A case report of native vertebral osteomyelitis caused by Cutibacterium modestum - BMC Infectious Diseases - BMC Infectious Diseases

Cutibacterium modestum was previously described as "Propionibacterium humerusii." The DNA sequence of C. modestum is 89% similar to that of C. acnes [1]. Previous studies have reported that this bacterium can be detected in human skin [3, 4]. This organism was formally termed as "Cutibacterium modestum" in 2020 [5].

MALDI-TOF MS is widely used for bacterial identification and allows for the relatively easy and quick identification of microorganisms, including C. acnes. However, the predominant peaks on mass spectrometry of C. modestum are different compared with those of C. acnes and its subspecies [6]. MALDI-TOF MS originally suggested that our isolate was C. acnes. However, the log score 1.62 of this species was not adequate to accurately identify the bacteria on either the species or genus level. In addition, the biochemical qualities of this isolate, in particular glycine arylamidase and indole levels, were not consisting with those of C. acnes and other Cutibacterim species [7]. We therefore performed 16SrRNA sequencing of the isolate. Biochemical analysis was very important for distinguishing C. modestum from other Cutibacterium species.

Since the description of Propionibacterium humerusii in 2011 and its new name C. modestum, no literature has reported a clinical C. modestum infection in humans. We were able to successfully treat this patient using antibiotics alone in accordance with the EUCAST breakpoint for C. acnes and Gram-positive anaerobes [8]. However, whether our choice of antibiotic was appropriate is uncertain. Accumulation of clinical experience of human infection caused by C. modestum is required to answer this question.

Recently, an implant-associated C. modestum infection was reported [9]. Our case patient was diagnosed as native vertebral osteomyelitis. Implant-associated C. acnes infections have been previously reported [10, 11], as well as Cutibacterium species-related native vertebral osteomyelitis.

Growth of Cutibacterium species depends on the bacterial inoculum size. It took 7 days for blood culture growth in our case. This suggests low inoculum of bacteremia in this case. When Cutibacterium species is considered as causative pathogen, prolonged blood culture incubation might be feasible.

In conclusion, we reported the native vertebral osteomyelitis due to C. modestum. C. modestum is very similar to C. acnes, and may be misidentified as C. acnes. The biochemical characteristics and inadequate results of MALDI-TOF were very important for distinguishing this bacterium from other Cutibacterium species. Further microbiological and clinical investigations are required to better describe the management of C. modestum infections.