Antimicrobial resistance is a global health problem. Among various antibiotic-resistant bacteria, Staphylococcus aureus , particularly methicillin-resistant S. aureus (MRSA), is a clinically important pathogen responsible for serious infections because of its multidrug resistance (MDR) and association with high mortality rates. The MDR nature of MRSA, including resistance to macrolides, aminoglycosides, fluoroquinolones, and tetracyclines, limits therapeutic choices and poses significant challenges in clinical management. This study aimed to analyze the correlation between mutations in the quinolone resistance-determining region (QRDR) and the minimum inhibitory concentration (MIC) of fluoroquinolone drugs, such as ciprofloxacin and levofloxacin, in MRSA and methicillin-sensitive S. aureus (MSSA). A total of 63 S . aureus clinical strains were isolated from blood samples of sepsis patients. DNA sequence analysis was performed using gDNA extracted from all S. aureus clinical isolates to identify mutations in the QRDR of gyr A, gyr B, par C, and par E. The MICs of antimicrobials were determined by the broth microdilution method. Among these genes, only mutations in par C showed a statistically significant positive correlation with elevated MIC levels, underscoring the primary role of par C in mediating resistance in our clinical isolates. Notably, all isolates exhibited a substitution at serine 80 (S80) in par C, and those harboring simultaneous substitutions at both S80 and glutamic acid 84 (E84) demonstrated markedly increased MIC values for both drugs. These findings reinforce previously reported associations between dual mutations and high-level fluoroquinolone resistance, while highlighting the distinct contribution of par C among the QRDR genes analyzed in this study. Furthermore, we found that the most frequent mutation in the QRDR was the cytosine-to-thymine mutation. IMPORTANCE: Antimicrobial resistance is a growing global health crisis, making bacterial infections harder to treat. Staphylococcus aureus , especially MRSA, is a major concern due to its resistance to multiple antibiotics, including fluoroquinolones like ciprofloxacin and levofloxacin. Our study highlights how specific genetic mutations in the quinolone resistance-determining region (QRDR) influence fluoroquinolone resistance. We found that mutations in the par C gene, particularly substitutions at serine 80 (S80) and glutamic acid 84 (E84), significantly increase resistance. Understanding these mutations helps predict antibiotic resistance and may guide more effective treatment strategies. By identifying key genetic changes that drive fluoroquinolone resistance, our research contributes to developing improved diagnostic tools and targeted therapies to combat drug-resistant S. aureus infections. This knowledge is crucial for clinicians and researchers working to control the spread of antibiotic-resistant bacteria and improve patient outcomes.

Post-mortem interval (PMI) analysis plays a crucial role in forensic investigations, providing essential insights into the time since death. This study examined histological changes and ribonucleic acid (RNA) quantification across major organs to identify molecular indicators for PMI estimation. Because RNA gradually degrades after death, understanding its stability under different tissue, temperature, and PMI conditions provides valuable forensic insights. We analyzed post-mortem changes in total RNA from the heart, kidney, liver, and lung tissues of Sprague Dawley rats stored at 4 °C and 26 °C. Tissue samples were collected at various PMIs and evaluated histologically for cellular integrity. Total RNA concentration and purity were measured, and complementary DNA (cDNA) was synthesized for molecular analysis. Expression levels of 5S rRNA, B2m, Gapdh, and Sort1 were quantified using reverse transcription quantitative PCR (RT-qPCR). The results showed that PMI and organ type significantly affected total RNA concentration, whereas temperature exerted only a minor effect. Among the four target genes, 5S rRNA exhibited the lowest Ct values, indicating the highest stability. Notably, RNA degradation patterns varied with temperature, particularly in kidney and liver tissues. These findings suggest that RNA-based molecular markers, particularly 5S rRNA, may serve as promising indicators for accurate PMI estimation.

The postmortem interval (PMI), defined as the time elapsed between death and the discovery or examination of the body, is a crucial parameter in forensic science for estimating the time of death. There are many ways to measure the PMI, such as Henssge’s nomogram, which uses rectal temperature measurement; livor mortis; rigor mortis; and forensic entomology. However, these methods are usually affected by various conditions in the surrounding environment. The purpose of the present study was to compare molecular genetics and histological changes in the brain and skeletal muscle tissues of SD rats over increasing periods of time after death. For the PMIs, we considered 0 h, 6 h, 12 h, 24 h, 36 h, 48 h, 4 days, 6 days, 8 days, 10 days, 14 days, and 21 days and compared them at 4 °C and 26 °C. Hematoxylin and Eosin (H&E) staining was performed to observe tissue changes. Morphological tissue changes were observed in cells for up to 21 days at 4 °C, and cell destruction was visually confirmed after 14 days at 26 °C. Total RNA (tRNA) was isolated from each tissue sample, and complementary DNA (cDNA) was synthesized. A reverse transcription quantitative PCR (RT-qPCR) SYBR Green assay targeting three types of housekeeping genes, including Gapdh, Sort1, B2m, and 5S rRNA, was performed. The results showed that Gapdh and 5S rRNA were highly stable and could be better RNA targets for estimating the PMI in brain and skeletal muscle tissues. Conversely, Sort1 and B2m showed poor stability and low expression levels. In conclusion, these molecular biomarkers could be used as auxiliary indicators of the PMI in human, depending on the stability of the marker.

The most common antibiotic-resistant bacteria in Korea are methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant enterococci (VRE). Pathogen identification in clinical laboratories can be divided into traditional phenotype- and genotype-based methods, both of which are complementary to each other. The genotype-based method using multiplex real-time polymerase chain reaction (PCR) is a rapid and accurate technique that analyzes material at the genetic level by targeting genes simultaneously. Accordingly, we aimed to develop a rapid method for studying the genetic characteristics of antibiotic-resistant bacteria and to provide an experimental guide for the efficient antibiotic resistance gene analysis of mecA detection for MRSA and vanA or vanB detection for VRE using a one-step multiplex qPCR assay at an early stage of infection. As a result, the sensitivity and specificity of the mecA gene for clinical S. aureus isolates, including MRSA and methicillin-susceptible S. aureus, were 97.44% (95% CI, 86.82–99.87%) and 96.15% (95% CI, 87.02–99.32%), respectively. The receiver operating characteristic area under the curve for the diagnosis of MRSA was 0.9798 (*** p < 0.0001). Therefore, the molecular diagnostic method using this newly developed one-step multiplex qPCR assay can provide accurate and rapid results for the treatment of patients with MRSA and VRE infections.

Jieun Lim, Minju Jung, Sohyeong Kim, Soyun Kim, Gaeun Woo, Heechul Park, and Sunghyun Kim. Biomed Sci Letters 2025;31:269-78. https://doi.org/10.15616/BSL.2025.31.4.269

Jung-Kon Moon, Sang-Ha Kim, Sohyeong Kim, Young-Kwon Kim, Young-Bin Yu, and Sunghyun Kim. BSL 2025;31:70-9. https://doi.org/10.15616/BSL.2025.31.1.70