Genome-wide identification of essential genes in the invasive Streptococcus anginosus strain

Streptococcus anginosus, part of the Streptococcus anginosus group (SAG), is a human commensal increasingly recognized as an opportunistic pathogen responsible for abscesses formation and infections, also invasive ones. Despite its growing clinical importance, the genetic determinants of its pathogenicity remain poorly understood. This study aimed to identify essential genes in S. anginosus 980/01, a bloodstream isolate, under nutrient-rich laboratory conditions using a transposon mutagenesis combined with Transposon-Directed Insertion Site Sequencing (TraDIS). A mutant library was generated using the ISS1 transposon delivered via the thermosensitive plasmid pGh9:ISS1. Following transposition, insertions were mapped using Illumina sequencing and subsequently analyzed. Essential genes were identified based on the absence of insertions and statistical filtering. The library exhibited 98% genome saturation with over 130,000 unique insertion sites. Among 1825 genes, 348 (19.1%) were essential, 1446 non-essential, and 30 non-conclusive. Comparative analyses were performed with S. pyogenes MGAS5005 and S. agalactiae A909. Similarly to the latter, essential genes were enriched in functions related to translation, transcription, and cell wall biosynthesis. However, 40 genes uniquely essential to S. anginosus 980/01 were identified, suggesting unique survival strategies in S. anginosus. This study presents the first genome-wide identification of essential genes for S. anginosus 980/01, highlighting conserved and unique essential genes. These findings provide a basis for understanding its physiology and key genetic determinants of bacterial viability, and may help to uncover the pathogenic potential of S. anginosus in future studies.