shigella

Sharq Elneil College

School of Medical Laboratory Sciences

Department of Microbiology

Medical Bacteriology course

U.Mahadi Hassan Mahmoud mahadi2010sd@yahoo.com

Bsc, Msc, MIBMS Microbiology

Kiyoshi Shiga

7 February 1871

Historical information

Discovered over 100 years ago by a

Japanese scientist named Kiyoshi Shiga,

Since its discovery, there have been several

epidemics of shigellosis (caused by the S.

dysenteriae bacterium

Outbreak in the Mexican-Guatemalan

border caused the deaths of 20,000 people

Reservoirs:

Human intestines are the

natural habitat and reservoir for

Shigella ,

thus the bacteria are present in

the stools of an infected person

up to a week or two after the

alleviation of symptoms

Morphology

Shigellae are Gram negative, rods.

Unlike salmonellae and many other

enterobacteria, shigellae are non-motile.

Non-sporing

Noncapsulate

Subgroup A: Shigella dysenteriae

Contains 12 distinct serotypes

Serotype 1 was formerly called S. shiga

Serotype 2 was formerly called S. schmitzii

Subgroup B: Shigella flexneri

Contains 6 related serotypes and 4 serotypes

divided into subserotypes.

Subgroup C: Shigella boydii

Contains 18 distinct serotypes

Subgroup D: Shigella sonnei

Contains one serotype

Medically Important spp

Culture &chaacteristics:

Shigellae are aerobes and facultative

anaerobes.

They grow between 10–45 ºC with an

optimum temperature of 37 ºC.

Specimens must be cultured with the

minimum of delay.

A selective medium is required to

isolate Shigella species from faeces.

XLD agar: Shigellae produce red-

pink

colonies, 2–4 mm in diameter,

without black centres

DCA and MacConkey agar:

Shigellae

produce non-lactose fermenting pale

coloured 1–2 mm diameter colonies.

On prolonged incubation, S. sonnei

forms pink colonies

.

Salmonella-Shigella (SS) agar:

Despite its name, this medium is

not suitable for isolating shigellae as

it is inhibitory to most strains

Routes of transmission

Faecal-oral route with poor sanitation,

unhygienic conditions, overcrowding,

facilitating the rapid spread of infection.

Only a few organisms are required to

cause disease.

Houseflies help to transfer shigellae

from faeces to food. The

Reactions of shigellae

● Lactose negative (S. sonnei is a late lactose and

sucrose fermenter)

● H2S negative

● Urease negative

● Oxidase negative

● Citrate negative

● Lysine decarboxylase (LDC) negative

● Ornithine decarboxylase (ODC) negative except

S. sonnei which is ODC positive

● Beta-galactosidase (ONPG) negative. S. sonnei

and up to 15% of Sd 1 strains and minority of

S. boydii strains are ONPG positive

Colicins are bactericidal macromolecules

which have narrow spectrum activity and they

are produced by Sh.sonneii (16)

colicins kill sensitive bacteria in 3 defined

steps :

1. Adsorption onto a specific receptor at the

surface of the bacterium.

2. Translocation across the outer membrane.

3. Killing activity.

Colicins typing

several toxins (endotoxin, enterotoxin, and

cytotoxin)

its ability to induce endocytosis into host cells, and

intracellular growth.

Each variation of the bacteria is able to penetrate

large intestine epithelial cells and multiply within.

S. dysenteriae, however, causes more severe

symptoms due to the production of the Shiga toxin.

This enables it to kill host cells by inhibiting protein

synthesis

Virulence factors

Pathology: Shigellosis

Developing countries:

Sh. flexneri is endemic (always present) in most

communities

Sh. dysenteriae type 1 often occurs in an epidemic pattern

These two species of Shigella generally produce the most

severe illness.

Developed countries:

Sh. sonnei is the most common and is the least virulent

Sh. boydii causes disease of intermediate severity is least

common, except in the Indian sub-continent.

Fecal-oral transmission

person-to-person, fomites, food, water,

Waterborne and water-washed

Infectious dose: low; as few as 10 cells to

infect

Incubation period: 1 to 7 days; typically,

1-3 days

Duration of illness:

untreated: severe symptoms for about two

weeks

Antibiotic treatment shortens illness and

prevent spread to others

Shigellosis - Complications severe anorexia (loss of appetite)

hypoproteinaemia (a low concentration of blood

protein)

hyponatraemia (a low concentration of blood

sodium)

dilation of the large intestine

seizures

anaemia

kidney damage

persistent diarrhoea

weight loss and malnutrition

Laboratory diagnosis

Specimens:

Collection of Spacimen

Culture

Identificaion

Serology

Molecular characterization

-

Ampicillin,

Septra,

Nalidixic acid

Ciprofloxacin. Since many

Shigella are becoming resistant to antibiotics, they

should be avoided as a means of treatment for mild

cases , as most will recover without help.

Antimicrobial suseptibility testing

Handwashing, especially after

defacation

Improved sanitation and hygiene

Improve water, waste

treatment/disposal and food sanitation

Reduce overcrowding, etc.

No effective vaccine

Prevention and Control:

THANKYOU FOR ATTENTION