Plesiomonas shigelloides

Microbiology

P. shigelloides is the only species in the genus Plesiomonas . Based on phenotypic characteristics, P. shigelloides was formerly classified in the Vibrionaceae family. However, sequence analysis indicates its phylogenetic ancestry is more closely aligned with the tribe Proteaceae in the Enterobacteriaceae family, with which it is now classified.

P. shigelloides is a facultative anaerobic, gram-negative rod, which is motile using polar flagella. Some isolates of P. shigelloides share the phase capsular O antigen with that of Shigella sonnei , thus antigenic cross-reactions with Shigella can occur; a phenomenon from which its species name “ shigelloides ” is derived.

The positive oxidase, lysine, and ornithine decarboxylase and arginine dihyrolase reactions and myo -inositol fermentation differentiates P. shigelloides from other bacteria. Its positive oxidase reaction distinguishes it from other Enterobacteriaceae. A positive ornithine decarboxylase differentiates it from most Aeromonas spp., and fermentation of myo -inositol differentiates it from both Aeromonas spp. and Vibrio spp. It is also positive for indole and catalase.

P. shigelloides grows well on traditional enteric media, including MacConkey, Hektoen enteric deoxycholate citrate (Leifson), and Salmonella-Shigella agars. Growth is enhanced when inoculated on selective media, such as trypticase soy broth with ampicillin. P. shigelloides produces gray, non-hemolytic colonies after 18–24 hours of incubation at 37°C; optimal growth occurs at temperatures of 40°C–44°C.

Pathogenesis and Virulence

P. shigelloides is epidemiologically associated with gastroenteritis, particularly in the first 2 years of life. However, some in vitro and in vivo studies have failed to confirm its enteropathogenicity. For example, ingested plesiomonads by human volunteers did not cause gastrointestinal disease, and these bacteria have been isolated from the stools of healthy individuals. Also, laboratory investigation suggested P. shigelloides to be of low pathogenic potential, and most standard assays of invasiveness yielded negative or variable results. ^, Furthermore, DNA probes for virulence genes of Shigella spp. and enteroinvasive Escherichia coli were also negative. ^, Nevertheless, based on case reports, epidemiologic studies of outbreaks and molecular studies, it is probable that certain strains of P. shigelloides are diarrhoeagenic. This is supported by a large clinical study in China in which P. shigelloides was isolated only from patients with diarrhea and not from patients without. Also, an in vitro experiment provided evidence for invasive capability. Comparative genomics of Plesiomonas strains revealed the presence of probable virulence traits that could not be determined using traditional approaches, further attesting to its pathogenic capacity. It is yet unclear whether there is age- and immunity-related susceptibility to this bacterium.

A variety of virulence factors have been associated with infections, such as enterotoxins, including forms that have cholera-like, heat-stable, and heat-labile properties, and a cytotoxin lipopolysaccharide complex (LPS). ^, Mutant studies have documented a crucial role of the LPS in pathogenicity through resistance to complement and adhesion to and invasion of some eukaryotic cells. In addition, a high-molecular-weight plasmid has been identified in most strains of P. shigelloides , that has been shown to assist in cell invasion.