salmonella

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SalmonellaeSalmonellae

Are often pathogenic for humans or animals when acquired by oral route

Transmitted from animals and animal products to humans, cause enteritis, systemic infection and enteric fever

ClassificationIs complex, organisms are a

continuum rather than a defined species

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Salmonellae, cont.Salmonellae, cont.

Family: enterobacteriaceaeOn the basis of epidemiology, host

range, biochemical reactions and structures of O, H, and Vi (when present) antigens Kauffmann-White classification system– E.g. Salmonella typhi, Salmonella typhimurium

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Salmonellae, cont.Salmonellae, cont.O Group Serotype Antig. FormulaD S. Typhi 9,12 (Vi):d:-A S. paratyphi A 1,2, 12:1-C1 S. choleraesuis6,7:c:1,5B S. Typhimurium1,4,5,12:i:1,2D S. Enteritidis 1,9, 12:g, m:-O antigens: boldface numerals(Vi): Vi antigen if presentPhase 1 H antigen: lower-case letterPhase 2 antigen: numeral

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Salmonellae, cont.Salmonellae, cont.

Based on DNA-DNA hybridization studies– 7 evolutionary groups

DNA hybridization Group I– Nearly all salmonella serotypes that

infect humansSalmonella enterica subspecies

enterica

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Salmonellae, cont.Salmonellae, cont.

Nucleic Acid HybridizationNucleic Acid Hybridization

• Ability of DNA strands from one organism to hybridize with the DNA strands of another organism

• Southern blotting

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Salmonellae, cont.Salmonellae, cont.

Species name Salmonella enterica widely acceptable nomenclature– Salmonella enterica subspecies

enterica serotype Typhimurium Salmonella Typhimurium

Rare human infections with groups IIIa and IIIb

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Salmonellae, cont.Salmonellae, cont.

> 2500 serotypes of salmonellaeIncluding >1400 in DNA hybridization

group I that can infect humans4 serotypes that cause enteric fever

identified in clinical laboratory by biochemical and serologic tests

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Salmonellae, cont. Salmonellae, cont. Should be routinely identified because

of clinical significance– Salmonella Paratyphi A (serogroup

A)– Salmonella Paratyphi B (serogroup

B)– Salmonella Choleraesuis (serogroup

C1) and– Salmonella Typhi (serogroup D)

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Salmonellae, cont.Salmonellae, cont.

>1400 other salmonellae are serogrouped by O antigens as A, B, C1, C2, D, and E; some nontypeable

Isolates sent to reference laboratories for definitive serologic identification

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Salmonellae, cont.Salmonellae, cont.

Morphology and IdentificationVary in lengthMost isolates are motile with peritrichous

flagellaGrow readily on simple mediaAlmost never ferment lactose or sucroseForm acid and sometimes gas from glucose

and mannose

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Salmonellae, cont.Salmonellae, cont.

Usually produce H2S

Survive freezing in water for long periods

Resistant to brilliant green, sodium tetrathionate, sodium deoxycholate that inhibit other enteric bacteria

Useful for inclusion in media to isolate salmonellae from feces

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Salmonellae, cont. Salmonellae, cont. VariationMay lose H antigens and become nonmotileLoss of antigen is associated with a change

from smooth to rough colony formVi antigen may be lost partially or

completelyAntigens may be acquired or lost in the

process of trunsduction

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Salmonellae, cont.Salmonellae, cont.Pathogenesis and Clinical FindingsS. Typhi, S. Choleraesuis, S.

Paratyphi A and S. Paratyphi B are primarily infective for humans– Infection implies acquisition from a

human sourceVast majority chiefly pathogenic in

animals that constitute reservoir for human infection– Poultry, pigs, rodents, cattle, pets

(turtles to parrots)

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Salmonellae, cont. Salmonellae, cont. Almost always enter via oral route

usually with contaminated food or drinkMean ID 105 – 108 salmonellae, perhaps

as few as 103

Host factors that contribute to resistance to infection– Gastric acidity– Normal intestinal microbial flora and – Local intestinal immunity

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Salmonellae, cont.Salmonellae, cont.

Produce 3 main types of disease in humans– Enteric fever (typhoid fever)– Bacteremia with focal lesions and – enterocolitis

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Salmonellae, cont.Salmonellae, cont.

Enteric Fevers Septicemias Enterocolitis

IP 7-20 days Variable 8-48 hrs

Onset Insidious Abrupt Abrupt

Fever Gradual, high Rapid rise, Usually low

plateau spiking septic T

Durn Several wks Variable 2-5 days

of dis.

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Salmonellae, cont. Salmonellae, cont. Enteric Fevers Septicemias Enterocolitis

GI sy. Early constipation, Often none Nausea, vomiting

bloody diarrhoea diarrhoea at onset

Blood +ve in 1st to 2nd +ve during Negative

Culture wk of dis. high fever

Stool -ve earlier dis.; Infrequently +ve soon after

Culture+ve 2nd wk on+ve onset

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Salmonellae, cont.Salmonellae, cont.

Diagnostic Laboratory TestsSpecimensBlood for culture must be taken

repeatedly– In enteric fevers and septicemia,

blood cultures often +ve in 1st wk of disease

Bone marrow cultures may be usefulUrine cultures may be +ve after 2nd wk

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Salmonellae, cont.Salmonellae, cont.

Stool specimens must be taken repeatedly

In enteric fevers, stools yield +ve results from 2nd or 3rd wk on– In enterocolitis, during 1st wk

A +ve culture of duodenal drainage establishes presence of salmonellae in the biliary tract in carriers

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Salmonellae, cont.Salmonellae, cont.

Bacteriologic Methods for Isolation

Differential medium culturesEMB, MacConkey’s or deoxycholate

medium permits rapid detection of lactose non-fermenters

Gram +ve organisms are somewhat inhibited

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Salmonellae, cont.Salmonellae, cont.Bismuth sulfite medium permits

rapid detection of salmonellae

– form black colonies because of H2S production

Many salmonellae produce H2S

Selective medium cultures On SSA, Hektoen agar, XLD or

DCA, favor growth of salmonellae and shigellae over other enterobacteriaceae

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Salmonellae, cont.Salmonellae, cont.

Hydrogen sulphide production

•Certain bacterial species have the ability to liberate sulphurfrom sulphur containing amino acids

•The end point is an insoluble, heavy metal sulphide which produces a black precipitate

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Salmonellae, cont.Salmonellae, cont.

H2S Production in SIM• How to Perform Test: Stab SIM media with inoculating needle.

• Property it tests for: This test is used to help differentiate species of the family Enterobacteriaceae. This test is used to determine the ability to reduce sulfur into H2S.

• Media and Reagents Used: SIM media contains the sulfur containing amino acid, cysteine, sodium thiosulfate, & peptonizediron or ferrous sulfate.

• Reading Results: H2S will react with the iron or ferrous sulfate and produce a black precipitate. A positive result has a black precipitate present and a negative result has no black precipitate.

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Salmonellae, cont.Salmonellae, cont.

Enrichment culturesOn selenite F or tetrathionate broth,

inhibit replication of normal intestinal bacteria

Permit multiplication of salmonellaeAfter incubation for 1-2 days, plated

on differential and selective media

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Salmonellae, cont.Salmonellae, cont.

Selective enrichment media

• Selenite cystine broth

• Gram negative broth (deoycholate-citrate/ mannitol)

• Shigella broth (No bile salts/novobiocin)

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Salmonellae, cont. Salmonellae, cont.

Final identificationSuspect colonies from solid media

identified by biochemical reaction patterns and slide agglutination tests with specific sera

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Salmonellae, cont.Salmonellae, cont.

Serologic MethodsAgglutination testKnown sera and unknown culture

mixed on a slideClumping, when occurs, can be

observed within a few minutesUseful for rapid preliminary

identification of cultures

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Salmonellae, cont.Salmonellae, cont.

Commercial kits available to agglutinate and serogroup salmonellae by their O antigens A, B, C1, C2, D and E

Tube dilution agglutination test (Widal test)

Serum agglutinins rise sharply during 2nd and 3rd wks of infection

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Salmonellae, cont.Salmonellae, cont.

At least 2 serum specimens at intervals 7-10 days needed to prove a rise in antibody titer

Serial (twofold) dilutions of unknown serum tested against antigens from representative salmonellae

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Salmonellae, cont.Salmonellae, cont.Results interpreted as

– High or rising titer of O (> 1:160) suggests that active infection is present

– High titer of H (> 1:160) suggests past immunization or past infection

– High titer of Ab to Vi antigen occurs in some carriers

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Salmonellae, cont.Salmonellae, cont.

Results must be interpreted cautiously

Possible presence of cross-reactive Ab limits use of serology in diagnosis of salmonella infection

ImmunityInfection of S Typhi or S Paratyphi

usually confer a certain degree of immunity

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Salmonellae, cont.Salmonellae, cont.

Reinfection may occur but often milder than 1st infection

Circulating Ab to O and Vi related to resistance to infection and disease

Relapses occur in 2-3 wks after recovery inspite of Ab

Secretory IgA Ab may prevent attachment to intestinal epithelium

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Salmonellae, cont.Salmonellae, cont.

Persons with S/S hemoglobin (sickle cell disease) are exceedingly susceptible to salmonella infections particularly osteomyelitis

Persons with A/S hemoglobin (sickle cell trait) may be more susceptible than normal individuals (those with A/A hemoglobin)

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Salmonellae, cont.Salmonellae, cont.

TreatmentVast majority of enterocolitis do not require

antimicrobial treatment– Antimicrobial treatment of salmonella

enteritis in neonates is importantIn enterocolitis, clinical symptoms and

excretion of salmonellae may be prolonged by antimicrobial therapy

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Salmonellae, cont.Salmonellae, cont.

In severe diarrhoea replacement of fluids and electrolytes is essential

Invasive salmonella infections– Ampicillin, trimethoprim-

sulfamethoxazole or 3rd generation cephalosporin

Multiple drug resistance transmitted genetically by plasmids among enteric bacteria is a problem

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Salmonellae, cont.Salmonellae, cont.

Susceptibility testing is important adjunct to select a proper antibiotic

In most carriers, organisms persist in gall bladder (part. if gallstones are present) and biliary tract

Some chronic carriers cured by ampicillin alone– Most cases cholecystectomy must be

combined with drug treatment

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Salmonellae, cont.Salmonellae, cont.

EpidemiologyFaeces of persons with unsuspected

subclinical disease or are carriers are a more important source of contamination than frank clinical cases that are promptly isolated– Carriers working as food handlers

shedding organisms

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Salmonellae, cont. Salmonellae, cont.

Many animals cattle, rodents, and fowl naturally infected with a variety of salmonellae– Have bacteria in their meat,

excreta or eggsHigh incidence in commercially

prepared chickens widely publicized

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Salmonellae, cont.Salmonellae, cont.

Incidence of TF but incidence of other salmonella infections markedly in USA– Aggravated by widespread use of

animal feeds containing antimicrobial drugs that favor proliferation of drug-resistant salmonellae and potential transmission to humans

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SalmonellaeSalmonellae

CarriersAfter manifest or subclinical

infection, some individuals continue to harbor salmonellae in their tissues for variable length of time

Convalescent or healthy permanent carriers

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Salmonellae, cont.Salmonellae, cont.

3% of survivors of typhoid become permanent carriers– Harboring organisms in gallbladder,

biliary tract or rarely intestine or urinary tract

Sources of InfectionAre food and drink contaminated with

salmonellae

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Salmonellae, cont. Salmonellae, cont.

Following sources are important:Water – contamination of feces often

results in explosive epidemicsMilk and other diary products (ice

cream, cheese, custard) – Contamination with feces and

inadequate pasteurization or improper handling

– Some outbreaks traceable to source of supply

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Salmonellae, cont.Salmonellae, cont.Shellfish – from contaminated waterDried or frozen eggs – from infected

fowl or contaminated during processing

Meats and meat products– From infected animals (poultry) or

contamination with feces by rodents or humans

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Salmonellae, cont.Salmonellae, cont.

Recreational drugs – Marijuana and other drugs

Animal dyes - Dyes such as carmine used in drugs, foods and cosmetics

Household pets – turtles, dogs, catsPrevention and ControlSanitary measures must be taken to

prevent contamination of food and water by rodents or other animals that excrete salmonellae

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Salmonellae, cont.Salmonellae, cont.Infected poultry, meats and eggs

must be thoroughly cookedCarriers must not be allowed to

work as food handlers and should observe strict hygienic precautions

2 inj. of acetone-killed bacteria suspensions of S. Typhi followed by a booster inj. some months later– Give partial resistance to small

infectious inocula of typhoid bacilli but not to large ones

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Salmonellae, cont.Salmonellae, cont.

Oral administration of a live avirulent mutant strain of S. Typhi significant protection in high endemicity

Vaccines against others not recommended