Shigella: Introduction, Morphology, Culture Characteristics, Pathogenesis

Shigella: Introduction, Morphology, Culture Characteristics, Pathogenesis, Lab Diagnosis and Treatment

Introduction of Shigella

Shigella are bacteria and causing an infectious disease called Shigellosis. Shigella is named after the Japanese microbiologist Kiyoshi Shiga who isolated the first member of the group in 1896 from epidemic dysentery in Japan which was then called Shigella shiga and is now called S. dysenteriae. Most who are infected with Shigella develop diarrhea, fever, and stomach cramps starting a day or two after they are exposed to the bacteria. Shigellosis usually resolves in 5 to 7 days. Some people who are infected may have no symptoms at all, but may still pass the Shigella bacteria to others. The spread of these organisms can be stopped by frequent and careful handwashing with soap and taking other hygiene measures because of being a mode of infection feco-oral route.

Morphology

Shigellae are short, Gram-negative rods measuring about 1-3 µm X 0.5 µm. They are non-motile, non-encapsulated, non-sporing, and non-acid fast.

Culture characteristics

They are aerobes and facultative anaerobes and can grow on ordinary media like nutrient agar. The optimal temperature and pH for growth are 37°C and 7.4 respectively. But they can grow at a temperature range of 10 to 40°C.

MacConkey agar: Colonies are non-lactose fermenting (except S. sonnei) large, circular, convex, smooth, and translucent.

Deoxycholate citrate agar (DCA): Colonies are colorless (non-lactose fermenting) except in the case of S. sonnei which forms pink colonies due to late lactose fermentation.

Xylose lysine deoxycholate (XLD) agar: Colonies are red due to the organism has ability to decarboxylate lysine without black centers

Salmonella-Shigella(SS) agar: It is a highly selective medium for the isolation of Salmonella and Shigella. Colorless colonies with no blackening whereas Salmonella colonies are colorless with black centers due to their ability to produce hydrogen sulfide.

Heaktoen Enteric Agar (HEA): They give green to blue-green colonies.

Selenite F broth ( enrichment medium): Sodium selenite of this medium inhibits coliform bacilli while permitting salmonellae and shigellae to grow. This is recommended medium for the isolation of these organisms from feces.

Resistance

They are killed at 56°C in an hour and by 1 % phenol in 30 minutes. They remain viable in water and ice for 1-6 months. Chlorination or boiling of water and pasteurization of milk is effective and destroys the bacilli.

Antigenic Structures

They possess a large number of antigens and they are-

somatic (O) antigens
Capsular(K) antigen ( some strains only)
Fimbrial antigens.

Classification

Scientific classification

Domain: Bacteria
Phylum: Proteobacteria
Class: Gammaproteobacteria
Order: Enterobacterales
Family: Enterobacteriaceae
Genus: Shigella

There are four species or serogroups based on biochemical and serological properties-

A. Shigella dysenterae: 12 Serotypes

B. Shigella flexneri: 6 serotypes

C. Shigella boydii: 18 serotypes and

D. Shigella sonnei : 1 serotype

On Basis of nannitol Fermentation-

Non-mannitol-fermenters: Shigella dysenteriae
Mannitol-fermenters: Shigella flexneri, Shigella boydii and Shigella sonnei

Toxins

Endotoxin
Exotoxin
Verotoxin

Habitat and Transmission

Shigella species are found only in the human intestinal tract. Carriers of pathogenic strains can excrete the organism up to two weeks after infection and occasionally for longer periods. They are killed by drying and are transmitted by the fecal-oral route. The highest incidence of Shigellosis occurs in areas of poor sanitation and where water supplies are polluted. Factors Contributing Spread-Spread is always from a human resource and generally involves one of the five Fs i.e. food, fingers, feces, flies, and fomites. This is in contrast to salmonellae, which are often spread to humans from infected animals. Transmission-feco-oral transmission is the main path of Shigella infection. Other modes of transmission include ingestion of contaminated food or water, contact with infected objects, or sexual contact. Outbreaks of Shigella infection are common in places where sanitation is poor.

Pathogenesis

The species of this genus cause a serious illness known as dysentery /shigellosis, which is an acute diarrheal disease characterized by the passage of pus, blood, or mucous through the stool. Infection mainly occurs because of the ingestion of contaminated food or water. The incubation period is 12-48 hours but may vary between 1-7 days. Through the ingestion, the bacilli will reach the large intestine of humans. The multiplication occurs in the epithelial cells of the large intestine. Then the bacteria spreads to adjacent cells and to the lamina propria (which is a thin layer of loose connective tissue which lies beneath the epithelium) where the colonization occurs. After the growth and multiplication, it starts to produce toxins. The lamina propria and submucosa develop an acute inflammatory reaction with the formation of abscess on the mucosal surface along with capillary thrombosis by the production of toxins. The necrosed epithelium becomes soft and sloughed out and causing superficial ulcers and bleeding. The toxin produced by the shigella bacteria has both enterotoxin effect and neurotoxic effect. Thus their combined action leads to severe diarrhea, polyneuritis, coma, and meningitis.

Clinical Syndromes (Shigellosis)

Ranges from asymptomatic infection to severe bacillary dysentery
Two-stage disease: watery diarrhea changing to dysentery with frequent small stools with blood and mucus, tenesmus, cramps, fever
Early-stage:- Watery diarrhea attributed to the enterotoxin activity of Shiga toxin
Fever attributed to the neurotoxic activity of the toxin

Clinical Features

Following are the clinical features of Shigellosis-

Fever
Bloody diarrhea
Abdominal cramps
Tenesmus
Mucus
Pus Convulsions
In mild infection-watery diarrhea
Bacteremia
Rare -Reiter Syndrome
Hemolytic-uremic syndrome.

Epidemiology

Shigellosis is a major cause of the diarrheal disease (developing nations). The major cause of bacillary dysentery (severe second stage form of shigellosis). Leading cause of infant diarrhea and mortality (death) in developing countries. They occur naturally in higher primates. Spread from human to human via the fecal-oral route. Less frequently, transmission by ingestion of contaminated food or water. Outbreaks usually occur in close communities; Secondary transmission occurs frequently. Low infectious dose (10-100 bacilli) with 1-3 day incubation period. Carriage of the organism persists for approximately one month following convalescence.

Laboratory Diagnosis

Diagnosis depends upon isolating the organisms, Shigella from feces.

Specimen: Collection of fresh stool

Transport: Specimens should be transported immediately otherwise, use a transport medium like Sach’s buffered glycerol saline.

Note: Avoid the use of an alkaline transport medium because it is inhibitory for shigellae ( but useful for Vibrios).

Direct Microscopy of feces

Saline and iodine wet mount: Fields are suggestive for pus cells, red blood cells, and absence of parasites.

Differences between Amoebic dysentery and bacillary dysentery
Amoebic dysentery

Trophozoites of Entamoeba histolytica
Cyst of E. histolytica
RBCs in clumps
Macrophages
pus cells
CL crystals

Bacillary dysentery

Non-motile bacteria
Shigella
Plenty pus cells
RBCs

Culture of specimens

MacConkey agar: Colonies are non-lactose fermenting (except S. sonnei) large, circular, convex, smooth, and translucent.

Deoxycholate citrate agar (DCA): Colonies are colorless (non-lactose fermenting) except in the case of S. sonnei which forms pink colonies due to late lactose fermentation.

Xylose lysine deoxycholate (XLD) agar: Colonies are red due to the organism has ability to decarboxylate lysine without black centers

Heaktoen Enteric Agar (HEA): They give green to blue-green colonies.

Shigella biochemical tests

Shigella species biochemical tests-

TSI test: Red/yellow, no production of hydrogen sulfide and gas formation

SIM test: The indole test is negative, non-motile, and has no production of hydrogen sulfide.

Urease test: The urea hydrolyzation test is negative.

Citrate test: The citrate utilization test is also negative as shown above image.

Slide Agglutination Test

Identification of Shigella is conformed by slide agglutination test using polyvalent and monovalent antisera. Then type-specific antisera belonging to subgroups A, B, or C is used for the agglutination test.

Colicin Typing

It is used for subgroup D (S. sonnei) strains.

Treatment

Tetracycline and chloramphenicol is the drug of choice against the Shigella dysenteriae. The treatment should be continued for 5-7 days.

Prevention

It may prevent by following ways-

Using pure water supply
Maintaining personal hygiene
Proper disposal of sewage
Controlling of insects ( flies).

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