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Other G-VE bacteria
Course: Medical Bacteriology Lec. No 7
4th Grade – Fall Semester 2021-2022
Dr. Salah Tofik Balaky
Ph.D. Medical Microbiology
12/12/2021
The Shigellae
The natural habitat of shigellae is limited to the intestinal
tracts of humans and other primates, where they produce
bacillary dysentery. Shigellae are slender G-ve rods
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Antigenic Structure
Shigellae have a complex antigenic pattern. There is great
overlapping in the serologic behaviour of different species,
and most of them share O antigens with other enteric bacilli.
The somatic O antigens of shigellae are lipopolysaccharides.
Their serologic specificity depends on the polysaccharide.
There are more than 40 serotypes.
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The classification of shigellae relies on biochemical
and antigenic characteristics.
Species Group and
Type
Mannitol Ornithine
Decarboxylase
S. dysenteriae A -VE -VE
S. flexneri B +VE -VE
S. boydii C +VE -VE
S. Sonnei D +VE +VE
Shigellae Pathogenesis: Shigella infections are almost
always limited to the gastrointestinal tract. The infective dose
is on the order of 10³ organisms (whereas it usually is 10⁵–
10⁸ for salmonellae and vibrios). The essential pathologic
process is invasion of the mucosal epithelial cells by induced
phagocytosis, escape from the phagocytic vacuole,
multiplication and spread within the epithelial cell cytoplasm,
and passage to adjacent cells.
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Shigellae Toxins
1. Endotoxins: Upon autolysis, all shigellae release their
toxic lipopolysaccharide. This endotoxin probably contributes
to the irritation of the bowel wall.
2. Shigella dysenteriae exotoxin: Acting as a neurotoxin,
this material may be contribute to the extreme severity and
fatal nature of S. dysenteriae infection and to the central
nervous system.
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Salmonellae are vary in length and most isolates
are motile with peritrichous flagella. They are small
Gram-negative bacillus. The genus Salmonella
contains over 2,000 sero-species and is one of the
most important pathogens in the family
Enterobacteriaceae. The most important
Salmonella species is S. enterica.
The Salmonellae
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Bacterial antigens
Usually motile (H antigen)
Possess polysaccharide capsule (K antigen on most
Salmonella spp. or Vi antigen of Salmonella typhi and
Salmonella paratyphi)
Specific O antigens
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Pathogenesis
Salmonellae are often pathogenic for humans or
animals. They are transmitted from animals and
animal products to humans.
1. Enteric fever (Typhoid and less severe
Paratyphoid fever)
2. Non-typhoid salmonellosis
3. Bacteremia
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Entric fever
(Typhoid and paratyphoid fever)
Typhoid is caused by Salmonella enterica subsp.
enterica serotype Typhi (Salmonella typhi) and
paratyphoid is caused by Salmonella enterica
serotypes S. paratyphi (A, B and C). Consumption
of contaminated food and water or contact with a
patient or carrier of the disease are common
vehicles of infection.
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Salmonella generally cause illness by localised
infection of the gastrointestinal tract. Infection is
characterised by colonisation and attachment of
the bacteria to epithelial cells, and subsequent
invasion of the intestinal tissue. During this
invasive process, a heat-labile enterotoxin is
secreted by the bacterium that precipitates a net
efflux of water and electrolytes into the intestinal
lumen resulting in diarrhoea.
Typhoid and paratyphoid fever
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Fever, enterocolitis with diarrhoea and abdominal
pain, hepatosplenomegaly, bradycardia and rose
spot on trunk
Symptoms:
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Non-Typhoid SalmonellosisThe disease caused by over 2,000 Salmonella
serovars. The most frequently reported one from
humans is S. serovar typhimurium and S. enteritidis.
The initial source of the bacterium is the intestinal
tracts of birds and other animals. Humans acquire the
bacteria from contaminated foods such as beef
products, poultry, eggs, egg products, or water. Once
the bacteria are in the body, the incubation time is
only about 8 to 48 hours.
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Non-Typhoid Salmonellosis …The disease results from a true food- borne infection
because the bacteria multiply and invade the
intestinal mucosa where they produce an enterotoxin
and cytotoxin that destroy the epithelial cells.
Abdominal pain, cramps, diarrhoea, nausea, vomiting,
and fever are the most prominent symptoms, which
usually persist for 2 to 5 days but can last for several
weeks.
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This is associated commonly with S. choleraesuis.
Following oral infection, there is early invasion of
the bloodstream (with possible focal lesions in
lungs, bones, meninges, etc), but intestinal
manifestations are often absent.
Bacteremia
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In the early 1900s there were thousands of typhoid
fever cases, and many died of the disease. Most of
these cases arose when people drank water
contaminated with sewage or ate food prepared by
individuals who were shedding the typhoid fever
bacterium (Salmonella typhi). The most famous carrier
of the typhoid bacterium was Mary Mallon.
“Typhoid Mary”
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Between 1896 and 1906 Mary Mallon worked as a cook
in seven homes in New York City. Twenty-eight cases
of typhoid fever occurred in these homes while she
worked in them. As a result the New York City Health
Department had Mary arrested and admitted to an
isolation hospital on North Brother Island in New York’s
East River.
“Typhoid Mary”
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Examination of Mary’s stools showed that she was
shedding large numbers of typhoid bacteria though she
exhibited no external symptoms of the disease. An
article published in 1908 in the Journal of the American
Medical Association referred to her as “Typhoid Mary,”
an epithet by which she is still known today. After being
released when she pledged not to cook for others or
serve food to them.
“Typhoid Mary”
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Mary changed her name and began to work as a cook
again. For five years she managed to avoid capture
while continuing to spread typhoid fever. Eventually the
authorities tracked her down. She was held in custody
for 23 years until she died in 1938. As a lifetime carrier,
Mary Mallon was positively linked with 10 outbreaks of
typhoid fever, 53 cases, and 3 deaths.
“Typhoid Mary”
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The pseudomonads are gram-negative, motile,
aerobic rods some of which produce water-soluble
pigments. Pseudomonads occur widely in soil, water,
plants, and animals. Pseudomonas aeruginosa is
frequently present in small numbers in the normal
intestinal flora and on the skin of humans and is the
major pathogen of the group.
The Pseudomonad Group
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P. aeruginosa is widely distributed in nature and is
commonly present in moist environments in hospitals.
It causes disease in humans with abnormal host
defences.
Pseudomonas aeruginosa
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P aeruginosa is motile and rod-shaped, G-ve and
occurs as single bacteria, in pairs, and
occasionally in short chains. P. aeruginosa is an
obligate aerobe that grows readily on many types
of culture media, sometimes producing a sweet
or grape-like odour. Some strains hemolyzes
blood.
P. aeruginosa
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P. aeruginosa forms smooth round colonies with
a fluorescent greenish colour. It often produces
the non-fluorescent bluish pigment pyocyanin,
which diffuses into the agar.
P. aeruginosa
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Pili (fimbriae) extend from the cell surface and
promote attachment to host epithelial cells.
The exopolysaccharide is responsible for the
mucoid colonies seen in bacterial cultures
The lipopolysaccharide, is responsible for many of
the endotoxic properties of the organism.
Antigenic Structure & Toxins
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Many strains of P. aeruginosa produce exotoxin A,
which causes tissue necrosis and is lethal for
animals when injected in purified form. Antitoxins
to exotoxin A are found in some human sera,
including those of patients who have recovered
from serious P. aeruginosa infections.
Antigenic Structure & Toxins
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P aeruginosa is pathogenic only when introduced
into areas devoid of normal defenses, eg, when
mucous membranes and skin are disrupted by
direct tissue damage. The bacterium attaches to
and colonizes the mucous membranes or skin,
invades locally, and produces systemic disease.
These processes are promoted by the pili,
enzymes, and toxins
P. aeruginosa pathogenesis
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Next lecture
H. pylori and Campylobacter
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Any questions or Comments?
