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Genus SALMONELLA Powered By Docstoc

Food poisoning, enteric fever
               Salmonella infections
                   in humans
• Enteric fever
   –   typhoid and paratyphoid fevers
   –   S. typhi, paratyphi A, B, C
   –   systemic infection
   –   infects only humans
   –   GI symptoms may not be evident
• Salmonella gastroenteritis
   – non-typhi serovars
   – zoonosis: predominantly food-borne
   – can be complicated by septicaemia
        • more common with some serovars, e.g. S. dublin (15% mortality rate
          when septicemic in the elderly)
        • Metastatic disease, e.g. osteomyelitis
• Salmonella enterica
   – one species, ~2000 serovars
   – Non standard nomenclature common
      • S. enterica serovar Typhimurium
      • or S. typhimurium
• rod-shaped, non-spore-forming Gram-negative
• belongs to the family Enterobacteriaceae
   – close relative of E. coli
• Motile by peritrichous flagella (H antigen).
   – nonmotile exceptions: S. gallinarum and S. pullorum
             Antigenic Structure
• Kauffmann-White antigenic scheme
   – agglutination reactions with specific antisera against
     Salmonella antigens
• O antigens
   – characteristic sequence of repeating polysaccharide units in
• H antigens
   – flagellar antigens (protein) and may occur in one of two
     phase variations.
• Vi antigen
   – a capsular polysaccharide homopolymer of N-acetyl
     galactosamineuronic acid
                Enteric fever
• person-to-person spread
  – no animal reservoir
• contamination with human faeces
  – usual vehicle contaminated water.
  – occasionally, contaminated food (usually handled
    by an individual who harbours S. typhi)
         Non-typhoidal serovars
• zoonosis with enormous animal reservoir
  – common animal reservoirs are chickens, turkeys,
    pigs, and cows
• contaminated food is major vehicle, usually:
  – red and white meats, raw eggs, milk & dairy
  – many other possibilities, from spices or chocolate
    to cannabis
  – can follow direct contact with infected animals
    (e.g. farm trip, reptiles as pets)
         Non-typhoidal serovars
• outbreaks common
• In food preparation establishments
• In Hospitals
  – now careful attention to hospital kitchen hygiene
         Non-typhoidal serovars
• Food-borne transmission by
  – contamination of cooked food by raw food
  – failing to achieve adequate cooking temperatures.
• secondary cases by person to person spread are
  common in outbreaks
• food handlers who practice good hygiene very
  rarely responsible for outbreaks
           Salmonella in eggs

• various Salmonella serovars isolated from the
  outside of egg shells
• S. enteritidis PT4 present inside the egg, in the yolk
• vertical transmission
   – deposition of the organism in the yolk by an infected
     layer hen prior to shell deposition.
              Infectious dose
• typically about 1,000,000 bacteria
• much lower if the stomach pH is raised
• much lower if the vehicle for infection is
  – protects the bacteria in their passage through the
  – an infectious dose of about 100 bacteria
                carrier states
• carrier state may last from many weeks to
  years with faecal shedding
  – convalescent carrier
• chronic carrier
  – ~3% of persons infected with S. typhi
  – ~0.1% of those infected with non-typhoidal
                  Clinical Features
                   Enteric Fever
• incubation period 10 to 14 days
• septicaemic illness
   –   myalgia and headache
   –   fever
   –   splenomegaly
   –   leukopenia
   –   abdominal pain
   –   Rose spots (macular rash on abdomen)
• 10% fatal
• positive blood, urine, and stool cultures
• Sequelae (secondary result ): intestinal haemorrhage and
                 Clinical features
• incubation period depends on dose
• symptoms usually begin within 6 to 48 hours
   –   Nausea and Vomiting
   –   Diarrhoea
   –   Abdominal pain
   –   Myalgia and headache
   –   Fever
• duration varies, usually 2 to 7 days
• seldom fatal, except in elderly or
   Pathogenesis Gastroenteritis
• Pathogenic salmonellae ingested in food
  survive passage through the gastric acid barrier
• invade intestinal mucosa
• invasion of epithelial cells stimulates the
  release of proinflammatory cytokines
• induces an inflammatory reaction
• causes diarrhoea and may lead to ulceration
  and destruction of the mucosa
               Enteric Fever
• Bacteria invade mucosa or Peyer's patches of
  small intestine (?M cells), pass into mesenteric
  lymph nodes where they multiply and then
  enter the blood stream via the thoracic duct
• Primary bacteraemia cleared by
  reticuloendothelial system (RES), bacteria
  multiply in RES cells and destroy them
• Facultative intracellular parasites
                  Enteric fever
• Secondary bacteraemia occurs and results in
  spread to other organs.
  – Infection of the biliary tract.
• Multiplication in biliary tract leads to seeding
  the intestine with large numbers of bacteria.
• Involvement of intestinal lymphoid tissue may
  lead to necrosis and ulceration.
• In untreated nonfatal cases, temperature drops
  in 3 to 4 weeks (onset on immunity?)
         Laboratory Diagnosis
• Biochemical tests and serological tests must be
  done in parallel
  – Some other bacteria, e.g. Citrobacter, may have
    similar serological profiles
  – Commercial kits commonly used, e.g. API20
• Phage typing done for epidemiological purposes
  – E.g. to find source of outbreak
  – Certain phage types predominate nationally
     • S. typhimurium PT4
     • S. enteritidis DT109
            Laboratory diagnosis

• For the diagnosis of enteric fever, specimens include
  blood, urine & feces for culture.
• Blood – detected in 75-90% of the patients in the first
  10 days of infection, and 30% of the patients in the
  3rd week.
• Faeces – can be isolated from 40-50% of patients in
  the 2nd week of infection &from about 80% of
  patients in the 3rd week.
• Urine – isolated from about 20% of patients
  after the 2nd week of infection [esp. pts with
• For Salmonella food poisoning, faeces are
  required for culture and blood during times of
• Gram negative motile rods except for S.
• Faeces may contain blood in the late stages of
  infection and few pus cells and red blood cells
  in Salmonella food poisoning cases
• Enrichment and selective media for
  Salmonella in faeces is Selenite broth.
• Differential media is XLD and SSA
• XLD-pink colonies with black centers
• SSA- black colonies with silver metallic sheen.
No Species                                Glu Lac/Suc H2S   Gas

1   P. aeruginosa                    K/K -    -       -     -
2   E. coli                          A/A +    +       -     +
3   Salmonella Typhimurium           K/A +    -       +     +
    Shigella flexneri                K/A +    -       -     -

                             1   2    3   4
• The Kaufmann-White system used to classify
  Salmonella based on identifying the O and H
  antigens possessed by different serovars.
• The detection of Vi antigen is also used in the
  detection of Salmonella typhi and other
O Antigens
• Cell wall heat stable antigens.
• Groups are designed A- Z, 51-61 AND 64-66
• Medically important Salmonella belong to groups A
  to G.
• Each group has a group factor, which is an O antigen
  common to all members of the group and not
  possessed by Salmonella belonging to other groups.
H Antigens
• Flagella, heat-stable antigens
• Serotyped by their H antigens
• Many Salmonella are diphasic, i.e. occur in 2 antigen
  forms referred to as phase l and phase ll.
• Phase l antigens are given in alphabetical letters and
  phase ll are either numbered or given a letter if
  known to occur in both phases.
Vi Antigens
• Surface or capsule K antigen can be found in S. typhi,
  S. paratyphi C and a few other Salmonella.
• It is associated with virulence and can be detected
  using Vi antiserum.
• It can interfere with O antigen testing
• If an isolate agglutinates the Vi antiserum but not an
  O antiserum, interference from Vi antigen should be
• Saline suspension of organism should be heated in
  water bath for 20 mins and after being allowed to
  cool, the bacterial cells should be retested with the O
Grouping and serotyping of Salmonella
• Antisera are costly, there laboratories need to stock up their
  polyvalent O antisera w/c covers locally important groups of
  Salmonella. Specific O, H and Vi antisera to identify the S.
• The following sera are required to identify S. typhi:
 Salmonella O antiserum Factor 9 [Group D]
 Salmonella H antiserum d
 Salmonella Vi antiserum
Widal Test
• This tests for O & H antibodies in the patients
  serum and comes in handy when culturing
  facilities are not available.
• Gastroenteritis
   – replace fluid loss by oral and intravenous routes
   – antibiotics are not recommended for uncomplicated
      • do not shorten illness
      • prolong excretion.
   – antibiotic therapy reserved for the septicaemic and
     metastatic disease
• Typhoid fever and enteric fevers should be treated with
   – usually ciprofloxacin
   – rise of resistance
• Remove source
  – Salmonella free life-stock
  – Vaccinate chicks
• Interrupt transmission
  – Good food hygiene
     • Cook food properly
     • Keep raw and cooked foods apart
  – Public Health: clean water
• Strengthen host
  – vaccination
             Salmonella vaccines
• Vaccination of travellers against typhoid recommended,
  but does not remove need for good hygiene
• Three licensed vaccines
   – Traditional heat-killed
      • very reactogenic
   – Vi subunit vaccine
   – live oral vaccine, S. typhi Ty21A
• Salmonellas can act as live attenuated carriers for other
   – So far only experimental
• No vaccines for gastroenteritis
Bacillary dysentery
• S. dysenteriae [Subgroup A, serovars 1-10]
• S. flexneri [Subgroup B, serovars 1-6, & X and
  Y variants]
• S. boydii [Subgroup C, serovars 1-15]
• S. sonnei [Subgroup D, only 1 serovar]
Normal habitat
• Found only in the human intestinal tract.
• Carriers of the pathogenic strain can excrete
  the organisms upto 2 wks after the infection
  &occasionally for longer periods.
• Killed by drying
• Transmitted by fecal-oral route
• High incidence of shigellosis occur in areas of poor sanitation
  and where water supplies are polluted.
• Lack of personal hygiene
• Young children are more frequently affected than adults [> 6
• Horseflies are also thought to be important in transferring
  Shigella from faeces to food.
• Epidemics can be caused by ingestion of contaminated milk
  and milk products.
• Shigella species cause bacillary dysentery
• Deaths in young children are caused by S. dysenteriae
• Clinical features include toxemia, sometimes
  bacteremia, and severe dysentery leading to marked
  dehydration &protein loss, inflammation & ulceration
  of LI, hemorrhage, abdominal pain & high fever.
• Death can occur from circulatory collapse or kidney
• Total WBCs is raised with neutrophilia
• Infection with S. dysenteriae can lead to leukemoid
  reaction developing 5-10 days after infection caused
  by an endotoxin.
• S. sonnei is not very pathogenic, therefore infections
  are rarely serious.
Laboratory diagnosis
• Faeces
• Specimen may be watery and contain little blood and
  mucus in the early stages of infection but consist
  mostly of pus and blood mixed with mucus in the
  later stages of infection.
• Specimens have an alkaline pH whereas faeces of
  amoebic dysentery are acidic.
• Gram negative non-motile rods
• Fecal microscopy will have red cells and large
  numbers of pus cells
• Faeces of amoebic dysentery contain red cells
  but very few pus cells
• SSA or XLD are required to isolate Shigella.
• XLD- colonies are pink
• Aerobic organism growing at 35-37ºC/24hrs
• Shigella are serotyped by their O antigens using
  polyvalent S. dysenteriae, S. flexneri, S. boydii & S.
  sonnei antisera using slide agglutination method.
• Some Shigella strains [S. dysenteriae and S. sonnei]
  possess surface or K antigens that can hide the O
  antigens being tested.
Antibiotic sensitivity
• Patient with severe bacillary dysentery require
  antibiotic therapy.
• Antimicrobials against Shigella species include
  A, D, C, cephalothin, nalidixic acid, SXT.
• Multidrug resistant strains have also been

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