High-Level Resistance of Staphylococcus aureus to β-Lactam Antibiotics Mediated by Penicillin-Binding Protein 4 (PBP4)

A MRSA mystery: how PBP4 and cyclic-di-AMP join forces against β-lactam antibiotics

The high-level resistance to next-generation β-lactams frequently found in Staphylococcus aureus isolates lacking mec, which encodes the transpeptidase PBP2a traditionally associated with methicillin-resistant Staphylococcus aureus (MRSA), has remained incompletely understood for decades. A new study by Lai et al. found that the co-occurrence of mutations in pbp4 and gdpP, which respectively cause increased PBP4-mediated cell wall crosslinking and elevated cyclic-di-AMP levels, produces synergistic β-lactam resistance rivaling that of PBP2a-producing MRSA (L.-Y. Lai, N. Satishkumar, S. Cardozo, V. Hemmadi, et al., mBio 15:e02889-23. 2024, https://doi.org/10.1128/mbio.02889-23). The combined mutations are sufficient to explain the high-level β-lactam resistance of some mec-lacking strains, but the mechanism of synergy remains elusive and an avenue for further research. Importantly, the authors establish that co-occurrence of these mutations leads to antibiotic therapy failure in a Caenorhabditis elegans infection model. These results underscore the need to consider this unique and novel β-lactam resistance mechanism during the clinical diagnosis of MRSA, rather than relying on mec as a diagnostic.

Laurocapram, a transdermal enhancer, boosts cephalosporin's antibacterial activity against Methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA), a notorious bacterium with high drug resistance and easy recurrence after surgery, has posed significant clinical treatment challenges. In the current scarcity of new antibiotics, the identification of adjuvants to existing antibiotics is a promising approach to combat infections caused by multidrug-resistant Gram-positive bacteria. The in vitro synergy test, which included a MIC assay, time-kill curve, antimicrobial susceptibility testing, and live/dead bacteria staining assay, revealed that laurocapram, a widely used chemical transdermal enhancer, could potentiate the antibacterial activity of cephalosporins against MRSA. In vitro, laurocapram combined with cefixime showed an excellent synergistic activity against MRSA (FICI = 0.28 ± 0.00). In addition, the combination of laurocapram and cefixime may inhibited the formation of MRSA biofilm and caused cell membrane damage. Following that, we discovered that combining laurocapram with cefixime could alleviate the symptoms of mice in the MRSA skin infection model and the MRSA pneumonia model. In conclusion, laurocapram is a promising and low-cost antibacterial adjuvant, providing a new strategy for further exploring the use of lower doses of cephalosporins to combat MRSA infection.

Decoding the Structural Diversity: A New Horizon in Antimicrobial Prospecting and Mechanistic Investigation

The escalating crisis of antimicrobial resistance (AMR) underscores the urgent need for novel antimicrobials. One promising strategy is the exploration of structural diversity, as diverse structures can lead to diverse biological activities and mechanisms of action. This review delves into the role of structural diversity in antimicrobial discovery, highlighting its influence on factors such as target selectivity, binding affinity, pharmacokinetic properties, and the ability to overcome resistance mechanisms. We discuss various approaches for exploring structural diversity, including combinatorial chemistry, diversity-oriented synthesis, and natural product screening, and provide an overview of the common mechanisms of action of antimicrobials. We also describe techniques for investigating these mechanisms, such as genomics, proteomics, and structural biology. Despite significant progress, several challenges remain, including the synthesis of diverse compound libraries, the identification of active compounds, the elucidation of complex mechanisms of action, the emergence of AMR, and the translation of laboratory discoveries to clinical applications. However, emerging trends and technologies, such as artificial intelligence, high-throughput screening, next-generation sequencing, and open-source drug discovery, offer new avenues to overcome these challenges. Looking ahead, we envisage an exciting future for structural diversity-oriented antimicrobial discovery, with opportunities for expanding the chemical space, harnessing the power of nature, deepening our understanding of mechanisms of action, and moving toward personalized medicine and collaborative drug discovery. As we face the continued challenge of AMR, the exploration of structural diversity will be crucial in our search for new and effective antimicrobials.

Altered PBP4 and GdpP functions synergistically mediate MRSA-like high-level, broad-spectrum β-lactam resistance in Staphylococcus aureus

Infections caused by Staphylococcus aureus are a leading cause of mortality worldwide. S. aureus infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are particularly difficult to treat due to their resistance to next-generation β-lactams (NGBs) such as methicillin, nafcillin, and oxacillin. Resistance to NGBs, which is alternatively known as broad-spectrum β-lactam resistance, is classically mediated by PBP2a, a penicillin-binding protein encoded by mecA (or mecC) in MRSA. Thus, presence of mec genes among S. aureus spp. serves as the predictor of resistance to NGBs and facilitates determination of the proper therapeutic strategy for a staphylococcal infection. Although far less appreciated, mecA-deficient S. aureus strains can also exhibit NGB resistance. These strains, which are collectively termed as methicillin-resistant lacking mec (MRLM), are currently being identified in increasing numbers among natural resistant isolates of S. aureus. The mechanism/s through which MRLMs produce resistance to NGBs remains unknown. In this study, we demonstrate that mutations that alter PBP4 and GdpP functions, which are often present among MRLMs, can synergistically mediate resistance to NGBs. Furthermore, our results unravel that this novel mechanism potentially enables MRLMs to produce resistance toward NGBs at levels comparable to those of MRSAs. Our study provides a fresh new perspective about alternative mechanisms of NGB resistance, challenging our current overall understanding of high-level, broad-spectrum β-lactam resistance in S. aureus. It thus suggests reconsideration of the current approach toward diagnosis and treatment of β-lactam-resistant S. aureus infections.

IMPORTANCE

In Staphylococcus aureus, high-level, broad-spectrum resistance to β-lactams such as methicillin, also referred to as methicillin resistance, is largely attributed to mecA. This study demonstrates that S. aureus strains that lack mecA but contain mutations that functionally alter PBP4 and GdpP can also mediate high-level, broad-spectrum resistance to β-lactams. Resistance brought about by the synergistic action of functionally altered PBP4 and GdpP was phenotypically comparable to that displayed by mecA, as seen by increased bacterial survival in the presence of β-lactams. An analysis of mutations detected in naturally isolated strains of S. aureus revealed that a significant proportion of them had similar pbp4 and GGDEF domain protein containing phosphodiesterase (gdpP) mutations, making this study clinically significant. This study not only identifies important players of non-classical mechanisms of β-lactam resistance but also indicates reconsideration of current clinical diagnosis and treatment protocols of S. aureus infections.

The Spx stress regulator confers high-level β-lactam resistance and decreases susceptibility to last-line antibiotics in methicillin-resistant Staphylococcus aureus

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a leading cause of mortality worldwide. MRSA has acquired resistance to next-generation β-lactam antibiotics through the horizontal acquisition of the mecA resistance gene. Development of high resistance is, however, often associated with additional mutations in a set of chromosomal core genes, known as potentiators, which, through poorly described mechanisms, enhance resistance. The yjbH gene was recently identified as a hot spot for adaptive mutations during severe infections. Here, we show that inactivation of yjbH increased β-lactam MICs up to 16-fold and transformed MRSA cells with low levels of resistance to being homogenously highly resistant to β-lactams. The yjbH gene encodes an adaptor protein that targets the transcriptional stress regulator Spx for degradation by the ClpXP protease. Using CRISPR interference (CRISPRi) to knock down spx transcription, we unambiguously linked hyper-resistance to the accumulation of Spx. Spx was previously proposed to be essential; however, our data suggest that Spx is dispensable for growth at 37°C but becomes essential in the presence of antibiotics with various targets. On the other hand, high Spx levels bypassed the role of PBP4 in β-lactam resistance and broadly decreased MRSA susceptibility to compounds targeting the cell wall or the cell membrane, including vancomycin, daptomycin, and nisin. Strikingly, Spx potentiated resistance independently of its redox-sensing switch. Collectively, our study identifies a general stress pathway that, in addition to promoting the development of high-level, broad-spectrum β-lactam resistance, also decreases MRSA susceptibility to critical antibiotics of last resort.

Structural Insights into the Penicillin-Binding Protein 4 (DacB) from Mycobacterium tuberculosis

Mycobacterium tuberculosis, a major cause of mortality from a single infectious agent, possesses a remarkable mycobacterial cell envelope. Penicillin-Binding Proteins (PBPs) are a family of bacterial enzymes involved in the biosynthesis of peptidoglycan. PBP4 (DacB) from M. tuberculosis (MtbPBP4) has been known to function as a carboxypeptidase, and the role and significance of carboxypeptidases as targets for anti-tuberculosis drugs or antibiotics have been extensively investigated over the past decade. However, their precise involvement remains incompletely understood. In this study, we employed predictive modeling and analyzed the three-dimensional structure of MtbPBP4. Interestingly, MtbPBP4 displayed a distinct domain structure compared to its homologs. Docking studies with meropenem verified the presence of active site residues conserved in PBPs. These findings establish a structural foundation for comprehending the molecular function of MtbPBP4 and offer a platform for the exploration of novel antibiotics.

C-di-AMP levels modulate Staphylococcus aureus cell wall thickness, response to oxidative stress, and antibiotic resistance and tolerance

IMPORTANCE

Antibiotic resistance and tolerance are substantial healthcare-related problems, hampering effective treatment of bacterial infections. Mutations in the phosphodiesterase GdpP, which degrades cyclic di-3', 5'-adenosine monophosphate (c-di-AMP), have recently been associated with resistance to beta-lactam antibiotics in clinical Staphylococcus aureus isolates. In this study, we show that high c-di-AMP levels decreased the cell size and increased the cell wall thickness in S. aureus mutant strains. As a consequence, an increase in resistance to cell wall targeting antibiotics, such as oxacillin and fosfomycin as well as in tolerance to ceftaroline, a cephalosporine used to treat methicillin-resistant S. aureus infections, was observed. These findings underline the importance of investigating the role of c-di-AMP in the development of tolerance and resistance to antibiotics in order to optimize treatment in the clinical setting.

Altered PBP4 and GdpP functions synergistically mediate MRSA-like high-level, broad-spectrum β-lactam resistance in Staphylococcus aureus

Infections caused by Staphylococcus aureus are a leading cause of mortality worldwide. S. aureus infections caused by Methicillin-Resistant Staphylococcus aureus (MRSA) are particularly difficult to treat due to their resistance to Next Generation β-lactams (NGB) such as Methicillin, Nafcillin, Oxacillin etc. Resistance to NGBs, which is alternatively known as broad-spectrum β-lactam resistance is classically mediated by PBP2a, a Penicillin-Binding Protein encoded by mecA (or mecC) in MRSA. Thus, presence of mec genes among S. aureus serves as the predictor of resistance to NGBs and facilitates determination of the proper therapeutic strategy for a staphylococcal infection. Although far less appreciated, mecA deficient S. aureus strains can also exhibit NGB resistance. These strains, which are collectively termed as Methicillin-Resistant Lacking mec (MRLM) are currently being identified in increasing numbers among natural resistant isolates of S. aureus. The mechanism/s through which MRLMs produce resistance to NGBs remains unknown. In this study, we demonstrate that mutations that alter PBP4 and GdpP functions, which are often present among MRLMs can synergistically mediate resistance to NGBs. Furthermore, our results unravel that this novel mechanism potentially enables MRLMs to produce resistance towards NGBs at levels comparable to that of MRSAs. Our study, provides a fresh new perspective about alternative mechanisms of NGBs resistance, challenging our current overall understanding of high-level, broad-spectrum β-lactam resistance in S. aureus. It thus suggests reconsideration of the current approach towards diagnosis and treatment of β-lactam resistant S. aureus infections.

Molecular Determinants of β-Lactam Resistance in Methicillin-Resistant Staphylococcus aureus (MRSA): An Updated Review

The development of antibiotic resistance in Staphylococcus aureus, particularly in methicillin-resistant S. aureus (MRSA), has become a significant health concern worldwide. The acquired mecA gene encodes penicillin-binding protein 2a (PBP2a), which takes over the activities of endogenous PBPs and, due to its low affinity for β-lactam antibiotics, is the main determinant of MRSA. In addition to PBP2a, other genetic factors that regulate cell wall synthesis, cell signaling pathways, and metabolism are required to develop high-level β-lactam resistance in MRSA. Although several genetic factors that modulate β-lactam resistance have been identified, it remains unclear how they alter PBP2a expression and affect antibiotic resistance. This review describes the molecular determinants of β-lactam resistance in MRSA, with a focus on recent developments in our understanding of the role of mecA-encoded PBP2a and on other genetic factors that modulate the level of β-lactam resistance. Understanding the molecular determinants of β-lactam resistance can aid in developing novel strategies to combat MRSA.

Synergistic Combinations of FDA-Approved Drugs with Ceftobiprole against Methicillin-Resistant Staphylococcus aureus

New strategies are urgently needed to address the public health threat of antimicrobial resistance. Synergistic agent combinations provide one possible pathway toward addressing this need and are also of fundamental mechanistic interest. Effective methods for comprehensively identifying synergistic agent combinations are required for such efforts. In this study, an FDA-approved drug library was screened against methicillin-resistant Staphylococcus aureus (MRSA) (ATCC 43300) in the absence and presence of sub-MIC levels of ceftobiprole, a PBP2a-targeted anti-MRSA β-lactam. This screening identified numerous potential synergistic agent combinations, which were then confirmed and characterized for synergy using checkerboard analyses. The initial group of synergistic agents (sum of the minimum fractional inhibitory concentration ∑FIC_min ≤0.5) were all β-lactamase-resistant β-lactams (cloxacillin, dicloxacillin, flucloxacillin, oxacillin, nafcillin, and cefotaxime). Cloxacillin-the agent with the greatest synergy with ceftobiprole-is also highly synergistic with ceftaroline, another PBP2a-targeted β-lactam. Further follow-up studies revealed a range of ceftobiprole synergies with other β-lactams, including with imipenem, meropenem, piperacillin, tazobactam, and cefoxitin. Interestingly, given that essentially all other ceftobiprole-β-lactam combinations showed synergy, ceftaroline and ceftobiprole showed no synergy. Modest to no synergy (0.5 < ∑FIC_min ≤ 1.0) was observed for several non-β-lactam agents, including vancomycin, daptomycin, balofloxacin, and floxuridine. Mupirocin had antagonistic activity with ceftobiprole. Flucloxacillin appeared particularly promising, with both a low intrinsic MIC and good synergy with ceftobiprole. That so many β-lactam combinations with ceftobiprole show synergy suggests that β-lactam combinations can generally increase β-lactam effectiveness and may also be useful in reducing resistance emergence and spread in MRSA. IMPORTANCE Antimicrobial resistance represents a serious threat to public health. Antibacterial agent combinations provide a potential approach to combating this problem, and synergistic agent combinations-in which each agent enhances the antimicrobial activity of the other-are particularly valuable in this regard. Ceftobiprole is a late-generation β-lactam antibiotic developed for MRSA infections. Resistance has emerged to ceftobiprole, jeopardizing this agent's effectiveness. To identify synergistic agent combinations with ceftobiprole, an FDA-approved drug library was screened for potential synergistic combinations with ceftobiprole. This screening and follow-up studies identified numerous β-lactams with ceftobiprole synergy.

A Nonclassical Mechanism of β-Lactam Resistance in Methicillin-Resistant Staphylococcus aureus and Its Effect on Virulence

Methicillin-resistant Staphylococcus aureus (MRSA) is a group of pathogenic bacteria that are infamously resistant to β-lactam antibiotics, a property attributed to the mecA gene. Recent studies have reported that mutations associated with the promoter region of pbp4 demonstrated high levels of β-lactam resistance, suggesting the role of PBP4 as an important non-mecA mediator of β-lactam resistance. The pbp4-promoter-associated mutations have been detected in strains with or without mecA. Our previous studies that were carried out in strains devoid of mecA described that pbp4-promoter-associated mutations lead to PBP4 overexpression and β-lactam resistance. In this study, by introducing various pbp4-promoter-associated mutations in the genome of a MRSA strain, we demonstrate that PBP4 overexpression can supplement mecA-associated resistance in S. aureus and can lead to increased β-lactam resistance. The promoter and regulatory region of pbp4 is shared with a divergently transcribed gene, abcA, which encodes a multidrug exporter. We demonstrate that the promoter mutations caused an upregulation of pbp4 and downregulation of abcA, confirming that the resistant phenotype is associated with PBP4 overexpression. PBP4 has also been associated with staphylococcal pathogenesis, however, its exact role remains unclear. Using a Caenorhabditis elegans model, we demonstrate that strains having increased PBP4 expression are less virulent than wild-type strains, suggesting that β-lactam resistance mediated via PBP4 likely comes at the cost of virulence. IMPORTANCE Our study demonstrates the ability of PBP4 to be an important mediator of β-lactam resistance in not only methicillin-susceptible Staphylococcus aureus (MSSA) background strains as previously demonstrated but also in MRSA strains. When present together, PBP2a and PBP4 overexpression can produce increased levels of β-lactam resistance, causing complications in treatment. Thus, this study suggests the importance of monitoring PBP4-associated resistance in clinical settings, as well as understanding the mechanistic basis of associated resistance, so that treatments targeting PBP4 may be developed. This study also demonstrates that S. aureus strains with increased PBP4 expression are less pathogenic, providing important hints about the role of PBP4 in S. aureus resistance and pathogenesis.

The cell cycle of Staphylococcus aureus: An updated review

As bacteria proliferate, DNA replication, chromosome segregation, cell wall synthesis, and cytokinesis occur concomitantly and need to be tightly regulated and coordinated. Although these cell cycle processes have been studied for decades, several mechanisms remain elusive, specifically in coccus-shaped cells such as Staphylococcus aureus. In recent years, major progress has been made in our understanding of how staphylococci divide, including new, fundamental insights into the mechanisms of cell wall synthesis and division site selection. Furthermore, several novel proteins and mechanisms involved in the regulation of replication initiation or progression of the cell cycle have been identified and partially characterized. In this review, we will summarize our current understanding of the cell cycle processes in the spheroid model bacterium S. aureus, with a focus on recent advances in the understanding of how these processes are regulated.

Targeting the Achilles’ Heel of Multidrug-Resistant Staphylococcus aureus by the Endocannabinoid Anandamide

Antibiotic-resistant Staphylococcus aureus is a major health issue that requires new therapeutic approaches. Accumulating data suggest that it is possible to sensitize these bacteria to antibiotics by combining them with inhibitors targeting efflux pumps, the low-affinity penicillin-binding protein PBP2a, cell wall teichoic acid, or the cell division protein FtsZ. We have previously shown that the endocannabinoid Anandamide (N-arachidonoylethanolamine; AEA) could sensitize drug-resistant S. aureus to a variety of antibiotics, among others, through growth arrest and inhibition of drug efflux. Here, we looked at biochemical alterations caused by AEA. We observed that AEA increased the intracellular drug concentration of a fluorescent penicillin and augmented its binding to membrane proteins with concomitant altered membrane distribution of these proteins. AEA also prevented the secretion of exopolysaccharides (EPS) and reduced the cell wall teichoic acid content, both processes known to require transporter proteins. Notably, AEA was found to inhibit membrane ATPase activity that is necessary for transmembrane transport. AEA did not affect the membrane GTPase activity, and the GTPase cell division protein FtsZ formed the Z-ring of the divisome normally in the presence of AEA. Rather, AEA caused a reduction in murein hydrolase activities involved in daughter cell separation. Altogether, this study shows that AEA affects several biochemical processes that culminate in the sensitization of the drug-resistant bacteria to antibiotics.

Targeting the Holy Triangle of Quorum Sensing, Biofilm Formation, and Antibiotic Resistance in Pathogenic Bacteria

Chronic and recurrent bacterial infections are frequently associated with the formation of biofilms on biotic or abiotic materials that are composed of mono- or multi-species cultures of bacteria/fungi embedded in an extracellular matrix produced by the microorganisms. Biofilm formation is, among others, regulated by quorum sensing (QS) which is an interbacterial communication system usually composed of two-component systems (TCSs) of secreted autoinducer compounds that activate signal transduction pathways through interaction with their respective receptors. Embedded in the biofilms, the bacteria are protected from environmental stress stimuli, and they often show reduced responses to antibiotics, making it difficult to eradicate the bacterial infection. Besides reduced penetration of antibiotics through the intricate structure of the biofilms, the sessile biofilm-embedded bacteria show reduced metabolic activity making them intrinsically less sensitive to antibiotics. Moreover, they frequently express elevated levels of efflux pumps that extrude antibiotics, thereby reducing their intracellular levels. Some efflux pumps are involved in the secretion of QS compounds and biofilm-related materials, besides being important for removing toxic substances from the bacteria. Some efflux pump inhibitors (EPIs) have been shown to both prevent biofilm formation and sensitize the bacteria to antibiotics, suggesting a relationship between these processes. Additionally, QS inhibitors or quenchers may affect antibiotic susceptibility. Thus, targeting elements that regulate QS and biofilm formation might be a promising approach to combat antibiotic-resistant biofilm-related bacterial infections.

High prevalence of MRSA and VRSA among inpatients of Mettu Karl referral hospital, southwest Ethiopia

OBJECTIVE

To assess the prevalence of methicillin and vancomycin-resistant Staphylococcus aureus among patients admitted to Mettu Karl referral hospital.

METHODS

A cross-sectional study was conducted to study the point prevalence of MRSA and VRSA. A total of 384 patients (male=201 and female=183) admitted to medical (109), pediatric (109), and surgical (166) wards of Mettu Karl referral hospital from November 2019 to April 2020 were included in the study. We studied 384 samples (166 wound swabs and 218 nasal swabs) collected from inpatients. Staphylococcus aureus was isolated, characterized, and identified based on morphological and biochemical features and confirmed by PCR amplification of the nuc gene. The isolates were checked against 12 antibiotics, and MRSA isolates were primarily identified using cefoxitin (30 μg) and confirmed by amplification of mecA gene. Staphylococcus aureus resistance to Vancomycin was tested by the broth microdilution method.

RESULTS

The rate of isolation of Staphylococcus aureus was 32.8% (126/384). The point prevalence of MRSA and VRSA from clinical specimens was 18.8% (72/384) and 2.6% (10/384) respectively. Of 126 Staphylococcus aureus isolated, 57.1% (72) were MRSA and 7.9% (10) were VRSA. Of the 166 samples collected from patients in the surgical ward, the rates of isolation of MRSA and VRSA were 21.1% (35/166) and 4.8% (8/166), respectively. A high rate of isolation of MRSA and VRSA was recorded among patients admitted to surgical wards compared to medical and pediatric wards.

CONCLUSIONS

This study showed a high prevalence of MRSA and VRSA in the hospital. Proper implementation of infection control practices and investigation of underlying risk factors are urgently needed to mitigate the further spread of the pathogen.

Impact of PBP4 Alterations on β-Lactam Resistance and Ceftobiprole Non-Susceptibility Among Enterococcus faecalis Clinical Isolates

Penicillin-resistance among Enterococcus faecalis clinical isolates has been recently associated with overexpression or aminoacidic substitutions in low-affinity PBP4. Ceftobiprole (BPR), a new-generation cephalosporin, is a therapeutic option against E. faecalis. Here, we present evidence that pbp4 gene sequence alterations may influence the expression level of the gene and ceftobiprole binding to PBP4 in E. faecalis clinical isolates showing remarkable MDR-phenotypes, and how this could interfere with BPR in vitro antibacterial and bactericidal activity. Seven E. faecalis strains from bloodstream infections were analyzed for their antibiotic and β-lactam resistance. BPR bactericidal activity was assessed by time-kill analysis; pbp4 genes were sequenced and pbp4 relative expression levels of transcription were performed by RT-qPCR. Five penicillin-resistant ampicillin-susceptible (PRAS) isolates were detected, 4 of which were also BPR non-susceptible (BPR-NS). In the time-kill experiments, BPR exposure resulted in a potent bactericidal activity (3-5 log₁₀ reduction) at the different concentrations tested. pbp4 gene sequence analysis revealed some mutations that may account for the changes in PBP4 affinity and MIC increase in the 4 BPR-NS strains (MICs 4-16 mg/L): the deletion of an adenine (delA) in the promoter region in all PRAS/BPR-NS strains; 12 different amino acid substitutions, 7 of which were next to the PBP catalytic-sites. The most significant were: T418A, located 6 amino acids (aa) upstream of the catalytic-serine included in the ⁴²⁴STFK⁴²⁷motif I; L475Q, 7 aa upstream of the ⁴⁸²SDN⁴⁸⁴motif II; V606A and the novel Y605H, 13/14 aa upstream of the ⁶¹⁹KTGT⁶²²motif III. Taken together, our data showed that elevated BPR MICs were attributable to increased transcription of pbp4 - associated with a single upstream adenine deletion and PBP4 alterations in the catalytic-site motifs – which might interfere with the formation of the BPR/PBP4 complex. pbp4 molecular alterations may account for the changes in PBP4 affinity and MIC increase, without affecting BPR cidal activity. Indeed, our in vitro dynamic analysis by time-kill assays showed that BPR exerted a bactericidal activity against E. faecalis clinical isolates, despite their MDR phenotypes.

Ftsh Sensitizes Methicillin-Resistant Staphylococcus aureus to β-Lactam Antibiotics by Degrading YpfP, a Lipoteichoic Acid Synthesis Enzyme

In the Gram-positive pathogen Staphylococcus aureus, FtsH, a membrane-bound metalloprotease, plays a critical role in bacterial virulence and stress resistance. This protease is also known to sensitize methicillin-resistant Staphylococcus aureus (MRSA) to β-lactam antibiotics; however, the molecular mechanism is not known. Here, by the analysis of FtsH substrate mutants, we found that FtsH sensitizes MRSA specifically to β-lactams by degrading YpfP, the enzyme synthesizing the anchor molecule for lipoteichoic acid (LTA). Both the overexpression of FtsH and the disruption of ypfP-sensitized MRSA to β-lactams were observed. The knockout mutation in ftsH and ypfP increased the thickness of the cell wall. The β-lactam sensitization coincided with the production of aberrantly large LTA molecules. The combination of three mutations in the rpoC, vraB, and SAUSA300_2133 genes blocked the β-lactam-sensitizing effect of FtsH. Murine infection with the ypfP mutant could be treated by oxacillin, a β-lactam antibiotic ineffective against MRSA; however, the effective concentration of oxacillin differed depending on the S. aureus strain. Our study demonstrated that the β-lactam sensitizing effect of FtsH is due to its digestion of YpfP. It also suggests that the larger LTA molecules are responsible for the β-lactam sensitization phenotype, and YpfP is a viable target for developing novel anti-MRSA drugs.

Mutations in the gdpP gene are a clinically relevant mechanism for β-lactam resistance in meticillin-resistant Staphylococcus aureus lacking mec determinants

In Staphylococcus aureus, resistance to β-lactamase stable β-lactam antibiotics is mediated by the penicillinbinding protein 2a, encoded by mecA or by its homologues mecB or mecC. However, a substantial number of meticillin-resistant isolates lack known mec genes and, thus, are called meticillin resistant lacking mec (MRLM). This study aims to identify the genetic mechanisms underlying the MRLM phenotype. A total of 141 MRLM isolates and 142 meticillin-susceptible controls were included in this study. Oxacillin and cefoxitin minimum inhibitory concentrations were determined by broth microdilution and the presence of mec genes was excluded by PCR. Comparative genomics and a genome-wide association study (GWAS) approach were applied to identify genetic polymorphisms associated with the MRLM phenotype. The potential impact of such mutations on the expression of PBP4, as well as on cell morphology and biofilm formation, was investigated. GWAS revealed that mutations in gdpP were significantly associated with the MRLM phenotype. GdpP is a phosphodiesterase enzyme involved in the degradation of the second messenger cyclic-di-AMP in S. aureus. A total of 131 MRLM isolates carried truncations, insertions or deletions as well as amino acid substitutions, mainly located in the functional DHH-domain of GdpP. We experimentally verified the contribution of these gdpP mutations to the MRLM phenotype by heterologous complementation experiments. The mutations in gdpP had no effect on transcription levels of pbp4; however, cell sizes of MRLM strains were reduced. The impact on biofilm formation was highly strain dependent. We report mutations in gdpP as a clinically relevant mechanism for β-lactam resistance in MRLM isolates. This observation is of particular clinical relevance, since MRLM are easily misclassified as MSSA (meticillin-susceptible S. aureus), which may lead to unnoticed spread of β-lactam-resistant isolates and subsequent treatment failure.

Genetic mutations in adaptive evolution of growth-independent vancomycin-tolerant Staphylococcus aureus

BACKGROUND

Antibiotic tolerance allows bacteria to overcome antibiotic treatment transiently and potentially accelerates the emergence of resistance. However, our understanding of antibiotic tolerance at the genetic level during adaptive evolution of Staphylococcus aureus remains incomplete. We sought to identify the mutated genes and verify the role of these genes in the formation of vancomycin tolerance in S. aureus.

METHODS

Vancomycin-susceptible S. aureus strain Newman was used to induce vancomycin-tolerant isolates in vitro by cyclic exposure under a high concentration of vancomycin (20× MIC). WGS and Sanger sequencing were performed to identify the genetic mutations. The function of mutated genes in vancomycin-tolerant isolates were verified by gene complementation. Other phenotypes of vancomycin-tolerant isolates were also determined, including mutation frequency, autolysis, lysostaphin susceptibility, cell wall thickness and cross-tolerance.

RESULTS

A series of vancomycin-tolerant S. aureus (VTSA) strains were isolated and 18 mutated genes were identified by WGS. Among these genes, pbp4, htrA, stp1, pth and NWMN_1068 were confirmed to play roles in VTSA formation. Mutation of mutL promoted the emergence of VTSA. All VTSA showed no changes in growth phenotype. Instead, they exhibited reduced autolysis, decreased lysostaphin susceptibility and thickened cell walls. In addition, all VTSA strains were cross-tolerant to antibiotics targeting cell wall synthesis but not to quinolones and lipopeptides.

CONCLUSIONS

Our results demonstrate that genetic mutations are responsible for emergence of phenotypic tolerance and formation of vancomycin tolerance may lie in cell wall changes in S. aureus.

PBP4-mediated β-lactam resistance among clinical strains of Staphylococcus aureus

BACKGROUND

PBP4, a low-molecular-weight PBP in Staphylococcus aureus, is not considered to be a classical mediator of β-lactam resistance. Previous studies carried out by our group with laboratory strains of S. aureus demonstrated the ability of PBP4 to produce β-lactam resistance through mutations associated with the pbp4 promoter and/or gene. Recent studies of β-lactam-resistant clinical isolates of S. aureus have reported similar mutations associated with pbp4.

OBJECTIVES

To determine if pbp4-associated mutations reported among clinical strains of S. aureus mediate β-lactam resistance.

METHODS

The pbp4 promoters and genes bearing mutations from clinical isolates were cloned into a heterologous host. Reporter, growth and Bocillin assays were performed to assess their role in β-lactam resistance. X-ray crystallography was used to obtain acyl-enzyme intermediate structures of the WT and mutant PBP4 with nafcillin and cefoxitin.

RESULTS

Of the five strains that contained pbp4 promoter mutations, three strains exhibited enhanced expression of PBP4. The R200L mutation in pbp4 resulted in increased survival in the presence of the β-lactams nafcillin and cefoxitin. Further, introduction of either a promoter or a gene mutation into the genome of a WT host increased the ability of the strains to resist the action of β-lactams. The four high-resolution X-ray structures presented demonstrate the binding pose of the β-lactams tested and provide hints for further drug development.

CONCLUSIONS

Mutations associated with the pbp4 promoter and pbp4 gene altered protein activity and mediated β-lactam resistance among the clinically isolated strains that were studied.

Exploitation of Antimicrobial Nanoparticles and Their Applications in Biomedical Engineering

Antibiotic resistance is a major threat to public health, which contributes largely to increased mortality rates and costs in hospitals. The severity and widespread nature of antibiotic resistance result in limited treatments to effectively combat antibiotic-resistant pathogens. Nanoparticles have different or enhanced properties in contrast to their bulk material, including antimicrobial efficacy towards a broad range of microorganisms. Their beneficial properties can be utilised in various bioengineering technologies. Thus, antimicrobial nanoparticles may provide an alternative to challenge antibiotic resistance. Currently, nanoparticles have been incorporated into materials, such as fibres, glass and paints. However, more research is required to elucidate the mechanisms of action fully and to advance biomedical applications further. This paper reviews the antimicrobial efficacies and the intrinsic properties of different metallic nanoparticles, their potential mechanisms of action against certain types of harmful pathogens and how these properties may be utilised in biomedical and healthcare products with the aim to reduce cross contaminations, disease transmissions and usage of antibiotics.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Inhibition of Efflux Pumps by Monoterpene (α-pinene) and Impact on Staphylococcus aureus Resistance to Tetracycline and Erythromycin

INTRODUCTION

Infectious diseases have been responsible for an increasing number of deaths worldwide. Staphylococcus aureus has been recognized as one of the most notable causative agents of severe infections, while efflux pump (EP) expression is one of the main mechanisms associated with S. aureus resistance to antibiotics.

OBJECTIVE

This study aimed to investigate the potential of &amp;#945;-pinene as an efflux pump inhibitor in species of S. aureus carrying the TetK and MrsA proteins.

METHODS

The minimum inhibitory concentrations (MIC) of &amp;#945;-pinene and other efflux pump inhibitors were assessed using serial dilutions of each compound at an initial concentration above 1024 μg/mL. Solutions containing culture medium and bacterial inoculums were prepared in test tubes and subsequently transferred to 96-well microdilution plates. The modulation of ethidium bromide (EtBr) and antibiotics (tetracycline and erythromycin) was investigated through analysis of the modification in their MICs in the presence of a subinhibitory concentration of &amp;#945;-pinene (MIC/8). Wells containing only culture medium and bacterial inoculums were used as negative control. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) was used as a positive control.

RESULTS

The MIC of ethidium bromide against S. aureus strains RN-4220 and IS-58 was reduced by association with α-pinene. This monoterpene potentiated the effect of tetracycline against the IS-58 strain but failed in modulating the antibacterial effect of erythromycin against RN-4220, suggesting a selective inhibitory effect on the TetK EP by &amp;#945;- pinene.

CONCLUSION

In conclusion, α-pinene has promising effects against S.aureus strains, which should be useful in the combat of antibacterial resistance associated with EP expression. Nevertheless, further research is required to fully characterize its molecular mechanism of action as an EP inhibitor.

Nosocomial Pneumonia in the Era of Multidrug-Resistance: Updates in Diagnosis and Management

Nosocomial pneumonia (NP), including hospital-acquired pneumonia in non-intubated patients and ventilator-associated pneumonia, is one of the most frequent hospital-acquired infections, especially in the intensive care unit. NP has a significant impact on morbidity, mortality and health care costs, especially when the implicated pathogens are multidrug-resistant ones. This narrative review aims to critically review what is new in the field of NP, specifically, diagnosis and antibiotic treatment. Regarding novel imaging modalities, the current role of lung ultrasound and low radiation computed tomography are discussed, while regarding etiological diagnosis, recent developments in rapid microbiological confirmation, such as syndromic rapid multiplex Polymerase Chain Reaction panels are presented and compared with conventional cultures. Additionally, the volatile compounds/electronic nose, a promising diagnostic tool for the future is briefly presented. With respect to NP management, antibiotics approved for the indication of NP during the last decade are discussed, namely, ceftobiprole medocaril, telavancin, ceftolozane/tazobactam, ceftazidime/avibactam, and meropenem/vaborbactam.

Truncation of GdpP mediates β-lactam resistance in clinical isolates of Staphylococcus aureus

OBJECTIVES

High-level β-lactam resistance in MRSA is mediated in the majority of strains by a mecA or mecC gene. In this study, we identified 10 mec gene-negative MRSA human isolates from Austria and 11 bovine isolates from the UK showing high levels of β-lactam resistance and sought to understand the molecular basis of the resistance observed.

METHODS

Different antimicrobial resistance testing methods (disc diffusion, Etest and VITEK® 2) were used to establish the β-lactam resistance profiles for the isolates and the isolates were further investigated by WGS.

RESULTS

A number of mutations (including novel ones) in PBPs, AcrB, YjbH and the pbp4 promoter were identified in the resistant isolates, but not in closely related susceptible isolates. Importantly, a truncation in the cyclic diadenosine monophosphate phosphodiesterase enzyme, GdpP, was identified in 7 of the 10 Austrian isolates and 10 of the 11 UK isolates. Complementation of four representative isolates with an intact copy of the gdpP gene restored susceptibility to penicillins and abolished the growth defects caused by the truncation.

CONCLUSIONS

This study reports naturally occurring inactivation of GdpP protein in Staphylococcus aureus of both human origin and animal origin, and demonstrates clinical relevance to a previously reported association between this truncation and increased β-lactam resistance and impaired bacterial growth in laboratory-generated mutants. It also highlights possible limitations of genomic determination of antibiotic susceptibility based on single gene presence or absence when choosing the appropriate antimicrobial treatment for patients.

The Novel Membrane-Associated Auxiliary Factors AuxA and AuxB Modulate β-lactam Resistance in MRSA by stabilizing Lipoteichoic Acids

A major determinant of β-lactam resistance in methicillin-resistant Staphylococcus aureus (MRSA) is the drug insensitive transpeptidase, PBP2a, encoded by mecA. Full expression of the resistance phenotype requires auxiliary factors. Two such factors, auxiliary factor A (auxA, SAUSA300_0980) and B (auxB, SAUSA300_1003), were identified in a screen against mutants with increased susceptibility to β-lactams in the MRSA strain, JE2. auxA and auxB encode transmembrane proteins, with AuxA predicted to be a transporter. Inactivation of auxA or auxB enhanced β-lactam susceptibility in community-, hospital- and livestock-associated MRSA strains without affecting PBP2a expression, peptidoglycan cross-linking or wall teichoic acid synthesis. Both mutants displayed increased susceptibility to inhibitors of lipoteichoic acid (LTA) synthesis and alanylation pathways and released LTA even in the absence of β-lactams. The β-lactam susceptibility of the aux mutants was suppressed by mutations inactivating gdpP, which was previously found to allow growth of mutants lacking the lipoteichoic synthase enzyme, LtaS. Using the Galleria mellonella infection model, enhanced survival of larvae inoculated with either auxA or auxB mutants was observed compared with the wild-type strain following treatment with amoxicillin. These results indicate that AuxA and AuxB are central for LTA stability and potential inhibitors can be tools to re-sensitize MRSA strains to β-lactams and combat MRSA infections.

Virulence Factors Found in Nasal Colonization and Infection of Methicillin-Resistant Staphylococcus aureus (MRSA) Isolates and Their Ability to Form a Biofilm

Hospitalizations related to Methicillin-resistant Staphylococcus aureus (MRSA) are frequent, increasing mortality and health costs. In this way, this study aimed to compare the genotypic and phenotypic characteristics of MRSA isolates that colonize and infect patients seen at two hospitals in the city of Niterói—Rio de Janeiro, Brazil. A total of 147 samples collected between March 2013 and December 2015 were phenotyped and genotyped to identify the protein A (SPA) gene, the mec staphylococcal chromosomal cassette (SCCmec), mecA, Panton-Valentine Leucocidin (PVL), icaC, icaR, ACME, and hla virulence genes. The strength of biofilm formation has also been exploited. The prevalence of SCCmec type IV (77.1%) was observed in the colonization group; however, in the invasive infection group, SCCmec type II was prevalent (62.9%). The Multilocus Sequence Typing (MLST), ST5/ST30, and ST5/ST239 analyses were the most frequent clones in colonization, and invasive infection isolates, respectively. Among the isolates selected to assess the ability to form a biofilm, 51.06% were classified as strong biofilm builders. Surprisingly, we observed that isolates other than the Brazilian Epidemic Clone (BEC) have appeared in Brazilian hospitals. The virulence profile has changed among these isolates since the ACME type I and II genes were also identified in this collection.

Constructing and deconstructing the bacterial cell wall

The history of modern medicine cannot be written apart from the history of the antibiotics. Antibiotics are cytotoxic secondary metabolites that are isolated from Nature. The antibacterial antibiotics disproportionately target bacterial protein structure that is distinct from eukaryotic protein structure, notably within the ribosome and within the pathways for bacterial cell-wall biosynthesis (for which there is not a eukaryotic counterpart). This review focuses on a pre-eminent class of antibiotics-the β-lactams, exemplified by the penicillins and cephalosporins-from the perspective of the evolving mechanisms for bacterial resistance. The mechanism of action of the β-lactams is bacterial cell-wall destruction. In the monoderm (single membrane, Gram-positive staining) pathogen Staphylococcus aureus the dominant resistance mechanism is expression of a β-lactam-unreactive transpeptidase enzyme that functions in cell-wall construction. In the diderm (dual membrane, Gram-negative staining) pathogen Pseudomonas aeruginosa a dominant resistance mechanism (among several) is expression of a hydrolytic enzyme that destroys the critical β-lactam ring of the antibiotic. The key sensing mechanism used by P. aeruginosa is monitoring the molecular difference between cell-wall construction and cell-wall deconstruction. In both bacteria, the resistance pathways are manifested only when the bacteria detect the presence of β-lactams. This review summarizes how the β-lactams are sensed and how the resistance mechanisms are manifested, with the expectation that preventing these processes will be critical to future chemotherapeutic control of multidrug resistant bacteria.

In Vitro Ceftaroline Resistance Selection of Methicillin-Resistant Staphylococcus aureus Involves Different Genetic Pathways

The pathways in the development of ceftaroline resistance of methicillin-resistant Staphylococcus aureus (MRSA) isolates belonging to the ST8, ST239, and ST228 were evaluated. Ceftaroline-resistant derivatives were isolated through selection during 40 passages. Ceftaroline MIC measurements and whole-genome sequencing were performed after 5, 20, and 40 passages. In two ST8 derivative isolates, ceftaroline MIC increased up to 128 mg/L. Mutations were acquired in gdpP and graS in one isolate after 20 passages and in gdpP in another after 40 passages. MIC for two ST239 derivatives increased to 128 mg/L. Substitutions in Pbp4 and polymorphisms in the upstream region of pbp4 were identified in both derivatives after 40 passages. In one isolate, additional mutation in gdpP and deletion in graR were detected. In an ST228 derivative, MIC increased to 32 mg/L with one mutation in penicillin-binding protein 2a (Y446N) detected after five passages and a second (E447K) after 20 passages. Three pathways in the development of ceftaroline resistance were identified. For ST8 and ST239 derivatives mutations were detected in gdpP and pbp4, respectively, whereas in ST228 - in mecA. Most derivatives harbored additional mutations whose potential role in the development of resistance has not been determined.

Antimicrobial resistance in methicillin-resistant Staphylococcus aureus to newer antimicrobial agents

Infections caused by methicillin-resistant Staphylococcus aureus (MRSA) result in significant morbidity and mortality for patients in both community and health care settings. This is primarily due to the difficulty in treating MRSA, which is often resistant to multiple classes of antibiotics. Understanding the mechanisms of antimicrobial resistance (AMR) in MRSA provides insight into the optimal use of antimicrobial agents in clinical practice and also underpins critical aspects of antimicrobial stewardship programs. In this review we delineate the mechanisms, prevalence, and clinical importance of resistance to antibiotics licensed in the past 20 years that target MRSA, as well as new drugs in the pipeline which are likely to be licensed soon. Current gaps in scientific knowledge about MRSA resistance mechanisms are discussed, and topics in the epidemiology of AMR in S. aureus that require further investigation are highlighted.

From Antihistamine to Anti-infective: Loratadine Inhibition of Regulatory PASTA Kinases in Staphylococci Reduces Biofilm Formation and Potentiates β-Lactam Antibiotics and Vancomycin in Resistant Strains of Staphylococcus aureus

Staphylococcus epidermidis and Staphylococcus aureus are important human pathogens responsible for two-thirds of all postsurgical infections of indwelling medical devices. Staphylococci form robust biofilms that provide a reservoir for chronic infection, and antibiotic-resistant isolates are increasingly common in both healthcare and community settings. Novel treatments that can simultaneously inhibit biofilm formation and antibiotic-resistance pathways are urgently needed to combat the increasing rates of antibiotic-resistant infections. Herein we report that loratadine, an FDA-approved antihistamine, significantly inhibits biofilm formation in both S. aureus and S. epidermidis. Furthermore, loratadine potentiates β-lactam antibiotics in methicillin-resistant strains of S. aureus and potentiates both β-lactam antibiotics and vancomycin in vancomycin-resistant strains of S. aureus. Additionally, we elucidate loratadine's mechanism of action as a novel inhibitor of the regulatory PASTA kinases Stk and Stk1 in S. epidermidis and S. aureus, respectively. Finally, we describe how Stk1 inhibition affects the expression of genes involved in both biofilm formation and antibiotic resistance in S. epidermidis and S. aureus.

PBP4 activity and its overexpression are necessary for PBP4-mediated high-level β-lactam resistance

Background

PBP4 is typically considered unimportant for conferring high-level β-lactam resistance in Staphylococcus aureus. Mutations in PBP4 have been associated with β-lactam non-susceptibility among natural strains of S. aureus. We have previously shown that PBP4 can mediate high-level β-lactam resistance in laboratory-generated strains passaged in β-lactam antibiotics. Mutations in the pbp4 promoter that up-regulate its expression and missense mutations that surround PBP4's active site were detected in high frequencies among passaged strains, suggesting PBP4 plays a key role in resistance. How these mutations participate in PBP4's ability to provide high-level β-lactam resistance is unknown.

Objectives

To determine whether enzymatic activity of PBP4 is required for high-level β-lactam resistance and to investigate how the pbp4-associated mutations provide β-lactam resistance.

Methods

The catalytic activity of PBP4 was disabled through introduction of a serine to alanine point mutation in its active site (Ser-75→Ala) in a representative and well-studied passaged strain, CRB. pbp4 promoter and missense mutations detected in CRB were reconstituted in a WT strain individually and in combination. β-Lactam resistance of the resultant strains was evaluated by population analysis. Bacterial peptidoglycan composition of the pbp4 mutants was evaluated with and without antibiotic treatment using LC.

Results

PBP4 inactivation imparted complete β-lactam susceptibility of CRB. Reconstitution of PBP4 missense mutations alone did not impart β-lactam resistance, but did so in synergism with pbp4 promoter mutation. A similar synergistic interaction of pbp4 mutations was observed in enhanced peptidoglycan cross-linking upon antibiotic treatment.

Conclusions

PBP4's activity and overexpression both contribute to high-level β-lactam resistance.

In vitro and in vivo activity of d-serine in combination with β-lactam antibiotics against methicillin-resistant Staphylococcus aureus

As d-amino acids play important roles in the physiological metabolism of bacteria, combination of d-amino acids with antibiotics may provide synergistic antibacterial activity. The aim of the study was to evaluate in vitro and in vivo activity of d-serine alone and in combination with β-lactams against methicillin-resistant Staphylococcus aureus (MRSA) strains, and to explore the possible sensitization mechanisms. The activity of d-serine, β-lactams alone and in combinations was evaluated both in vitro by standard MICs, time-kill curves and checkerboard assays, and in vivo by murine systemic infection model as well as neutropenic thigh infection model. An in vitro synergistic effect was demonstrated with the combination of d-serine and β-lactams against MRSA standard and clinical strains. Importantly, the combinations enhanced the therapeutic efficacy in the animal models as compared to β-lactam alone groups. Initial mechanism study suggested possible revision of d-alanine-d-alanine residue to d-alanine-d-serine in peptidoglycan by adding of d-alanine in the medium, which may cause decreased affinity to PBPs during transpeptidation. In conclusion, d-serine had synergistic activity in combination with β-lactams against MRSA strains both in vitro and in vivo. Considering the relatively good safety of d-serine alone or in combination with β-lactams, d-serine is worth following up as new anti-MRSA infection strategies.

Identification of a Novel Gene Associated with High-Level β-Lactam Resistance in Heterogeneous Vancomycin-Intermediate Staphylococcus aureus Strain Mu3 and Methicillin-Resistant S. aureus Strain N315

β-Lactam resistance levels vary among methicillin-resistant Staphylococcus aureus (MRSA) clinical isolates, mediated by chromosomal mutations and exogenous resistance gene mecA However, MRSA resistance mechanisms are incompletely understood. A P440L mutation in the RNA polymerase β' subunit (RpoC) in slow-vancomycin-intermediate S. aureus (sVISA) strain V6-5 is associated with conversion of heterogeneous VISA (hVISA) to sVISA. In this study, we found a V6-5-derivative strain (L4) with significantly decreased MICs to oxacillin (OX) and vancomycin. Whole-genome sequencing revealed that L4 has nonsense mutations in two genes, relQ, encoding (p)ppGpp synthetase, an alarmone of the stringent response, and a gene of unknown function. relQ deletion in the hVISA strain Mu3 did not affect OX MIC. However, introducing nonsense mutation of the unknown gene into Mu3 decreased OX MIC, whereas wild-type gene recovered high-level resistance. Thus, mutation of this unknown gene (ehoM) decreased β-lactam resistance in Mu3 and L4. Presence of relQ in a multicopy plasmid restored high-level resistance in strain L4 but not in the ehoM mutant Mu3 strain, indicating a genetic interaction between ehoM and relQ depending on the L4 genetic background. While mupirocin (a stringent response inducer) can increase the β-lactam resistance of MRSA, mupirocin supplementation in an ehoM deletion mutant of N315 did not elevate resistance. ehoM expression in N315 was induced by mupirocin, and the relative amount of ehoM transcript in Mu3 was higher than in N315 induced by the stringent response. Our findings indicate that ehoM plays an essential role in high-level β-lactam resistance in MRSA via the stringent response.

Recognition of Peptidoglycan Fragments by the Transpeptidase PBP4 From Staphylococcus aureus

Peptidoglycan (PG) is an essential component of the cell envelope, maintaining bacterial cell shape and protecting it from bursting due to turgor pressure. The monoderm bacterium Staphylococcus aureus has a highly cross-linked PG, with ~90% of peptide stems participating in DD-cross-links and up to 15 peptide stems connected with each other. These cross-links are formed in transpeptidation reactions catalyzed by penicillin-binding proteins (PBPs) of classes A and B. Most S. aureus strains have three housekeeping PBPs with this function (PBP1, PBP2, and PBP3) but MRSA strains have acquired a third class B PBP, PBP2a, which is encoded by the mecA gene and required for the expression of high-level resistance to β-lactams. Another housekeeping PBP of S. aureus is PBP4, which belongs to the class C PBPs, and hence would be expected to have PG hydrolase (DD-carboxypeptidase or DD-endopeptidase) activity. However, previous works showed that, unexpectedly, PBP4 has transpeptidase activity that significantly contributes to both the high level of cross-linking in the PG of S. aureus and to the low level of β-lactam resistance in the absence of PBP2a. To gain insights into this unusual activity of PBP4, we studied by NMR spectroscopy its interaction in vitro with different substrates, including intact peptidoglycan, synthetic peptide stems, muropeptides, and long glycan chains with uncross-linked peptide stems. PBP4 showed no affinity for the complex, intact peptidoglycan or the smallest isolated peptide stems. Transpeptidase activity of PBP4 was verified with the disaccharide peptide subunits (muropeptides) in vitro, producing cyclic dimer and multimer products; these assays also showed a designed PBP4(S75C) nucleophile mutant to be inactive. Using this inactive but structurally highly similar variant, liquid-state NMR identified two interaction surfaces in close proximity to the central nucleophile position that can accommodate the potential donor and acceptor stems for the transpeptidation reaction. A PBP4:muropeptide model structure was built from these experimental restraints, which provides new mechanistic insights into mecA independent resistance to β-lactams in S. aureus.

Mechanisms of action and antimicrobial activity of ceftobiprole

Ceftobiprole, a novel last generation parenteral cephalosporin, has an extended spectrum of activity, notably against methicillin-resistant Staphylococcus aureus (MRSA), ampicillin-susceptible enterococci, penicillin-resistant pneumococci, Enterobacterales and susceptible Pseudomonas aeruginosa. It exerts an inhibitory action on essential peptidoglycan transpeptidases, interfering with cell wall synthesis. The inhibitory action of ceftobiprole through binding to abnormal PBPs like PBP2a in methicillin-resistant staphylococci and PBP2b and PBP2x in the case of β-lactam-resistant pneumococci, ultimately leads to rapid bacterial cell death. In the case of Enterobacterales, ceftobiprole retains activity against narrow spectrum β-lactamases but is hydrolysed by their extended-spectrum counterparts, overexpressed Amp C, and carbapenemases. It is also affected by certain efflux pumps from P. aeruginosa. For anaerobic bacteria, ceftobiprole is active against Gram-positive Clostridioides difficile and Peptococcus spp. and Gram-negative Fusobacterium nucleatum but not against Bacteroides group or other anaerobic Gram-negatives. In in vitro studies, a low propensity to select for resistant subpopulations has been demonstrated. Currently, ceftobiprole is approved for the treatment of community-acquired pneumonia and hospital-acquired pneumonia with the exception of ventilator-associated pneumonia. Ceftobiprole's place in therapy appears to lie mainly in its combined activity against Gram-positive organisms, such as S. aureus and S. pneumoniae alongside that against Gram-negative organisms such as P. aeruginosa.

Structural and kinetic analyses of penicillin-binding protein 4 (PBP4)-mediated antibiotic resistance in Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) causes serious community-acquired and nosocomial infections worldwide. MRSA strains are resistant to a variety of antibiotics, including the classic penicillin and cephalosporin classes of β-lactams, making them intractable to treatment. Although β-lactam resistance in MRSA has been ascribed to the acquisition and activity of penicillin-binding protein 2a (PBP2a, encoded by mecA), it has recently been observed that resistance can also be mediated by penicillin-binding protein 4 (PBP4). Previously, we have shown that broad-spectrum β-lactam resistance can arise following serial passaging of a mecA-negative COL strain of S. aureus, creating the CRB strain. This strain has two missense mutations in pbp4 and a mutation in the pbp4 promoter, both of which play an instrumental role in β-lactam resistance. To better understand PBP4's role in resistance, here we have characterized its kinetics and structure with clinically relevant β-lactam antibiotics. We present the first crystallographic PBP4 structures of apo and acyl-enzyme intermediate forms complexed with three late-generation β-lactam antibiotics: ceftobiprole, ceftaroline, and nafcillin. In parallel, we characterized the structural and kinetic effects of the PBP4 mutations present in the CRB strain. Localized within the transpeptidase active-site cleft, the two substitutions appear to have different effects depending on the drug. With ceftobiprole, the missense mutations impaired the K_m value 150-fold, decreasing the proportion of inhibited PBP4. However, ceftaroline resistance appeared to be mediated by other factors, possibly including mutation of the pbp4 promoter. Our findings provide evidence that S. aureus CRB has at least two PBP4-mediated resistance mechanisms.

Genetic Diversity among Staphylococcus aureus Isolates Showing Oxacillin and/or Cefoxitin Resistance Not Linked to the Presence of mec Genes

Methicillin-resistant Staphylococcus aureus isolates lacking mec genes (n = 32), collected from Belgian hospitals, were characterized for their β-lactamase production and the presence of mutations in pbp genes, the pbp4 promoter, and genes involved in penicillin-binding protein 4 overproduction (gdpP and yjbH). Twelve isolates were β-lactamase hyperproducers (BHPs), while 12 non-BHP isolates might produce an incomplete GdpP protein. Most isolates showed nucleotide missense mutations in pbp genes. A few isolates also showed mutations in the pbp4 promoter.

PBP4: A New Perspective on Staphylococcus aureus β-Lactam Resistance

β-lactam antibiotics are excellent drugs for treatment of staphylococcal infections, due to their superior efficacy and safety compared to other drugs. Effectiveness of β-lactams is severely compromised due to resistance, which is widespread among clinical strains of Staphylococcus aureus. β-lactams inhibit bacterial cells by binding to penicillin binding proteins (PBPs), which perform the penultimate steps of bacterial cell wall synthesis. Among PBPs of S. aureus, PBP2a has received the most attention for the past several decades due to its preeminent role in conferring both high-level and broad-spectrum resistance to the entire class of β-lactam drugs. Studies on PBP2a have thus unraveled incredible details of its mechanism of action. We have recently identified that an uncanonical, low molecular weight PBP of S. aureus, PBP4, can also provide high-level and broad-spectrum resistance to the entire class of β-lactam drugs at a level similar to that of PBP2a. The role of PBP4 has typically been considered not so important for β-lactam resistance of S. aureus, and as a result its mode of action remains largely unknown. In this article, we review our current knowledge of PBP4 mediating β-lactam resistance in S. aureus.

A Retrospective Analysis of Treatment and Clinical Outcomes among Patients with Methicillin-Susceptible Staphylococcus aureus Bloodstream Isolates Possessing Detectable mecA by a Commercial PCR Assay Compared to Patients with Methicillin-Resistant Staphylococcus aureus Bloodstream Isolates

mecA-positive Staphylococcus aureus isolates phenotypically susceptible to cefoxitin (mecA-methicillin-sensitive S. aureus [MSSA]) have been identified. We describe the treatment and outcomes among patients with mecA-MSSA bloodstream infections (BSI) and MRSA BSI matched 1:1 for age, BSI origin, and BSI type (n = 17 per group). Compared to MRSA BSI patients, mecA-MSSA BSI patients more often experienced clinical failure (58.8% and 11.8%, P = 0.010), driven largely by persistent bacteremia (35.3% and 11.8%). mecA-MSSA BSI patients may be at higher risk for poor clinical outcomes.

PBP4 Mediates β-Lactam Resistance by Altered Function

Penicillin binding protein 4 (PBP4) can provide high-level β-lactam resistance in Staphylococcus aureus A series of missense and promoter mutations associated with pbp4 were detected in strains that displayed high-level resistance. We show here that the missense mutations facilitate the β-lactam resistance mediated by PBP4 and the promoter mutations lead to overexpression of pbp4 Our results also suggest a cooperative interplay among PBPs for β-lactam resistance.