Mother-to-child transmission of and multiple-strain colonization by Bacteroides fragilis in a cohort of mothers and their children

Abstract

Bacteroides fragilis represents an early infant colonizer with important host interactions. Our knowledge about the diversity, transmission, and persistence of this bacterium, however, is limited. Here, we addressed these questions using a combination of multilocus sequence typing (MLST) and variable-number tandem repeat (VNTR) sequence analyses. We used both culture-dependent and -independent typing. We genotyped B. fragilis in fecal samples from a cohort of 93 mothers and their children, with samples taken from the mothers and from the children at the ages 1 to 10 days, 4 months, 1 year, and 2 years. By MLST we found two main B. fragilis groups, which we denoted clades A and B. Direct typing of stool samples using the icd gene revealed seven sequence types, five within clade A and two within clade B. A single clade A sequence type, however, represented 79% of all the sequences. This sequence type was further subtyped using VNTR. VNTR subtyping revealed 16 different VNTR types. Based on the distribution patterns of these, we show mother-to-child transmission and multiple-strain colonization. We argue that negative host selection promotes the coexistence of multiple strains. The significance of our findings is that we have started unraveling the transmission and persistence patterns of one of the most important human gut colonizers.

Fig. 1.

SplitsTree for full MLST and icd genotype data. (A) The full MLST was determined for the pure strains included in this work, in addition to the genome-sequenced strains. (B) All identified icd genotypes were included in the tree. Numbering at the branches shows the bootstrap support in percentage. Distance bars represent uncorrected p-distances for neighbor-joining trees.

Fig. 2.

Distribution of icd genotypes among mothers and infants analyzed in this study. A color code and intensity scale are used to indicate genotype and signal intensity, respectively. The Roman numbers (I and II) indicate whether cepA- or cfiA-positive strains were isolated from the samples, respectively. IgE sensitization information is given for children 2 years of age: +, sensitized; −, nonsensitized; /, lack of information about sensitization. The 8-bit pixel values for the inverse band signal intensities were defined as follows: high, >200 (maximum, 256); medium, 200 to 170; weak, 170 to 150; and very weak, <150. The background signals were about 120. Id, identification.

Fig. 3.

Relative icd genotype composition (A) and proportion of B. fragilis-positive samples (B) for the different age categories investigated. The representations are summaries of the data presented in Fig. 2.

Fig. 4.

Relative TRS1 composition directly from the stool samples for the different age categories investigated. The percentages of the different TRS1 types identified are shown.