A defect in cell wall recycling confers antibiotic resistance and sensitivity in Staphylococcus aureus

Oxacillin promotes membrane vesicle secretion in Staphylococcus aureus via a SarA-Sle1 regulatory cascade

Membrane vesicles (MVs) are nanoscale particles secreted by living bacteria in vitro and in vivo. Bacterial MVs encapsulate various proteins, making them promising candidates for developing vaccines, drug carriers, and cancer immunotherapy agents. However, the mechanisms underlying MV secretion in Gram-positive bacteria remain unclear. Here, we showed that the subinhibitory concentration of oxacillin (OXA) stimulated MV production in Staphylococcus aureus with diverse genetic backgrounds. OXA treatment remarkably increased the expression of sle1, which encodes a main peptidoglycan hydrolase for adjusting peptidoglycan cross-linking. Deletion of sle1 decreased the OXA-mediated MV yield, whereas overexpression of sle1 considerably increased MV production. The accessory regulator SarA increased in response to OXA treatment, and SarA inactivation substantially attenuated OXA-stimulated MV production. We also demonstrated that SarA controlled sle1 expression by directly binding to its promoter region. Thus, the SarA-Sle1 regulatory axis was formed to mediate OXA-induced MV production in S. aureus. MVs derived from OXA-treated S. aureus RN4220 (MVs/OXA) exhibited a smaller particle size compared with those purified from wild-type RN4220; however, proteomic analysis revealed a comparable protein profile between MVs and MVs/OXA. Overall, our research reveals a mechanism underlying OXA-promoted S. aureus MV secretion and highlights the potential application of OXA-induced MVs.

When the Host Encounters the Cell Wall and Vice Versa

Peptidoglycan (PGN) and associated surface structures such as secondary polymers and capsules have a central role in the physiology of bacteria. The exoskeletal PGN heteropolymer is the major determinant of cell shape and allows bacteria to withstand cytoplasmic turgor pressure. Thus, its assembly, expansion, and remodeling during cell growth and division need to be highly regulated to avoid compromising cell survival. Similarly, regulation of the assembly impacts bacterial cell shape; distinct shapes enhance fitness in different ecological niches, such as the host. Because bacterial cell wall components, in particular PGN, are exposed to the environment and unique to bacteria, these have been coopted during evolution by eukaryotes to detect bacteria. Furthermore, the essential role of the cell wall in bacterial survival has made PGN an important signaling molecule in the dialog between host and microbes and a target of many host responses. Millions of years of coevolution have resulted in a pivotal role for PGN fragments in shaping host physiology and in establishing a long-lasting symbiosis between microbes and the host. Thus, perturbations of this dialog can lead to pathologies such as chronic inflammatory diseases. Similarly, pathogens have devised sophisticated strategies to manipulate the system to enhance their survival and growth.

Functional profiling of CHAP domain-containing peptidoglycan hydrolases of Staphylococcus aureus USA300 uncovers potential targets for anti-staphylococcal therapies

The bacterial pathogen Staphylococcus aureus employs a thick cell wall for protection against physical and chemical insults. This wall requires continuous maintenance to ensure strength and barrier integrity, but also to permit bacterial growth and division. The main cell wall component is peptidoglycan. Accordingly, the bacteria produce so-called peptidoglycan hydrolases (PGHs) that cleave glycan strands to facilitate growth, cell wall remodelling, separation of divided cells and release of exported proteins into the extracellular milieu. A special class of PGHs contains so-called 'cysteine, histidine-dependent amidohydrolase/peptidase' (CHAP) domains. In the present study, we profiled the roles of 11 CHAP PGHs encoded by the core genome of S. aureus USA300 LAC. Mutant strains lacking individual CHAP PGHs were analysed for growth, cell morphology, autolysis, and invasion and replication inside human lung epithelial cells. The results show that several investigated CHAP PGHs contribute to different extents to extracellular and intracellular growth and replication of S. aureus, septation of dividing cells, daughter cell separation once the division process is completed, autolysis and biofilm formation. In particular, the CHAP PGHs Sle1 and SAUSA300_2253 control intracellular staphylococcal replication and the resistance to β-lactam antibiotics like oxacillin. This makes the S. aureus PGHs in general, and the Sle1 and SAUSA300_2253 proteins in particular, attractive targets for future prophylactic or therapeutic anti-staphylococcal interventions. Alternatively, these cell surface-exposed enzymes, or particular domains of these enzymes, could be applied in innovative anti-staphylococcal therapies.

Isocyanides inhibit bacterial pathogens by covalent targeting of essential metabolic enzymes

Isonitrile natural products, also known as isocyanides, demonstrate potent antimicrobial activities, yet our understanding of their molecular targets remains limited. Here, we focus on the so far neglected group of monoisonitriles to gain further insights into their antimicrobial mode of action (MoA). Screening a focused monoisonitrile library revealed a potent S. aureus growth inhibitor with a different MoA compared to previously described isonitrile antibiotics. Chemical proteomics via competitive cysteine reactivity profiling, uncovered covalent modifications of two essential metabolic enzymes involved in the fatty acid biosynthetic process (FabF) and the hexosamine pathway (GlmS) at their active site cysteines. In-depth studies with the recombinant enzymes demonstrated concentration-dependent labeling, covalent binding to the catalytic site and corresponding functional inhibition by the isocyanide. Thermal proteome profiling and full proteome studies of compound-treated S. aureus further highlighted the destabilization and dysregulation of proteins related to the targeted pathways. Cytotoxicity and the inhibition of cytochrome P450 enzymes require optimization of the hit molecule prior to therapeutic application. The here described novel, covalent isocyanide MoA highlights the versatility of the functional group, making it a useful tool and out-of-the-box starting point for the development of innovative antibiotics.

Transcriptomic analysis of cell envelope inhibition by prodigiosin in methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading threat to public health as it is resistant to most currently available antibiotics. Prodigiosin is a secondary metabolite of microorganisms with broad-spectrum antibacterial activity. This study identified a significant antibacterial effect of prodigiosin against MRSA with a minimum inhibitory concentration as low as 2.5 mg/L. The results of scanning electron microscopy, crystal violet staining, and confocal laser scanning microscopy indicated that prodigiosin inhibited biofilm formation in S. aureus USA300, while also destroying the structure of the cell wall and cell membrane, which was confirmed by transmission electron microscopy. At a prodigiosin concentration of 1.25 mg/L, biofilm formation was inhibited by 76.24%, while 2.5 mg/L prodigiosin significantly reduced the vitality of MRSA cells in the biofilm. Furthermore, the transcriptomic results obtained at 1/8 MIC of prodigiosin indicated that 235and 387 genes of S. aureus USA300 were significantly up- and downregulated, respectively. The downregulated genes were related to two-component systems, including the transcriptional regulator LytS, quorum sensing histidine kinases SrrB, NreA and NreB, peptidoglycan biosynthesis enzymes (MurQ and GlmU), iron-sulfur cluster repair protein ScdA, microbial surface components recognizing adaptive matrix molecules, as well as the key arginine synthesis enzymes ArcC and ArgF. The upregulated genes were mainly related to cell wall biosynthesis, as well as two-component systems including vancomycin resistance-associated regulator, lipoteichoic acid biosynthesis related proteins DltD and DltB, as well as the 9 capsular polysaccharide biosynthesis proteins. This study elucidated the molecular mechanisms through which prodigiosin affects the cell envelope of MRSA from the perspectives of cell wall synthesis, cell membrane and biofilm formation, providing new potential targets for the development of antimicrobials for the treatment of MRSA.

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.