NCCP15736 was isolated from a symptomatic human carrier but NCCP15737 was isolated from an asymptomatic human carrier. To determine the causal mechanisms underlying the observed pathogenicity, we investigated the virulence factors of NCCP15736 and compared these factors with those of NCCP15737. Using a BLAST search against VFDB, we identified the same number, 25, of virulence factors from NCCP15736 and NCCP15737, respectively (Additional file
2: Table S2). The 25 virulence genes present in NCCP15736 were also present in NCCP15737. The virulence genes of NCCP15736 and NCCP15737 can be classified into five categories: adherence, invasion, iron uptake, secretion system, and toxins. In the adherence category,
E. coli common pilus (ECP)-related genes (
ecpA, B, C, D, E, and
ecpR) F1C fimbriae (
focC), and type I fimbriae genes (
fimA, B, C, D, E, F, G, H, and
I) were identified. Tia invasion determinant (
tia) [
26], which belongs to the invasion category and originates from
E. coli O1:K1, was identified in both strains. In the iron uptake category, iron-regulated element gene (
ireA) and salmochelin siderophore-related gene (
iroN) were identified in NCCP15736 and NCCP15737. Neither strain contained all of the genes in the LEE-encoded TTSS effectors category, harboring only one secretion gene,
escR [
27]. In the toxins category, alpha-hemolysin–related genes (
hlyA, B and
D) [
28] were identified. Alpha-hemolysin is a major virulence factor present in ETEC, STEC, and EPEC strains. It is acquired by horizontal gene transfer via conjugative plasmids [
29]. Shiga-like toxin-related genes (
stx1A and
1B) [
30] were present in both of the strains and exhibited 100% sequence conservation. In summary, the NCCP15736 and NCCP15737 strains showed the same virulence factors, although NCCP15736 was isolated from a symptomatic carrier and NCCP15737 was isolated from an asymptomatic carrier. In a previous report [
12], the expression of type I fimbriae genes was found to be significantly repressed, and the repression was hypothesized to be the main cause of the asymptomatic nature of NCCP15737.