Differential occurrence of Streptococcus pneumoniae serotype 11E between asymptomatic carriage and invasive pneumococcal disease isolates reflects a unique model of pathogen microevolution

Abstract

Background: Streptococcus pneumoniae is a commensal colonizer of the human nasopharynx (NP) that causes disease after evasion of host defenses and dissemination. Pneumococcal strains expressing the newly identified serotype 11E arise from antigenically similar 11A progenitors by genetic inactivation of the O-acetyltransferase gene wcjE. Each 11E strain contains a distinct mutation to wcjE, suggesting that 11E strains are not transmitted among hosts despite their recovery from multiple patients with pneumococcal disease. We investigated whether the presumed lack of transmission of serotype 11E is consistent with its inability to survive in the NP.

Methods: More than 400 pneumococcal carriage, middle ear, conjunctiva, and blood isolates, serotyped as 11A by Quellung reaction, were reexamined for reactivity to 11A- and 11E-specific antibodies. We confirmed serotyping of isolates with sequencing of wcjE alleles.

Results: Serotype 11E strains were statistically more likely to occur among blood (4 of 15), conjunctiva (1 of 14), or middle ear (2 of 21) isolates than among carriage isolates (2 of 355). All 11E isolates contained unique mutations that putatively decrease wcjE expression.

Conclusions: The lack of a circulating 11E clone and the increased occurrence of 11E strains among disease isolates supports the idea that serotype 11E independently arises during infection after initial colonization with a serotype 11A progenitor. Factors encountered in the NP likely contribute to relative rarity of 11E among carriage isolates, whereas selective pressures in deeper tissues possibly promote 11E emergence. These findings illustrate a novel model of microevolution that transpires during the span of a single encounter with serotype 11A, highlighting the adaptability of bacterial pathogens within hosts.

Figure 1.

Two isolates express polysaccharide that competitively inhibits both monoclonal antibodies. Serial fold dilutions of an isolate’s lysate (x-axis) were examined for inhibition of Hyp11AM1 binding to 11E PS or Hyp11AM9 binding to 11A PS. Inhibition of binding is recorded on the y-axis as the percentage of signal for optical density at 450 nm (OD₄₅₀) compared with negative control, where 100% signal signifies no inhibition. Inhibition curves represent the average of duplicate samples. Reference inhibition curves (black lines) are shown for lysates of MNZ2291 (11A) (white diamond) and MNZ2301 (11E) (black circle). A, Inhibition curves for initial lysates from MNZ2292 (white square) and MNZ2293 (gray triangle). B, Inhibition curves from MNZ2293 subclones MNZ2293-A (gray triangle) and MNZ2293-C (white triangle). C, Inhibition curves from 3 separate MNZ2292 subclones (black, white, and gray squares). Abbreviation: mAb, monoclonal antibody.

Figure 2.

Occurrence of wcjE-disrupted serotype by anatomical site of isolation. Odds ratios represent proportion of 11E:11A compared with equivalent proportion of nasopharyngeal isolates.