Trimethoprim sulfamethoxazole

Eleven carbapenem resistance genes (bla_IMP, bla_VIM, bla_NDM, bla_SPM, bla_AIM, bla_DIM, bla_GIM, bla_SIM, bla_KPC, bla_BIC, and bla_OXA–48) (Poirel et al., 2011 (link)) were screened using PCR detection from the presumptive carbapenemase-producing bacteria. The amplicons were sequenced (Macrogen) and identified using NCBI BLAST¹. For the screened carbapenemase-producing bacteria, MDR to 16 antibiotics was determined using Kirby-Bauer disk diffusion, and resistance to colistin was determined using broth dilution methods. For MDR, the following antibiotic disks were used: ampicillin-sulbactam (10/10 μg), cefotaxime (30 μg), ceftazidime (30 μg), chloramphenicol (30 μg) ciprofloxacin (5 μg), colistin (2 mg/L), doripenem (10 μg), fosfomycin (200 μg), gentamicin (10 μg), levofloxacin (5 μg), meropenem (10 μg), netilmicin (10 μg), piperacillin (100 μg), tetracycline (30 μg), tobramycin (10 μg), and trimethoprim-sulfamethoxazole (1.25/23.75 μg) (Liofilchem, Roseto degli Abruzzi, Italy). Resistance to the antibiotics was determined according to the Clinical and Laboratory Standards Institute (CLSI) guideline (Clinical Laboratory Standars and Institue, 2016 ). Subsequently, MICs of 16 antibiotics for E. coli strain N7 were evaluated using the broth dilution method (Hasselmann, 2003 (link)).

Antibiotic susceptibility was tested in all S. aureus isolates by disc diffusion method (Figure 1), following the European Committee of Antimicrobial Susceptibility Testing guidelines (2022) [28 ] or, when not possible, the Clinical and Laboratory Standards Institute guidelines (2022) [29 ]. S. aureus ATCC^® 29213 was used as a quality control strain. The susceptibility to several clinically relevant antibiotics was studied, including those commonly prescribed in general dental practice, namely amoxicillin (10 μg), cefoxitin (30 μg), chloramphenicol (30 μg), ciprofloxacin (5 μg), clindamycin (2 μg), erythromycin (15 μg), gentamicin (10 μg), quinupristin-dalfopristin (15 μg), tetracycline (30 μg), and trimethoprim-sulfamethoxazole (25 μg) (Liofilchem^®, Roseto degli Abruzzi, Italy). Cefoxitin was used to predict the presence of MRSA strains [28 ,29 ]. Multidrug-resistance (MDR) was considered when the isolates were resistant to three or more antibiotics of different classes.

The Kirby Bauer disk diffusion method was used with Muller Hinton agar (Oxoid Ltd. Basingstoke, Hampshire, England) to determine antimicrobial susceptibility patterns of the isolates and CLSI M100 2020 was used to interpret the results [30 ].
The following antimicrobial discs were used: ampicillin (10μl), amoxicillin/clavulanic acid (20/10μl), piperacillin/tazobactam (100/10μl), cefazolin (30μl), cefuroxime (30μl), ceftazidime (30μl) obtained from Hardy Diagnostics, Santa Maria, CA, USA. Ceftriaxone (30μl), cefotaxime (30μl), cefepime (30μl), imipenem (10μl), meropenem (10μl), amikacin (30μl), gentamicin (10μl), and tobramycin(10μl) were obtained from OXOID LTD., Basingstoke, Hampshire, England. Nalidixic acid(30μl), ciprofloxacin(5μl), trimethoprim/sulfamethoxazole (1.25/23.75μl), nitrofurantoin(300μl), and tetracycline(30μl) from Liofilchem, Roseto degli Abruzzi, Italy.