SALMONELLASALMONELLA

SalmonellaSalmonella   is a Gram-negative facultative is a Gram-negative facultative rod-shaped bacterium belonging to family rod-shaped bacterium belonging to family Enterobacteriaceae,Enterobacteriaceae,

SalmonellaeSalmonellae live in the intestinal tracts of warm live in the intestinal tracts of warm and cold blooded animals. Some species are and cold blooded animals. Some species are ubiquitous. Other species are specifically ubiquitous. Other species are specifically adapted to a particular host. adapted to a particular host.

In humans, In humans, SalmonellaSalmonella are the cause of two are the cause of two diseases called diseases called salmonellosissalmonellosis: : – enteric feverenteric fever ( (typhoidtyphoid), ), resulting from bacterial resulting from bacterial

invasion of the bloodstream, andinvasion of the bloodstream, and – acute gastroenteritisacute gastroenteritis, , resulting from a foodborne resulting from a foodborne

infection/intoxicationinfection/intoxication. .

The taxonomy of the salmonellae has been The taxonomy of the salmonellae has been in flux for many years, and it is in flux for many years, and it is problematic, with more than 2463 problematic, with more than 2463 serotypes. serotypes.

Under the current Under the current American CDC (Center American CDC (Center for Disease Control)for Disease Control) classification scheme classification scheme there are two species: there are two species: Salmonella entericaSalmonella enterica and and Salmonella bongori.Salmonella bongori. S. enterica S. enterica is is further divided into 6 subspecies.further divided into 6 subspecies.

ClassificationClassification

Earlier classification system included Earlier classification system included – (1) the Kaufmanns-White system(1) the Kaufmanns-White system, which , which

identified each serotype as an individual identified each serotype as an individual Salmonella species, Salmonella species,

– (2) the Edwards-Ewing system(2) the Edwards-Ewing system, which , which divided the salmonellae into 3 species (divided the salmonellae into 3 species (S. S. choleraesuis, S. enteritidis, choleraesuis, S. enteritidis, and and S. typhiS. typhi) ) and hundred of serotypes, and and hundred of serotypes, and

– (3)(3) a DNA hybridization schemea DNA hybridization scheme that that lumped the salmonelae into two species lumped the salmonelae into two species known as known as S. enteritidis S. enteritidis and and S. bongori. S. bongori. S. enteritidisS. enteritidis is then subdivided this species into is then subdivided this species into

the subspecies the subspecies arizonae, diarizonae, enterica, arizonae, diarizonae, enterica, houtanae, indica and salamae.houtanae, indica and salamae.

S. enterica contains more than 2500 serotypes (2541 in l 2004) differentiated on the O and H- Antigens

Salmonella serotype (serovar) Typhimurium,

Salmonella serotype Enteritidis, Salmonella serotype Typhi, Salmonella serotype Paratyphi, Salmonella serotype Cholerae suis etc.

Ex.: Salmonella enterica subspecies enterica serovar Typhi or Salmonella Typhi

Although the classification of Although the classification of salmonellae relies primarily on salmonellae relies primarily on serotyping of surface antigens, the serotyping of surface antigens, the typhi serotypetyphi serotype can be differentiated can be differentiated from other serotypes on the basis of from other serotypes on the basis of its relatively inert biochemical its relatively inert biochemical behavior. behavior.

The The typhi serotypetyphi serotype is negative for is negative for Simmons citrate, gas from glucose, Simmons citrate, gas from glucose, acetate utilization, etc. acetate utilization, etc.

MorphologyMorphology

Gram – negativGram – negative rodse rods ununcapsulacapsulatedted ( (exceptexcept S. typhi) S. typhi) unsporulatedunsporulatedsporulaţisporulaţi Peritrichous flagella (ensure Peritrichous flagella (ensure

motility)motility)

Cultural propertiesCultural properties

Aerobe-anaerobe facultative Grow easily on simple culture

media Onto selective and differential

media that contain biliary salts and lactose grow like lactose-negative “S” colonies.

produce de H2S, colonies have a “cat-eye” appearance.

Biochemical propertiesBiochemical properties Motile,Motile, Lactose negative; Lactose negative; acid and gas from glucose, mannitol, maltose, and sorbitol;acid and gas from glucose, mannitol, maltose, and sorbitol; no Acid from adonitol, no Acid from adonitol, sucrose, salicin, lactosesucrose, salicin, lactose ONPG test negative (lactose ONPG test negative (lactose

negative)negative) Indole test negativeIndole test negative Methyl red test positiveMethyl red test positive Voges-Proskauer test negativeVoges-Proskauer test negative Citrate positive (growth on Simmon's citrate agar)Citrate positive (growth on Simmon's citrate agar) Lysine decarboxylase positiveLysine decarboxylase positive Urease negativeUrease negative OOrnithine decarboxylase positivernithine decarboxylase positive 2S produced from thiosulfate2S produced from thiosulfate Phenylalanine and tryptophan deaminase negativePhenylalanine and tryptophan deaminase negative Gelatin hydrolysis negativeGelatin hydrolysis negative

TSI (Triple Sugar Iron) TSI (Triple Sugar Iron) MIU (Motility Indole UreaMIU (Motility Indole Urea)). . Simmons Citrate mediumSimmons Citrate medium

Mechanisms of Mechanisms of PathogenicityPathogenicity

(1) Bacterial products involved in virulence:(1) Bacterial products involved in virulence: Salmonellae owe their pathogenicity largely to Salmonellae owe their pathogenicity largely to

their ability to their ability to invadeinvade tissue and to survive within tissue and to survive within macrophages. macrophages.

The Vi antigen is a capsule that affords The Vi antigen is a capsule that affords salmonellae some protection from phagocytosis.salmonellae some protection from phagocytosis.

Once phagocytosed, S. typhi inhibits generation Once phagocytosed, S. typhi inhibits generation of oxidative free radicals and intraphagosomal of oxidative free radicals and intraphagosomal killing. killing.

Additionally, salmonellae have endotoxic Additionally, salmonellae have endotoxic lipopolysaccharide, which is responsible for lipopolysaccharide, which is responsible for septic shock in patients with bacteriemia.septic shock in patients with bacteriemia.

Salmonellae that cause enteritis produce at least Salmonellae that cause enteritis produce at least two enterotoxinstwo enterotoxins that are responsible for many of that are responsible for many of the clinical signs of enteritis. the clinical signs of enteritis.

The firstThe first of these is a of these is a smallsmall (25-30kD) protein that (25-30kD) protein that binds to GM1 gangliosides and cause binds to GM1 gangliosides and cause hypersecretion of fluids and electrolytes by hypersecretion of fluids and electrolytes by elevating levels of c AMP. elevating levels of c AMP.

It appears that both proteinkinase C and It appears that both proteinkinase C and prostaglandin E2 are involved in this process. prostaglandin E2 are involved in this process.

The second enterotoxinThe second enterotoxin is is largerlarger (about 100 kD) (about 100 kD) and is unrelated in structure and mechanism of and is unrelated in structure and mechanism of activity to the first enterotoxinactivity to the first enterotoxin

Salmonella strains that produce enterotoxins have Salmonella strains that produce enterotoxins have been reported to invade the intestinal wall more been reported to invade the intestinal wall more effectively and to be more virulent than their effectively and to be more virulent than their nontoxigenic counterparts.nontoxigenic counterparts.

(2) The Salmonella infection cycle.(2) The Salmonella infection cycle.

Intestinal infection with salmonellae can Intestinal infection with salmonellae can follow one of two infection cycle. One follow one of two infection cycle. One cycle causes enteritis, other causes cycle causes enteritis, other causes typhoidtyphoid

(a) Enteritis(a) Enteritis. . Most serotypes cause enteritis, an Most serotypes cause enteritis, an

infection that is limited to the terminal infection that is limited to the terminal ileum. The salmonellae invade the ileum. The salmonellae invade the intestinal wall and produce enterotoxins intestinal wall and produce enterotoxins that cause nausea, vomiting and that cause nausea, vomiting and diarrhea. Bacteria rarely spread beyond diarrhea. Bacteria rarely spread beyond the gastrointestinal wall.the gastrointestinal wall.

(b) Enteric fever (Typhoid).(b) Enteric fever (Typhoid). Two serotypes Two serotypes typhi typhi and and paratyphi paratyphi can cause can cause

typhoid.typhoid. The salmonella invade the wall of the terminal The salmonella invade the wall of the terminal

ileum and than spread to the intestinal ileum and than spread to the intestinal lymphatics, where they are phagocytosed by lymphatics, where they are phagocytosed by PMNs and macrophages. PMNs and macrophages.

Salmonella phagocytosed by PMNs are killed, Salmonella phagocytosed by PMNs are killed, but those phagocytosed by macrophages but those phagocytosed by macrophages survive and multiply within phagocytic vacuoles. survive and multiply within phagocytic vacuoles.

Wandering macrophages that contain Wandering macrophages that contain salmonellae act as “taxicabs” that deliver salmonellae act as “taxicabs” that deliver salmonellae to various reticuloendothelial salmonellae to various reticuloendothelial tissues. tissues.

Infected macrophages are eventually destroyed Infected macrophages are eventually destroyed and salmonellae released from lysed and salmonellae released from lysed macrophages cause septicemia.macrophages cause septicemia.

Some salmonellae begin to Some salmonellae begin to disseminate hematogenously to a disseminate hematogenously to a variety of ectopic sites, including the variety of ectopic sites, including the bones, lungs, liver, brain where they bones, lungs, liver, brain where they cause osteomyelitis, pyelonephritis, cause osteomyelitis, pyelonephritis, empyema, hepatic necrosis, empyema, hepatic necrosis, meningitis. meningitis.

Other salmonella remain in the Other salmonella remain in the intestine, where they invade the gut intestine, where they invade the gut wall and may cause ulceration, wall and may cause ulceration, perforation and hemorrhage. perforation and hemorrhage.

Salmonellae multiply avidly in the Salmonellae multiply avidly in the gallbladder and bile, and the infected gallbladder and bile, and the infected bile continues to circulate salmonellae bile continues to circulate salmonellae to the intestine. to the intestine.

Salmonellae also multiply well in gut Salmonellae also multiply well in gut associated lymphoid tissue and may associated lymphoid tissue and may ulcerate Payer’s patches ulcerate Payer’s patches

Epidemiology:Epidemiology:

In many countries Salmonella enteritis In many countries Salmonella enteritis is the third most commonly reported is the third most commonly reported form of “food poisoning”. form of “food poisoning”.

The infection is zoonotic, and the The infection is zoonotic, and the poultry is the source of infection. poultry is the source of infection.

Other sources of infection include milk Other sources of infection include milk products, food and water products, food and water contaminated with animal feces or contaminated with animal feces or urine urine

(1) Enteritis(1) Enteritis

8-48 hours after the ingestion of food or drink 8-48 hours after the ingestion of food or drink contaminated with Salmonella, enterocolitis contaminated with Salmonella, enterocolitis begins with nausea, vomiting, abdominal begins with nausea, vomiting, abdominal pain, diarrhea which can vary from mild to pain, diarrhea which can vary from mild to severe.severe.

In some cases manifestation include fever, In some cases manifestation include fever, headache and chills. headache and chills.

Salmonella enteritis last about 5 days, but Salmonella enteritis last about 5 days, but severe loss of fluids and electrolytes may be severe loss of fluids and electrolytes may be lifethreatening in infants and elderly patients. lifethreatening in infants and elderly patients.

Recovery from enteritis does not confer Recovery from enteritis does not confer immunity against reinfectionimmunity against reinfection

(2) Enteric fever (2) Enteric fever (typhoid) (typhoid)

About 7-14 days after ingesting salmonellae, About 7-14 days after ingesting salmonellae, patients begin to develop symptoms and signs patients begin to develop symptoms and signs of typhoid, includingof typhoid, including– anorexia,anorexia,– lethargy, lethargy, – a dull frontal headache, a dull frontal headache, – a nonproductive cough, a nonproductive cough, – abdominal pain, abdominal pain, – fever up to 40 C, fever up to 40 C,

At this time, there are no salmonellae detected At this time, there are no salmonellae detected in the blood, and leukocyte count is normal.in the blood, and leukocyte count is normal.

By the second or third week of disease, By the second or third week of disease, salmonellae have escaped macrophages and the salmonellae have escaped macrophages and the patient is severely ill.patient is severely ill.

Rose spots may appear on the trunk and they Rose spots may appear on the trunk and they contain salmonellae. In some cases, patients contain salmonellae. In some cases, patients suffer from delirium. suffer from delirium.

If peyer pathes become perforated, peritonitis If peyer pathes become perforated, peritonitis may develop. may develop.

Dissemination of salmonellae in ectopic foci may Dissemination of salmonellae in ectopic foci may result in liver necrosis, empyema, meningitis, result in liver necrosis, empyema, meningitis, osteomyelitis, endocarditis. osteomyelitis, endocarditis.

The fatality rate is 2-10%The fatality rate is 2-10% Recovery is a prolonged process, that last for a Recovery is a prolonged process, that last for a

month or longer, and it confers a lifelong month or longer, and it confers a lifelong immunity against typhoid.immunity against typhoid.

(3) Primary septicemia(3) Primary septicemia

Patients with anemia may Patients with anemia may develop septicemia after develop septicemia after asymptomatic ileal infection with asymptomatic ileal infection with S. choleraesiusS. choleraesius..

Manifestation include spiking Manifestation include spiking fever, weight loss, anorexia, fever, weight loss, anorexia, anemia, bacteriemia, bacteremia, anemia, bacteriemia, bacteremia, hepatosplenomegaly. hepatosplenomegaly.

DiagnosisDiagnosis The diagnosis of salmonellosis requires The diagnosis of salmonellosis requires

bacteriologic isolation of the organisms from bacteriologic isolation of the organisms from appropriate clinical specimens. appropriate clinical specimens.

Laboratory identification of the genus Laboratory identification of the genus Salmonella is done by biochemical tests; the Salmonella is done by biochemical tests; the serologic type is confirmed by serologic serologic type is confirmed by serologic testing. testing.

Feces, blood, or other specimens should be Feces, blood, or other specimens should be plated on several nonselective and selective plated on several nonselective and selective agar media (blood, MacConkey, eosin-agar media (blood, MacConkey, eosin-methylene blue, bismuth sulfite, Salmonella-methylene blue, bismuth sulfite, Salmonella-Shigella, Hektoen agars) as well as Shigella, Hektoen agars) as well as intoenrichment broth such as selenite or intoenrichment broth such as selenite or tetrathionate. tetrathionate.

The biochemical reactions of suspicious The biochemical reactions of suspicious colonies are then determined on triple sugar colonies are then determined on triple sugar iron agar and lysine-iron agar, and a iron agar and lysine-iron agar, and a presumptive identification is made. presumptive identification is made.

Biochemical identification of salmonellae Biochemical identification of salmonellae has been simplified by systems that permit has been simplified by systems that permit the rapid testing of 10-20 different the rapid testing of 10-20 different biochemical parameters simultaneously. biochemical parameters simultaneously.

The presumptive biochemical identification The presumptive biochemical identification of Salmonella then can be confirmed by of Salmonella then can be confirmed by antigenic analysis of O and H antigens using antigenic analysis of O and H antigens using polyvalent and specific antisera. polyvalent and specific antisera.

Salmonella isolates then should be sent to a Salmonella isolates then should be sent to a central or reference laboratory for more central or reference laboratory for more comprehensive serologic testing and comprehensive serologic testing and confirmation. confirmation.