C. acnes strains were previously classified into two main types, I and II, on the basis of their cell wall carbohydrate content and serum lectin responses
[12][13]. It was then suggested that
C. acnes strains could be distinguished on the basis of random amplification of polymorphic DNA (RAPD) analysis
[14]. Analyses of the sequences of the non-ribosomal housekeeping genes
RecA and
tly, and the use of the QUBPa1 and QUBPa2 mAbs, specific for type I and II strains, respectively, subsequently showed that the two types corresponded to evolutionary lineages of
C. acnes displaying both genetic and phenotypic differences. An additional phylotype, type III, corresponding to strains with filamentous appendages, was then added to the classification
[15][16]. Based on 16S rRNA gene analysis,
C. acnes strains were divided into ribotypes (RTs), which could be used to differentiate between types I, II, and III, associated with healthy skin or acne
[17]. RT4, RT5 and RT8 appeared to be found in acne lesions, whereas RT2 and RT6 were mostly found on healthy skin
[18]. Multi-locus sequence typing (MLST) and single-locus sequence typing (SLST) methods have been developed for this species, to increase typing resolution. Depending on the number and nature of the housekeeping genes selected, two schemes for discriminating between
C. acnes strains have been described. The Belfast scheme, using seven target genes, can differentiate type I into clades IA
1, IA
2, IB, and IC
[19][20]. A multiplex touchdown PCR typing method has also been developed that can discriminate between the IA
1, IA
2, IB, IC, type II, and type III phylotypes, providing a rapid overview of the
C. acnes types present in a population
[21]. The other scheme, the Aarhus scheme, uses nine target genes to split type I into clades I-1a, I-1b, and I-2
[22]. This scheme appears to be the most discriminant of these two schemes
[23]. SLST and whole-genome sequencing (WGS) methods were subsequently used to develop a new scheme capable of differentiating
C. acnes strains into clades IA-1, IA-2, IB-1, IB-2, IB-3, IC, II, and III
[24], but some
C. acnes strains were not identified with this scheme
[25] (). MS-based typing for routine analysis has been evaluated with the MALDI-TOF MS method, to characterize
C. acnes strains in diverse samples
[26]. This method is very useful for rapidly discriminating
C. acnes phylotypes, although missing some of them
[26]. However, this approach was improved using profiling of identified biomarkers, such as ribosomal subunit proteins, and named MALDI-MS prototyping, which allowed discrimination of all of the
C. acnes phylotypes. Although this method needs to ultrafiltrate bacteria whole pellets in order to analyze the concentrated protein fraction, it is nonetheless compatible with the analysis of a large number of
C. acnes strains
[27]. Another attempt to facilitate
C. acnes typing involved the use of the MLVA method to analyze the polymorphism of 13 VNTRs; the results of this method were well-correlated with those for the MLST and SLST methods
[28]. Nomenclatures highlight the need for standardization. WGS is of potential interest for this purpose because it should provide a higher-resolution phylogeny more rapidly and at lower cost
[29][30].