Propionibacterium acnes and its phages

University dissertation from Department of Clinical Sciences, Lund University

Abstract: Microorganisms are everywhere! They can tolerate many diverse extreme environments, such as the human body. Even though many of us might associate the word microorganism with infections and disease, most are actually either harmless or even beneficial for us. Those commensals often fight off more dangerous bacteria and might also directly benefit their host. The human skin harbors several different microorganisms, with Propionibacterium acnes being one of the most common bacteria. This Gram-positive anaerobe has long been attributed as the cause of acne vulgaris, and is known to cause severe inflammations on orthopedic implants. However, like most bacteria, P. acnes can be infected by specific bacterial viruses, eg. bacteriophages. If those can contribute to the progress of the diseases mentioned is unknown.

In this thesis we have investigated the role of P. acnes as both a pathogen and a commensal, and characterized the phages infecting P. acnes. To undertake those studies, we first had to develop a genetic toolbox to better be able to characterize the bacteria and their phages, since there is a huge lack of molecular tools for the study of P. acnes (Paper I). Furthermore, we found that P. acnes that caused inflammations on orthopedic implants had a higher capacity to form biofilms, than did strains isolated from the skin. Thus, the ability to form biofilm seems to be a characteristic of invasive isolates (Paper II). Even though unwanted on orthopedic implants, we found that colonization by P. acnes on the human skin is beneficial for its host. This is due to that P. acnes secretes a heme-oxygenase that protects our cells against free radicals, and increase the viability of the skin cells (Paper III). Furthermore, we characterized several bacteriophages that could infect P. acnes. Those phages had a high capacity to infect and lyse P. acnes (Paper IV). Finally, the sequencing of two of the phages revealed that the phages were not able to integrate their DNA into its host chromosome, but instead, most likely had a pseudolysogenic relation with their host (Paper V).

In summary, this thesis can conclude that P. acnes is commonly infected by phages, living in a pseudolysogenic relation. Furthermore, colonization by P. acnes might prove both beneficial and harmful for the host, all depending on the site of colonization.