Extraintestinal pathogenic Escherichia coli strains of avian and human origin: link between phylogenetic relationships and common virulence patterns

Abstract

Extraintestinal pathogenic Escherichia coli (ExPEC) strains of human and avian origin show similarities that suggest that the avian strains potentially have zoonotic properties. However, the phylogenetic relationships between avian and human ExPEC strains are poorly documented, so this possibility is difficult to assess. We used PCR-based phylotyping and multilocus sequence typing (MLST) to determine the phylogenetic relationships between 39 avian pathogenic E. coli (APEC) strains of serogroups O1, O2, O18, and O78 and 51 human ExPEC strains. We also compared the virulence genotype and pathogenicity for chickens of APEC strains and human ExPEC strains. Twenty-eight of the 30 APEC strains of serogroups O1, O2, and O18 were classified by MLST into the same subcluster (B2-1) of phylogenetic group B2, whereas the 9 APEC strains of serogroup O78 were in phylogenetic groups D (3 strains) and B1 (6 strains). Human ExPEC strains were closely related to APEC strains in each of these three subclusters. The 28 avian and 25 human strains belonging to phylogenetic subcluster B2-1 all expressed the K1 antigen and presented no significant differences concerning the presence of other virulence factors. Moreover, human strains of this phylogenetic subcluster were highly virulent for chicks, so no host specificity was identified. Thus, APEC strains of serotypes O1:K1, O2:K1, and O18:K1 belong to the same highly pathogenic clonal group as human E. coli strains of the same serotypes isolated from cases of neonatal meningitis, urinary tract infections, and septicemia. These APEC strains constitute a potential zoonotic risk.

FIG. 1.

Phylogenetic relationships between 102 E. coli isolates originating from human clinical extraintestinal sources (n = 51), avian colibacillosis (n = 39), nonpoultry animal cases of septicemia (n = 4), healthy hosts (n = 6), and hemolytic-uremic syndrome (n = 2). The phylogenetic tree was constructed by neighbor-joining analysis based on the sequence analysis of six housekeeping genes. The phylogenetic groups and subclusters are given on the right. Bootstrap values are given above the nodes where values are higher than 60% (bootstrap values from the maximum-parsimony analysis are in parentheses). The serogroup of each strain (when known) is given in parentheses. The clinical origins of ExPEC strains are coded by colors and letters as follows: A (red), avian colibacillosis; M (purple), neonatal meningitis; S (blue), septicemia; U (green), urinary tract infection; no letter (black), strain from a healthy host. The animals from which the six nonpoultry isolates originated are given. E. coli EDL933 and SAKAI are EHEC strains.

FIG. 2.

Association of VFs with phylogenetic groups in E. coli strains originating from human clinical extraintestinal sources (n = 51), feces of healthy humans (n = 4), or avian colibacillosis (n = 39). In this schematic representation of the FCA, the distributions of VFs and major serotypes are represented on a factorial plane defined by the most discriminating axes: F1 and F2. Factorial axis F1 accounts for 29.49% of the variance, and factorial axis F2 accounts for 16.51% of the remaining variance. The strength of the associations is indicated by the distance between the symbols and the sizes of the symbols (the larger the size of the symbol, the stronger the association). The species of the host (avian or human), phylogenetic group (A, B1, B2, or D), clinical origin (avian colibacillosis, neonatal meningitis, septicemia, urinary tract infection, or healthy individual), O serogroup, and VFs are indicated. The presence or absence of a virulence factor is indicated by “1” or “0,” respectively.

FIG. 3.

LD₅₀s for chicks of human (n = 22) and avian (n = 3) ExPEC strains of phylogenetic group B2. Decimal dilutions of bacterial cultures of ExPEC strains of human and avian origins were inoculated into 1-day-old chicks, and the lethality was recorded. The virulence of the E. coli strains was expressed as the LD₅₀ for chicks; the log LD₅₀ is represented. The O serogroups and origins (Hu, human; Av, avian) of the strains are given.