Salmonellosis is a
serious intestinal infection caused by Salmonella,
which is rod-shaped Gram-negative bacteria. The bacteria mainly infects
humans and domestic animals. It is an important foodborne pathogen which
overburden economy of both developed and underdeveloped countries through the
costs associated with diagnosis, prevention, control and treatment. Salmonella is predominantly present in
eggs, dairy and poultry, it can also be transmitted through fruits and
vegetables which includes mangoes, apples, tomato, celery, cantaloupe and
lettuce. Mainly gastroenteritis is caused to healthy adults by Salmonella Typhimurium and Enteritidis.
This bacteria could transmit through uncooked food products; Salmonella is predominantly present is slaughterhouses
where it can contaminate organs of food animals.
Discovery and isolation
of Salmonella was first conducted by
Theobald Smith in 1855 from the intestines of pigs that were infected by swine
fever. An American Pathologist, Dr Daniel Elmer Salmon named this bacteria and
he also worked with Smith. Salmonella
can be classified into two species this nomenclatural system is manipulated by
Centers for Disease Control and Prevention, the species comprise of Salmonella enterica (type species) and Salmonella bongori. There are four
subspecies of S. enterica which can
be be represented by Roman numerals, I, S. enterica subsp. enterica; II, S.
enterica subsp. salamae; IIIa, S. enterica subsp. arizonae; IIIb, S. enterica
subsp. diarizonae; IV, S. enterica subsp. houtenae; and VI, S. enterica subsp. indica.
The most prevalent is enterica (I) which causes infections in warm-blooded
animals and humans, on the other hand S.
bongori and the rest of the five subspecies of Salmonella does not affect humans, but is widely present in
By using Kauffman-White
scheme 2600 serotypes of Salmonella are
identified but they can easily adapt to animal hosts and humans. Kauffmann-White
classification formulated a system to classify Salmonella according to serotype by using three antigenic
determinants which includes, somatic (O), flagellar (H) and capsular (K). O
antigen plays essential role in protecting membrane from heat, it is composed
of oligosaccharide constituent of lipopolysaccharide, and this antigen is
situated at bacterial cell membrane. In contrast the bacterial flagella known
as H antigens is instable in heat, it helps to stimulate host immune responses.
Some flagellar proteins are diphasic which means they have unique capability to
express specific protein at a time. The rarest antigens of Salmonella is the K antigens which is positioned at bacterial
capsule. Particular subtype of K antigen known as Virulence (Vi) which is found
in serotypes: Dublin, Paratyphi C and Typhi. In 1934 the International
Association of Microbiologists adopted this. A simple technique used for genus
identification of Salmonella and epidemiological
study is Agglutination by antibodies particularly employing surface O antigens
resulting in six serogroups of Salmonellae;
A, B, C1, C2, D and E. Presently, the nomenclature system which is adopted
excludes the information about Salmonella
subspecies, for example S.enterica serovar
Typhimurium is shortened to S.
Salmonellosis targets to
elderly, children who are below 5 years or individuals which are
immunosuppressed are presumably to develop Salmonella
infection. However, the infection extremity depends on the serotype and the
health condition of patient, patients who are suffering Salmonellosis are
likely to get burdened with reactive arthritis (it is painful arthritis). Salmonella invade host cells, where it
replicates and survives, consequently causing lethal and life threatening
disease in humans.
begins when the bacteria is live and then it gains entry in gastrointestinal
tract. The localized form initiates when living Salmonella invades small intestine and proliferates in tissues. The
chromosomal DNA of bacteria consists of gene collections which are responsible
for invasion of epithelial, macrophagic and dendritic cells that are known as Salmonella Pathogenicity Islands (SPIs).
Salmonella contaminated food or water
enters digestive tract, and tries to perforate the epithelial cells of
intestinal wall. The bacteria travels from the lymphatic system into the blood
which is known as the Typhoid form, and then the bacteria resides in different
organs which includes, kidneys, liver and spleen, to produce Septic form. The
endotoxins which were produced by dead bacteria damage affected organs showing
increase in permeability, disturbed thermal regulation moreover vomiting and
diarrhea can also be manifested.
The effectors of Salmonella are introduced into the
cytoplasm through type III secretion system which are encoded
by SPIs. Signal transduction pathway is triggered by effectors which causes reassembly
of actin cytoskeleton of the host cell, when cell membrane is disheveled the
bacteria is engulfed. This process of membrane disarrangement is similar to
Phagocytosis. After engulfment of Salmonella
into the host cell, it is
enclosed in a membrane compartment i.e. a vacuole. When a foreign particle
enters human body, it activates host cell immune response, this causes in the
amalgamation of lysosome and the secretion of digesting enzymes, this will
degrade intracellular bacteria. The effector proteins are inserted in vacuole
through type III secretion system which causes disarrangement of the enclosed
structure. Salmonella now can survive
and replicate inside host cells because the reconstructed vacuole stops the
fusion of the lysosomes. This efficiency of bacteria to survive and replicate
inside macrophages guides it to transport to the Reticuloendothelial system. The
control and spread of bacteria is constrained by reticuloendothelial system.
Some serovars may cause infection in liver, spleen, gallbladder, bones or other
organs, but gastroenteritis which is common human infection is restricted to
After successfully invading the intestine, Salmonella may cause ulceration which
induces an acute inflammatory response. This may cause the production of
cytotoxins that inhibit protein synthesis. Various proinflammatory cytokines
are produced when the mucosa of the epithelial cells is invaded, these
inflammatory cytokines includes, IL-1, IL-6, IL-8, TNF-2, MCP-1, and GM-CSF.
These cytokines stimulates inflammatory response leading direct damage to the
intestines. Inflammation of intestine may contribute to symptoms such as
chills, fever, leukocytosis, abdominal pain and diarrhea. Patient’s stool may
contain blood, mucus, polymorphonuclear leukocytes.
The strains which infect intestinal mucosa causes
severe inflammatory response and patient may suffer from diarrhea. Small and
large intestines discharge fluid and electrolytes, resulting in diarrhea. The
bacteria may spread and cause systemic infection after passing from the basal
side of the epithelial cells into the lamina propria. This unusual spread of
organism gives rise to enteric fever. The activation of adenylate cyclase
occurs leading to higher levels of cyclic AMP secretion. Adenylate cyclase
stimulates the production of prostaglandins (activates inflammatory reaction)
or other elements of inflammatory response. Furthermore, intestinal secretion
is also possible due to other enterotoxin-like substances produced from some
strains of Salmonella. In
pathogenesis of Salmonella the basic
principle of these toxins is unknown.
Figure 1: Pathogenesis of Salmonella
Figure 2: Flowchart of Salmonella enterocolitis and diarrhea
Figure 3 Invasion of intestinal
mucosa by Salmonella
The strains of Salmonella
can be classified into Typhoid Salmonella
and Non-Typhoidal Salmonella (NTS).
The different types of human infections caused by NTS can be categorized into,
gastroenteritis, bacteremia, endovascular and localized infections.
NTS strains differ from S. Typhi
and S. Paratyphi
Salmonella does not require complex nutritional requirement for
growth i.e. they are non-fastidious. They can grow and multiply outside host
cells rapidly. The optimum temperature for some serotypes of Salmonella ranges from 35°C-37°C, but
they can also grow between 2°C-4°C or at 54°C. The bacteria can grow in the
presence of 0.4%-4% sodium chloride. Salmonella
cannot withstand extreme temperatures, they can be killed at 70°C or above.
The pH range for bacteria to growth is from 4-9 and optimum pH is between
6.5-7.5. The water activity (aw) of Salmonella is 0.99 and 0.94, though they can subsist the water
activity of <0.2 for example in dried foods. The growth of bacteria is restricted at temperatures of <7°C, when pH is <3.8 and the water activity is <0.94.