Shigella

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• Shigellae are enterobacterieceae

Answer 1

They are non sporing and non capsulated
Non motile and non flagellate

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• Possess capsule (K antigen) and O antigen

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Also non lactose fermenting

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• Reduce Nitrates and are Oxidase

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• Aerobes and facultative anaerobes

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with a growth

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temperature range of 10-40oC and optima of 37oC and pH 7.4.

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They grow well on conventional media:

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• Nutrient Agar and Blood Agar

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• MacConkey Agar:

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• Deoxycholate Citrate Agar (DCA)

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• Xylose Lysine Deoxycholate (XLD):

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best selective medium for shigellae.

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Colonies are red

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Salmonella-Shigella (SS) Agar:

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•Colorless colonies with no blackening

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•While those of Salmonella are colorless with

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black centers.

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Hektoen Enteric (HE) Agar:

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•Direct plating medium for fecal specimen.

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•Colonies of Shigella are green while those of

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Salmonella are blue green with black centers

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due to H2S production.

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Peptone Water and Nutrient Broth:

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•Growth with uniform turbidity

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Classification of Shigella

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• Based on somatic O polysaccharide antigen

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Shigella

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species are classified into 4 species:

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•S. dysenteriae

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•S. flexneri

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•S. boydii and

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•S. sonnei

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• Which are also designated as serogroups A

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B

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respectively.

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1. S. dysenteriae (group A):

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• They are mannitol non-fermenting bacilli

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• Consists of 12 serotypes

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each characterized by a different type antigen.

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• Among all

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S. shigae (Serotype 1) is the only Shigella which produces a

Question 37

powerful exotoxin (Shiga toxin)

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• Shiga toxin acts as enterotoxin as well as neurotoxin.

Question 39

As enterotoxin

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it acts on the intestinal mucosa causing transdation of

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fluid in the lumen

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As neurotoxin

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it damages endothelial cells of small blood vessels of the

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CNS which results in neurological complications like polyneuritis

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coma and

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meningism.

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• S. dysenteriae also produces an cytotoxin which is active on vero cells

Question 45

and is known as Verotoxin.

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2. S. flexneri (group B):

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• The most complex species antigenically.

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• Typed into 6 serotypes

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• Serotype 6 is always indole negative

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3. S. boydii (group C):

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• It has 19 serotypes.

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• Isolated least frequently from cases of bacillary dysentery.

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4. S. sonnei (group D):

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• It is antigenically homogeneous and has only 1 serotype.

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• It can be typed by colicin typing into 26 colicin types.

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Other Antigens of Shigellae

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• K antigen may be present in some serotypes which may not be

Question 58

visible as capsule

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but it covers the O antigen and makes it

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inagglutinable by homologous antisera.

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• Fimbrial antigens may be found in some strains

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especially in

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S. flexneri.

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• Crossreactivity:

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o Antigens of many Shigella serotype (other than S. sonnei)

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cross-react with serotypes of E. coli.

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o Antigens of S. sonnei cross-react with Plesiomonas shigelloides.

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Mode of transmission:

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• Infection occurs by ingestion through contaminated fingers

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(most common)

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food

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• It can also be transmitted sexually homosexuals.

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• The infection is highly communicable because of:

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✔ The low infective dose(10 - 100 bacilli initiate the disease) is

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required to produce the disease (Highly infectious)

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✔ Their ability to survive the gastric acidity.

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✔ Shigella can survive up to 30 days in milk

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eggs and cheese

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▪ Outbreak of shigella can are common in place where sanitation is poor

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Pathogenesis

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• Shigellae cause bacillary dysentery.

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• Humans are the only reservoir.

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• Bacteria are acquired by fecal-oral spread.

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• Penetrate the mucosa and epithelial cells of the colon (but do not

Question 81

invade beyond the epithelium).

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• Survive the passage through the host defences due to the O antigens

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and Lipopolysaccharide (LPS)

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Invasiveness:

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• Exotoxin (shiga toxin): help to penetrate and multiply in colonic mucosa

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• Large plasmids: responsible for cell penetration.

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These proteins are called ‘virulence marker antigens’ (VMA).

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Pathogenesis ctn

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…

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• Bacilli enter the mucosa via M cells and cross M cells to reach

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the sub-mucosa where they are engulfed by macrophages and

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cause the released of cytokines which attract and increased the

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number of inflammatory cells to the infected site and cell death

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(due to cytotoxic properties of shiga toxin).

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Polymorphonuclear leukocytes which can be seen on microscopic

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examination of stool

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together with red cells.

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• Leading to the acute colitis that characterizes shigellosis.

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o Most common form is an initial watery diarrhea

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o Severe form of disease is caused by S. dysenteriae.

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Clinical manifestation

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• Bacillary dysentery has a short incubation period (1-7 days

Question 101

usually 48 hours).

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• The clinical manifestations of shigellosis vary from asymptomatic

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to severe forms of the disease.

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• The main clinical features are: frequent passage of loose

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scanty

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feces containing blood and mucus

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along with abdominal cramps.

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• Fever and vomiting may be present.

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▪ Infection is usually self-limited

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▪ Although antibiotic treatment is recommended to reduce the

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risk of secondary spread to family members and other contacts

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Diagnosis

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• To identify the causative agent; stool samples can be cultured.

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• Fresh feces should be inoculated without delay or

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• Transported in a suitable medium such as cary-Blair medium

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• The best sample for culture is rectal swab.

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• Microscopy: show numerous erythrocytes and polymorphs and

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some macrophages.

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• Commonly used primary isolation media include MacConkey

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Hektoen Enteric Agar and Salmonela-Shigella(SS) agar

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• Biochemical test: urease

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citrate

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• Identification is confirmed by slide agglutination

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Treatment

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• Routine antibacterial treatment is not indicated in dysentery.

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• Treatment with a suitable antibiotic is necessary in the very

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young

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the aged or the debilitated

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▪ Ampicillin

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cotrimoxazole

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antibiotics such as nalidixic acid and ciprofloxacin are

Question 127

appropriate choices.

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• Multiple drug resistance plasmids are widely prevalent in shigellae.

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The choice of antibiotic should be based on the sensitivity

Question 130

of the prevailing strain.

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Control

Question 132

• Improving personal hygiene (hands)

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• Environmental sanitation (proper sewage disposal)

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• Water chlorination

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• Insect control (flies)

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• No effective vaccine is available