Effectiveness of the standard and an alternative set of Streptococcus pneumoniae multi locus sequence typing primers

Abstract

Background: Multi-locus sequence typing (MLST) is a portable, broadly applicable method for classifying bacterial isolates at an intra-species level. This methodology provides clinical and scientific investigators with a standardized means of monitoring evolution within bacterial populations. MLST uses the DNA sequences from a set of genes such that each unique combination of sequences defines an isolate's sequence type. In order to reliably determine the sequence of a typing gene, matching sequence reads for both strands of the gene must be obtained. This study assesses the ability of both the standard, and an alternative set of, Streptococcus pneumoniae MLST primers to completely sequence, in both directions, the required typing alleles.

Results: The results demonstrated that for five (aroE, recP, spi, xpt, ddl) of the seven S. pneumoniae typing alleles, the standard primers were unable to obtain the complete forward and reverse sequences. This is due to the standard primers annealing too closely to the target regions, and current sequencing technology failing to sequence the bases that are too close to the primer. The alternative primer set described here, which includes a combination of primers proposed by the CDC and several designed as part of this study, addresses this limitation by annealing to highly conserved segments further from the target region. This primer set was subsequently employed to sequence type 105 S. pneumoniae isolates collected by the Canadian Immunization Monitoring Program ACTive (IMPACT) over a period of 18 years.

Conclusions: The inability of several of the standard S. pneumoniae MLST primers to fully sequence the required region was consistently observed and is the result of a shift in sequencing technology occurring after the original primers were designed. The results presented here introduce clear documentation describing this phenomenon into the literature, and provide additional guidance, through the introduction of a widely validated set of alternative primers, to research groups seeking to undertake S. pneumoniae MLST based studies.

Figure 1

S. pneumoniae MLST typing regions for each of the segments not fully sequenced by the standard primers aligned with a section of the corresponding genomic DNA. Panels (A) through (F) identify each individual gene and direction combination, for which the complete typing region is not obtained. The black arrows depict the binding sites of the standard primers to the up or downstream genomic DNA. The line marked boxes identify the segment that is consistently not obtained by sequencing with the standard primers. The angle bracket and top sequence identify either the 5’ or 3’ end of the typing region depending on the specific MLST gene.

Figure 2

5’ or 3’ end of the S. pneumoniae MLST typing region that is not obtained by both the forward and reverse standard primers aligned with the sequencing results from both the forward and reverse alternative primers. Panels (A) through (F) depict the sequencing results of the alternative primers in relation to their corresponding typing region. The angle bracket and top sequence identify the 5’ or 3’ end of the typing region, the middle sequence is the result from sequencing with the forward alternative primer, and the bottom sequence is the result from sequencing with the reverse alternative primer.