Use of Multilocus Sequence Typing to Investigate the Population Biology of Propionibacterium Acnes in Ocular Infections

Abstract

Purpose:: Multilocus sequence typing (MLST) is a bacterial typing system, characterising isolates on the basis of the sequences of ~450 bp internal fragments of several housekeeping genes. We describe the development of an MLST scheme to study the population structure of Propionibacterium acnes isolates to determine if strains from patients with ocular infections represent a particular subgroup.

Methods:: 131 P. acnes isolates were examined using an in-house MLST scheme. The collection included one reference strain, 31 isolates from keratitis and endophthalmitis infections, 99 from other infections. Seven housekeeping genes distributed around the P. acnes genome were identified as targets for PCR amplification. Nested primers were designed to amplify 400-500 bp fragments of each gene. For each distinct allele within a locus an arbitrary number was assigned. Each isolate was therefore given seven numbers (the allelic profile) that represented its sequence type (ST). Isolates with an identical allelic profile were assigned the same ST. Each ST was given an arbitrary number in order of analysis. Phylogenetic analyses of the strains were carried out using the START programme.

Results:: Based on BURST analysis, 39 STs were assigned to the 131 P. acnes isolates that grouped into 5 distinct clonal complexes. ST-6 was the dominant strain with 67/131 (50.1%) of clinical isolates belonging to this ST. Also, 10/29 (34.5%) isolates from eye infections belonged to ST-6. Isolates obtained from corneal scrapes were present in BURST groups 1, 3 and 5. Vitreous humour isolates were present in groups 1 and 4. Aqueous humour isolates were present in group 1 only. There were no ocular infection isolates in group 2 although the total number of isolates in this group was relatively small.

Conclusions:: The MLST scheme developed in this study enables comparative characterisation of P. acnes isolates at the molecular level. We have confirmed that P. acnes has a clonal population structure and there appears to be no correlation between the clinical source and the particular clonal complexes.

Keywords: microbial pathogenesis: experimental studies • bacterial disease