The AbcA transporter of Staphylococcus aureus affects cell autolysis

Staphylococcus aureus AbcA transporter enhances persister formation under β-lactam exposure

We evaluated the role of Staphylococcus aureus AbcA transporter in bacterial persistence and survival following exposure to the bactericidal agents nafcillin and oxacillin at both the population and single-cell levels. We show that AbcA overexpression resulted in resistance to nafcillin but not oxacillin. Using distinct fluorescent reporters of cell viability and AbcA expression, we found that over 6-14 hours of persistence formation, the proportion of AbcA reporter-expressing cells assessed by confocal microscopy increased sixfold as cell viability reporters decreased. Similarly, single-cell analysis in a high-throughput microfluidic system found a strong correspondence between antibiotic exposure and AbcA reporter expression. Persister cells grown in the absence of antibiotics showed neither an increase in nafcillin MIC nor in abcA transcript levels, indicating that survival was not associated with stable mutational resistance or abcA overexpression. Furthermore, persister cell levels on exposure to 1×MIC and 25×MIC of nafcillin decreased in an abcA knockout mutant. Survivors of nafcillin and oxacillin treatment overexpressed transporter AbcA, contributing to an enrichment of the number of persisters during treatment with pump-substrate nafcillin but not with pump-non-substrate oxacillin, indicating that efflux pump expression can contribute selectively to the survival of a persister population.

Two transporters cooperate to secrete amphipathic peptides from the cytoplasmic and membranous milieus

Diverse organisms secrete amphipathic biomolecules for competitive gains. However, how cells cope with producing these membrane-permeabilizing molecules is unclear. We focused on the PSM family of secreted amphipathic peptides in the pathogen Staphylococcus aureus that uses two ABC transporters, PmtCD and AbcA, to export peptides across the bacterial cell membrane. We found that increased peptide hydrophobicity favors PSM secretion through PmtCD over AbcA and that only PmtCD protected cells against amphipathic peptides. We propose a two-system model in which PmtCD and AbcA independently export PSMs from either membrane or cytosolic environments, respectively. Our model provides a rationale for the encoding of multiple transport systems on diverse biosynthetic gene clusters used to produce distinct amphipathic molecules. In addition, our data serve as a guide for selectively blocking PSM secretion to achieve antimicrobial or antivirulence approaches and to disrupt established roles of PSM-mediated virulence.

Bacterial resistance to antibacterial agents: Mechanisms, control strategies, and implications for global health

The spread of bacterial drug resistance has posed a severe threat to public health globally. Here, we cover bacterial resistance to current antibacterial drugs, including traditional herbal medicines, conventional antibiotics, and antimicrobial peptides. We summarize the influence of bacterial drug resistance on global health and its economic burden while highlighting the resistance mechanisms developed by bacteria. Based on the One Health concept, we propose 4A strategies to combat bacterial resistance, including prudent Application of antibacterial agents, Administration, Assays, and Alternatives to antibiotics. Finally, we identify several opportunities and unsolved questions warranting future exploration for combating bacterial resistance, such as predicting genetic bacterial resistance through the use of more effective techniques, surveying both genetic determinants of bacterial resistance and the transmission dynamics of antibiotic resistance genes (ARGs).

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.

Efflux Pump Mediated Antimicrobial Resistance by Staphylococci in Health-Related Environments: Challenges and the Quest for Inhibition

The increasing emergence of antimicrobial resistance in staphylococcal bacteria is a major health threat worldwide due to significant morbidity and mortality resulting from their associated hospital- or community-acquired infections. Dramatic decrease in the discovery of new antibiotics from the pharmaceutical industry coupled with increased use of sanitisers and disinfectants due to the ongoing COVID-19 pandemic can further aggravate the problem of antimicrobial resistance. Staphylococci utilise multiple mechanisms to circumvent the effects of antimicrobials. One of these resistance mechanisms is the export of antimicrobial agents through the activity of membrane-embedded multidrug efflux pump proteins. The use of efflux pump inhibitors in combination with currently approved antimicrobials is a promising strategy to potentiate their clinical efficacy against resistant strains of staphylococci, and simultaneously reduce the selection of resistant mutants. This review presents an overview of the current knowledge of staphylococcal efflux pumps, discusses their clinical impact, and summarises compounds found in the last decade from plant and synthetic origin that have the potential to be used as adjuvants to antibiotic therapy against multidrug resistant staphylococci. Critically, future high-resolution structures of staphylococcal efflux pumps could aid in design and development of safer, more target-specific and highly potent efflux pump inhibitors to progress into clinical use.

Multidrug-Resistance Transporter AbcA Secretes Staphylococcus aureus Cytolytic Toxins

Phenol-soluble modulins (PSMs) are Staphylococcus aureus cytolytic toxins that lyse erythrocytes and neutrophils and have important functions in the S. aureus infectious process. The molecular mechanisms of PSM secretion, however, are not well understood. Here we report that knockout of the multidrug-resistance ABC transporter AbcA, which contributes to S. aureus resistance against antibiotics and chemicals, diminished the secreted amount of PSM, leading to the accumulation of PSM in the intracellular fraction. The amount of PSM in the culture supernatants of the abcA knockout mutants was restored by introduction of the wild-type abcA gene, whereas it was not completely restored by introduction of mutant abcA genes encoding AbcA mutant proteins carrying amino acid substitutions in the adenosine triphosphate binding motifs. The abcA knockout mutant exhibited attenuated virulence in a mouse systemic infection model. These findings suggest that the multidrug resistance transporter AbcA secretes PSMs and contributes to S. aureus virulence.

Regulation of expression of abcA and its response to environmental conditions

The ATP-dependent transporter gene abcA in Staphylococcus aureus confers resistance to hydrophobic β-lactams. In strain ISP794, abcA is regulated by the transcriptional regulators MgrA and NorG and shares a 420-nucleotide intercistronic region with the divergently transcribed pbp4 gene, which encodes the transpeptidase Pbp4. Exposure of exponentially growing cells to iron-limited media, oxidative stress, and acidic pH (5.5) for 0.5 to 2 h had no effect on abcA expression. In contrast, nutrient limitation produced a significant increase in abcA transcripts. We identified three additional regulators (SarA, SarZ, and Rot) that bind to the overlapping promoter region of abcA and pbp4 in strain MW2 and investigated their role in the regulation of abcA expression. Expression of abcA is decreased by 10.0-fold in vivo in a subcutaneous abscess model. In vitro, abcA expression depends on rot and sarZ regulators. Moenomycin A exposure of strain MW2 produced an increase in abcA transcripts. Relative to MW2, the MIC of moenomycin was decreased 8-fold for MW2ΔabcA and increased 10-fold for the MW2 abcA overexpresser, suggesting that moenomycin is a substrate of AbcA.

Polyoxometalates active against tumors, viruses, and bacteria

Polyoxometalates (PMs) as discrete metal-oxide cluster anions with high solubility in water and photochemically and electrochemically active property have a wide variety of structures not only in molecular size from sub-nano to sub-micrometers with a various combination of metals but also in symmetry and highly negative charge. One of the reasons for such a structural variety originates from their conformation change (due to the condensed aggregation and the structural assembly) which strongly depends on environmental parameters such as solution pH, concentration, and coexistent foreign inorganic and/or organic substances. In the course of the application of the physicochemical properties of such PMs to the medical fields, antitumoral, antiviral, and antibacterial activities have been developed for realization of a novel inorganic medicine which provides a biologically excellent activity never replaced by other approved medicines. Several PMs as a candidate for clinical uses have been licensed toward the chemotherapy of solid tumors (such as human gastric cancer and pancreatic cancer), DNA and RNA viruses (such as HSV, HIV, influenza, and SARS), and drug-resistant bacteria (such as MRSA and VRSA) in recent years: [NH3Pr(i)]6[Mo7O24]∙3H2O (PM-8) and [Me3NH]6[H2Mo(V) 12O28(OH)12(Mo(VI)O3)4]∙2H2O (PM-17) for solid tumors; K7[PTi2W10O40]∙6H2O (PM-19), [Pr(i)NH3]6H[PTi2W10O38(O2)2]∙H2O (PM-523), and K11H[(VO)3(SbW9O33)2]∙27H2O (PM-1002) for viruses; and K6[P2W18O62]∙14H2O (PM-27), K4[SiMo12O40]∙3H2O (SiMo12), and PM-19 for MRSA and VRSA. The results are discussed from a point of view of the chemotherapeutic clarification in this review.

Thioridazine affects transcription of genes involved in cell wall biosynthesis in methicillin-resistant Staphylococcus aureus

The antipsychotic drug thioridazine is a candidate drug for an alternative treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA) in combination with the β-lactam antibiotic oxacillin. The drug has been shown to have the capability to resensitize MRSA to oxacillin. We have previously shown that the expression of some resistance genes is abolished after treatment with thioridazine and oxacillin. To further understand the mechanism underlying the reversal of resistance, we tested the expression of genes involved in antibiotic resistance and cell wall biosynthesis in response to thioridazine in combination with oxacillin. We observed that the oxacillin-induced expression of genes belonging to the VraSR regulon is reduced by the addition of thioridazine. The exclusion of such key factors involved in cell wall biosynthesis will most likely lead to a weakened cell wall and affect the ability of the bacteria to sustain oxacillin treatment. Furthermore, we found that thioridazine itself reduces the expression level of selected virulence genes and that selected toxin genes are not induced by thioridazine. In the present study, we find indications that the mechanism underlying reversal of resistance by thioridazine relies on decreased expression of specific genes involved in cell wall biosynthesis.

Efflux-mediated drug resistance in bacteria: an update

Drug efflux pumps play a key role in drug resistance and also serve other functions in bacteria. There has been a growing list of multidrug and drug-specific efflux pumps characterized from bacteria of human, animal, plant and environmental origins. These pumps are mostly encoded on the chromosome, although they can also be plasmid-encoded. A previous article in this journal provided a comprehensive review regarding efflux-mediated drug resistance in bacteria. In the past 5 years, significant progress has been achieved in further understanding of drug resistance-related efflux transporters and this review focuses on the latest studies in this field since 2003. This has been demonstrated in multiple aspects that include but are not limited to: further molecular and biochemical characterization of the known drug efflux pumps and identification of novel drug efflux pumps; structural elucidation of the transport mechanisms of drug transporters; regulatory mechanisms of drug efflux pumps; determining the role of the drug efflux pumps in other functions such as stress responses, virulence and cell communication; and development of efflux pump inhibitors. Overall, the multifaceted implications of drug efflux transporters warrant novel strategies to combat multidrug resistance in bacteria.

The Staphylococcus aureus GGDEF domain-containing protein, GdpS, influences protein A gene expression in a cyclic diguanylic acid-independent manner

Staphylococcus aureus is an important human pathogen that is the principal cause of a variety of diseases, ranging from localized skin infections to life-threatening systemic infections. The success of the organism as a pathogen and its ability to cause such a wide range of infections are due to its extensive virulence factors. In this study, we identified the role of the only GGDEF domain protein (GdpS [GGDEF domain protein from Staphylococcus]) in the virulence of S. aureus NCTC8325. Inactivation of gdpS results in an alteration in the production of a range of virulence factors, such as serine and cysteine proteases, fibrinogen-binding proteins, and, specifically, protein A (Spa), a major surface protein of S. aureus. The transcript level of spa decreases eightfold in the gdpS mutant compared with the parental NCTC8325 strain. Furthermore, the transcript level of sarS, which encodes a direct positive regulator of spa, also decreases in the gdpS mutant compared with the wild type, while the transcript levels of agr, sarA, sarT, and rot display no apparent changes in the gdpS mutant, suggesting that GdpS affects the expression of spa through interaction with SarS by unknown mechanisms. Furthermore, the complementation assays show that the influences of GdpS on spa and sarS depend on its N-terminal domain, which is predicted to be the sensor of a two-component system, rather than its C-terminal GGDEF domain with conserved GGDEF, suggesting that GdpS functions in S. aureus by an unknown mechanism independent of 3',5'-cyclic diguanylic acid signaling.

A new family of cyanobacterial penicillin-binding proteins. A missing link in the evolution of class A beta-lactamases

It is largely accepted that serine beta-lactamases evolved from some ancestral DD-peptidases involved in the biosynthesis and maintenance of the bacterial peptidoglycan. DD-peptidases are also called penicillin-binding proteins (PBPs), since they form stable acyl-enzymes with beta-lactam antibiotics, such as penicillins. On the other hand, beta-lactamases react similarly with these antibiotics, but the acyl-enzymes are unstable and rapidly hydrolyzed. Besides, all known PBPs and beta-lactamases share very low sequence similarities, thus rendering it difficult to understand how a PBP could evolve into a beta-lactamase. In this study, we identified a new family of cyanobacterial PBPs featuring the highest sequence similarity with the most widespread class A beta-lactamases. Interestingly, the Omega-loop, which, in the beta-lactamases, carries an essential glutamate involved in the deacylation process, is six amino acids shorter and does not contain any glutamate residue. From this new family of proteins, we characterized PBP-A from Thermosynechococcus elongatus and discovered hydrolytic activity with synthetic thiolesters that are usually good substrates of DD-peptidases. Penicillin degradation pathways as well as acylation and deacylation rates are characteristic of PBPs. In a first attempt to generate beta-lactamase activity, a 90-fold increase in deacylation rate was obtained by introducing a glutamate in the shorter Omega-loop.

The transcriptional regulators NorG and MgrA modulate resistance to both quinolones and beta-lactams in Staphylococcus aureus

MgrA is a known regulator of the expression of several multidrug transporters in Staphylococcus aureus. We identified another regulator of multiple efflux pumps, NorG, by its ability, like that of MgrA, to bind specifically to the promoter of the gene encoding the NorA efflux pump. NorG is a member of the family of the GntR-like transcriptional regulators, and it binds specifically to the putative promoters of the genes encoding multidrug efflux pumps NorA, NorB, NorC, and AbcA. Overexpression of norG produces a threefold increase in norB transcripts associated with a fourfold increase in the level of resistance to quinolones. In contrast, disruption of norG produces no change in the level of transcripts of norA, norB, and norC but causes an increase of at least threefold in the transcript level of abcA, associated with a fourfold increase in resistance to methicillin, cefotaxime, penicillin G, and nafcillin. Overexpression of cloned abcA caused an 8- to 128-fold increase in the level of resistance to all four beta-lactam antibiotics. Furthermore, MgrA and NorG have opposite effects on norB and abcA expression. MgrA acts as an indirect repressor for norB and a direct activator for abcA, whereas NorG acts as a direct activator for norB and a direct repressor for abcA.

How antibiotics can make us sick: the less obvious adverse effects of antimicrobial chemotherapy

Antimicrobial agents are associated with side-effects, which are usually tolerated because the benefits of treatment outweigh the toxic effects. Clinicians know about these side-effects but are less likely to understand additional adverse events, such as the overgrowth of resistant microorganisms. Overgrowth can itself precipitate a secondary infection, which can be more difficult to treat. Resistant organisms then spread to other patients and the environment, and contribute to increasing antimicrobial resistance worldwide. Organisms exposed to antibiotics undergo molecular changes that might enhance virulence. Enhanced pathogenicity would affect patients, particularly if the organism is also multiply resistant. Clinicians have a responsibility to select the correct antibiotic as soon as they have diagnosed infection, but an absence of microbiological understanding and ignorance of the potential environmental effects have contributed to inappropriate prescribing. The less obvious results of antimicrobial consumption probably go unrecognised in routine clinical care.

Identification of sarV (SA2062), a new transcriptional regulator, is repressed by SarA and MgrA (SA0641) and involved in the regulation of autolysis in Staphylococcus aureus

The expression of genes involved in the pathogenesis of Staphylococcus aureus is known to be controlled by global regulatory loci, including agr, sarA, sae, arlRS, lytSR, and sarA-like genes. Here we described a novel transcriptional regulator called sarV of the SarA protein family. The transcription of sarV is low or undetectable under in vitro conditions but is significantly augmented in sarA and mgrA (norR or rat) (SA0641) mutants. The sarA and mgrA genes act as repressors of sarV expression, as confirmed by transcriptional fusion and Northern analysis data. Purified SarA and MgrA proteins bound specifically to separate regions of the sarV promoter as determined by gel shift and DNase I footprinting assays. The expression of 19 potential target genes involved in autolysis and virulence, phenotypes affected by sarA and mgrA, was evaluated in an isogenic sarV mutant pair. Our data indicated that the sarV gene product played a role regulating some virulence genes and more genes involved in autolysis. The sarV mutant was more resistant to Triton X-100 and penicillin-induced lysis compared to the wild type and the sarA mutant, whereas hyperexpression of sarV in the parental strain or the sarV mutant rendered the resultant strain highly susceptible to lysis. Zymographic analysis of murein hydrolase activity revealed that inactivation of the sarV gene results in decreased extracellular murein hydrolase activity compared to that of wild-type S. aureus. We propose that sarV may be part of the common pathway by which mgrA and sarA gene products control autolysis in S. aureus.

Effect of disruption of Staphylococcus aureus PBP4 gene on resistance to beta-lactam antibiotics

Penicillin-binding proteins (PBPs) mediate susceptibility to beta-lactam antibiotics. PBP 4, although not essential for survival, has been associated with low-level resistance to beta-lactam antibiotics. To determine its contribution to survival of Staphylococcus aureus cells exposed to beta-lactams, the PBP 4 gene (pbp4) was disrupted and then complemented in the methicillin-susceptible strain RN4220 and the homogeneous methicillin-resistant strain COL. Depending on the antibiotic tested, the presence or absence of an intact pbp4 has no effect or only a modest effect on growth measured by population analysis. These data indicate that PBP 4 is a relatively unimportant target of beta-lactams not only in methicillin-susceptible but also methicillin-resistant S. aureus.

Characterization of RAT, an autolysis regulator in Staphylococcus aureus

In trying to identify genetic loci involved in the regulation of cap5 genes in Staphylococcus aureus, we isolated a transposon mutant that exhibited a growth defect, enhanced autolysis and increased sensitivity to Triton X-100 and penicillin, attributable in part to increased murein hydrolase activity. Analysis of the chromosomal sequence flanking the transposon insertion site revealed that the gene disrupted in the mutant encodes an open reading frame of 147 amino acids. We named this gene rat, which stands for regulator of autolytic activity. Sequence analysis indicated that Rat is homologous to the MarR and, to a lesser extent, the SarA protein families. Mutations in rat resulted in decreased expression of known autolytic regulators lytSR, lrgAB and arlRS. Gel shift studies indicated that Rat binds to the lytRS and arlRS promoters, thus confirming Rat as a DNA-binding protein to these known repressors of autolytic activity. As anticipated, rat appears to be a negative regulator of autolysin genes including lytM and lytN. These data suggest that the rat gene product is an important regulator of autolytic activity in S. aureus.

The prevalence and mechanisms of vancomycin resistance in Staphylococcus aureus

The emergence of Staphylococcus aureus resistant to vancomycin has caused considerable concern. Such strains are currently rare, although they have been isolated from many areas of the world. Considerable controversy surrounds strains of S. aureus displaying heterogeneous resistance to vancomycin regarding their definition and methods for detection. This has led to considerable variance in estimates of prevalence (0-1.3%-20% in Japan) and has hindered efforts to define the clinical relevance of these strains. The mechanism of resistance involves a complex reorganization of cell wall metabolism, leading to a grossly thickened cell wall with reduced peptidoglycan cross-linking. There may be many different ways in which strains achieve this endpoint. Current knowledge and theories are summarized.