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Phylogenetic Study IDs Bacterial Sub-group Associated with Acne

By a GenomeWeb staff reporter

NEW YORK (GenomeWeb News) – A Propionibacterium acnes sub-group linked to moderate to severe acne belongs to a genetic lineage distinct from P. acnes bacteria on healthy skin, according to a population genetic study appearing online this week in PLoS ONE.

A pair of researchers from Aarhus University in Denmark did phylogenetic analyses on hundreds of P. acnes isolates, comparing those in acne sufferers, individuals with opportunistic infections, and unaffected controls. From phylogenetic analyses and whole-genome data for five P. acnes isolates, the team identified a P. acnes sub-population tied to acne and began unraveling genetic features within this group — insights that they say could offer clues about preventing and treating acne.

"The results of the study indicate that particular clones of P. acnes play an etiologic role in acne while others are associated with health," senior author Mogens Kilian, medical bacteriology chair at Aarhus University, and co-author Hans Lomholt, a medical microbiology and immunology researcher at Aarhus University, wrote.

P. acnes bacteria have turned up in a range of opportunistic infections, the researchers noted, and past studies suggest the microbes can prompt inflammatory responses in some skin, gland, and white blood cells. But, they added, not everyone is convinced that the bug should be considered an acne culprit, since it often shows up on the skin of those without acne or other infections.

"The involvement of Propionibacterium acnes in the pathogenesis of acne is controversial, mainly owing to its dominance as an inhabitant of healthy skin," the duo wrote, noting that previous genetic, phenotypic, serological, and other studies have identified three P. acnes types and two sub-types.

To further explore the population genetic structure within the species and potential ties to acne, Lomholt and Kilian used multi-locus sequence analysis to assess nine housekeeping genes from 210 P. acnes skin isolates. They also compared two potential virulence factor genes — tly and camp5 — in the isolates.

The samples came from Denmark, China, the US, Sweden, Norway, Germany, and the UK and included isolates from the skin of healthy individuals, individuals with moderate to severe acne, and individuals with opportunistic infections.

The researchers found dozens of sequence types, though they noted that isolates collected in China and Scandinavia were quite similar to one another overall.

Though they did not see much genomic recombination within the isolates sampled, the researchers' phylogenetic analyses identified three P. acnes sub-populations, including one linked to moderate to severe acne that was not found in those with other infections or unaffected controls.

When the researchers compared five previously sequenced P. acnes genomes they found relatively little genetic diversity in core genomes. Even so, the pair did find clusters of genes that varied from one strain to the next and between sub-groups.

Among the differences between acne-associated and other strains were changes to genes coding for potential virulence factors as well as some enzymes thought to contribute to host cell inflammation.

"While some evolutionary lineages of the species are associated with health, others, including an epidemic clone, are strongly associated with moderate to severe acne," the team concluded. "The fact that this dichotomy correlates with previously observed differences in the in vitro expression of putative virulence factors strongly support the conclusion that acne is an infectious disease affecting genetically susceptible individuals."

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