Food poisoning, enteric fever
• 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
• Kauffmann-White antigenic scheme
– agglutination reactions with specific antisera against
• O antigens
– characteristic sequence of repeating polysaccharide units in
• H antigens
– flagellar antigens (protein) and may occur in one of two
• Vi antigen
– a capsular polysaccharide homopolymer of N-acetyl
• 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)
• 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
– can follow direct contact with infected animals
(e.g. farm trip, reptiles as pets)
• outbreaks common
• In food preparation establishments
• In Hospitals
– now careful attention to hospital kitchen hygiene
• 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.
• 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 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
• incubation period 10 to 14 days
• septicaemic illness
– myalgia and headache
– abdominal pain
– Rose spots (macular rash on abdomen)
• 10% fatal
• positive blood, urine, and stool cultures
• Sequelae (secondary result ): intestinal haemorrhage and
• incubation period depends on dose
• symptoms usually begin within 6 to 48 hours
– Nausea and Vomiting
– Abdominal pain
– Myalgia and headache
• duration varies, usually 2 to 7 days
• seldom fatal, except in elderly or
• 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
• 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
• 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?)
• 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
• 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
• 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
• Cell wall heat stable antigens.
• Groups are designed A- Z, 51-61 AND 64-66
• Medically important Salmonella belong to groups A
• 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.
• 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.
• 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
• This tests for O & H antibodies in the patients
serum and comes in handy when culturing
facilities are not available.
– 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
• 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 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
• S. dysenteriae [Subgroup A, serovars 1-10]
• S. flexneri [Subgroup B, serovars 1-6, & X and
• S. boydii [Subgroup C, serovars 1-15]
• S. sonnei [Subgroup D, only 1 serovar]
• 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.
• 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.
• Patient with severe bacillary dysentery require
• Antimicrobials against Shigella species include
A, D, C, cephalothin, nalidixic acid, SXT.
• Multidrug resistant strains have also been