Impact of the Stringent Stress Response on the Expression of Methicillin Resistance in Staphylococcaceae Strains Carrying mecA, mecA1 and mecC

Progress in the Prevalence, Classification and Drug Resistance Mechanisms of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus is a common human pathogen with a variety of virulence factors, which can cause multiple infectious diseases. In recent decades, due to the constant evolution and the abuse of antibiotics, Staphylococcus aureus was becoming more resistant, the infection rate of MRSA remained high, and clinical treatment of MRSA became more difficult. The genetic diversity of MRSA was mainly represented by the continuous emergence of epidemic strains, resulting in the constant changes of epidemic clones. Different classes of MRSA resulted in different epidemics and resistance characteristics, which could affect the clinical symptoms and treatments. MRSA had also spread from traditional hospitals to community and livestock environments, and the new clones established a relationship between animals and humans, promoting further evolution of MRSA. Since the resistance mechanism of MRSA is very complex, it is important to clarify these resistance mechanisms at the molecular level for the treatment of infectious diseases. We firstly described the diversity of SCCmec elements, and discussed the types of SCCmec, its drug resistance mechanisms and expression regulations. Then, we described how the vanA operon makes Staphylococcus aureus resistant to vancomycin and its expression regulation. Finally, a brief introduction was given to the drug resistance mechanisms of biofilms and efflux pump systems. Analyzing the resistance mechanism of MRSA can help study new anti-infective drugs and alleviate the evolution of MRSA. At the end of the review, we summarized the treatment strategies for MRSA infection, including antibiotics, anti-biofilm agents and efflux pump inhibitors. To sum up, here we reviewed the epidemic characteristics of Staphylococcus aureus, summarized its classifications, drug resistance mechanisms of MRSA (SCCmec element, vanA operon, biofilm and active efflux pump system) and novel therapy strategies, so as to provide a theoretical basis for the treatment of MRSA infection.

Study of the antibacterial effects of the starch-based zinc oxide nanoparticles on methicillin resistance Staphylococcus aureus isolates from different clinical specimens of patients from Basrah, Iraq

This study aimed to assess the efficacy of starch-based zinc oxide nanoparticles (ZnO-NPs) against methicillin-resistant Staphylococcus aureus (MRSA) isolates from clinical specimens in Basrah, Iraq. In this cross-sectional study, 61 MRSA were collected from different clinical specimens of patients in Basrah city, Iraq. MRSA isolates were identified using standard microbiology tests, cefoxitin disc diffusion and oxacillin salt agar. ZnO-NPs were synthesized in three different concentrations (0.1 M, 0.05 M, 0.02 M) by the chemical method using starch as the stabilizer. Starch-based ZnO-NPs were characterized using ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM). The antibacterial effects of particles were investigated by the disc diffusion method. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the most effective starch-based ZnO-NPs were determined using a broth microdilution assay. The UV-Vis of all concentrations of starch-based ZnO-NPs exhibited a strong absorption band at 360 nm which was characteristic of the ZnO-NPs. XRD assay confirmed the representative hexagonal wurtzite phase of the starch-based ZnO-NPs, and their purity and high crystallinity. The spherical shape with a diameter of 21.56 ± 3.42 and 22.87 ± 3.91 was revealed for the particles by FE-SEM and TEM, respectively. EDS analysis confirmed the presence of zinc (Zn) (61.4 ± 0.54%) and oxygen (O) (36 ± 0.14%). The 0.1 M concentration had the highest antibacterial effects (mean ± SD of inhibition zone = 17.62 ± 2.65 mm) followed by the 0.05 M concentration (16.03 ± 2.24 mm) and the 0.02 M concentration (12.7 ± 2.57 mm). The MIC and the MBC of the 0.1 M concentration were in the range of 25-50 µg/mL and 50-100 µg/mL, respectively. Infections caused by MRSA can be treated with biopolymer-based ZnO-NPs as effective antimicrobials.

Molecular Characterization of Clinical Rel Mutations and Consequences for Resistance Expression and Fitness in Staphylococcus aureus

The stringent response (SR) is a universal stress response that acts as a global regulator of bacterial physiology and virulence, and is a contributor to antibiotic tolerance and resistance. In most bacteria, the SR is controlled by a bifunctional enzyme, Rel, which both synthesizes and hydrolyzes the alarmone (p)ppGpp via two distinct catalytic domains. The balance between these antagonistic activities is fine-tuned to the needs of the cell and, in a "relaxed" state, the hydrolase activity of Rel dominates. We have previously shown that two single amino acid substitutions in Rel (that were identified in clinical isolates from persistent infections) confer elevated basal concentrations of (p)ppGpp and consequent multidrug tolerance in Staphylococcus aureus. Here, we explore the molecular details of how these mutations bring about this increase in cellular (p)ppGpp and investigate the wider cellular consequences in terms of resistance expression, resistance development, and bacterial fitness. Using enzyme assays, we show that both these mutations drastically reduce the hydrolase activity of Rel, thereby shifting the balance of Rel activity in favor of (p)ppGpp synthesis. We also demonstrate that these mutations induce high-level, homogeneous expression of β-lactam resistance and confer a significant fitness advantage in the presence of bactericidal antibiotics (but a fitness cost in the absence of antibiotic). In contrast, these mutations do not appear to accelerate the emergence of endogenous resistance mutations in vitro. Overall, our findings reveal the complex nature of Rel regulation and the multifaceted implications of clinical Rel mutations in terms of antibiotic efficacy and bacteria survival.