Propionibacterium acnes affects the cellular properties of cultured human keratinocytes in a strain-specific and dose-dependent manner

Propionibacterium acnes (P. acnes) bacterium is a member of the skin microflora, but may also serve as an opportunistic pathogen contributing to the pathogenesis of different skin diseases. Earlier we have shown that various P. acnes strains (889, 6609, ATCC 11828) belonging to different phylogroups within the species differentially affected the proliferation and viability of cultured immortalized human keratinocytes (HPV-KER). We found that apart from this strain specificity, the extent of the induced cell biological changes greatly depended on the dose of the bacterial treatment; high doses of the pathogenic 889 and ATCC 11828 strains resulted the death of the HPV-KER cells. In order to analyze this effect in more detail, we performed a fluorescent microscopic analysis of the P. acnes treated cultures, and found cells exhibiting altered morphology and extensive membrane blebbing, characteristic of membrane damage. This was also demonstrated by measuring the quantity of free lactate-dehydrogenase enzyme released to the supernatant from the damaged, bacterial-treated HPV-KER cells using an LDH assay. To demonstrate that the P. acnes induced cell damage is not a keratinocyte specific effect, we also treated human erythrocytes and quantified the rate of membrane damage by measuring the amount of free hemoglobin in the supernatant of the treated cells by spectrophotometry. We found, that in response to the high dose treatment of the same pathogenic strains (889 and ATCC 11828) the amount of free hemoglobin increased. The same treatment conditions also caused marked pH changes (acidification) of the culture supernatants. These results suggest that P. acnes modifies the proliferation and viability of cultured HPV-KER cells in a strain-specific and dose-dependent manner, and this effect may be dependent on the production of an acidic factor possibly generated by the bacterium