**2.** *Salmonella* **as a pathogenic Bacteria**

*Salmonella* is a Gram-negative facultative rod-shaped bacterium in the same family as *Escherichia coli,* Enterobacteriaceae. *Salmonella* live in the intestinal tracts of warm and cold blooded animals. In humans, *Salmonella* are the cause of two diseases called salmonellosis: enteric fever (typhoid), resulting from bacterial invasion of the bloodstream, and acute gastroenteritis, resulting from a foodborne infection/ intoxication. Most common species related to human infections are *Salmonella Typhi* and *Salmonella Paratyphi* cause host-specific infections, being human the host. But other serotypes as *S.Typhimurium* and *S. Enteritidis*, mainly related to food products are also important serotypes that cause human diseases [5, 6].

*Salmonella* are found in the natural environment (water, soil, sometimes plants used as food). These bacteria are zoonotic, human or animal can excrete *Salmonella* either when they are infected with disease Salmonellosis and when they remain carriers. Also this disease is called food handling born disease due to the infected or carrier food handling workers [7].

*Salmonella* is intracellular parasite pathogen, do not multiply significantly in the natural environment (out of digestive tracts), but they can survive several weeks in water and several years in soil if conditions of temperature, humidity, and pH are favorable [5].

#### **2.1 Antigenic structure and virulence factors**

The bacterial antigens are the components or products of pathogens that are able to induce the immune defenses of the host to defend against, and to eliminate, the pathogen or disease. As with all *Enterobacteriaceae*, the genus *Salmonella* has three kinds of major antigens with diagnostic or identifying applications: somatic, surface, and flagellar [8–10].

#### *2.1.1 Somatic (O) or cell wall antigens*

Somatic antigens are the O side chain of LPS; they are heat stable and alcohol resistant. Cross-absorption studies individualize a large number of antigenic factors, 67 of which are used for serological identification. O factors labeled with the same number are closely related.

#### *2.1.2 Surface (envelope) antigens*

Surface antigens, commonly in enteric bacteria (e.g., *Escherichia coli and Klebsiella*), may be found in some *Salmonella*. Surface antigens in *Salmonella* may mask O antigens, and the bacteria will not be agglutinated with O antisera. One specific surface antigen is well known: the Vi antigen. The Vi antigen occurs in only three species *Salmonella Typhi*, *Salmonella Paratyphi* C, and *Salmonella dublin*.

#### *2.1.3 Flagellar (H) antigens*

Flagellar antigens are heat-labile proteins. Mixing *Salmonella* cells with flagellaspecific antisera gives agglutination. Also, anti-flagellar antibodies can immobilize bacteria H antigens. Antigenic changes of the flagella known as the phase variation of H1 and H2 occurs in *Salmonella Typhimurium*.

#### *2.1.4 Exotoxins*

*Salmonella* strains may produce a thermos-labile enterotoxin that which has a limited relatedness to cholera toxin and *E. coli* (enterotoxin LT) in both structurally and antigenically characters. Additionally, a cytotoxin that inhibits protein synthesis. Both of these toxins play a role in the diarrheal symptoms of Salmonellosis.

#### **2.2** *Salmonella* **between pathogenicity and immunity**

Innate immunity barriers play a good role in defense against *Salmonella* adhesion and colonization. But, upon infection specific immunity come to act; both humoral and cellular specific immune response will be activated to control this infection.

Primary infections with *S. Typhi* or *Salmonella ParaTyphi* usually induce a degree of immunity. Reinfection may occur but is often milder than the first infection. Circulating antibodies to O and Vi are related to resistance to infection and disease [11, 12].

*Salmonella* infections in humans vary with the bacterial species, the infectious dose upon ingested contaminated food, and the host health. The oral dose of at least 105 *Salmonella Typhi* cells are the most effective dose to cause typhoid in 50% of human volunteers as agreed by many references, whereas at least 109 *S. Typhimurium* cells (oral dose) are needed to cause symptoms of a toxic infection. Infants, immunosuppressed patients, and those affected with blood disease are more susceptible to *Salmonella* infection than healthy adults [12].

#### **2.3 Salmonellosis (Typhoid fever)**

In the **pathogenesis** of typhoid the bacteria enter the human digestive tract, penetrate the intestinal mucosa (causing no lesion), and stope in the mesenteric lymph nodes. Enteric Fever, Salmonellosis or Enterocolitis occurs after attachment to enterocytes of the ileum and colon. About 12-24 hours following ingestion of contaminated food (containing a sufficient number of *Salmonella*), the ingested *Salmonella* reach the small intestine, from which they enter the lymphatics and then the bloodstream. They are carried by the blood to many organs, including the intestine. Then *Salmonella* cells will attach to Microfold cells (M cells) and dendric cells of the jejunum. These specialized epithelial cells are found in the Peyer's patches and initiate mucosal immunity by endocytosis process for these bacteria antigens to the Macrophages and B-Lymphocytes to form APC specific for these antigens. Invasion can occur in this stage via the means of endocytosis, transfer, and exocytosis. Phagocytosis in the subserosa by macrophages and translocation into the mesenteric lymph nodes. Lymphogenous and hematogenous dissemination combined by immune cells proliferation and specific immune response is integrated. Of course the complement system upon these events is already activated, since LPS layer can activate the alternative pathway as soon as this endotoxin liberated from bacterial cells due to destruction leading to more inflammatory reactions at the site of invasion as described in the complement system roles. Moreover, the mannose residues that are found on the surface of *Salmonella* undergo lectin pathway activation of the complement system [5–12].

The organisms usually multiply in intestinal lymphoid tissue and are excreted in stools. However, in the case of *S. Typhi*, the bacteria survive ingestion by the phagocytes, and multiply within these cells. This period of time, during which the bacteria are multiplying within the phagocytes, is the 10–14 day is known as the incubation period. When huge numbers of bacteria fill an individual phagocyte, the bacteria are discharged out of the cell and into the bloodstream, where their presence begins to cause symptoms. Secondary foci in the spleen, liver, bone marrow, bile ducts, skin (roseola), and Peyer's patches then develop [5].

The presence of increasingly large numbers of bacteria in the bloodstream (called bacteremia) is responsible for an increasingly high fever, rising in stages throughout the first week to 39/40/41°C and may last throughout the four to eight weeks of the disease, in untreated individuals. Other symptoms include constipation (initially), extreme fatigue, headache and joint pain. Further symptoms:

leukopenia, bradycardia, splenic swelling, abdominal roseola, beginning in the third-week diarrhea, sometimes with intestinal bleeding due to ulceration of the Peyer's patches and inflammation of the gallbladder, severe irritation and inflammation of the lining of the abdominal cavity, called peritonitis, which is frequently a fatal outcome of typhoid fever [5, 12].

From the mesenteric lymph nodes, viable bacteria and LPS (endotoxin) will be released into the bloodstream resulting in septicemia. Moreover the effect of LPS as pyrogenic toxin, it causes activation of the complement alternative pathway which ends with membrane attack complex MAC, and that will increase LPS levels in bloodstream due to breakage of more bacterial cells leading to more harmful pyrogenic effects. The fever rises to a high plateau, and the spleen and liver become enlarged. Rose spots or rash usually on the skin of the abdomen or chest may be seen in some cases. Another scientific fact, LPS can induce both T-Dependent and T-Independent specific immune response. Specific antibodies against *Salmonella* antigens will be formed after primary infection occurrence, but, T-independent specific antibodies are with no memory B-cells formation. And that is the cause of short time specific immunity resultant after *Samonella* infections according to many scientists' opinions [7].

The complications of typhoid fever include liver and spleen enlargement (sometimes so extreme that the spleen ruptures), anemia (low red blood cell count due to blood loss from the intestinal bleeding), joint infections (especially frequent in patients with sickle cell anemia and immune system disorders), pneumonia (due to a superimposed infection, usually by *Streptococcus pneumoniae*), heart infections, meningitis, and infections of the brain (causing confusion and even coma). Untreated typhoid fever may take several months for full resolve. Spontaneous cure usually occurs [12].

#### **2.4 Immune response features of** *Samonella*

Due to that *Salmonella* behave as intracellular parasite inside host, these bacteria can survive inside phagocytic cells and escape the immune system meeting. Escape of destruction inside phagocytic cells like macrophage referred to the resistance of *Salmonella* to the oxidative burst used by these immune cells to kill and digest invading bacterial cells. Phagocytosis is the key process for induction of specific immune response but in the condition of contact with invading microbes and process the antigens to become APCs. Hence encountering of these bacteria sometimes can be late due to lack facing with immune cells and bacteria hide inside phagocytic cells. Also in certain circumstances as in immunocompromised patients like diabetic and old people, they usually suffer from late or weak immunological response against *Salmonella* and almost become carriers when the diagnosis and treatment are late [7, 8].

*Salmonella* induce both Th1 immunity (T-helper 1 or immunity or cellular mediated immunity) and Th2 immunity (T-helper 2 or humoral mediated immunity). When APC formed, then the immune response will be turned from innate or non-specific immunity to the specific humoral and cellular immune response, APC will present the processed Antigens of *Salmonella* to the cells of the specific immunity. Concerning Humoral mediated Immunity, specific IgG, IgM and IgA antibodies are formed against *Samonella* antigens, LPS- O antigen Vi antigen and H-antigen. Agglutinating antibodies can give positive reaction after one week post symptoms rise according to Gruber-Widal against H and O *Salmonella* antigens then the antibodies titer continue to elevate with infection time going. The white blood cell count can be found as normal or low at these stages [7, 8].

Salmonella *and the Immune System DOI: http://dx.doi.org/10.5772/intechopen.95673*

Antigenic variation can occur due to that *Salmonella* is able to generate genetic exchange and mutation abilities leading to the flagellar phase during infection course. This phenomena will cause the sero-variation and disease phases properties that is usually a characteristic of the infectious disease resultant from *Salmonella*.

Cellular mediated immunity is induced after APC formation, since *Salmonella* act as intracellular parasite and multiply inside macrophages. Then specific activated Cytotoxic T-cells will be produced and specific T-memory cells are released. Some scientists attribute joints inflammation that combined with Typhoid infections to the cellular immune response and due to accumulation of antigen-antibody complexes in patients' joints mainly during high bacterial load infections [7–12].

Cytokines of both Th1 and Th2 levels increase during Salmonellosis, Interlukins (IL-1, IL-2, IL-4, IL-6, IL-8, IL-9, IL-10, IL-13, IL-15, IL-17). Also Interferongamma (IFN-γ) play a great role during cellular immune response and its levels elevates in patients' blood even after cure. Another important cytokine is Tumor necrosis factor-alpha (TNF-α), its levels raise upon infection start and stay elevated along the disease time [5, 12].

#### **2.5 Carriers**

After manifest or subclinical infection, some individuals continue to harbor *Salmonella* in their tissues for different times (for example convalescent carriers or healthy permanent carriers). Three percent of survivors of typhoid become permanent carriers, harboring the organisms in the gallbladder; biliary tract; or, rarely, the intestine or urinary tract. Carriers of *S. Typhi* must be treated even when asymptomatic, as they are responsible for the majority of new cases of typhoid fever. Eliminating the carrier state is actually a difficult and require two different medications for four to six weeks at least.

In the case of carriers with gall stones, surgery may need to be performed to remove the gall bladder, because the *S. Typhi* bacteria are often housed in the gall bladder, where they may survive despite antibiotic treatment [7, 8, 12].

#### **2.6 Re-infections and healthy carriers**

Despite of that some patients with *Salmonella* will get spontaneous cure, *Salmonella* excretion by human patients may continue long after clinical cure. About 5% of patients clinically cured from typhoid remain carriers for months or even years. Antibiotics are sometimes ineffective on *Salmonella* but can reduce mortality which may reach was 10% [9].

However, relapses may occur in 2–3 weeks after recovery despite specific antibodies titer rise. Secretory IgA antibodies may prevent attachment of *Salmonella* to intestinal epithelium during next time exposure and avoid secondary infection establishment.

Some genetic factors can make person susceptible host for re-infection easier like persons with S/S hemoglobin (sickle cell disease) are susceptible to *Salmonella* infections. Persons with A/S hemoglobin (sickle cell trait) may be more susceptible than normal individuals (those with A/A hemoglobin) [9, 10].

The incidence of human disease decreases when the level of development of a country increases (like controlled water sewage systems, improve hygiene, pasteurization of milk and dairy products). Bad ways in having food like eating raw or undercooked egg can cause illness due to these bacteria called *Salmonella* Enteritidis Infection or Egg-associated salmonellosis which is an important public health problem.

Plasmid-borne antibiotic resistance is very frequent among and can be considered as a virulence factor upon ongoing infections. *Salmonella* strains can get resistance against ampicillin, streptomycin, kanamycin, chloramphenicol, tetracycline, and sulfonamides [11–13].

#### **2.7 Vaccination against Typhoid fever**

Vaccination is very good health measure in eradication of *Salmonella* infections. Tourists and visitors for countries endemic with *Salmonella* must be vaccinated with *Salmonella* vaccines as a prophylactic health measure.

Early research produced two vaccines made from the entire (whole-cell) bacterium. The first one became available in the 1890s, the second in 1952. Both protected about 65% of recipients. However, the frequency and severity of the adverse effects they caused dissuaded many countries from using them. These shortcomings, combined with drug treatment failures, as a result of increasingly widespread resistance to antibiotic therapy [12–14].

Before the end of the 20th century, two new-generation typhoid vaccines had entered the scene. One, named (Ty21) and first licensed in 1983, is given in three to four oral doses and consists of a live but genetically modified *S. Typhi* strain. The second, named "Vi" and licensed in 1994, is given by injection and consists of a sugar molecule (polysaccharide) located on the surface of the bacterium. In clinical trials and early field use, the duration of efficacy of both vaccines varied to some degree. Moreover, no evidence of efficacy has been reported in children under two years of age [15]. Both vaccines are licensed, internationally available, and safe, and both are effective enough not only to reduce the incidence of typhoid fever in endemic areas but also to control outbreaks [9–11].

Meanwhile, third-generation typhoid vaccines are under trial. One is a Vi conjugate vaccine that protects about 85% of recipients, according to late-stage clinical trials, and appears to be effective in children under two years of age. A second candidate vaccine, is, like Ty21a, a live attenuated vaccine but, unlike Ty21a, can be given in a single oral dose [15].

Three types of typhoid vaccines are currently available for use nowadays:

1.Oral live-attenuated vaccine.

2.Heat-phenol-inactivated vaccine; killed bacterial vaccine.

3.The Vi capsular polysaccharide vaccine for intramuscular use.

A fourth vaccine, an acetone-inactivated parenteral vaccine, is currently available only to the armed forces. While Typhoid fever vaccinations for tourists and travelers to the endemic areas is best be done with the oral attenuated vaccine Virotif Ty 21a.

Despite of that; No typhoid vaccine is 100% effective and provide only shortterm protection (sometimes for a few months), it is not a substitute for being careful and elevate hygiene [15].

#### **3. Conclusions**

*Salmonella*, whatever species, is dangerous microbe that is able to invade human body due to many weapons owned. Good and healthy immune system can stand against these bacteria. But with bad nutrition, low hygiene and immunosuppression;

#### Salmonella *and the Immune System DOI: http://dx.doi.org/10.5772/intechopen.95673*

infection with *Salmonella* will occur upon exposure and may develop to a systemic disease. *Salmonella* induce human immunity with different types of resistance processes, either specific (adaptive immune response) or non-specific (Innate Immune Response) that act against these bacteria and lead to cure during treatment course. Despite of many effective vaccines that have been produced; elevation personal hygiene is still the best way to eradicate this infectious disease. Healthy carriers of *Salmonella* are a public health problem.
