Salmonella and Salmonellosis (page 4)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Pathogenesis of Salmomella
Salmonella infections in humans vary with the serovar, the
the infectious dose, the nature of the contaminated food, and the host
status. Certain serovars are highly pathogenic for humans; the
of more rare serovars is unknown. Strains of the same serovar are also
known to differ in their pathogenicity. An oral dose of at least 105Salmonella
Typhi cells are needed to cause typhoid in 50% of human volunteers,
at least 109 S. Typhimurium cells (oral dose) are
to cause symptoms of a toxic infection. Infants, immunosuppressed
and those affected with blood disease are more susceptible to Salmonella
infection than healthy adults.
In the pathogenesis of typhoid the bacteria enter the human
tract, penetrate the intestinal mucosa (causing no lesion), and are
in the mesenteric lymph nodes. There, bacterial multiplication occurs,
and part of the bacterial population lyses. From the mesenteric lymph
viable bacteria and LPS (endotoxin) may be released into the
resulting in septicemia Release of endotoxin is responsible for
�collapsus and tuphos� (a stuporous state�origin of the name typhoid)
to action on the ventriculus neurovegetative centers.
Salmonella excretion by human patients may continue long
clinical cure. Asymptomatic carriers are potentially dangerous when
About 5% of patients clinically cured from typhoid remain carriers for
months or even years. Antibiotics are usually ineffective on Salmonella
carriage (even if salmonellae are susceptible to them) because the site
of carriage may not allow penetration by the antibiotic.
Salmonellae survive sewage treatments if suitable germicides are not
used in sewage processing. In a typical cycle of typhoid, sewage from a
community is directed to a sewage plant. Effluent from the sewage plant
passes into a coastal river where edible shellfish (mussels, oysters)
Shellfish concentrate bacteria as they filter several liters of water
hour. Ingestion by humans of these seafoods (uncooked or superficially
cooked) may cause typhoid or other salmonellosis. Salmonellae do not
or multiply in contaminated shellfish.
Typhoid is strictly a human disease.The incidence of human disease
when the level of development of a country increases (i.e., controlled
water sewage systems, pasteurization of milk and dairy products). Where
these hygienic conditions are missing, the probability of fecal
of water and food remains high and so is the incidence of typhoid.
Foodborne Salmonella toxic infections are caused by
serovars (e.g., Typhimurium). About 12-24 hours following ingestion of
contaminated food (containing a sufficient number of Salmonella),
symptoms appear (diarrhea, vomiting, fever) and last 2-5 days.
cure usually occurs.
Salmonella may be associated with all kinds of food.
of meat (cattle, pigs, goats, chicken, etc.) may originate from animal
salmonellosis, but most often it results from contamination of muscles
with the intestinal contents during evisceration of animals, washing,
transportation of carcasses. Surface contamination of meat is usually
little consequence, as proper cooking will sterilize it (although
of contaminated meat may result in contamination of hands, tables,
towels, other foods, etc.). However, when contaminated meat is ground,
multiplication of Salmonella may occur within the ground meat
if cooking is superficial, ingestion of this highly contaminated food
produce a Salmonellainfection. Infection may follow ingestion
any food that supports multiplication of Salmonella such as
cream, mayonnaise, creamed foods, etc.), as a large number of ingested
salmonellae are needed to give symptoms. Prevention of Salmonella
toxic infection relies on avoiding contamination (improvement of
preventing multiplication of Salmonella in food (constant
of food at 4°C), and use of pasteurized and sterilized milk and
products. Vegetables and fruits may carry Salmonella when
with fertilizers of fecal origin, or when washed with polluted water.
The incidence of foodborne Salmonella infection/toxication
reletavely high in developed countries because of commercially prepared
food or ingredients for food. Any contamination of commercially
food will result in a large-scale infection. In underdeveloped
foodborne Salmonella intoxications are less spectacular because
of the smaller number of individuals simultaneously infected, but also
because the bacteriological diagnosis of Salmonella toxic
may not be available. However, the incidence of Salmonella
in underdeveloped countries is known to be high.
Salmonella epidemics may occur among infants in pediatric
The frequency and gravity of these epidemics are affected by hygienic
malnutrition, and the excessive use of antibiotics that select for
Salmonella Enteritidis Infection
Egg-associated salmonellosis is an important public health problem
in the United States and several European countries. Salmonella
Enteritidis, can be inside perfectly normal-appearing eggs, and if the
eggs are eaten raw or undercooked, the bacterium can cause illness.
the 1980s, illness related to contaminated eggs occurred mosy
in the northeastern United States, but now illness caused by S. Enteritidis
is increasing in other parts of the country as well.
Unlike eggborne salmonellosis of past decades, the current epidemic
is due to intact and disinfected grade A eggs. Salmonella
silently infects the ovaries of healthy appearing hens and contaminates
the eggs before the shells are formed. Most types of Salmonella
live in the intestinal tracts of animals and birds and are transmitted
to humans by contaminated foods of animal origin. Stringent procedures
for cleaning and inspecting eggs were implemented in the 1970s and have
made salmonellosis caused by external fecal contamination of egg shells
extremely rare. However, unlike eggborne salmonellosis of past decades,
the current epidemic is due to intact and disinfected grade A eggs. The
reason for this is that Salmonella Enteritidis silently infects
the ovaries of hens and contaminates the eggs before the shells are
Although most infected hens have been found in the northeastern
States, the infection also occurs in hens in other areas of the
In the Northeast, approximately one in 10,000 eggs may be internally
In other parts of the United States, contaminated eggs appear less
Only a small number of hens seem to be infected at any given time, and
an infected hen can lay many normal eggs while only occasionally laying
an egg contaminated with Salmonella Enteritidis.
A person infected with the Salmonella Enteritidis
has fever, abdominal cramps, and diarrhea beginning 12 to 72 hours
consuming a contaminated food or beverage. The illness usually lasts 4
to 7 days, and most persons recover without antibiotic treatment.
the diarrhea can be severe, and the person may be ill enough to require
hospitalization. The elderly, infants, and those with impaired
systems (including HIV) may have a more severe illness. In these
the infection may spread from the intestines to the bloodstream, and
to other body sites and can cause death unless the person is treated
Salmonella strains may produce a thermolabile enterotoxin
bears a limited relatedness to cholera toxin both structurally and
This enterotoxin causes water secretion in rat ileal loop and is
by antibodies against both cholera toxin and the thermolabile
(LT) of enterotoxinogenic E. coli, but it does not bind in
to ganglioside GM1 (the receptor for E. coli LT and cholera ctx).
a cytotoxin that inhibits protein synthesis and is immunologically
from Shiga toxin has been demonstrated. Both of these toxins are
to play a role in the diarrheal symptoms of salmonellosis.