Pathogenicity Islands of Shigella

Evolutionary and genomic insights into the long-term colonization of in animals

The enteroinvasive bacterium Shigella flexneri is known as a highly host-adapted human pathogen. There had been no known other reservoirs reported until recently. Here 34 isolates obtained from animals (yaks, dairy cows and beef cattle) from 2016-2017 and 268 human S. flexneri isolates from China were sequenced to determine the relationships between animal and human isolates and infer the evolutionary history of animal-associated S. flexneri. The 18 animal isolates (15 yak and 3 beef cattle isolates) in PG1 were separated into 4 lineages, and the 16 animal isolates (1 yak, 5 beef cattle and 10 dairy cow isolates) in PG3 were clustered in 8 lineages. The most recent human isolates from China belonged to PG3 whereas Chinese isolates from the 1950s-1960s belonged to PG1. PG1 S. flexneri may has been transmitted to the yaks during PG1 circulation in the human population in China and has remained in the yak population since, while PG3 S. flexneri in animals were likely recent transmissions from the human population. Increased stability of the large virulence plasmid and acquisition of abundant antimicrobial resistance determinants may have enabled PG3 to expand globally and replaced PG1 in China. Our study confirms that animals may act as a reservoir for S. flexneri. Genomic analysis revealed the evolutionary history of multiple S. flexneri lineages in animals and humans in China. However, further studies are required to determine the public health threat of S. flexneri from animals.

Intracellular survival of Shigella

Bacterial invasion of eukaryotic cells and host recognition and killing of the invading bacteria are a key issue in determining the fate of bacterial infection. Once inside host cells, pathogenic bacteria often modify the phagosomal compartment or enter the host cytosol to escape from the lytic compartment and gain a replicative niche. Cytosolic invaders, however, are monitored by host innate immune systems, such as mediated by Nod/CARD family proteins, which induce inflammatory responses via activation of NF-kappaB. Furthermore, recent studies indicate that autophagy, a major cytoplasmic degradation system that eliminates cytosolic protein and organelles, also recognizes invading bacteria. Indeed, unless they are able to circumvent entrapping by autophagic membranes, bacteria targeted by autophagy ultimately undergo degradation by delivery into autolysosomes. In this article, we review recent advances in understanding of Shigella strategies to infect epithelial cells, and then focus on recent studies of an intriguing bacterial survival strategy against autophagic degradation.

Shigella infections in children: new insights

Shigellosis , the acute enteric infection caused by bacteria of the genus Shigella , has a worldwide distribution with an estimated annual incidence of 164.7 million cases, of which 163.2 million occur in developing countries, and 1.1 million deaths. Sixty-nine percent of all episodes and 61 percent of all Shigella -related deaths involve children younger than 5 years old. In the United States, 10,000 to 15,000 cases of shigellosis are reported each year. Although usually confined to the colonic mucosa, shigellosis sometimes can cause extraintestinal complications. Recent publications have shed light on the clinical characteristics of Shigella -induced bacteremia, surgical complications, urogenital symptoms, and neurologic manifestations, and on the unique manifestations in the neonatal period. The mainstay of treatment of shigellosis in children is correction of the fluid and electrolyte loss, which often is achieved by the administration of oral rehydration solutions. Appropriate antibiotic therapy shortens the duration of both clinical symptoms and fecal excretion of the pathogen. However, the increasing antimicrobial resistance of shigellae worldwide constitutes a major problem. Regarding the pathophysiology of shigellosis and its complications, recent data not only elucidated the molecular mechanisms involved but also linked manifestations of disease to the interplay of bacterial virulence factors and host responses. The improved understanding of the pathophysiology is hoped to lead to innovative therapeutic approaches against shigellosis and new generations of vaccine candidates.