Cutibacterium acnes molecular typing: time to standardize the method

The Gram-positive, anaerobic/aerotolerant bacterium Cutibacterium acnes is a commensal of healthy human skin; it is subdivided into six main phylogenetic groups or phylotypes: IA1, IA2, IB, IC, II and III. To decipher how far specific subgroups of C. acnes are involved in disease physiopathology, different molecular typing methods have been developed to identify these subgroups: i.e. phylotypes, clonal complexes, and types defined by single-locus sequence typing (SLST). However, as several molecular typing methods have been developed over the last decade, it has become a difficult task to compare the results from one article to another. Based on the scientific literature, the aim of this narrative review is to propose a standardized method to perform molecular typing of C. acnes, according to the degree of resolution needed (phylotypes, clonal complexes, or SLST types). We discuss the existing different typing methods from a critical point of view, emphasizing their advantages and drawbacks, and we identify the most frequently used methods. We propose a consensus algorithm according to the needed phylogeny resolution level. We first propose to use multiplex PCR for phylotype identification, MLST9 for clonal complex determination, and SLST for phylogeny investigation including numerous isolates. There is an obvious need to create a consensus about molecular typing methods for C. acnes. This standardization will facilitate the comparison of results between one article and another, and also the interpretation of clinical data.