Cloning and expression of Staphylococcus aureus plasmid-mediated quaternary ammonium resistance in Escherichia coli

The mobilome landscape of biocide-resistance in Brazilian ESKAPE isolates

The increasing frequency of antibiotic-resistant bacteria is a constant threat to global human health. Therefore, the pathogens of the ESKAPE group (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, and Enterobacter spp.) are among the most relevant causes of hospital infections responsible for millions of deaths every year. However, little has been explored about the danger of microorganisms resistant to biocides such as antiseptics and disinfectants. Widely used in domestic, industrial, and hospital environments, these substances reach the environment and can cause selective pressure for resistance genes and induce cross-resistance to antibiotics, further aggravating the problem. Therefore, it is necessary to use innovative and efficient strategies to monitor the spread of genes related to resistance to biocides. Whole genome sequencing and bioinformatics analysis aiming to search for sequences encoding resistance mechanisms are essential to help monitor and combat these pathogens. Thus, this work describes the construction of a bioinformatics tool that integrates different databases to identify gene sequences that may confer some resistance advantage about biocides. Furthermore, the tool analyzed all the genomes of Brazilian ESKAPE isolates deposited at NCBI and found a series of different genes related to resistance to benzalkonium chloride, chlorhexidine, and triclosan, which were the focus of this work. As a result, the presence of resistance genes was identified in different types of biological samples, environments, and hosts. Regarding mobile genetic elements (MGEs), around 52% of isolates containing genes related to resistance to these compounds had their genes identified in plasmids, and 48.7% in prophages. These data show that resistance to biocides can be a silent, underestimated danger spreading across different environments and, therefore, requires greater attention.

Efflux Pump (QacA, QacB, and QacC) and β-Lactamase Inhibitors? An Evaluation of 1,8-Naphthyridines against Staphylococcus aureus Strains

The bacterial species Staphylococcus aureus presents a variety of resistance mechanisms, among which the expression of β-lactamases and efflux pumps stand out for providing a significant degree of resistance to clinically relevant antibiotics. The 1,8-naphthyridines are nitrogen heterocycles with a broad spectrum of biological activities and, as such, are promising research targets. However, the potential roles of these compounds on bacterial resistance management remain to be better investigated. Therefore, the present study evaluated the antibacterial activity of 1,8-naphthyridine sulfonamides, addressing their ability to act as inhibitors of β-lactamases and efflux pump (QacA/B and QacC) against the strains SA-K4414 and SA-K4100 of S. aureus. All substances were prepared at an initial concentration of 1024 μg/mL, and their minimum inhibitory concentrations (MIC) were determined by the broth microdilution method. Subsequently, their effects on β-lactamase- and efflux pump-mediated antibiotic resistance was evaluated from the reduction of the MIC of ethidium bromide (EtBr) and β-lactam antibiotics, respectively. The 1,8-naphthyridines did not present direct antibacterial activity against the strains SA-K4414 and SA-K4100 of S. aureus. On the other hand, when associated with antibiotics against both strains, the compounds reduced the MIC of EtBr and β-lactam antibiotics, suggesting that they may act by inhibiting β-lactamases and efflux pumps such as QacC and QacA/B. However, further research is required to elucidate the molecular mechanisms underlying these observed effects.

Quaternary Phosphonium Compounds: An Examination of Non-Nitrogenous Cationic Amphiphiles That Evade Disinfectant Resistance

Quaternary ammonium compounds (QACs) serve as mainstays in the formulation of disinfectants and antiseptics. However, an over-reliance and misuse of our limited QAC arsenal has driven the development and spread of resistance to these compounds, as well as co-resistance to common antibiotics. Extensive use of these compounds throughout the COVID-19 pandemic thus raises concern for the accelerated proliferation of antimicrobial resistance and demands for next-generation antimicrobials with divergent architectures that may evade resistance. To this end, we endeavored to expand beyond canonical ammonium scaffolds and examine quaternary phosphonium compounds (QPCs). Accordingly, a synthetic and biological investigation into a library of novel QPCs unveiled biscationic QPCs to be effective antimicrobial scaffolds with improved broad-spectrum activities compared to commercial QACs. Notably, a subset of these compounds was found to be less effective against a known QAC-resistant strain of MRSA. Bioinformatic analysis revealed the unique presence of a family of small multiresistant transporter proteins, hypothesized to enable efflux-mediated resistance to QACs and QPCs. Further investigation of this resistance mechanism through efflux-pump inhibition and membrane depolarization assays illustrated the superior ability of P6P-10,10 to perturb the cell membrane and exert the observed broad-spectrum potency compared to its commercial counterparts. Collectively, this work highlights the promise of biscationic phosphonium compounds as next-generation disinfectant molecules with potent bioactivities, thereby laying the foundation for future studies into the synthesis and biological investigation of this nascent antimicrobial class.

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.

An Efficient Synthesis of 3-Alkylpyridine Alkaloids Enables Their Biological Evaluation

3-Alkylpyridine alkaloids (3-APAs) isolated from the arctic sponge Haliclona viscosa are a promising group of bioactive marine alkaloids. However, due to limited bioavailability, investigations of their bioactivity have been hampered. Additionally, synthesis of a common intermediate requires the use of protecting groups and harsh conditions. In this work, we developed a simple and concise two-step route to nine different natural and synthetic haliclocyclins. These compounds displayed modest antibiotic activity against several Gram-positive bacterial strains.

Physiological Functions of Bacterial "Multidrug" Efflux Pumps

Bacterial multidrug efflux pumps have come to prominence in human and veterinary pathogenesis because they help bacteria protect themselves against the antimicrobials used to overcome their infections. However, it is increasingly realized that many, probably most, such pumps have physiological roles that are distinct from protection of bacteria against antimicrobials administered by humans. Here we undertake a broad survey of the proteins involved, allied to detailed examples of their evolution, energetics, structures, chemical recognition, and molecular mechanisms, together with the experimental strategies that enable rapid and economical progress in understanding their true physiological roles. Once these roles are established, the knowledge can be harnessed to design more effective drugs, improve existing microbial production of drugs for clinical practice and of feedstocks for commercial exploitation, and even develop more sustainable biological processes that avoid, for example, utilization of petroleum.

Efflux identification and engineering for ansamitocin P-3 production in Actinosynnema pretiosum

Ansamitocin P-3 (AP-3) exhibits potent biological activities against various tumor cells. As an important drug precursor, reliable supply of AP-3 is limited by low fermentation yield. Although different strategies have been implemented to improve AP-3 yield, few have investigated the impact of efflux on AP-3 production. In this study, AP-3 efflux genes were identified through combined analysis of two sets of transcriptomes. The production-based transcriptome was implemented to search for efflux genes highly expressed in response to AP-3 accumulation during the fermentation process, while the resistance-based transcriptome was designed to screen for genes actively expressed in response to the exogenous supplementation of AP-3. After comprehensive analysis of two transcriptomes, six efflux genes outside the ansamitocin BGC were identified. Among the six genes, individual deletion of APASM_2704, APASM_6861, APASM_3193, and APASM_2805 resulted in decreased AP-3 production, and alternative overexpression led to AP-3 yield increase from 264.6 to 302.4, 320.4, 330.6, and 320.6 mg/L, respectively. Surprisingly, APASM_2704 was found to be responsible for exportation of AP-3 and another macro-lactam antibiotic pretilactam. Furthermore, growth of APASM_2704, APASM_3193, or APASM_2805 overexpression mutants was obviously improved under 300 mg/L AP-3 supplementation. In summary, our study has identified AP-3 efflux genes outside the ansamitocin BGC by comparative transcriptomic analysis, and has shown that enhancing the transcription of transporter genes can improve AP-3 production, shedding light on strategies used for exporter screening and antibiotic production improvement. KEY POINTS: • AP-3-related efflux genes were identified by transcriptomic analysis. • Deletion of the identified efflux genes led in AP-3 yield decrease. • Overexpression of the efflux genes resulted in increased AP-3 production.

Advancements in the Development of Non-Nitrogen-Based Amphiphilic Antiseptics to Overcome Pathogenic Bacterial Resistance

The prevalence of quaternary ammonium compounds (QACs) as common disinfecting agents for the past century has led bacteria to develop resistance to such compounds. Given the alarming increase in resistant strains, new strategies are required to combat this rise in resistance. Recent efforts to probe and combat bacterial resistance have focused on studies of multiQACs. Relatively unexplored, however, have been changes to the primary atom bearing positive charge in these antiseptics. Here we review the current state of the field of both phosphonium and sulfonium amphiphilic antiseptics, both of which hold promise as novel means to address bacterial resistance.

The Varied Role of Efflux Pumps of the MFS Family in the Interplay of Bacteria with Animal and Plant Cells

Efflux pumps represent an important and large group of transporter proteins found in all organisms. The importance of efflux pumps resides in their ability to extrude a wide range of antibiotics, resulting in the emergence of multidrug resistance in many bacteria. Besides antibiotics, multidrug efflux pumps can also extrude a large variety of compounds: Bacterial metabolites, plant-produced compounds, quorum-sensing molecules, and virulence factors. This versatility makes efflux pumps relevant players in interactions not only with other bacteria, but also with plant or animal cells. The multidrug efflux pumps belonging to the major facilitator superfamily (MFS) are widely distributed in microbial genomes and exhibit a large spectrum of substrate specificities. Multidrug MFS efflux pumps are present either as single-component transporters or as tripartite complexes. In this review, we will summarize how the multidrug MFS efflux pumps contribute to the interplay between bacteria and targeted host cells, with emphasis on their role in bacterial virulence, in the colonization of plant and animal host cells and in biofilm formation. We will also address the complexity of these interactions in the light of the underlying regulatory networks required for the effective activation of efflux pump genes.

A Transcriptomic Approach to Identify Novel Drug Efflux Pumps in Bacteria

The core genomes of most bacterial species include a large number of genes encoding putative efflux pumps. The functional roles of most of these pumps are unknown, however, they are often under tight regulatory control and expressed in response to their substrates. Therefore, one way to identify pumps that function in antimicrobial resistance is to examine the transcriptional responses of efflux pump genes to antimicrobial shock. By conducting complete transcriptomic experiments following antimicrobial shock treatments, it may be possible to identify novel drug efflux pumps encoded in bacterial genomes. In this chapter we describe a complete workflow for conducting transcriptomic analyses by RNA sequencing, to determine transcriptional changes in bacteria responding to antimicrobials.

Drug resistance of oral bacteria to new antibacterial dental monomer dimethylaminohexadecyl methacrylate

Only two reports exist on drug-resistance of quaternary ammonium monomers against oral bacteria; both studies tested planktonic bacteria for 10 passages, and neither study tested biofilms or resins. The objectives of this study were to investigate the drug-resistance of Streptococcus mutans, Streptococcus sanguinis and Streptococcus gordonii against dimethylaminohexadecyl methacrylate (DMAHDM), and to evaluate biofilms on resins with repeated exposures for 20 passages for the first time. DMAHDM, dimethylaminododecyl methacrylate (DMADDM) and chlorhexidine (CHX) were tested with planktonic bacteria. Biofilms were grown on a resin containing 3% DMAHDM. Minimum-inhibitory concentrations were measured. To detect drug-resistance, the survived bacteria from the previous passage were used as inoculum for the next passage for repeated exposures. S. gordonii developed drug-resistance against DMADDM and CHX, but not against DMAHDM. Biofilm colony-forming units (CFU) on DMAHDM-resin was reduced by 3–4 log; there was no difference from passages 1 to 20 (p > 0.1). No drug-resistance to DMAHDM was detected for all three bacterial species. In conclusion, this study showed that DMAHDM induced no drug-resistance, and DMAHDM-resin reduced biofilm CFU by 3–4 log, with no significant change from 1 to 20 passages. DMAHDM with potent antibacterial activities and no drug-resistance is promising for dental applications.

Current and Emerging Topical Antibacterials and Antiseptics: Agents, Action, and Resistance Patterns

Bacterial skin infections represent some of the most common infectious diseases globally. Prevention and treatment of skin infections can involve application of a topical antimicrobial, which may be an antibiotic (such as mupirocin or fusidic acid) or an antiseptic (such as chlorhexidine or alcohol). However, there is limited evidence to support the widespread prophylactic or therapeutic use of topical agents. Challenges involved in the use of topical antimicrobials include increasing rates of bacterial resistance, local hypersensitivity reactions (particularly to older agents, such as bacitracin), and concerns about the indiscriminate use of antiseptics potentially coselecting for antibiotic resistance. We review the evidence for the major clinical uses of topical antibiotics and antiseptics. In addition, we review the mechanisms of action of common topical agents and define the clinical and molecular epidemiology of antimicrobial resistance in these agents. Moreover, we review the potential use of newer and emerging agents, such as retapamulin and ebselen, and discuss the role of antiseptic agents in preventing bacterial skin infections. A comprehensive understanding of the clinical efficacy and drivers of resistance to topical agents will inform the optimal use of these agents to preserve their activity in the future.

Scaffold-Hopping of Multicationic Amphiphiles Yields Three New Classes of Antimicrobials

Quaternary ammonium compounds (QACs) are a vital class of antiseptics. Recent investigations into their construction are uncovering novel and potent multicationic variants. Based on a trisQAC precedent, we have implemented a scaffold-hopping approach to develop alternative QAC architectures that display 1-3 long alkyl chains in specific projections from cyclic and branched core structures bearing 3-4 nitrogen atoms. The preparation of 30 QAC structures allowed for correlation of scaffold structure with antimicrobial activity. We identified QACs with limited conformational flexibility that have improved bioactivity against planktonic bacteria as compared to their linear counterparts. We also confirmed that resistance, as evidenced by an increased minimum inhibitory concentration (MIC) for methicillin-resistant Staphylococcus aureus (MRSA) compared to methicillin-susceptible Staphylococcus aureus (MSSA), can reduce efficacy up to 64-fold for monocationic QACs. Differentiation of antimicrobial and anti-biofilm activity, however, was not observed, suggesting that these compounds utilize a non-specific mode of eradication.

Quaternary Ammonium Compounds: An Antimicrobial Mainstay and Platform for Innovation to Address Bacterial Resistance

Quaternary ammonium compounds (QACs) have represented one of the most visible and effective classes of disinfectants for nearly a century. With simple preparation, wide structural variety, and versatile incorporation into consumer products, there have been manifold developments and applications of these structures. Generally operating via disruption of one of the most fundamental structures in bacteria-the cell membrane-leading to cell lysis and bacterial death, the QACs were once thought to be impervious to resistance. Developments over the past decades, however, have shown this to be far from the truth. It is now known that a large family of bacterial genes (generally termed qac genes) encode efflux pumps capable of expelling many QAC structures from bacterial cells, leading to a decrease in susceptibility to QACs; methods of regulation of qac transcription are also understood. Importantly, qac genes can be horizontally transferred via plasmids to other bacteria and are often transmitted alongside other antibiotic-resistant genes; this dual threat represents a significant danger to human health. In this review, both QAC development and QAC resistance are documented, and possible strategies for addressing and overcoming QAC-resistant bacteria are discussed.

Biocide Susceptibility of Staphylococcus aureus CC398 and CC30 Isolates from Pigs and Identification of the Biocide Resistance Genes, qacG and qacC

OBJECTIVES

Methicillin-resistant Staphylococcus aureus (MRSA), in particular clonal complex (CC) 398, is increasingly found in livestock. Recently, MRSA CC30 was identified in Danish pigs. We determined the susceptibility of porcine S. aureus isolates of CC398 and CC30 to disinfectants used in pig farming (benzalkonium chloride, hydrogen peroxide, formaldehyde, sodium hypochlorite, and caustic soda). Furthermore, efflux pump activity, antimicrobial resistance profiles, hemolysis properties, and the presence of toxic shock syndrome toxin-1 (TSST-1) and Panton-Valentine Leukocidin (PVL)-encoding virulence factors were investigated.

METHODS

Susceptibilities to biocides and antimicrobial agents of 79 porcine S. aureus isolates were determined by the microdilution method. Isolates comprised 21 methicillin-sensitive S. aureus (MSSA) and 40 MRSA isolates belonging to CC398 and 13 MSSA and 5 MRSA isolates belonging to CC30. The presence of quaternary ammonium compound (QAC) resistance efflux pumps was analyzed using an ethidium bromide accumulation assay. The presence of qac resistance genes in active efflux pump positive isolates was determined by whole-genome sequencing data. All isolates were screened for lukPV and tst genes with PCR, and hemolytic activities were determined using an agar plate assay.

RESULTS

S. aureus isolates did not show reduced susceptibility to the biocides tested. However, the QAC resistance gene, qacG, was detected in three MRSA CC30 isolates and the qacC in one MRSA CC30 isolate. CC30 isolates were generally more susceptible to non-beta-lactam antibiotics than CC398. Isolates generally had low hemolytic activity and none encoded PVL or TSST-1.

CONCLUSION

The presence of qac genes in European porcine S. aureus isolates and in livestock-associated MRSA CC30 is for the first time described in this study. This finding is concerning as it ultimately may compromise disinfection with QACs and thereby contribute to the selection and spread of MRSA CC30.

Recently introduced qacA/B genes in Staphylococcus epidermidis do not increase chlorhexidine MIC/MBC

OBJECTIVES

Chlorhexidine is used as a disinfectant to prevent surgical infections. Recently, studies have indicated that chlorhexidine usage has selected methicillin-resistant Staphylococcus aureus strains that are tolerant to chlorhexidine and that this may be related to the presence of the qacA/B-encoded efflux pumps. Here, we evaluated if high-level exposure to chlorhexidine selects for tolerant colonizing Staphylococcus epidermidis and we addressed the consequences of long-term exposure to chlorhexidine.

METHODS

Chlorhexidine susceptibility and carriage of qacA/B was determined for colonizing S. epidermidis isolated from scrub nurses heavily exposed to chlorhexidine and were compared with isolates from non-users of chlorhexidine hand rubs. S. epidermidis blood isolates from the 1960s, before the wider introduction of chlorhexidine to the market, were also tested and compared with recently collected S. epidermidis blood isolates.

RESULTS

There was no correlation between the use of chlorhexidine in scrub nurses and the presence of qacA/B genes in S. epidermidis isolates or increased MICs/MBCs of chlorhexidine for S. epidermidis isolates. While 55% of current blood isolates harboured the qacA/B genes, none of the 33 historical S. epidermidis isolates did, although their MICs and MBCs of chlorhexidine were comparable to those for current isolates.

CONCLUSIONS

Chlorhexidine used as a hand rub does not select for S. epidermidis isolates with increased MICs or MBCs of chlorhexidine. However, the absence of qacA/B genes in S. epidermidis isolates obtained in the 1960s suggests that long-term use of biocides like chlorhexidine or related compounds may select for the presence of qacA/B genes.

Multidrug Efflux Pumps in Staphylococcus aureus: an Update

The emergence of infections caused by multi- or pan-resistant bacteria in the hospital or in the community settings is an increasing health concern. Albeit there is no single resistance mechanism behind multiresistance, multidrug efflux pumps, proteins that cells use to detoxify from noxious compounds, seem to play a key role in the emergence of these multidrug resistant (MDR) bacteria. During the last decades, experimental data has established their contribution to low level resistance to antimicrobials in bacteria and their potential role in the appearance of MDR phenotypes, by the extrusion of multiple, unrelated compounds. Recent studies suggest that efflux pumps may be used by the cell as a first-line defense mechanism, avoiding the drug to reach lethal concentrations, until a stable, more efficient alteration occurs, that allows survival in the presence of that agent. In this paper we review the current knowledge on MDR efflux pumps and their intricate regulatory network in Staphylococcus aureus, a major pathogen, responsible from mild to life-threatening infections. Particular emphasis will be given to the potential role that S. aureus MDR efflux pumps, either chromosomal or plasmid-encoded, have on resistance towards different antimicrobial agents and on the selection of drug - resistant strains. We will also discuss the many questions that still remain on the role of each specific efflux pump and the need to establish appropriate methodological approaches to address all these questions.

New biocide with both N-chloramine and quaternary ammonium moieties exerts enhanced bactericidal activity

Considering the rise of antibiotic resistance, the development of new antibacterial agents with improved biocidal functions is urgently required. In this study, ionic 5,5-dimethylhydantoin (DMH) analogues containing either a quaternary ammonium moiety (2)-4) or a phosphonate functional group (5),-6), were designed and synthesized to investigate the possible enhancing effect of quaternary ammonium moieties on the antibacterial performance of N-chloramines. These ionic DMH analogues were converted to their N-chloramine counterparts either in free form or after being covalently immobilized on a polymer surface via the "click" chemistry method. In the subsequent antimicrobial assessment against multi-drug-resistant Escherichia coli (MDR-E. coli) and methicillin-resistant Staphylococcus aureus (MRSA), chlorinated 2 and 3, the cyclic N-chloramines with a structural cation, exhibited distinctly enhanced biocidal functions in solution and after immobilization on surfaces.

LmrR-mediated gene regulation of multidrug resistance in Lactococcus lactis

Multidrug resistance (MDR) in Lactococcus lactis is due to the expression of the membrane ATP-binding cassette (ABC) transporter LmrCD. In the absence of drugs, the transcriptional regulator LmrR prevents expression of the lmrCD operon by binding to its operator site. Through an autoregulatory mechanism LmrR also suppresses its own expression. Although the lmrR and lmrCD genes have their own promoters, primer extension analysis showed the presence of a long transcript spanning the entire lmrR-lmrCD cluster, in addition to various shorter transcripts harbouring the lmrCD genes only. 'In-gel' Cu-phenanthroline footprinting analysis indicated an extensive interaction between LmrR and the lmrR promoter/operator region. Atomic force microscopy imaging of the binding of LmrR to the control region of lmrR DNA showed severe deformations indicative of DNA wrapping and looping, while LmrR binding to a fragment containing the lmrCD control region induced DNA bending. The results further suggest a drug-dependent regulation mechanism in which the lmrCD genes are co-transcribed with lmrR as a polycistronic messenger. This leads to an LmrR-mediated regulation of lmrCD expression that is exerted from two different locations and by distinct regulatory mechanisms.

Genetics of antimicrobial resistance in Staphylococcus aureus

Strains of Staphylococcus aureus that are resistant to multiple antimicrobial compounds, including most available classes of antibiotics and some antiseptics, are a major threat to patient care owing to their stubborn intransigence to chemotherapy and disinfection. This reality has stimulated extensive efforts to understand the genetic nature of the determinants encoding antimicrobial resistance, together with the mechanisms by which these determinants evolve over time and are spread within bacterial populations. Such studies have benefited from the application of molecular genetics and in recent years, the sequencing of over a dozen complete staphylococcal genomes. It is now evident that the evolution of multiresistance is driven by the acquisition of discrete preformed antimicrobial resistance genes that are exchanged between organisms via horizontal gene transfer. Nonetheless, chromosomal mutation is the catalyst of novel resistance determinants and is likely to have an enhanced influence with the ongoing introduction of synthetic antibiotics.

Bacterial multidrug transport through the lens of the major facilitator superfamily

Multidrug transporters are membrane proteins that expel a wide spectrum of cytotoxic compounds from the cell. Through this function, they render cells resistant to multiple drugs. These transporters are found in many different families of transport proteins, of which the largest is the major facilitator superfamily. Multidrug transporters from this family are highly represented in bacteria and studies of them have provided important insight into the mechanism underlying multidrug transport. This review summarizes the work carried out on these interesting proteins and underscores the differences and similarities to other transport systems.

Efflux-mediated response of Staphylococcus aureus exposed to ethidium bromide

OBJECTIVES

By adapting an antibiotic-susceptible Staphylococcus aureus strain to increasing concentrations of ethidium bromide, a known substrate of efflux pumps (EPs), and by phenotypically and genotypically analysing the resulting progeny, we characterized the molecular mechanisms of S. aureus adaptation to ethidium bromide.

METHODS

S. aureus ATCC 25923 was grown in increasing concentrations of ethidium bromide. The MICs of representatives of eight classes of antibiotics, eight biocides and two dyes against ATCC 25923 and its ethidium bromide-resistant progeny ATCC 25923(EtBr) were determined with or without six efflux pump inhibitors (EPIs). Efflux activity in the presence/absence of EPIs was evaluated by real-time fluorometry. The presence and expression of eight EP genes were assayed by PCR and quantitative RT-PCR (qRT-PCR), respectively. Mutations in grlA, gyrA and norA promoter regions were screened by DNA sequencing.

RESULTS

Compared with its parental strain, ATCC 25923(EtBr) was 32-fold more resistant to ethidium bromide and also more resistant to biocides and hydrophilic fluoroquinolones. Resistance to these could be reduced by the EPIs chlorpromazine, thioridazine and reserpine. Increased efflux of ethidium bromide by ATCC 25923(EtBr) could be inhibited by the same EPIs. qRT-PCR showed that norA was 35-fold over-expressed in ATCC 25923(EtBr), whereas the remaining EP genes showed no significant increase in their expression. Sequencing of the norA promoter region revealed a 70 bp deletion in ATCC 25923(EtBr).

CONCLUSIONS

Exposure of S. aureus to quaternary compounds such as ethidium bromide results in decreased susceptibility of the organism to a wide variety of compounds, including quinolones and biocides through an efflux-mediated response, which for strain ATCC 25923 is mainly NorA-mediated. This altered expression may result from alterations in the norA promoter region.

Analysis of tryptophan residues in the staphylococcal multidrug transporter QacA reveals long-distance functional associations of residues on opposite sides of the membrane

Tryptophan residues can possess a multitude of functions within a multidrug transport protein, e.g., mediating interactions with substrates or distal parts of the protein, or fulfilling a structural requirement, such as guiding the depth of membrane insertion. In this study, the nine tryptophan residues of the staphylococcal QacA multidrug efflux protein were individually mutated to alanine and phenylalanine, and the functional consequences of these changes were determined. Phenylalanine substitutions for each tryptophan residue were functionally tolerated. However, alanine modifications revealed an important functional role for three tryptophan residues, W58, W149, and W173, each of which is well conserved among QacA-related transport proteins in the major facilitator superfamily. The most functionally compromising mutation, an alanine substitution for W58, likely to be located at the extracellular interface of transmembrane segment 2, abolished all detectable QacA-mediated resistance and transport function. Second-site suppressor analyses identified several mutations that rescued the function of the W58A QacA mutant. Remarkably, all of these suppressor mutations were shown to be located in cytoplasmic loops between transmembrane helices 2 and 3 or 12 and 13, demonstrating novel functional associations between amino acid positions on opposite sides of the membrane and in distal N- and C-terminal regions of the QacA protein.

VceR negatively regulates the vceCAB MDR efflux operon and positively regulates its own synthesis in Vibrio cholerae 569B

The vceCAB (vce) operon encodes the multidrug resistance pump VceCAB (VCE), which contributes to resistance of Vibrio cholerae to carbonyl cyanide m-chlorophenylhydrazine (CCCP), deoxycholate, and pentachlorophenol by several-fold. vceR, which encodes the TetR-type repressor VceR and is divergently transcribed from vce, has been characterized in Escherichia coli. Detailed characterization of vceR in V. cholerae 569B confirmed the repressive effect of VceR on VCE function and indicated several novel features of VceR. Deletion of vceR increased resistance of strain 569B to CCCP and deoxycholate modestly, but did not affect resistance to pentachlorophenol. Transcriptional analysis revealed that vce expression was not only increased in strain 569BDeltavceR::Omega by 2-fold but continued to rise throughout the growth cycle. Using a vceR-lux transcriptional fusion plasmid, we examined whether vceR is autoregulated in strain 569B. Expression of vceR from the vceR-lux fusion was significantly lower in strain 569BDeltavceR::Omega than in strain 569B. In addition, exposure to CCCP reduced vceR expression from the vceR-lux fusion in strain 569B but not in strain 569BDeltavceR::Omega. Despite differences in the VceR binding site in strain 569B from the previously recognized 28 bp sequence in V. cholerae CVD101, purified recombinant VceR bound to the 24 bp sequence from strain 569B. We propose that VceR modulates vce expression by binding in vivo to the 24 bp sequence within the vceR-vce intergenic region; unlike many TetR repressors that are negatively autoregulated, VceR positively regulates vceR expression in trans.

Active Export Proteins Mediating Drug Resistance in Staphylococci

Drug resistance mediated by integral membrane transporters is an important mode of cellular resistance to cytotoxic agents across all classes of living organisms. Gram-positive bacteria, such as staphylococcal species, are not encapsulated by a selective outer membrane permeability barrier. Therefore, these organisms often employ integral membrane drug transport systems to maintain cellular concentrations of antimicrobials at subtoxic levels. Staphylococcal species, including the opportunistic human pathogen Staphylococcus aureus, encode a multitude of drug exporters, encompassing transporters from each of the five currently recognized families of bacterial drug resistance transporters. A number of these transporters are chromosomally encoded and allow the host cell to realize clinically significant levels of drug resistance after minor mutations to regulatory regions. Others are plasmid-encoded and can be easily passed between staphylococcal strains and species, or acquired from other Gram-positive genera. In combination, staphylococcal drug transporters potentiate resistance to a vast array of antimicrobial compounds, including macrolide, quinolone, tetracycline and streptogramin antibiotics, as well as a broad range of biocides, such as quaternary ammonium compounds, biguanidines and diamidines. An understanding of the genetic and molecular properties of drug transporters will lead to effective treatments of staphylococcal infections. Here we provide a detailed review of the active drug transporters of the staphylococci.

Functional effects of intramembranous proline substitutions in the staphylococcal multidrug transporter QacA

The QacA multidrug transporter is encoded on Staphylococcus aureus multidrug resistance plasmids and confers broad-range antimicrobial resistance to more than 30 monovalent and bivalent lipophilic, cationic compounds from at least 12 different chemical classes. QacA contains 10 proline residues predicted to be within transmembrane regions, several of which are conserved in related export proteins. Proline residues are classically known as helix-breakers and are highly represented within the transmembrane helices of membrane transport proteins, where they can mediate the formation of structures essential for protein stability and transport function. The importance of these 10 intramembranous proline residues for QacA-mediated transport function was determined by examining the functional effect of substituting these residues with glycine, alanine or serine. Several proline-substituted QacA mutants failed to confer high-level resistance to selected QacA substrates. However, no single proline mutation, including those at conserved positions, significantly disrupted QacA protein expression or QacA-mediated resistance to all representative substrates, suggesting that these residues are not essential for the formation of structures requisite to the QacA substrate transport mechanism.

Does microbial resistance to biocides create a hazard to food hygiene?

Numerous reports are available on microbial resistance to antibiotics as well as to biocides. Instances of cross-resistance between these substance groups have been reported. Resistance, which is a genetically determined phenomenon, has to be distinguished from phenotypic adaptation processes, which are not hereditary. Adaptation can be avoided by rigorous cleaning and disinfection, avoiding concentrations of disinfectants below the microbicidal concentration. Resistance phenomena have to be divided into intrinsic and acquired resistance. Intrinsic resistance is the naturally greater resistance of certain microbial species compared to others. The term acquired resistance is used if certain strains of a microbial species differ significantly in their susceptibility to biocides compared to the average of this species. An overview of existing reports of resistance to different biocidal substances is given. In most of these reports, resistance is defined as an elevated minimum inhibitory concentration. The relevance of these data for disinfection processes, where microbicidal concentrations are applied, is discussed. Rotational use of different types of disinfectants, to avoid development of resistance, has been discussed controversially. Because of the unspecific mechanism of action of biocides, and the lack of scientific evidence for its need, rotational use of disinfectants is not recommended. In conclusion the risk of hazards in food production and processing caused by resistance to biocides is regarded as low.

Guidelines for the prophylaxis and treatment of methicillin-resistant Staphylococcus aureus (MRSA) infections in the UK

These evidence-based guidelines have been produced after a literature review of the treatment and prophylaxis of methicillin-resistant Staphylococcus aureus (MRSA) infection. The guidelines were further informed by antibiotic susceptibility data on MRSA from the UK. Recommendations are given for the treatment of common infections caused by MRSA, elimination of MRSA from carriage sites and prophylaxis of surgical site infection. There are several antibiotics currently available that are suitable for use in the management of this problem and potentially useful new agents are continuing to emerge.

The TetR family of transcriptional repressors

We have developed a general profile for the proteins of the TetR family of repressors. The stretch that best defines the profile of this family is made up of 47 amino acid residues that correspond to the helix-turn-helix DNA binding motif and adjacent regions in the three-dimensional structures of TetR, QacR, CprB, and EthR, four family members for which the function and three-dimensional structure are known. We have detected a set of 2,353 nonredundant proteins belonging to this family by screening genome and protein databases with the TetR profile. Proteins of the TetR family have been found in 115 genera of gram-positive, alpha-, beta-, and gamma-proteobacteria, cyanobacteria, and archaea. The set of genes they regulate is known for 85 out of the 2,353 members of the family. These proteins are involved in the transcriptional control of multidrug efflux pumps, pathways for the biosynthesis of antibiotics, response to osmotic stress and toxic chemicals, control of catabolic pathways, differentiation processes, and pathogenicity. The regulatory network in which the family member is involved can be simple, as in TetR (i.e., TetR bound to the target operator represses tetA transcription and is released in the presence of tetracycline), or more complex, involving a series of regulatory cascades in which either the expression of the TetR family member is modulated by another regulator or the TetR family member triggers a cell response to react to environmental insults. Based on what has been learned from the cocrystals of TetR and QacR with their target operators and from their three-dimensional structures in the absence and in the presence of ligands, and based on multialignment analyses of the conserved stretch of 47 amino acids in the 2,353 TetR family members, two groups of residues have been identified. One group includes highly conserved positions involved in the proper orientation of the helix-turn-helix motif and hence seems to play a structural role. The other set of less conserved residues are involved in establishing contacts with the phosphate backbone and target bases in the operator. Information related to the TetR family of regulators has been updated in a database that can be accessed at www.bactregulators.org.

Susceptibilities to antiseptic agents and distribution of antiseptic-resistance genes qacA/B and smr of methicillin-resistant Staphylococcus aureus isolated in Asia during 1998 and 1999

Many antiseptic agents are used in hygienic handwashes in the prevention of nosocomial infections by methicillin-resistant Staphylococcus aureus (MRSA). The plasmid-borne genes qacA/B and smr confer resistance to cationic antiseptic agents in S. aureus. In this study, the susceptibilities for dyes and antiseptic agents (e.g. acriflavine, acrinol, benzalkonium chloride, benzethonium chloride, chlorhexidine digluconate and alkyldiaminoethylglycine hydrochloride) of 894 isolates of MRSA collected from 11 Asian countries (South Korea, China, the Philippines, Singapore, Vietnam, Thailand, Indonesia, India, Sri Lanka, Saudi Arabia and Japan) between 1998 and 1999 were examined. In addition, the distributions of the antiseptic-resistance genes qacA/B and smr were studied by PCR. Among the Asian MRSA isolates 57.7% (516/894) were acriflavine-resistant. The smr gene was detected in 31.6% (12/38) of MRSA isolates from India but only in 1.9% (16/856) of all the isolates from other Asian countries. MRSA with qacA/B comprised 41.6% (372/894) of the isolates across Asia. In addition, PFGE was performed to type the MRSA and grouped the tested 30 MRSA isolates with qacA/B into 21 PFGE types. The results indicated that qacA/B is functionally the most important gene mediating antiseptic resistance in the MRSA strains of Asia and that a specific MRSA with qacA/B was not prevalent in Asia but qacA/B were widely spread among MRSA of Asia, while the geographical distribution of smr is more limited.

Bacterial resistance to silver in wound care

Ionic silver exhibits antimicrobial activity against a broad range of micro-organisms. As a consequence, silver is included in many commercially available healthcare products. The use of silver is increasing rapidly in the field of wound care, and a wide variety of silver-containing dressings are now commonplace (e.g. Hydrofiber dressing, polyurethane foams and gauzes). However, concerns associated with the overuse of silver and the consequent emergence of bacterial resistance are being raised. The current understanding of the biochemical and molecular basis behind silver resistance has been documented since 1998. Despite the sporadic evidence of bacterial resistance to silver, there have been very few studies undertaken and documented to ascertain its prevalence. The risks of antibacterial resistance developing from the use of biocides may well have been overstated. It is proposed that hygiene should be emphasized and targeted towards those applications that have demonstrable benefits in wound care. It is the purpose of this review to assess the likelihood of widespread resistance to silver and the potential for silver to induce cross-resistance to antibiotics, in light of its increasing usage within the healthcare setting.

Interactions of the QacR multidrug-binding protein with structurally diverse ligands: implications for the evolution of the binding pocket

The QacR multidrug-binding repressor protein regulates the expression of the Staphylococcus aureus qacA gene, a multidrug resistance (MDR) locus that is prevalent in clinical isolates of this important human pathogen. In this paper we demonstrate that the range of structurally diverse compounds capable of inducing qacA transcription is significantly more varied than previously appreciated, particularly in relation to bivalent cations. For all of the newly identified inducing compounds, induction of qacAexpression was correlated with a matching ability to dissociate QacR from operator DNA. Development of a ligand-binding assay based on intrinsic tryptophan fluorescence permitted dissociation constants to be determined for the majority of known QacR ligands, with values ranging from 0.2 to 82 microM. High-affinity binding of a compound to QacR in vitro was not found to correlate very strongly with either its in vivo inducing abilities or its structure. The latter observation indicated that the QacR ligand-binding pocket appears to have evolved to accommodate a wide range of toxic hydrophobic cations, rather than a specific class of compound. Importantly, the antimicrobial ligands of QacR included several plant alkaloids that share structural similarities with synthetic MDR substrates. This is consistent with the suggestion that the qacA-qacR MDR locus was recently derived from genes that protect against natural antimicrobial compounds.

Analysis on distribution and genomic diversity of high-level antiseptic resistance genes qacA and qacB in human clinical isolates of Staphylococcus aureus

High-level antiseptic resistance of Staphylococcus aureus is mediated by multidrug efflux pumps encoded by qacA and qacB genes. We investigated distribution and genomic diversity of these antiseptic resistance genes in a total of 522 clinical strains of S. aureus isolated recently in a Japanese hospital. The qacA/B gene was detected in 32.6% of methicillin-resistant S. aureus (MRSA) and 7.5% of methicillin-susceptible S. aureus (MSSA), whereas the low-level resistance gene smr, which was examined simultaneously, was detected at lower frequencies in both MRSA (3.3%) and MSSA (5.9%). Epidemiologic typing of S. aureus isolates suggested that higher prevalence of qacA/B in MRSA may be due to spread of a single predominant MRSA strain carrying qacA/B in the hospital. Restriction fragment length polymorphism (RFLP) analysis indicated higher prevalence of the qacB-type gene (59.3%) than the qacA-type gene (40.7%) among the qacA/B genes detected. Nucleotide sequencing analysis revealed the presence of two genetic variants in qacA (V1 and V2) and four variants in qacB (V1-V4) that differ from the qacA prototype in pSK1 by 1-5 nucleotides and 7-9 nucleotides, respectively. Although most strains with qacA-V1, qacA-V2, qacB-V3, and qacB-V4 showed high-level resistance to ethidium bromide (EB)(MIC > 100 microg/ml), all of the S. aureus isolates carrying qacB-V1 and qacB-V2 showed lower MICs of EB and some monovalent cationic antiseptic substances. By analysis of the genomic organization of the qacA/B downstream region, divergent forms of this region rearranged with an insertion of IS256 or IS257 were found primarily for qacB. The downstream region of qacA-V1 was suggested to be an evolutionary origin for other divergent forms. These findings indicated that both qacA and qacB are prevalent in recent clinical isolates, especially in MRSA, and these genes consist of variable genetic variants that may be responsible for different resistance levels against antiseptic substances.

Potential impact of increased use of biocides in consumer products on prevalence of antibiotic resistance

There has recently been much controversy surrounding the increased use of antibacterial substances in a wide range of consumer products and the possibility that, as with antibiotics, indiscriminate use of biocides might contribute to the overall pattern of susceptibility in the general environment and in the clinic. Such speculation, based on the isolation of resistant mutants from in vitro monoculture experiments, is not reflected by an emergence of biocide-resistant strains in vivo. This review provides a broad coverage of the biocide and resistance literature and evaluates the potential risks, perceived from such laboratory monoculture experiments, against evidence gathered over 50 years of field studies. An explanation for the continued effectiveness of broad-spectrum biocidal agents against the decline in efficacy of therapeutic agents is provided based on the fitness costs of resistance and the ubiquity of naturally occurring substances that possess antibacterial effect. While we conclude from this review of the literature that the incorporation of antibacterial agents into a widening sphere of personal products has had little or no impact on the patterns of microbial susceptibility observed in the environment, the associated risks remain finite. The use of such products should therefore be associated with a clear demonstration of added value either to consumer health or to the product life. Hygienic products should therefore be targeted to applications for which the risks have been established.

Regulation of bacterial drug export systems

The active transport of toxic compounds by membrane-bound efflux proteins is becoming an increasingly frequent mechanism by which cells exhibit resistance to therapeutic drugs. This review examines the regulation of bacterial drug efflux systems, which occurs primarily at the level of transcription. Investigations into these regulatory networks have yielded a substantial volume of information that has either not been forthcoming from or complements that obtained by analysis of the transport proteins themselves. Several local regulatory proteins, including the activator BmrR from Bacillus subtilis and the repressors QacR from Staphylococcus aureus and TetR and EmrR from Escherichia coli, have been shown to mediate increases in the expression of drug efflux genes by directly sensing the presence of the toxic substrates exported by their cognate pump. This ability to bind transporter substrates has permitted detailed structural information to be gathered on protein-antimicrobial agent-ligand interactions. In addition, bacterial multidrug efflux determinants are frequently controlled at a global level and may belong to stress response regulons such as E. coli mar, expression of which is controlled by the MarA and MarR proteins. However, many regulatory systems are ill-adapted for detecting the presence of toxic pump substrates and instead are likely to respond to alternative signals related to unidentified physiological roles of the transporter. Hence, in a number of important pathogens, regulatory mutations that result in drug transporter overexpression and concomitant elevated antimicrobial resistance are often observed.

Disinfectant and antibiotic resistance of lactic acid bacteria isolated from the food industry

Quaternary ammonium compounds (QACs) are widely used as disinfectant in medical and food environments. There is a growing concern about the increasing incidence of disinfectant-resistant microorganisms from food. Disinfectant-resistant lactic acid bacteria (LAB) may survive disinfection and cause spoilage problems. Moreover, resistant LAB may potentially act as a reservoir for resistance genes. A total number of 320 LAB from food industry and meat were screened for resistance to the QAC benzalkonium chloride (BC). Out of 320 strains, five strains (1.5%) were considered to be resistant and 56 (17.5%) were tolerant to BC. The resistant strains were isolated from food processing equipment after disinfection. The resistant, tolerant, and some sensitive control bacteria were examined for susceptibility to 18 different antibiotics, disinfectants, and dyes using disc agar diffusion test and microdilution method. Little systematic cross-resistance between BC and any of the antimicrobial agents tested were detected except for gentamycin and chlorhexidine. A BC-tolerant strain was much easier to adapt to higher levels of BC as compared to a BC-sensitive strain. No known gram-positive QAC resistance genes (qacA/B, qacC, qacG, and qacH) were detected in the BC-resistant strains. Identification to species level of the BC-resistant isolates was carried out by comparative analysis of 16S-rDNA sequencing. In conclusion, resistance to BC is not frequent in LAB isolated from food and food environments. Resistance may occur after exposure to BC. The BC resistant isolates showed no cross-resistance with other antimicrobial compounds, except for gentamycin and chlorhexidine. Nevertheless, BC-resistant LAB may be isolated after disinfection and may contribute to the dissemination of resistance.

Biocides, drug resistance and microbial evolution

Antimicrobial biocides are widely used in critical human health situations in which rigorous infection control is needed. Increasingly, biocidal agents are being marketed for home use, although there is little evidence that they significantly improve home hygiene. Biocide resistance mechanisms share many themes with antibiotic resistance mechanisms.

In vitro resistance of Staphylococcus aureus to thrombin-induced platelet microbicidal protein is associated with alterations in cytoplasmic membrane fluidity

Platelet microbicidal proteins (PMPs) are small, cationic peptides which possess potent microbicidal activities against common bloodstream pathogens, such as Staphylococcus aureus. We previously showed that S. aureus strains exhibiting resistance to thrombin-induced PMP (tPMP-1) in vitro have an enhanced capacity to cause human and experimental endocarditis (T. Wu, M. R. Yeaman, and A. S. Bayer, Antimicrob. Agents Chemother. 38:729-732, 1994; A. S. Bayer et al., Antimicrob. Agents Chemother. 42:3169-3172, 1998; V. K. Dhawan et al., Infect. Immun. 65:3293-3299, 1997). However, the mechanisms mediating tPMP-1 resistance in S. aureus are not fully delineated. The S. aureus cell membrane appears to be a principal target for the action of tPMP-1. To gain insight into the basis of tPMP-1 resistance, we compared several parameters of membrane structure and function in three tPMP-1-resistant (tPMP-1(r)) strains and their genetically related, tPMP-1-susceptible (tPMP-1(s)) counterpart strains. The tPMP-1(r) strains were derived by three distinct methods: transposon mutagenesis, serial passage in the presence of tPMP-1 in vitro, or carriage of a naturally occurring multiresistance plasmid (pSK1). All tPMP-1(r) strains were found to possess elevated levels of longer-chain, unsaturated membrane lipids, in comparison to their tPMP-1(s) counterparts. This was reflected in corresponding differences in cell membrane fluidity in the strain pairs, with tPMP-1(r) strains exhibiting significantly higher degrees of fluidity as assessed by fluorescence polarization. These data provide further support for the concept that specific alterations in the cytoplasmic membrane of S. aureus strains are associated with tPMP-1 resistance in vitro.

Antibiotic and biocide resistance in methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococcus

Concern has been growing regarding the potential of antibiotic and disinfectant co-resistance in clinically important bacteria. In this study, the susceptibilities of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive Staphylococcus aureus (MSSA) to chlorhexidine (CHX), the quaternary ammonium compounds cetylpyridinium chloride (CPC) and benzalkonium chloride (BC), triclosan, dibromopropamidine isethionate (DBPI) and triclocarban were compared. MRSA exhibited low-level resistance to CHX and the QACs, with MICs of 1.5 to 3-fold (CHX), and 2 to 4-fold (QACs) higher than MSSA. However, the MIC values for MRSA ranged between 0.025 (the MIC of MSSA) and 1 microg/mL with triclosan, and between <5 (the MIC of MSSA) and 75 microg/mL with DPBI. Nevertheless, these strains remain relatively sensitive to most of these antimicrobial agents. The bactericidal efficacy of CHX, CPC and DBPI (with the exception of one strain) correlated with their MIC value. This was not observed using triclosan; MRSA and MSSA strains were equally susceptible to its killing effect, regardless of MIC. The permeabilizing agent, ethylenediamine tetraacetic acid (EDTA) was unable to potentiate the antibacterial activities of the biocides against any of the strains tested. Attempts to select for staphylococcal strains with increased resistance to triclosan, CPC or CHX, using disc diffusion, step-wise broth, or repeated exposure/recovery technique, were only partially successful, and resistance was found to be unstable. The susceptibilities of vancomycin-resistant enterococcus (VRE) and vancomycin-sensitive enterococcus (VSE) to the biocides were also compared and found to be similar both in terms of MIC testing and time-kill studies.

Plasmid-mediated resistance to thrombin-induced platelet microbicidal protein in staphylococci: role of the qacA locus

Thrombin-induced platelet microbicidal protein 1 (tPMP-1) is a small, cationic peptide released from rabbit platelets following thrombin stimulation. In vitro resistance to this peptide among strains of Staphylococcus aureus correlates with the survival advantage of such strains at sites of endothelial damage in humans as well as in experimental endovascular infections. The mechanisms involved in the phenotypic resistance of S. aureus to tPMP-1 are not fully delineated. The plasmid-encoded staphylococcal gene qacA mediates multidrug resistance to multiple organic cations via a proton motive force-dependent efflux pump. We studied whether the qacA gene might also confer resistance to cationic tPMP-1. Staphylococcal plasmids encoding qacA were found to confer resistance to tPMP-1 in an otherwise susceptible parental strain. Deletions which removed the region containing the qacA gene in the S. aureus multiresistance plasmid pSK1 abolished tPMP-1 resistance. Resistance to tPMP-1 in the qacA-bearing strains was inoculum independent but peptide concentration dependent, with the level of resistance decreasing at higher peptide concentrations for a given inoculum. There was no apparent cross-resistance in qacA-bearing strains to other endogenous cationic antimicrobial peptides which are structurally distinct from tPMP-1, including human neutrophil defensin 1, protamine, or the staphylococcal lantibiotics pep5 and nisin. These data demonstrate that the staphylococcal multidrug resistance gene qacA also mediates in vitro resistance to cationic tPMP-1.

Bacterial resistance and topical antimicrobial wash products

Current scientific evidence has not shown that a link exists between the use of topical antimicrobial formulations and antiseptic or antibiotic resistance. As a result of the extensive history and varied use of antiseptic products and ingredients, any selective pressure for antibiotic resistance that may be occurring or may be uncovered in the future because of antiseptic use would be expected to be insignificant compared with the selective pressure because of antibiotic use. This review illustrates the effectiveness of topical antimicrobial wash products against antibiotic-resistant and antiseptic-resistant bacteria in use settings as well as the studies performed (antiseptic, deodorant, and oral care) demonstrating the lack of development of resistance in long-term clinical studies. Although these studies illustrate that the use of topical antimicrobial products have not been shown to play a role in the fluctuations of the specific composition or resistance of the skin flora, changes in skin flora have been shown to occur. Based on current knowledge, the benefit from use of topical antimicrobial wash products in combination with standard infection control and personal hygiene practices far outweighs the risk of increased antibiotic resistance.

Identification and characterization of quaternary ammonium compound resistant staphylococci from the food industry

The distribution of known genes conferring resistance to quaternary ammonium compounds (QACs) among different species of staphylococci isolated from the food industry was investigated. Twenty-four isolates hosting one of the genes qacA/qacB, smr, qacG or qacH, were subjected to species identification. Species determination was performed by biochemical analyses (API STAPH), comparative 16S rDNA sequence analysis and tDNA intergenic spacer length polymorphism analysis. Good correlation was obtained between the different methods. The isolates belonged to six different species of coagulase-negative Staphylococcus. The most commonly found species were Staphylococcus epidermidis and Staphylococcus saprophyticus. The results also indicated the possible spread of specific isolates of staphylococci which may reflect the dominance of certain strains in environments were QACs are used on a regular basis. The isolates were further characterized by the resistance phenotype to antimicrobial agents including antibiotics and disinfectants. Resistance to ampicillin, penicillin G and dyes was prevalent in strains harbouring the qacA or qacB genes, features also common among clinical staphylococci containing qacA/qacB. One QAC resistant strain harbouring the smr gene showed resistance to ampicillin, penicillin, tetracycline, erythromycin and trimethoprim. No enterotoxin production was detected among the QAC resistant strains.

Antiseptics and disinfectants: activity, action, and resistance

Antiseptics and disinfectants are extensively used in hospitals and other health care settings for a variety of topical and hard-surface applications. A wide variety of active chemical agents (biocides) are found in these products, many of which have been used for hundreds of years, including alcohols, phenols, iodine, and chlorine. Most of these active agents demonstrate broad-spectrum antimicrobial activity; however, little is known about the mode of action of these agents in comparison to antibiotics. This review considers what is known about the mode of action and spectrum of activity of antiseptics and disinfectants. The widespread use of these products has prompted some speculation on the development of microbial resistance, in particular whether antibiotic resistance is induced by antiseptics or disinfectants. Known mechanisms of microbial resistance (both intrinsic and acquired) to biocides are reviewed, with emphasis on the clinical implications of these reports.

Complete nucleotide sequence of pSK41: evolution of staphylococcal conjugative multiresistance plasmids

The 46.4-kb nucleotide sequence of pSK41, a prototypical multiresistance plasmid from Staphylococcus aureus, has been determined, representing the first completely sequenced conjugative plasmid from a gram-positive organism. Analysis of the sequence has enabled the identification of the probable replication, maintenance, and transfer functions of the plasmid and has provided insights into the evolution of a clinically significant group of plasmids. The basis of deletions commonly associated with pSK41 family plasmids has been investigated, as has the observed insertion site specificity of Tn552-like beta-lactamase transposons within them. Several of the resistance determinants carried by pSK41-like plasmids were found to be located on up to four smaller cointegrated plasmids. pSK41 and related plasmids appear to represent a consolidation of antimicrobial resistance functions, collected by a preexisting conjugative plasmid via transposon insertion and IS257-mediated cointegrative capture of other plasmids.