food-borne Salmonella infections:
(I) avoid
contamination of the food with salmonellae from sources such as diseased
human beings and animals and carriers and ingredients carrying the organisms,
e.g., contaminated eggs
(2) destroy the
organisms in foods by heat (or other means) when possible, as by cooking or
pasteurization, paying special attention to held-over foods
(3) prevent the growth
of Salmonella in foods by adequate refrigeration or by other means.
In the prevention of
contamination, care and cleanliness in food handling and preparation are
important. The food handlers should be healthy (and not be carriers) and clean.
Rats and other vermin and insects should be kept away from the food.
Ingredients used in foods should be free of salmonellae, if possible.
Foods should not be allowed to stand at room
temperature for any length of time, but if this happens, thorough cooking will
destroy the Salmonella organisms unlike Staphylococcus enterotoxin.
Warmed-over leftovers, held without
refrigeration, often support the growth of Salmonella, as may canned foods that
have been contaminated and held after the cans were opened. Inspection of
animals and meats at packing houses may remove some Salmonella-infected meats
but is not in itself a successful method for the prevention of human
salmonellosis.
Staphylococcus Food
Intoxication
One of the most commonly
occurring food poisonings is caused by the ingestion of the enterotoxin formed
in food during growth of certain strains of Staphylococcus aureus. The
toxin is termed an enterotoxin because it causes gastro- enteritis or
inflammation of the lining of the intestinal tract.
Staphylococcus, typically
appear as clusters of grapes or in pairs and short chains. Growth on solid
media usually is golden or yellow but may be unpigmented in some strains. Most
enterotoxin-producing S. aureus cultures are coagulase-positive
(coagulating blood plasma), produce a thermal stable nuclease, and are
facultative in their oxygen requirements in a complex glucose medium but grow
better aerobically than anaerobically. Some of the toxigenic cocci are very
salt-tolerant (10 to 20 percent NaCl), and also tolerate nitrites fairly well
and therefore can grow in curing solutions and on curing and cured meats if
other environmental conditions are favourable. They also are fairly tolerant of
dissolved sugars (50 to 60 percent sucrose). They are fermentative and
proteolytic but usually do not produce obnoxious odors in most foods or make
them appear unattractive.
S. aureus
produces six enterotoxins (A, B, Cl , C2 , D, and E) that differ in toxicity;
most food poisoning is from type A. The range of conditions permitting
growth of the staphylococcus, and hence toxin production, varies with the food
involved. The better medium the food is for the coccus, the wider the range of
temperature, pH, or aw over which growth can take place. The temperature range
for growth and toxin production is about 4 to 46oC, depending on the
food. S. aureus grows most rapidly between 20 and 45oC. A and
D are more often associated with food-poisoning outbreaks. Toxin production and
growth of the Staphylococcus is best at 40oC.
The sources from which
the food-poisoning staphylococci enter foods are human or animal. The nasal
passages of many persons are laden with these organisms, which are a common
cause of sinus infections. Also, boils and infected wounds may be sources. Staphylococci
are becoming increasingly important in causing mastitis in cows, and some of
these cocci can form enterotoxin in milk or milk products.
Production of enterotoxin
by the Staphylococci is more likely when competing microorganisms are absent,
few, or inhibited for some reason. Therefore, a food that had been contaminated
with the Staphylococci after a heat process would be favourable for toxin
production. The type of food has an influence on the amount of enterotoxin
produced; much is produced in meat products and custard-filled bakery goods.
The presence of starch and protein in considerable amounts enhance toxin
production by the staphylococci. Type B enterotoxin is the most heat-resistant.
The normal cooking of foods will not destroy the toxin formed therein before
the heat process. Such foods might cause poisoning, although no live
staphylococci could be demonstrated.
About 75 percent of all
staphylococcal food-poisoning outbreaks occur because of inadequate cooling of
foods. Other foods incriminated include other meats and meat products, fish and
fish products, milk and milk products, cream sauces, salads, puddings, custards,
pies, and salad dressings. The fillings in bakery goods usually are good
culture media in which the staphylococci can grow during the time that these
foods are held at room temperatures. Toxin production has even been reported in
imitation cream filling. The contaminated leftover turkey, or other fowl, along
with the gravy and dressing, is kept out of the refrigerator, it may cause
poisoning.
Foods that ordinarily are
too acid for good growth of the staphylococci may have this acidity reduced by
added ingredients, such as eggs or cream, and then become dangerous. Growth and
toxin production by staphylococci may take place in the steam tables in
cafeterias and restaurants and in food-vending machines that keep foods heated
for extended periods if temperatures and times are not properly controlled.
Individuals differ in
their susceptibility to staphylococcus poisoning, so that of a group of people
eating food containing toxin some may become very ill and few may be affected
little or not at all. The incubation period for this kind of poisoning usually
is brief, 2 or 4 hr unlike the other common food poisonings and infections,
which usually have longer incubation periods.
The most common human
symptoms are nausea, vomiting, retching, abdominal cramping of varying
severity, and diarrhea. Blood and mucus may be found in stools and vomitus in
severe cases. Headache, muscular cramping, sweating, chills, prostration, weak
pulse, shock, and shallow respiration may occur. Usually a subnormal body
temperature is found rather than fever. The duration is brief, usually only a
day or two, and recovery ordinarily is uneventful and complete. The mortality
is extremely low.
For the most part no
treatment is given, except in extreme cases, when saline solutions may be given
parenterally to restore the salt balance and counteract dehydration. Diagnosis
of the poisoning would depend, on isolation of staphylococci and demonstration
that this produce enterotoxin or isolation and detection of the enterotoxin.
The means of prevention
of outbreaks of staphylococcus food poisoning include
(1) prevention of
contamination of the food with the staphylococci (2) prevention of the growth of the
staphylococci and (3) killing staphylococci in foods.
Contamination of foods
can be reduced by general methods of sanitation, by using ingredients free from
the cocci, e.g., pasteurized rather than raw milk, and by keeping employees
away from foods when these workers have staphylococcal infections in the form
of colds, boils, carbuncles, etc. Growth of the cocci can be prevented by
adequate refrigeration of foods and, in some instances, by adjustment to a more
acid pH. Also the addition of a bacteriostatic substance, such as serine or an
antibiotic, has been suggested. Some foods may be pasteurized to kill the
staphylococci before exposure of the foods to ordinary temperatures, e.g.,
pasteurization of custard filled puffs and eclairs for 30 min at 190.6 to 218.3
C oven temperature.
Enteropathogenic Escherichia
Coli
E. coli
is generally regarded as part of the normal flora of the human intestinal tract
and that of many animals. Serotypes of E. coli have been implicated in
human diarrheal diseases or foodpoisoning outbreaks are designated as
enteropathogenic E. coli (EEC).
The human disease
syndromes resulting from the ingestion of EEC have been divided into two main
groups. The first group consists of strains which produce an enterotoxin and
result in a choleralike or enterotoxigenic illness in humans. These
enterotoxigenic strains usually produce two enterotoxins, a heat-stable (ST)
and a heat-labile (LT) toxin, and are thought to be responsible for infantile
diarrheal diseases and traveller’s diarrhea. EEC serotypes capable of
elaborating the enterotoxins if ingested, are colonized in the upper small
intestine and produce the enterotoxins. The enterotoxins cause fluid
accumulation in the intestinal lumen.
The second major group
consists of invasive strains which produce a cytotoxin and result in the
invasive illness, colitis, or dysentery like syndrome. These serotypes are non-enterotoxigenic,
grow in the colon, and invade or penetrate the epithelial cells of colonic
mucosa, resulting in fever, chills, headache, abdominal cramps etc
A large infective dose of
EEC is required for either the enterotoxigenic or invasive illness to occur.
Therefore, foods must be highly contaminated or inadequately preserved or
refrigerated to allow for prolific growth. The optimal temperature for growth
is 37oC, with a temperature range for growth of 10 to 40oC.
The optimal pH for growth is 7.0 to 7.5, with the minimum at pH 4.0 and the
maximum at pH 8.5. The organism is relatively heat sensitive and can readily be
destroyed at pasteurization temperatures and by the proper cooking of foods.
In addition, the
hemorrhagic E. coli (EHEC) strains can result in illness in humans as
manifested by bloody diarrhea and severe abdominal pain.
Thorough and sanitary
methods of cooking, chilling foods after use, maintaining personal hygiene,
treating water and ensuring sanitary disposal of sewage can control E. coli
infections
