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Congo red binding and salt aggregation as indicators of virulence in Shigella species
F Qadri1, S A Hossain, I Ciznár
1International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka.
Abstract:
Smooth strains of Shigella dysenteriae type 1, Shigella flexneri, Shigella boydii, and Shigella sonnei which form pigmented colonies (Pcr+) on Congo red agar were virulent in the Sereny test. Smooth variants unable to bind Congo red (Pcr-) were avirulent. Measurements of dye uptake from solution showed that S. dysenteriae type 1 bound the most dye, followed in order of uptake by S. flexneri, S. boydii, and S. sonnei. Using the salt aggregation test (SAT) to determine cell surface hydrophobicity, we found the same order of species. The SAT could not, however, detect differences in surface properties between Pcr+ and Pcr- pairs of isogenic smooth strains. Enteroinvasive Escherichia coli strains used in the study showed SAT and Congo red-binding properties which were similar to those of the S. flexneri strains. A direct correlation was found between pigment-binding ability and the presence of the large 140-megadalton plasmid in S. flexneri, enteroinvasive E. coli, and S. boydii but not in S. dysenteriae type 1 or S. sonnei strains. Congo red interacted with outer membranes and outer membrane proteins of S. dysenteriae type 1 but not with lipopolysaccharides. However, rough mutants of Shigella species deficient in lipopolysaccharides bound Congo red and formed pigmented colonies, showing that dye binding as a virulence assay may be misinterpreted in such cases. There was complete correlation of the Pcr+ phenotype with virulence in the smooth strains in this study, suggesting that Congo red binding can be utilized as a quick and reliable alternative to the Sereny test.
Insights
The ability of Shigella bacteria to bind Congo red dye correlates with their virulence. This Congo red binding (Pcr+) phenotype offers a quick and reliable alternative to the Sereny test for assessing bacterial virulence.
Area of Science:
- Microbiology
- Bacterial Pathogenesis
- Molecular Biology
Background:
- Shigella species are major causes of bacterial dysentery.
- Assessing Shigella virulence is crucial for disease control.
- The Sereny test is a standard but laborious method for evaluating virulence.
Purpose of the Study:
- To investigate the correlation between Congo red binding and Shigella virulence.
- To explore the utility of Congo red binding as a rapid virulence assay.
- To understand the molecular basis of Congo red interaction with Shigella.
Main Methods:
- Utilized Congo red agar plates to assess pigment binding (Pcr) in Shigella strains.
- Performed the Sereny test to determine bacterial virulence.
- Measured dye uptake and cell surface hydrophobicity using the salt aggregation test (SAT).
- Analyzed the role of plasmids and outer membrane components in dye binding.
Main Results:
- Smooth virulent Shigella strains formed pigmented colonies (Pcr+) on Congo red agar, while avirulent strains did not.
- Congo red binding ability correlated with bacterial hydrophobicity and virulence across Shigella species and enteroinvasive E. coli.
- The 140-megadalton plasmid was associated with Congo red binding in S. flexneri, E. coli, and S. boydii.
- Rough mutants lacking lipopolysaccharides could bind Congo red, highlighting potential assay misinterpretation.
Conclusions:
- Congo red binding (Pcr+) is a reliable indicator of virulence in smooth Shigella strains.
- Congo red binding serves as a rapid and effective alternative to the Sereny test.
- Understanding dye-binding mechanisms is important for accurate virulence assessment.