Phoenix m50 system

The resistance profiles of S. xiamenensis strains were determined with the BD Phoenix™ M50 system (Becton, Dickinson and Company, Franklin Lakes, NJ, USA); the following 13 agents were included: ampicillin, ampicillin-sulbactam, aztreonam, cefotaxime, cefoxitin, ceftazidime, ceftazidime-avibactam, chloramphenicol, ciprofloxacin, ertapenem, imipenem, meropenem, and tetracycline. The minimal inhibitory concentrations (MICs) of 9 β-lactam antibiotics of cloned strains were obtained via broth microdilution methods.

A 5-mL broth was aspirated from the flagged positive blood culture bottle using a sterile disposable syringe. The broth was then dispensed into a sterile BD Vacutainer SST II Advance tube and centrifuged at 2,000 × g for 15 minutes in the Neya 16R Centrifuge instrument (REMI, India). After centrifugation, the supernatant was discarded, and the bacterial pellets trapped in the gel layer were carefully harvested with StabiFlexLoop (HiMedia Laboratories, India) of 1.25-mm diameter. A 0.5 McFarland bacterial inoculum was prepared from the harvested bacterial pellets using BD PhoenixSpec nephelometer (Becton Dickinson, USA). NMIC/ID panel (Becton Dickinson, USA) was inoculated with the standard bacterial suspension per the manufacturer’s instructions. NMIC/ID panel is used for Gram-negative organisms only. Each panel contains a set of wells (five for each antibiotic) with varying concentrations of different antimicrobial agents. Additionally, there are 51 wells containing different identification substrates. The panel was loaded into the BD Phoenix M50 system (Becton Dickinson, USA) for automated ID and AST of the bacterial isolate. The system tracks the growth of microorganisms in all wells and displays identification and susceptibility results using the patterns of microbial growth.

The BD Phoenix™ M50 System (Becton, Dickinson and Company, New Jersey, USA) and K-B (K-B) disk-diffusion method (Oxoid, Hampshire, United Kingdom) was used to test the susceptibility of the strains to 24 antimicrobial agents commonly used in clinical practice. The susceptibility criteria were determined in accordance with the Clinical and Laboratory Standards Institute (CLSI) guidelines (2021) [14 ] (https://clsi.org/). Escherichia coli ATCC 25,922 served as the quality control strain.

Once a bacterial ID was confirmed, AST was performed on the isolates using the BD Phoenix M50 system (Becton, Dickinson and Company). Each bacterium identified was used to prepare a 0.5 McFarland standard in BD Phoenix ID broth (Becton, Dickinson and Company). Depending on the bacterium’s identity, it was tested on positive minimal inhibitory concentration (PMIC), negative minimal inhibitory concentration (NMIC), or streptococcus minimal inhibitory concentration (SMIC) panels. MIC and AST interpretations were available after approximately 24 h and were recorded to calculate the categorical agreement (CA; proportion of APAS AST interpretation with the same categorical SIR [susceptible, intermediate, resistant] interpretation as the SOC method) and essential agreement (EA; proportion of APAS MIC values within one dilution of those obtained using the SOC method). AST differences were also categorized into 3 different error types: minor errors (mEs), major errors (MEs), and very major errors (VMEs). mEs were defined as results that were susceptible or resistant in one system but intermediate in the other. MEs were defined as susceptible by the SOC results but resistant by the APAS results. Finally, VMEs were defined as resistant by the SOC results but susceptible by the APAS results.

Antimicrobial drug susceptibility testing was performed according to the manufacturer’s instruction using a BD Phoenix M50 system (BD Diagnostic Systems, Sparks, MD, USA). Briefly, a few colonies of each isolate were resuspended in the ID broth (BD Diagnostic Systems) to a concentration of 0.5 McFarland. The adjusted inoculum (25 µl) was then added to antimicrobial susceptibility test (AST) broth, containing methylene blue and resazurin (BD Diagnostic Systems). The suspension was then added to the BD Phoenix GP-PMIC 84 panels to test if the bacteria were susceptible to amoxicillin, cefepime, cefotaxime, chloramphenicol, clindamycin, erythromycin, levofloxacin, linezolid, meropenem, penicillin G, tetracycline, and vancomycin. Quality controls were performed according to the manufacturer’s recommendations using the following reference isolates; Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853. All of the twelve bacterial isolates in this study were verified as GBS using the BD Phoenix M50 system.