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Horizontal acquisition of novel chromosomal genes is considered to be a key process in the evolution of bacterial pathogens. However, the identification of gene presence or absence could be hindered by the inconsistencies in bacterial genome annotations. Here, we performed a cross-annotation of omnipresent core and mosaic accessory genes in the chromosome ofSalmonella enterica serovar Typhimurium across a total of 20 fully assembled genomes deposited into GenBank. Cross-annotation resulted in a 32% increase in the number of core genes and a 3-fold decrease in the number of genes identified as mosaic genes (i.e., genes present in some strains only) by the original annotation. Of the remaining noncore genes, the vast majority were prophage genes, and 255 of the nonphage genes were actually of core origin but lost in some strains upon the emergence of theS . Typhimurium serovar, suggesting that the chromosomal portion of theS . Typhimurium genome acquired a very limited number of novel genes other than prophages. Only horizontally acquired nonphage genes related to bacterial fitness or virulence were found in four recently sequenced isolates, all located on three different genomic islands that harbor multidrug resistance determinants. Thus, the extensive use of antimicrobials could be the main selection force behind the new fitness gene acquisition and the emergence of novelSalmonella pathotypes.IMPORTANCE Significant discrepancies in the annotations of bacterial genomes could mislead the conclusions about evolutionary origin of chromosomal genes, as we demonstrate here via a cross-annotation-based analysis ofSalmonella Typhimurium genomes from GenBank. We conclude that despite being able to infect a broad range of vertebrate hosts, the genomic diversity ofS . Typhimurium strains is almost exclusively limited to gene loss and the transfer of prophage DNA. Only nonphage chromosomal genes acquired after the emergence of the serovar are linked to the genomic islands harboring multidrug resistance factors. Since the fitness factors could lead to increased virulence, this poses an important research question: could overuse or misuse of antimicrobials act as selection forces for the emergence of more pathogenic strains ofSalmonella ?

Corrected Genome Annotations Reveal Gene Loss and Antibiotic Resistance as Drivers in the Fitness Evolution of Salmonella enterica Serovar Typhimurium