Sensititretm semi automated antimicrobial susceptibility system

Fourteen Salmonella isolates were collected by the National Antimicrobial Resistance Monitoring System (NARMS) in year 2004–2005. All isolates were streaked onto Mueller-Hinton (MH) agar (Oxoid, Cambridge, UK). A single colony was selected and subsequently inoculated in MH broth (Oxoid, Cambridge, UK), and incubated for 16–18 h at 37°C with shaking (250 rpm). All isolates were subjected to susceptibility testing via the Sensititre^TM semi-automated antimicrobial susceptibility system (TREK Diagnostic Systems, Inc.) using a custom-made panel including amikacin, gentamicin, kanamycin, streptomycin, ampicillin, amoxicillin-clavulanic acid, ceftiofur, ceftriaxone, cefoxitin, sulfamethoxazole/sulfisoxazole, trimethoprim-sulfamethoxazole, chloramphenicol, ciprofloxacin, nalidixic acid, and tetracycline [21 ]. The isolates were subjected to preliminary biochemical screening to distinguish the different serogroups using serogroup-specific antisera (Difco Laboratories, Detroit, MI) and serotyping was used to identify serovars at the National Veterinary Services Laboratories, APHIS, USDA (Ames, IA).

Minimum inhibitory concentrations (MIC; μg/mL) of enterococci were determined by broth microdilution according to the manufacturer’s directions using the Sensititre^TM semi-automated antimicrobial susceptibility system (Trek Diagnostic Systems, Inc., Cleveland, OH, USA) and the Sensititre^TM Gram-Positive Custom Plate CVM2AGPF or CMV3AGPF (chloramphenicol, ciprofloxacin, daptomycin, erythromycin, flavomycin, gentamicin, kanamycin, lincomycin, linezolid, nitrofurantoin, penicillin, streptomycin, Synercid (Quinupristin/Dalfopristin), tetracycline, tigecycline, tylosin, and vancomycin). Results were interpreted according to the Clinical and Laboratory Standards Institute (CLSI) guidelines, when defined [12 ,13 ], and NARMS (https://www.fda.gov/media/108180/download, accessed on 13 June 2022). No CLSI interpretive criteria were defined for kanamycin, lincomycin, and tylosin, and only susceptible breakpoints were established for tigecycline (≤0.25 μg/mL). E. faecalis ATCC 29212, E. faecalis ATCC 51299, S. aureus ATCC 29213, and Escherichia coli ATCC 25922 were used as quality controls for the determination of MIC.

Minimum inhibitory concentration (MIC, µg/mL) for S. aureus was determined by broth microdilution with the Sensititre^TM semi-automated antimicrobial susceptibility system (Trek Diagnostic Systems, Inc., Cleveland, OH, USA) and the Sensititre^TM Gram-Positive Plate GPN3F. Antimicrobials and breakpoints were: ampicillin (≥5 µg/mL), ceftriaxone (≥64 µg/mL), ciprofloxacin (≥4 µg/mL), clindamycin (≥4 µg/mL), daptomycin (>1 µg/mL), erythromycin (≥8 µg/mL), gatifloxacin (≥2 µg/mL), gentamicin (≥16 µg/mL), levofloxacin (≥4 µg/mL), linezolid (≥8 µg/mL), oxacillin (≥4 µg/mL), penicillin G (≥0.25 µg/mL), rifampin (≥4 µg/mL), streptomycin (≥1000 µg/mL), Synercid (Quinupristin/Dalfopristin (Q/D)) (≥4 µg/mL), tetracycline (≥16 µg/mL), trimethoprim/sulfamethoxazole (≥4/76 µg/mL) and vancomycin (≥16 µg/mL). MIC values were manually recorded using the Sensitouch system. Clinical and Laboratory Standards Institute (CLSI) standards were used to determine resistance [20 ,21 ]. Only susceptible breakpoints for daptomycin (<1 µg/mL) have been established by CLSI; resistance for this drug was defined as MICs greater than that value. S. aureus ATCC 29213 was used as a quality control strain [20 ,21 ].