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Shigella flexneri serotype 6 ( Sf 6) is one of the most common serotypes recovered from surveillance studies of moderate to severe diarrhea. Despite the clinical significance of Sf 6, this serotype is understudied. In this work, we have performed both serotype-specific genomic and phenotypic comparisons of Sf 6 isolates to one another and non- S . flexneri serotypes. Comparative genomic analyses identified significant nucleotide homology between Sf 6 strains ( n = 325), despite a broad range of collection timeframes and geographic locations. We identified Sf 6 specific factors, including a potential novel Shigella virulence factor (type II secretion system). Additionally, we identified established Shigella virulence genes ( ospG ) and metabolic genes ( rutABCDEFGR ) that were absent in Sf 6 strains while present in the majority of 728 non- Sf 6 strains. Complete sequencing of 11 clinical Sf 6 strains, demonstrated that the Sf 6 virulence plasmid (pINV) is ~38 kb smaller than the average non- Sf 6 pINV (~228 kb). Comparisons of S. flexneri species level antibiotic susceptibility highlighted that clinical Sf 6 isolates from Africa in the Global Enteric Multicenter Study (GEMS) and Vaccine Impact on Diarrhea in Africa (VIDA) study demonstrated geographic, serotype-specific susceptibility pattern. Phenotypic analyses of Sf 6 identified reduced intracellular invasion and cytokine induction from HT-29 cells, as well as reduced Ipa protein effector secretion, compared with S. flexneri serotype 2a strain 2457T. Together these data highlight conserved and unique serotype-specific genotypic and phenotypic features for Sf 6. This level of conservation has not been noted for other S. flexneri serotypes and is promising for vaccine and diagnostic assays to provide global Sf 6-specific coverage. IMPORTANCE Shigellosis is an ongoing global public health crisis with >270 million annual episodes among all age groups; however, the greatest disease burden is among children in low- and middle-income countries (LMIC). The lack of a licensed Shigella vaccine and the observed rise in antimicrobial-resistant Shigella spp. highlights the urgency for effective preventative and interventional strategies. The inclusion of S. flexneri serotype 6 ( Sf 6) is a necessary component of a multivalent vaccine strategies based on its clinical and epidemiological importance. Given the genomic diversity of Sf 6 compared with other S. flexneri serotypes and Sf 6 unique O-antigen core structure, serotype-specific characterization of Sf 6 is a critical step to inform Shigella -directed vaccine and alternative therapeutic designs. Herein, we identified conserved genomic content among a large collection of temporally and geographically diverse Sf 6 clinical isolates and characterized genotypic and phenotypic properties that separate Sf 6 from non- Sf 6 S. flexneri serotypes.
Shigella flexneri serotype 6 ( Sf 6) is one of the most common serotypes recovered from surveillance studies of moderate to severe diarrhea. Despite the clinical significance of Sf 6, this serotype is understudied. In this work, we have performed both serotype-specific genomic and phenotypic comparisons of Sf 6 isolates to one another and non- S . flexneri serotypes. Comparative genomic analyses identified significant nucleotide homology between Sf 6 strains ( n = 325), despite a broad range of collection timeframes and geographic locations. We identified Sf 6 specific factors, including a potential novel Shigella virulence factor (type II secretion system). Additionally, we identified established Shigella virulence genes ( ospG ) and metabolic genes ( rutABCDEFGR ) that were absent in Sf 6 strains while present in the majority of 728 non- Sf 6 strains. Complete sequencing of 11 clinical Sf 6 strains, demonstrated that the Sf 6 virulence plasmid (pINV) is ~38 kb smaller than the average non- Sf 6 pINV (~228 kb). Comparisons of S. flexneri species level antibiotic susceptibility highlighted that clinical Sf 6 isolates from Africa in the Global Enteric Multicenter Study (GEMS) and Vaccine Impact on Diarrhea in Africa (VIDA) study demonstrated geographic, serotype-specific susceptibility pattern. Phenotypic analyses of Sf 6 identified reduced intracellular invasion and cytokine induction from HT-29 cells, as well as reduced Ipa protein effector secretion, compared with S. flexneri serotype 2a strain 2457T. Together these data highlight conserved and unique serotype-specific genotypic and phenotypic features for Sf 6. This level of conservation has not been noted for other S. flexneri serotypes and is promising for vaccine and diagnostic assays to provide global Sf 6-specific coverage. IMPORTANCE Shigellosis is an ongoing global public health crisis with >270 million annual episodes among all age groups; however, the greatest disease burden is among children in low- and middle-income countries (LMIC). The lack of a licensed Shigella vaccine and the observed rise in antimicrobial-resistant Shigella spp. highlights the urgency for effective preventative and interventional strategies. The inclusion of S. flexneri serotype 6 ( Sf 6) is a necessary component of a multivalent vaccine strategies based on its clinical and epidemiological importance. Given the genomic diversity of Sf 6 compared with other S. flexneri serotypes and Sf 6 unique O-antigen core structure, serotype-specific characterization of Sf 6 is a critical step to inform Shigella -directed vaccine and alternative therapeutic designs. Herein, we identified conserved genomic content among a large collection of temporally and geographically diverse Sf 6 clinical isolates and characterized genotypic and phenotypic properties that separate Sf 6 from non- Sf 6 S. flexneri serotypes.
Diarrheal diseases caused by Shigella and enterotoxigenic Escherichia coli (ETEC) are significant health burdens, especially in resource-limited regions with high child mortality. In response to the lack of licensed vaccines and rising antibiotic resistance for these pathogens, this study developed a recombinant Shigella flexneri strain with the novel incorporation of the eltb gene for the heat-labile enterotoxin B (LTB) subunit of ETEC directly into Shigella’s genome, enhancing stability and consistent production. This approach combines the immunogenic potential of LTB with the antigen delivery properties of S. flexneri outer membrane vesicles (OMVs), aiming to provide cross-protection against both bacterial pathogens in a stable, non-replicating vaccine platform. We confirmed successful expression through GM1-capture ELISA, achieving levels comparable to ETEC. Additionally, proteomic analysis verified that the isolated vesicles from the recombinant strains contain the LTB protein and the main outer membrane proteins and virulence factors from Shigella, including OmpA, OmpC, IcsA, SepA, and Ipa proteins, and increased expression of Slp and OmpX. Thus, our newly designed S. flexneri OMVs, engineered to carry ETEC’s LTB toxin, represent a promising strategy to be considered as a subunit vaccine candidate against S. flexneri and ETEC.
Background: Viable but non-culturable (VBNC) is a specific physiological state in which living bacteria lose the ability to grow and form colonies on conventional bacterial growth media; however, these cells remain metabolically active. Objectives: Considering the importance of the pathogenicity of Shigella flexneri and the possibility of its transmission through contaminated water and food, we aimed to study the possibility of this bacterium entering the VBNC state under osmotic and nutritional stresses at low temperature (4°C). Methods: Shigella flexneri was inoculated in distilled water and NaCl solutions with concentrations of 0, 5, 10, 15, 20, 25, and 30% at 4°C. The cultivability of the bacteria was checked daily. After observing the loss of cultivability, entry into the VBNC state was assessed using RT-PCR, flow cytometry, and fluorescence microscopy. Results: After cultivation of S. flexneri in different concentrations of salt solution, the time of non-cultivability was observed at concentrations of 0, 5, 10, 15, 20, 25, and 30%, respectively, on days 19, 20, 21, 22, 22, 24, and 24. Expression results of ipaH and ipaD genes using RT-PCR, along with the presence of live cells detected by flow cytometry and fluorescent staining, indicated that the cells had entered the VBNC state. Conclusions: Shigella flexneri can survive and enter the VBNC state under nutritional and osmotic stresses at low temperatures (4°C). In this state, although the bacterium is alive, it cannot be detected by conventional culture-dependent methods. Due to the potential risk of bacterial recovery, conventional techniques may be insufficient to detect the presence of pathogens in the VBNC state when evaluating water and food.
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