Improvement of isolation and drug testing methods for Corynebacterium spp. in clinical specimens of granulomatous mastitis.

OBJECTIVE

This study aimed to address the low clinical isolation rate, prolonged antimicrobial susceptibility testing (AST) time, and inconsistent results for Corynebacterium kroppenstedtii, the primary pathogens of granulomatous mastitis (GM). We optimized culture media and cultivation methods to enhance isolation efficiency and shorten AST turnaround time.

METHODS

An optimized culture medium (cation-adjusted Mueller-Hinton broth CAMHB with 3.3% horse serum, β-NAD, and 1% Tween 80) combined with a sealed culture system was developed. We analyzed bacterial isolation rates and culture times using 44 GM clinical specimens. The optimized culture medium was compared with MH-F medium in microbroth dilution AST, validated against the E-test method for consistency. Antibiotic susceptibility profiles of 15 isolates to 13 agents were also assessed, including inducible resistance.

RESULTS

The optimized culture medium combined with sealed culture increased the isolation rate of C. kroppenstedtii to 22.72% and reduced average culture time to 36.8 h. AST time was shortened to 20 h, showing higher consistency with the E-test method (Kappa 0.916 vs. 0.744). All isolates exhibited 100% sensitivity to vancomycin and rifampicin, while 8.72% of strains showed induced resistance to penicillin and cefuroxime over time.

CONCLUSION

Optimized culture media and sealed-culture methods significantly improved isolation efficiency and enabled rapid AST within 20 h for C. kroppenstedtii, yielding more reliable results than traditional approaches