# Optimization of the microbiological quality control validation of corneal medium using a clinical C. acnes isolate

## TL;DR

This study shows that using a clinical isolate of C. acnes improves the validation of corneal medium quality control, leading to more reliable results for cornea transplant patients.

## Contribution

The study demonstrates that a specific clinical C. acnes isolate (DSM 117854) is more reliable for matrix validation than the reference strain.

## Key findings

- Clinical C. acnes isolates showed varied growth characteristics, with some being more reliable in detecting growth in corneal medium.
- Strain DSM 117854 consistently showed blood-culture-detectable growth in the presence of corneal medium, unlike the reference strain.
- Three clinical isolates (CA02, CA10, CA22) showed reproducible growth signals in the presence of corneal medium.

## Abstract

The microbiological quality of corneal tissue for transplantation must be monitored in a validated procedure complying with the European Pharmacopoeia (Ph. Eur.). An automated culture system is used to detect test strains in the presence of the medium. The Ph. Eur. mentioned C. acnes ATCC 11827 strain has allegedly been isolated from skin. This strain is known to display variable growth in routine matrix validation. As the Ph. Eur. also opens the use of clinical isolates, this study analyzed the use of clinical isolates for matrix validation.

We compared the time-to-detection (TTD) in absence and presence of cornea organ culture medium (cocm) of 24 C. acnes strains, preferring clinical isolates, in an automated culture system.

In the first step clinical strains (n=23) were examined, thus 4 slow-growing (TTD 305,1–336 h), 5 intermediate-fast-growing (TTD 250,1-305 h), 5 fast-growing strains (TTD 164,3–250 h) and nine non-blood-culture-detectable strains (TTD>336 h) were identified. Seven strains showed a reproducible growth signal in cocm-absence. In presence of cocm, three strains (CA02, CA10, CA22) showed a reproducible growth signal, while the reference strain was not blood-culture-detectable (bcd) in four of five experiments. The strain with the preferred growth characteristics (DSM 117854) was tested according to the Ph. Eur. conditions and showed bcd growth in all measurements (n=5) in presence of medium, in contrast to the reference strain.

The use of a clinical isolate of C. acnes could improve matrix validation of cocm quality control testing and thus the care of cornea-transplant-patients. C. acnes DSM 117854 provides an isolate that promises more reliable results in matrix validation.

The online version contains supplementary material available at 10.1007/s10561-026-10211-9.