The emerging strains of Shigella dysenteriae type 2 in Bangladesh are clonal

Abstract

A total of 113 strains of Shigella dysenteriae type 2 isolated from patients attending the Dhaka diarrhoea treatment centre of ICDDR,B: Centre for Health and Population Research during the period 1999-2004 were studied. Serotype of the isolates was confirmed using commercially available antisera. Except for arabinose fermentation, all the strains had similar biochemical reactions. More than 60% of the strains were sensitive to commonly used antibiotics; only 6% (n=7) of the strains were resistant to nalidixic acid, and none of the strains were resistant to mecillinam and ciprofloxacin. All strains were invasive as demonstrated by the presence of a 140 MDa plasmid, ial, sen and ipaH genes, Congo Red absorption ability and by the Sereny test performed on representative strains. Plasmid patterns were heterogeneous but more than 50% of strains were confined to a single pattern. All strains possessed a 1.6 MDa plasmid and 87% of the strains contained a 4 MDa plasmid. Middle-range plasmids (90 MDa to 30 MDa) present in 36% of the strains were not associated with antibiotic resistance. All the strains were clustered within a single type with four subtypes by pulsed-field gel electrophoresis while ribotyping patterns of all the strains were identical.

Fig. 1

Agarose gel electrophoresis of plasmid DNA showing the representative patterns among the isolates of S. dysenteriae type 2. Lanes A, B, S. dysenteriae type 2 (pattern P1); lane C, S. dysenteriae type 2 (P2); lane D, S. dysenteriae type 2 (P3); lane E, S. dysenteriae type 2 (P4); lane F, S. dysenteriae type 2 (P5); lane G, S. dysenteriae type 2 (P6); lane H, S. dysenteriae type 2 (P7); lane I, S. dysenteriae type 2 (P8); lane J, S. dysenteriae type 2 (P9); lane K, S. dysenteriae type 2 (P10); lanes L, M, N, E. coli R-1, PDK-9, and V-517 respectively.

Fig. 2

PFGE banding patterns of NotI-digested chromosomal DNA of representative strains of S. dysenteriae type 2. Lane A, λ ladder (marker); lanes B–L, S. dysenteriae type 2, K-467 (PFGE type A2); K-490 (A1); K-1075 (A1); K-1157 (A1); K-1455 (A3); K-1913 (A1); K-1141 (A1); K-1639 (A4); K-727 (A1); K-1822 (A1); K-1827 (A1) respectively; lane M, λ ladder (marker).

Fig. 3

Ribotyping patterns of representative strains of S. dysenteriae type 2. Lanes A–M, S. dysenteriae type 2 (K-467, K-490, K-727, K-774, K-1075, K-1083, K-1141, K-1157, K-1158, K-1455, K-1639, K-1827 and KD-11), all strains showed same ribotype (R1).