Takuya Hosoda

Abstract Background Streptococcus dysgalactiae subsp. equisimilis (SDSE) is an emerging cause of invasive infection, particularly in aging populations. We investigated the clinical and genomic characteristics of SDSE bacteremia at a Japanese medical center serving a large older-adult population. Method We retrospectively analyzed 73 consecutive episodes of SDSE bacteremia, including 63 initial and 10 recurrent episodes, from April 2020 through September 2024. Clinical features were compared with those of Streptococcus pyogenes (GAS) and Streptococcus agalactiae (GBS) bacteremia. Whole-genome sequencing was performed to assess strain relatedness, emm types, sequence types (STs), and virulence gene profiles. Results The median age of patients with initial SDSE bacteremia was 86.0 years, the highest among 10 major bloodstream pathogens evaluated, predominantly affecting super-elderly patients compared with GAS and GBS. Lower-extremity skin and soft tissue infections were the most common portal of entry. Among 10 recurrent episodes, 5 were caused by strains with STs and emm types identical to those of the initial isolates, consistent with probable relapse; the remaining episodes involved different genotypes, suggesting reinfection. The all-cause 30-day mortality rate was 4.8%. Phylogenetic analysis demonstrated genetic diversity, including 15 emm types and 14 STs. No specific lineage or virulence gene was significantly associated with clinical outcomes. Conclusions In our cohort, SDSE bacteremia predominantly affected super-elderly adults and frequently arose from lower-extremity portals of entry. Recurrent episodes involved both probable relapse and potential reinfection. These findings suggest that localized interventions, such as skin care and edema management, may be important adjuncts to antimicrobial therapy for preventing recurrence.

ABSTRACT Carbapenem-resistant Enterobacterales (CRE) pose a global threat due to limited treatment options and high mortality. Difficult-to-treat resistance (DTR), defined as non-susceptibility to all conventional β-lactams and fluoroquinolones, has primarily been applied to Pseudomonas aeruginosa . However, its relevance for Enterobacterales remains unclear, particularly in regions with a distinct carbapenemase landscape, such as Japan where IMP-type metallo-β-lactamases predominate. We analyzed the MultiDrug-Resistant organisms clinical research network (MDRnet) cohort, a multicenter prospective study conducted at 13 Japanese hospitals between April 2019 and March 2024. This analysis included patients with clinically indicated cultures yielding CRE. Clinical characteristics and outcomes assessed using the desirability of outcome ranking (DOOR) framework and genomic epidemiology characterized by whole-genome sequencing were compared between DTR and non-DTR groups. Among 196 CRE cases, 64 (32.7%) represented infections, including 12 DTR cases (18.8%). Carbapenemase genes were detected in 34/64 infections (53.1%), with similar prevalence in the DTR and non-DTR groups (50.0% vs 53.8%). bla _IMP-1 was the most frequently identified carbapenemase gene ( n = 28). The overall 30-day mortality rate was 21.9%, with 16.7% (95% confidence interval [CI], 2.1%–48.4%) in the DTR group and 23.1% (95% CI, 12.5%–36.8%) in the non-DTR group. DOOR outcomes were similar between groups. Appropriate empiric and definitive therapy was less frequently administered in the DTR group. In this cohort, where IMP-type carbapenemases and non-DTR CRE are prevalent, DTR classification did not appear to correlate with 30-day mortality or DOOR outcomes. These findings underscore the importance of regional molecular epidemiology when interpreting clinical outcomes of CRE infections. IMPORTANCE Difficult-to-treat resistance (DTR) is increasingly used to assess the clinical impact of antimicrobial resistance, but its significance in carbapenem-resistant Enterobacterales (CRE) may vary across molecular epidemiologic settings. In this prospective multicenter study from Japan, where IMP-type carbapenemases predominated, most CRE infections were classified as non-DTR. Whole-genome sequencing also revealed a distinct local clonal structure. DTR was not associated with 30-day mortality or desirability of outcome ranking despite lower rates of appropriate empiric and definitive therapy. Our study highlights the importance of interpreting DTR in CRE within the context of regional molecular epidemiology and supports the need for continued regional multicenter studies to evaluate its clinical utility.

INTRODUCTION: Carbapenem-resistant Gram-negative bacteria (CRGNB) pose a major clinical threat. This study evaluated the in vitro activity of cefiderocol and other recently approved β-lactam/β-lactamase inhibitor combinations against major CRGNB. MATERIALS AND METHODS: A total of 292 CRGNB clinical isolates were analyzed, comprising 146 Enterobacterales, 106 Pseudomonas aeruginosa, and 40 Stenotrophomonas maltophilia, all collected from hospitals across Japan. Antimicrobial susceptibility testing was performed by broth microdilution (BMD). Disk diffusion testing was also conducted for cefiderocol, and categorical agreement with BMD was assessed. Whole-genome sequencing (WGS) was used for species confirmation and characterization of resistance determinants. RESULTS: Carbapenemase producers accounted for 64.4 % of Enterobacterales (94/146) and 8.5 % of P. aeruginosa (9/106), with metallo-β-lactamase (MBL) producers comprising 92.6 % (87/94) and 77.8 % (7/9), respectively. Based on CLSI breakpoints, 94.5 % (276/292) of isolates were susceptible to cefiderocol, including 91.8 % of Enterobacterales, 99.1 % of P. aeruginosa, and 92.5 % of S. maltophilia. Ceftolozane-tazobactam, ceftazidime-avibactam, and imipenem-relebactam were active against 12.3 %, 44.5 % and 45.9 % of Enterobacterales, and 89.6 %, 86.8 % and 72.6 % of P. aeruginosa, respectively. Categorical agreement between cefiderocol disk diffusion and BMD exceeded 92 % across all groups, although very major errors occurred in Enterobacterales (n = 2) and S. maltophilia (n = 3). Cefiderocol-non-susceptible Enterobacterales isolates frequently harbored carbapenemase and extended-spectrum β-lactamase (ESBL) genes, together with mutations in ftsI (encoding PBP3), ompK35, or siderophore receptor genes (cirA, tonB). DISCUSSION: Cefiderocol showed potent in vitro activity against CRGNB in Japan, including MBL producers. Disk diffusion correlated well with BMD results; however, confirmatory BMD testing should be considered when resistance is clinically suspected.

With the widespread use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS), the number of identifiable bacterial species has increased. However, anaerobic bacteremia remains challenging to accurately diagnose due to the diverse range of anaerobic bacteria and the frequent occurrence of polymicrobial infections. Consequently, MALDI-TOF MS often fails to achieve accurate species-level identification in such cases. To address this limitation, we evaluated whole-genome sequencing (WGS) as an alternative method for identifying anaerobic bacteria in blood cultures. Over a 4-year period (April 2020 to March 2024), 69 cases of anaerobic bacteremia were identified, involving 85 bacterial strains. WGS successfully identified 73 strains (89%) at the species level. MALDI-TOF MS accurately identified 43 strains (59%) at the species level and 6 strains (8.2%) at the genus level. Among the 24 discordant strains, 9 species were not included in the database, and 6 species had limited prior reports of bloodstream infections. Additionally, 21 of the 69 cases (30%) were polymicrobial, and WGS revealed 9 cases (13%) in which multiple species had not been identified by MALDI-TOF MS. These results highlight the limitations of MALDI-TOF MS in anaerobic bacterial identification, particularly in polymicrobial infections, and suggest that alternative molecular approaches are necessary to improve diagnostic accuracy.IMPORTANCEAccurate identification of anaerobic bacteria remains a significant challenge despite the widespread use of matrix-assisted laser desorption ionization time-of-flight mass spectrometry. While this technology has improved the detection of many bacterial species, some anaerobes remain unidentified due to their absence from reference databases and the difficulties associated with their isolation, particularly in polymicrobial infections. Whole-genome sequencing (WGS) has revealed previously unreported anaerobes and identified polymicrobial infections that were initially misclassified as monomicrobial. Our findings underscore the importance of implementing molecular approaches such as WGS- or PCR-based methods in clinical diagnostics to improve the detection of anaerobic pathogens.

Rapid detection and reporting of carbapenemase-producing Enterobacterales (CPE) is one of the top priorities of clinical microbiology laboratories. The Clinical and Laboratory Standards Institute recommends the modified carbapenem inactivation method (mCIM) as the preferred method for this purpose, but it requires a broth incubation process which can be cumbersome. Here, we compared the performance of mCIM with three alternative rapid CPE detection methods against a collection of genetically defined CPE, with most carrying blaIMP, and non-CPE clinical isolates. The sensitivities of mCIM, simplified carbapenem inactivation method (sCIM), Rapidec Carba NP, and NG-Test Carba 5 were 98.0%, 54.9%, 90.2%, and 72.5%, whereas the specificities were 89.5%, 84.2%, 89.5%, and 100%, respectively. Modification of the interpretive criteria of sCIM increased its sensitivity to 88.2% and specificity to 89.5%. The results suggest that mCIM is currently the optimal method for CPE detection in an epidemiological setting where CPE-producing IMP group carbapenemase is predominant. While sCIM is easier to perform, it requires further validation before it can be widely adopted as an alternative to mCIM in the clinical laboratory. IMPORTANCE Simple identification methods for carbapenemase-producing Enterobacterales are required for the clinical laboratory. The simplified carbapenem inactivation method (sCIM) is a carbapenemase detection method that can be performed with less hands-on time than mCIM, but its sensitivity and specificity were suboptimal compared with other phenotypic detection methods when tested against a collection of IMP-producing CPE. Insufficient inactivation of imipenem from inadequate inoculation was suspected as the cause. While sCIM is easier to perform, it requires optimization before it can be widely adopted as an alternative to mCIM in the clinical laboratory.