Type: Process Essays
Sample donated: Pearl Poole
Last updated: September 28, 2019
INRODUCTIONSalmonellosis is aserious intestinal infection caused by Salmonella,which is rod-shaped Gram-negative bacteria. The bacteria mainly infectshumans and domestic animals. It is an important foodborne pathogen whichoverburden economy of both developed and underdeveloped countries through thecosts associated with diagnosis, prevention, control and treatment.
Salmonella is predominantly present ineggs, dairy and poultry, it can also be transmitted through fruits andvegetables which includes mangoes, apples, tomato, celery, cantaloupe andlettuce. 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 slaughterhouseswhere it can contaminate organs of food animals.CLASSIFICATIONAND NOMENCLATUREDiscovery and isolationof Salmonella was first conducted byTheobald Smith in 1855 from the intestines of pigs that were infected by swinefever.
An American Pathologist, Dr Daniel Elmer Salmon named this bacteria andhe also worked with Smith. Salmonellacan be classified into two species this nomenclatural system is manipulated byCenters for Disease Control and Prevention, the species comprise of Salmonella enterica (type species) and Salmonella bongori. There are foursubspecies of S. enterica which canbe be represented by Roman numerals, I, S.
enterica subsp. enterica; II, S.enterica subsp. salamae; IIIa, S. enterica subsp. arizonae; IIIb, S.
entericasubsp. diarizonae; IV, S. enterica subsp. houtenae; and VI, S. enterica subsp.
indica.The most prevalent is enterica (I) which causes infections in warm-bloodedanimals 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 incold-blooded animals.By using Kauffman-Whitescheme 2600 serotypes of Salmonella areidentified but they can easily adapt to animal hosts and humans. Kauffmann-Whiteclassification formulated a system to classify Salmonella according to serotype by using three antigenicdeterminants which includes, somatic (O), flagellar (H) and capsular (K). Oantigen plays essential role in protecting membrane from heat, it is composedof oligosaccharide constituent of lipopolysaccharide, and this antigen issituated at bacterial cell membrane.
In contrast the bacterial flagella knownas H antigens is instable in heat, it helps to stimulate host immune responses.Some flagellar proteins are diphasic which means they have unique capability toexpress specific protein at a time. The rarest antigens of Salmonella is the K antigens which is positioned at bacterialcapsule. Particular subtype of K antigen known as Virulence (Vi) which is foundin serotypes: Dublin, Paratyphi C and Typhi. In 1934 the InternationalAssociation of Microbiologists adopted this. A simple technique used for genusidentification of Salmonella and epidemiologicalstudy is Agglutination by antibodies particularly employing surface O antigensresulting in six serogroups of Salmonellae;A, B, C1, C2, D and E. Presently, the nomenclature system which is adoptedexcludes the information about Salmonellasubspecies, for example S.enterica serovarTyphimurium is shortened to S.
Typhimurium. PATHOGENESISSalmonellosis targets toelderly, children who are below 5 years or individuals which areimmunosuppressed are presumably to develop Salmonellainfection. However, the infection extremity depends on the serotype and thehealth condition of patient, patients who are suffering Salmonellosis arelikely to get burdened with reactive arthritis (it is painful arthritis). Salmonella invade host cells, where itreplicates and survives, consequently causing lethal and life threateningdisease in humans.Infectious cyclebegins when the bacteria is live and then it gains entry in gastrointestinaltract. The localized form initiates when living Salmonella invades small intestine and proliferates in tissues. Thechromosomal DNA of bacteria consists of gene collections which are responsiblefor invasion of epithelial, macrophagic and dendritic cells that are known as Salmonella Pathogenicity Islands (SPIs).
Salmonella contaminated food or waterenters digestive tract, and tries to perforate the epithelial cells ofintestinal wall. The bacteria travels from the lymphatic system into the bloodwhich is known as the Typhoid form, and then the bacteria resides in differentorgans which includes, kidneys, liver and spleen, to produce Septic form. Theendotoxins which were produced by dead bacteria damage affected organs showingincrease in permeability, disturbed thermal regulation moreover vomiting anddiarrhea can also be manifested. The effectors of Salmonella are introduced into thecytoplasm through type III secretion system which are encodedby SPIs. Signal transduction pathway is triggered by effectors which causes reassemblyof actin cytoskeleton of the host cell, when cell membrane is disheveled thebacteria is engulfed. This process of membrane disarrangement is similar toPhagocytosis.
After engulfment of Salmonellainto the host cell, it isenclosed in a membrane compartment i.e. a vacuole. When a foreign particleenters human body, it activates host cell immune response, this causes in theamalgamation of lysosome and the secretion of digesting enzymes, this willdegrade intracellular bacteria.
The effector proteins are inserted in vacuolethrough type III secretion system which causes disarrangement of the enclosedstructure. Salmonella now can surviveand replicate inside host cells because the reconstructed vacuole stops thefusion of the lysosomes. This efficiency of bacteria to survive and replicateinside macrophages guides it to transport to the Reticuloendothelial system. Thecontrol and spread of bacteria is constrained by reticuloendothelial system.Some serovars may cause infection in liver, spleen, gallbladder, bones or otherorgans, but gastroenteritis which is common human infection is restricted tothe intestine. After successfully invading the intestine, Salmonella may cause ulceration whichinduces an acute inflammatory response.
This may cause the production ofcytotoxins that inhibit protein synthesis. Various proinflammatory cytokinesare produced when the mucosa of the epithelial cells is invaded, theseinflammatory cytokines includes, IL-1, IL-6, IL-8, TNF-2, MCP-1, and GM-CSF.These cytokines stimulates inflammatory response leading direct damage to theintestines. Inflammation of intestine may contribute to symptoms such aschills, fever, leukocytosis, abdominal pain and diarrhea. Patient’s stool maycontain blood, mucus, polymorphonuclear leukocytes. The strains which infect intestinal mucosa causessevere inflammatory response and patient may suffer from diarrhea.
Small andlarge intestines discharge fluid and electrolytes, resulting in diarrhea. Thebacteria may spread and cause systemic infection after passing from the basalside of the epithelial cells into the lamina propria. This unusual spread oforganism gives rise to enteric fever. The activation of adenylate cyclaseoccurs leading to higher levels of cyclic AMP secretion. Adenylate cyclasestimulates the production of prostaglandins (activates inflammatory reaction)or other elements of inflammatory response. Furthermore, intestinal secretionis also possible due to other enterotoxin-like substances produced from somestrains of Salmonella. Inpathogenesis of Salmonella the basicprinciple of these toxins is unknown. Figure 1: Pathogenesis of Salmonella Figure 2: Flowchart of Salmonella enterocolitis and diarrhea Figure 3 Invasion of intestinalmucosa by Salmonella CLINICALMANIFESTATIONSThe strains of Salmonellacan be classified into Typhoid Salmonellaand Non-Typhoidal Salmonella (NTS).
The different types of human infections caused by NTS can be categorized into,gastroenteritis, bacteremia, endovascular and localized infections. Gastroenteritis NTS strains differ from S. Typhiand S. Paratyphi CHARACTERISTIC Salmonella does not require complex nutritional requirement forgrowth i.e. they are non-fastidious.
They can grow and multiply outside hostcells rapidly. The optimum temperature for some serotypes of Salmonella ranges from 35°C-37°C, butthey can also grow between 2°C-4°C or at 54°C. The bacteria can grow in thepresence of 0.
4%-4% sodium chloride. Salmonellacannot 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 between6.5-7.
5. The water activity (aw) of Salmonella is 0.99 and 0.94, though they can subsist the wateractivity of <0.2 for example in dried foods. The growth of bacteria isrestricted at temperatures of <7°C, when pH is <3.8 and the wateractivity is <0.