What is the Difference Between Shigella and Salmonella

The main difference between Shigella and salmonella is that Shigella causes shigellosis with the symptoms of diarrhea, fever, and stomach cramps, whereas Salmonella causes salmonellosis with less intense diarrhea, fever, and stomach cramps than shigellosis.

Shigella and Salmonella are two types of disease-causing bacteria. Both are Gram-negative bacteria. The major symptoms of both diseases are diarrhea, fever, and stomach cramps.

Key Areas Covered

1. What is Shigella
   • Definition, Characteristics, Importance
2. What is Salmonella
   • Definition, Characteristics, Importance
3. Similarities Between Shigella and Salmonella
   • Outline of Common Features
4. Difference Between Shigella and Salmonella
   • Comparison of Key Differences                

Key Terms

Salmonella, Shigella

What is Shigella

Shigella is a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to E. coli. The genus is named after Kiyoshi Shiga, who first discovered it in 1897. Generally, Shigella is the causative agent of human shigellosis. Shigella causes disease in primates, but not in other mammals. Shigellosis is a bacterial infection affecting the small and large intestines. However, the spreading of bacteria is by food and water contamination. Diarrhea is the primary symptom of shigellosis. In addition, Shigella release toxins that irritate the intestine.

Figure 1: Shigella

Furthermore, shigellosis is the disease of the bacterium, Shigella. Normally, the symptoms of the disease begin 1 to 2 days after infection. The symptoms last for 7 days. People can recover from the disease, shigellosis without antibiotics. Only persons with the weak immune system are given antibiotics for shigellosis.

What is Salmonella

Salmonella is a genus of Gram-negative bacteria of the family Enterobacteriaceae. Salmonella species are rod-shaped, non-spore-forming, and predominantly motile. Generally, the cell diameter is between about 0.7 and 1.5 μm, with lengths from 2 to 5 μm. Peritrichous flagella occur all around the cell body of Salmonella. More importantly, Salmonella is a chemotroph, obtaining its energy from oxidation and reduction reactions using organic sources. It is also a facultative anaerobe capable of generating ATP with oxygen when it is available; when oxygen is not available, it uses other electron acceptors or fermentation.

Figure 2: Salmonella

Moreover, scientists named Salmonella after Daniel Elmer Salmon (1850–1914), an American veterinary surgeon. Salmonella is the most common cause of diarrhea and Salmonella Typhi and Salmonella Paratyphi are the other disease-causing pathogens of the Salmonella genus that cause typhoid fever and paratyphoid fever.

Similarities Between Shigella and Salmonella

• Shigella and Salmonella are two types of Gram-negative bacteria.
• Both are rod-shaped bacteria.
• They are disease-causing bacteria.

Difference Between Shigella and Salmonella

Definition

Shigella refers to a genus of bacteria that is Gram-negative, facultative anaerobic, non-spore-forming, nonmotile, rod-shaped, and genetically closely related to E. coli while Salmonella refers to a genus of rod-shaped Gram-negative bacteria of the family Enterobacteriaceae that occurs mainly in the gut, especially a serotype causing food poisoning.

Motility

Shigella is non-motile while Salmonella is motile.

Shape

Moreover, Shigella is slender, bacilli-shaped while Salmonella is rod-shaped.

Colony Color

Shigella appears in green color colonies in Hektoen agar while Salmonella appears in black color colonies in Hektoen agar.

H2S production

Shigella does not produce H2S while Salmonella produces H2S.

Disease

In addition, Shigella causes Shigellosis while Salmonella produces salmonellosis.

Major Pathogen

The major Shigella pathogen is Shigella dysenteriae while the major Salmonella pathogen is Salmonella typhi.

Toxin Production

Shigella produces heat liable toxin called shiga toxin while Salmonella produces enterotoxin.

Effect on Human Intestine

Furthermore, Shigella affects small and large intestines while Salmonella affects colon.

Incubation Period

The incubation period of Shigella is 1-2 days while the incubation period of Salmonella is 6-72 hours.

Symptoms

Shigella shows severe symptoms while Salmonella causes less severe symptoms.

Conclusion

In brief, Shigella and Salmonella are two disease-causing bacteria. They are Gram-negative and rod-shaped bacteria. However, Shigella causes shigellosis while Salmonella causes salmonellosis. Major symptoms of both diseases are diarrhea, fever, and stomach pain. But symptoms are more severe in shigellosis than in salmonellosis. Therefore, the main difference between Shigella and Salmonella is the type of disease caused by each pathogen.

References:

1. Hale TL, Keusch GT. Shigella. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 22.
2. “Salmonella Homepage.” Centers for Disease Control and Prevention, Centers for Disease Control and Prevention, 27 July 2022.

Image Courtesy:

1. “Shigella stool” By CDC – Own work (Public Domain) via Commons Wikimedia
2. “SalmonellaNIAID” By NIAID – Own Work (Public Domain) via Commons Wikimedia