PI-2 Pili of Streptococcus pneumoniae | CDC EID

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Volume 16, Number 6–June 2010
Research
Increase in Pilus Islet 2–encoded Pili among Streptococcus pneumoniae Isolates, Atlanta, Georgia, USA
Dorothea Zähner, Aditya Gudlavalleti, and David S. Stephens
Author affiliations: Emory University School of Medicine, Atlanta, Georgia, USA (D. Zähner, A. Gudlavalleti, D.S. Stephens); and Department of Veterans Affairs Medical Center, Atlanta (D. Zähner, D.S. Stephens)

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Abstract
To define the prevalence of pilus islet 2 (PI-2)–encoded pili in Streptococcus pneumoniae in a geographically defined area, we examined 590 S. pneumoniae isolates from population-based surveillance of invasive pneumococcal disease in Atlanta, Georgia, USA, 1994–2006. In 2006, PI-2 was present in 21% of all invasive isolates, including serotypes 1 (100%), 7F (89%), 11A (21%), 19A (40%), and 19F (75%). Only serotype 19F is included in the 7-valent pneumococcal conjugate vaccine that is in use worldwide. In 1999, PI-2-containing isolates were of the same serotypes but accounted for only 3.6% of all invasive isolates. The increase of PI-2 in 2006 resulted predominantly from the emergence of serotype 19A isolates of sequence type 320 and the expansion of serotype 7F isolates. The increase in PI-2-containing isolates and the finding that isolates of all identified serotypes expressed highly conserved PI-2 pili supports their potential as a vaccine candidate.
Streptococcus pneumoniae is a major human pathogen, which causes pneumonia, sinusitis, otitis, bacterial meningitis, and septicemia. Introduction of the 7-valent pneumococcal conjugate vaccine (PCV7, Prevnar; Wyeth, Madison, NJ, USA) in 2000 has dramatically decreased invasive pneumococcal disease in the United States in children and in adults (1) and is now in use worldwide. PCV7 includes capsule polysaccharide serotypes 4, 6B, 9V, 14, 18C, 19F, and 23F, serotypes that were responsible for >80% of cases of invasive pneumococcal disease in the United States before the introduction of the conjugate vaccine (1). In recent years other non-PCV7 serotypes, including 7F and 19A, have been emerging in the United States, (2–4).

Pili have recently been identified on several gram-positive bacteria (5–7). To date, 2 different pilus islets (PIs) have been described in S. pneumoniae that encode the structural and biosynthetic genes for 2 antigenically different types of pili, PI-1 (8) and recently described PI-2 (9). The so-called rlrA pathogenicity islet (10) or PI-1 is a 14-kb genetic region present in 21%–27% of clinical isolates, depending on the geographic region analyzed. It is most prevalent in, but not restricted to, the PCV7-vaccine serotypes 4, 6B, 9V, 14, and 19F (11–13).

PI-2 is a 7-kb region located between the genes that encode peptidase T (PepT) and ferrochelatase (HemH) (9). The region is composed of 5 genes, which encode 2 surface proteins, PitA and PitB, with pitA a pseudogene due to a stop-codon in the N terminus, a signal peptidase-like protein (SipA), and 2 sortases (SrtG1 and SrtG2); the latter is nonfunctional in most of the strains (9). This PI has been reported to be present in ≈16% of the analyzed isolates belonging to serotypes 1, 2, 7F, 19A, and 19F (9). The presence of PI-2 pili appears to be a clonal property, rather than serotype associated, an observation that has been described for PI-1 as well (11–13). For isolates of serotype 19A, PI-2–containing isolates have been shown to be associated with sequence type (ST) ST320, a clone related to the worldwide distributed multidrug-resistant serotype 19F clone Taiwan19F-14 (4,14). ST320 belongs to clonal complex (CC) CC271 (with ST271 as the predicted founder of this CC); isolates of this CC have been shown to encode both PIs and express both pili concomitantly on their surface (9).

The PI-2 pilus is composed of polymers of the major pilus protein PitB (9). In contrast to other pili in gram-positive bacteria that contain accessory pilus proteins, that serve as adhesins (15–17) or adaptors for pilus attachment to the cell wall (18,19), PI-2 pili appear to consist solely of PitB polymers, and these polymers themselves have been shown to mediate adhesion of S. pneumoniae to eukaryotic cells (9).

Bagnoli et al. identified PI-2–containing isolates in a random, worldwide collection of pneumococcal isolates (9). We describe the distribution of PI-2 pili in a well-documented, comprehensive collection from a defined geographic region, i.e., in invasive pneumococcal isolates collected as part of a population-based surveillance program conducted in the metropolitan area of Atlanta, Georgia, USA.

Materials and Methods
Strain Collection
A set of 590 strains of S. pneumoniae from population-based surveillance of invasive pneumococcal disease isolated between 1994 and 2006 in the 8-county metropolitan Atlanta area, Georgia Health District 3 formed the basis for this study. The isolates were obtained as a part of the Centers for Disease Control and Prevention–sponsored Active Bacterial Core surveillance of the Georgia Emerging Infections Program (1,20). Information about serotype and antimicrobial drug susceptibility was determined for all isolates as described (21). Only viable strains with documented serotype and available antibiogram were included in the study. Laboratory strain S. pneumoniae R6 is a nonencapsulated derivative of the serotype 2 strain D39 (22).

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