Identification of a gene in Mycoplasma hominis associated with preterm birth and microbial burden in intraamniotic infection

Abstract

Objective: Microbial invasion of the amniotic cavity is associated with spontaneous preterm labor and adverse pregnancy outcome, and Mycoplasma hominis often is present. However, the pathogenic process by which M hominis invades the amniotic cavity and gestational tissues, often resulting in chorioamnionitis and preterm birth, remains unknown. We hypothesized that strains of M hominis vary genetically with regards to their potential to invade and colonize the amniotic cavity and placenta.

Study design: We sequenced the entire genomes of 2 amniotic fluid isolates and a placental isolate of M hominis from pregnancies that resulted in preterm births and compared them with the previously sequenced genome of the type strain PG21. We identified genes that were specific to the amniotic fluid/placental isolates. We then determined the microbial burden and the presence of these genes in another set of subjects from whom samples of amniotic fluid had been collected and were positive for M hominis.

Results: We identified 2 genes that encode surface-located membrane proteins (Lmp1 and Lmp-like) in the sequenced amniotic fluid/placental isolates that were truncated severely in PG21. We also identified, for the first time, a microbial gene of unknown function that is referred to in this study as gene of interest C that was associated significantly with bacterial burden in amniotic fluid and the risk of preterm delivery in patients with preterm labor.

Conclusion: A gene in M hominis was identified that is associated significantly with colonization and/or infection of the upper reproductive tract during pregnancy and with preterm birth.

Keywords: chorioamnionitis; genital mycoplasmas; genome sequencing; microbial invasion of the amniotic cavity; pathogenicity.

Figure 1. Aggregative properties of isolates from intra-amniotic infection in liquid culture

The M. hominis strains were cultured overnight in polystyrene culture tubes and the wall of the tube was imaged using a light microscope with a 10X objective. The rectal isolate PG21 aggregated and adhered to the walls of the polystyrene culture tubes (visible in the first panel as clusters of bacteria), whereas all three amniotic fluid/placental isolates failed to aggregate or adhere to the culture vessel (bare polystyrene), suggesting differences in the expression of surface proteins.

Figure 2. Variable membrane protein loci

A. The variable membrane protein loci from the 3 amniotic fluid/placental strains and PG21 are depicted. The scale bar at the top indicates the number of nucleotides within the longest loci (AF1 and PF5). The loci within the genomes of AF1 and PF5 each contained two vmp genes whereas the AF3 and PG21 genomes lacked vmp. B. The Vaa amino acid sequences were compared using Geneious. The heatmaps indicate amnio acid similarity and range from 100% similar (black) to <60% similar (light grey).

Figure 3. Whole genome comparison

The chromosome of the amniotic fluid isolate AF1 (red) was used as the benchmark, and the chromosomes from the amniotic fluid/placental isolates AF3 (blue), PF5 (purple), and the rectal isolate PG21 (green) were compared to it. Genes that are absent within a given chromosome appear as gaps. The genes present in all three MIAC strains, but absent in PG21 (alr, goiB, goiC) appear as gaps exclusive to the green circle and are denoted with black arrows. A prophage that was exclusive to AF1 and absent in the other 3 strains in noted with a grey arrow. The membrane protein Lmp1 is present in PG21 but appears as a gap because it is in a different location on the chromosome, and it is noted with a grey arrow.

Figure 4. Genetic locus of M hominis goiC

The variable locus surrounding the goiC gene is shown for all 4 strains. Strains AF1 and AF3 contained highly similar loci, with 3 sets of direct repeats (shown in the consensus map at the top as colored rectangles). PL5 lacked one set of repeats and 10 of the genes present in AF1 and AF3. The PG21 genome lacked all 3 sets of repeats and goiC.

Figure 5. M hominis goiC is associated with preterm delivery and bacterial burden

The y-axis (logarithmic scale) represents the microbial burden in amniotic fluid in colony forming units per mL amniotic fluid. The open square represents amniotic fluid from a case of spontaneous preterm labor and preterm birth in which the M. hominis goiC gene was absent. The open diamonds represent amniotic fluid from cases of spontaneous preterm labor and term birth in which goiC was absent. The black squares represent amniotic fluid from cases of spontaneous preterm labor and preterm birth in which goiC gene was present.