# Emerging mastitis-associated Corynebacterium parakroppenstedtii and Corynebacterium pseudokroppenstedtii: clinical, microbiological, and epidemiological features from a two-year study in Guangdong, China

## TL;DR

A two-year study in China found that two emerging bacteria, Corynebacterium parakroppenstedtii and C. pseudokroppenstedtii, are the main causes of mastitis, challenging previous assumptions about the disease's causes and treatment.

## Contribution

This study provides new clinical, microbiological, and epidemiological insights into emerging Corynebacterium species causing mastitis, emphasizing the importance of species-level identification for effective treatment.

## Key findings

- C. parakroppenstedtii was responsible for 86% of mastitis cases, while C. pseudokroppenstedtii accounted for 12%.
- C. pseudokroppenstedtii showed higher resistance to ceftriaxone and ciprofloxacin compared to C. parakroppenstedtii.
- Granulomatous lobular mastitis was the most common clinical presentation, occurring in 72% of cases.

## Abstract

Corynebacterium parakroppenstedtii, and C. pseudokroppenstedtii are emerging as significant pathogens in mastitis. Despite its clinical significance, data on its epidemiological, microbiological, and clinical features remain limited.

We conducted a comprehensive study on mastitis cases associated with Corynebacterium kroppenstedtii complex (CKC) in Guangdong, China (September 2021–September 2023). A total of 101 bacterial isolates were collected and initially identified as CKC using MALDI-TOF MS. Species-level confirmation was achieved through partial sequencing of the 16S rRNA, ropB, and fusA genes. Unclassified isolates were further characterized by whole genome sequencing (WGS). Clinical information was collected, and antimicrobial susceptibility testing was performed.

Among the 101cases, C. parakroppenstedtii accounted for 86% of infections, while C. pseudokroppenstedtii for 12%, and a potential novel species for the remaining 2%. Notably, no C. kroppenstedtii infections were detected. Granulomatous lobular mastitis (GLM) was the predominant presentation, occurring in 72% of all cases. Comparative analysis revealed that C. pseudokroppenstedtii infections were associated with higher rates of pus formation and recurrence, whereas C. parakroppenstedtii infections were more prevalent among parous women. Moreover, C. pseudokroppenstedtii exhibited higher resistance rates to ceftriaxone (32.15% vs. 20.00%) and ciprofloxacin (75.00% vs. 19.09%) compared with C. parakroppenstedtii.

These findings challenge the prevailing understanding that C. kroppenstedtii is the main pathogen in mastitis and underscore the need for species-level identification to guide diagnosis and optimize antibiotic therapy for CKC-related mastitis. These insights are vital for improving clinical management and informing treatment strategies.