A comparative study of virulence in Shigella isolates from Lagos, Nigeria.

B A Iwalokun, G O Gbenle, T A Adewole, S I Smith, K Akinsinde


Shigellosis and virulence in 513 patients presenting with mild and severe cases of diarrhoea in Lagoswas investigated. A total of 49 Shigella strains were isolated with 33 grouped as mild shigellosis and 16 classed as severe shigellosis. 20 mild and 3 severe shigellosis strains were typed avirulent by congo red assay and sereny test. Plasmid analysis showed that 28 of the mild shigellosis strains lacked the 2 and 6Mdal plasmids, while 11 severe shigellosis strains harboured these plasmids. All 2 and 6Mdal plasmids’ strains were virulent by conventional methods, deficient in glucose, contained rhamnose and elicited a high extracellular protease activity. Although all the strains produced extracellular protease with marked activity at 8 h of growth, the activity of this enzyme at 12 h was significantly higher (t = 8.6; P < 0.05) in virulent than avirulent strains. However, at 20 h, the difference in enzyme activity became non-significant (t = 0.8; P > 0.05). Similar pattern was observed in the controls. None of the mild shigellosis strains of Sh. Sonnei harboured the 120 and 140 Mdal plasmids. Strains that were avirulent by conventional methods but harboured 120 and 140 Mdal plasmids displayed protease kinetics that did not follow a clearly defined pattern. They also contained rhamnose but the amount (3-5%) was significantly lower (X2 =117.9; P < 0.05) than the rhamnose content (23-40%) of the virulent strains.

      These results indicate that (1) the 2 and 6 Mdal plasmids are the primary accessories of virulence in bacterial pathogens of shigellosis, while the 120 and 140 Mdal plasmids play a secondary role; (2) plasmid profiling, cell wall polysaccharide sugar analysis and protease kinetic study identifies virulence in shigellosis better than the conventional methods and (3) the severity of shigellosis is a function of many other factors, which are beyond the scope of this study.

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