Methods: Twenty-seven null-capsule isolates from 430 pneumococci that were isolated between 2010–2014 were used for this study. The capsular type was identified by DNA sequence-based methods, and genetic backgrounds were compared by multilocus sequence typing. Among the null-capsule isolates, the SP2852 strain was employed for non-encapsulation analysis. The pspK gene of this strain was replaced with ermB by homologous recombination (SP2852 ΔpspK::ermB). Then, genomic DNA from SP2852 ΔpspK::ermB was transformed into encapsulated isolates via natural transformation. Clindamycin-resistant isolates were further analyzed by sequence.
Results: The proportion of null-capsule isolates tended to increase from 5% in 2010–2011 to 12.3% in 2014. These null-capsule isolates were classified into 14 STs that included STs previously identified as capsule-positive isolates. To assess non-encapsulation via natural transformation, 2 encapsulated strains (serotype 19F and 14) were cultured with genomic DNA from SP2852 ΔpspK::ermB. Subsequently, clindamycin-resistant null-capsule isolates were detected with high frequency (2.5×10-4–8.7×10-5). Sequence analysis showed capsular coding regions of these null-capsule isolates were replaced with that of ΔpspK::ermB. Furthermore, these isolates grew significant faster than their parent strains.
Conclusion: Null-capsule isolates with various genetic backgrounds were revealed gradually after introduction of vaccine. Moreover, encapsulated strains could take up genomic DNA of null-capsule isolates more easily and become a null-capsule strain by homologous recombination, suggesting that non-encapsulation and acquiring PspK resulted in the emergence of null-capsule strains by natural transformation. Furthermore, non-encapsulation could be beneficial for pneumococci as an evasion mechanism from vaccines.
S. Suzuki, None
A. Matsuzawa, None
H. Nakaminami, None
N. Noguchi, None