Detection of VISA and hVISA in Staphylococcus aureus isolates and investigation of the in vitro efficacy of delafloxacin

Staphylococcus aureus (S. aureus) is an opportunistic gram-positive bacterium that can be found in the skin and nasal flora of healthy people and can cause serious infections, including skin, soft tissue, bone, bloodstream, respiratory tract, toxic shock syndrome, and food poisoning. Vancomycin and teicoplanin are the most commonly preferred antibiotics for the treatment of methicillin resistant S. aureus (MRSA) infections. However, resistance to vancomycin and teicoplanin is increasing, resulting in a need for new antibiotics. Delafloxacin is a novel antibiotic, a dual-target anionic fluoroquinolone with in vitro activity against methicillin sensitive S. aureus (MSSA) and MRSA. It is Food and Drug Administration (FDA)-approved for the treatment of susceptible pathogens in adult patients with acute bacterial skin and skin structure infections (ABSSSI) and community-acquired bacterial pneumonia (CABP). The aims of this study are to compare the performance of different methods in detecting methicillin resistance in S. aureus isolates, to investigate (population analysis profile-area under the curve) PAP-AUC using heteroresistant vancomycin intermediate S. aureus (hVISA), and to evaluate in vitro activity of delafloxacin with the broth microdilution (BMD) method. A total of 105 S. aureus isolates isolated from wound, tissue, deep tracheal aspiration and sputum samples between June 2021 and October 2023 were included in the study. Identification was performed by conventional methods and matrix assisted laser desorption ionization- time of flight mass spectrometry (MALDI-TOF MS) (Bruker Daltonics, Germany). Methicillin resistance was determined by investigating the mecA and mecC gene regions using cefoxitin (30 μg) (Bioanalyse, Turkey) disk, VITEK 2 automated system (BioMerieux, France), and polymerase chain reaction (PCR). Vancomycin, teicoplanin, and delafloxacin minimum inhibitory concentration (MIC) values in S. aureus isolates were determined by the BMD method. The presence of hVISA was investigated by the teicoplanin agar screening method and the macrogradient test method, and confirmated by using PAP-AUC as the gold standard method. The mecA gene was detected in 64 (61%) isolates by PCR. The mecC gene was not detected in any isolate. Using mecA PCR as the reference method for detecting methicillin resistance, the specificity, sensitivity, positive predictive value, and negative predictive value of the cefoxitin disk diffusion method were 97.6%, 100%, 98.5%, and 100%, respectively; the specificity, sensitivity, positive predictive value, and negative predictive value for the VITEK 2 oxacillin MIC were 92.7%, 100%, 95.5%, and 100%, respectively; and the specificity, sensitivity, positive predictive value, and negative predictive values of the VITEK 2 cefoxitin screening were 87.8%, 100%, 92.7%, and 100%, respectively. The cefoxitin disk diffusion method had the best performance. S. aureus isolates were found susceptible to vancomycin at MIC values between 0.5 and 2 mg/L and teicoplanin at MIC values between 0.125 and 2 mg/L, while one isolate was found resistant to teicoplanin at MIC values of 4 mg/L. The MIC50 for vancomycin was 1 mg/L and the MIC90 was 2 mg/L. The MIC50 for teicoplanin was 0.5 mg/L and the MIC90 was 1 mg/L. There were no isolates suspected of hVISA using the macrogradient test method. The teicoplanin agar screening method identified 12 isolates as suspected of hVISA. The presence of hVISA was confirmed in only two isolates (1 MRSA isolate and 1 MSSA isolate) using the PAP-AUC method. Delafloxacin resistance was found in 19 (18.1%) of the isolates. The delafloxacin susceptibility rate in S. aureus isolates was 81.9% (75% in MRSA isolates, 92.7% in MSSA isolates). Levofloxacin resistance was 19% in S. aureus isolates (26.6% in MRSA isolates, 7.3% in MSSA isolates). The MIC50/90 value for delafloxacin was found to be 0.008 mg/L/0.5 mg/L in all isolates; 0.008 mg/L /0.016 mg/L in MSSA isolates; and 0.008 mg/L/0.5 mg/L in MRSA isolates. In conclusion, the cefoxitin disk diffusion method had the best performance in detecting methicillin resistance in S. aureus isolates; the presence of hVISA, which is clinically important due to its role in infection control and vancomycin treatment failure, was detected in 1.9% of the isolates; the delafloxacin susceptibility rate, especially in levofloxacin-resistant isolates, was found to be lower than in other studies. Keywords: S. aureus, Methicillin resistance, hVISA, PAP-AUC, Delafloxacin