The isoleucyl-tRNA synthetase mutation V588F conferring mupirocin resistance in glycopeptide-intermediate Staphylococcus aureus is not associated with a significant fitness burden

Exploring mechanisms of mupirocin resistance and hyper-resistance

Mupirocin is a broad-spectrum antibiotic that acts predominantly against Gram-positive bacteria. It is produced by Pseudomonas fluorescens NCIMB 10586 and has been clinically used to treat primary and secondary skin infections and to eradicate nasal colonisation of methicillin-resistant Staphylococcus aureus strains. Mupirocin inhibits protein synthesis by blocking the active site of isoleucyl-tRNA synthetase (IleRS), which prevents the enzyme from binding isoleucine and ATP for Ile-tRNAIle synthesis. Two types of IleRS are found in bacteria - while IleRS1 is susceptible to mupirocin inhibition, IleRS2 provides resistance to cells. These two types belong to distinct evolutionary clades which likely emerged from an early gene duplication in bacteria. Resistance in IleRS2 is based on the loss of interactions that govern mupirocin binding to IleRS1, such as hydrogen bonding to the carboxylate moiety of mupirocin. IleRS2 enzymes with Ki in the millimolar range have recently been discovered. These hyper-resistant IleRS2 variants surprisingly have a non-canonical version of the catalytic motif, which serves as a signature motif of class I aminoacyl-tRNA synthetases to which IleRS belongs. The non-canonical motif, in which the 1st and 3rd positions are swapped, is key for hyper-resistance and can be accommodated without abolishing enzyme activity in IleRS2 but not in IleRS1. Clinical use of mupirocin led to the emergence of resistance in S. aureus. Low-level resistance arises by mutations of the housekeeping IleRS1, while high-level resistance develops by the acquisition of the resistant IleRS2 on a plasmid. There is no evidence that hyper-resistant variants have been found in clinical isolates.

Molecular Mechanisms of Drug Resistance in Staphylococcus aureus

This paper discusses the mechanisms of S. aureus drug resistance including: (1) introduction. (2) resistance to beta-lactam antibiotics, with particular emphasis on the mec genes found in the Staphylococcaceae family, the structure and occurrence of SCCmec cassettes, as well as differences in the presence of some virulence genes and its expression in major epidemiological types and clones of HA-MRSA, CA-MRSA, and LA-MRSA strains. Other mechanisms of resistance to beta-lactam antibiotics will also be discussed, such as mutations in the gdpP gene, BORSA or MODSA phenotypes, as well as resistance to ceftobiprole and ceftaroline. (3) Resistance to glycopeptides (VRSA, VISA, hVISA strains, vancomycin tolerance). (4) Resistance to oxazolidinones (mutational and enzymatic resistance to linezolid). (5) Resistance to MLS-B (macrolides, lincosamides, ketolides, and streptogramin B). (6) Aminoglycosides and spectinomicin, including resistance genes, their regulation and localization (plasmids, transposons, class I integrons, SCCmec), and types and spectrum of enzymes that inactivate aminoglycosides. (7). Fluoroquinolones (8) Tetracyclines, including the mechanisms of active protection of the drug target site and active efflux of the drug from the bacterial cell. (9) Mupirocin. (10) Fusidic acid. (11) Daptomycin. (12) Resistance to other antibiotics and chemioterapeutics (e.g., streptogramins A, quinupristin/dalfopristin, chloramphenicol, rifampicin, fosfomycin, trimethoprim) (13) Molecular epidemiology of MRSA.

Whole Genome Sequencing of Methicillin-Resistant Staphylococcus epidermidis Clinical Isolates Reveals Variable Composite SCCmec ACME among Different STs in a Tertiary Care Hospital in Oman

Staphylococcus epidermidis has been recently recognized as an emerging nosocomial pathogen. There are concerns over the increasing virulence potential of this commensal due to the capabilities of transferring mobile genetic elements to Staphylococcus aureus through staphylococcal chromosomal cassette (SCCmec) and the closely related arginine catabolic mobile element (ACME) and the copper and mercury resistance island (COMER). The potential pathogenicity of S. epidermidis, particularly from blood stream infections, has been poorly investigated. In this study, 24 S. epidermidis isolated from blood stream infections from Oman were investigated using whole genome sequence analysis. Core genome phylogenetic trees revealed one third of the isolates belong to the multidrug resistance ST-2. Genomic analysis unraveled a common occurrence of SCCmec type IV and ACME element predominantly type I arranged in a composite island. The genetic composition of ACME was highly variable among isolates of same or different STs. The COMER-like island was absent in all of our isolates. Reduced copper susceptibility was observed among isolates of ST-2 and ACME type I, followed by ACME type V. In conclusion, in this work, we identify a prevalent occurrence of highly variable ACME elements in different hospital STs of S. epidermidis in Oman, thus strongly suggesting the hypothesis that ACME types evolved from closely related STs.

Fitness Cost of Antibiotic Resistance in Staphylococcus aureus: A Systematic Review

Background: Recent reports have shown the potential of Staphylococcus aureus for acquiring resistance to last-resort antibiotics. However, most antibiotic resistance mechanisms were associated with a fitness cost that was typically observed as a reduced bacterial growth rate. This systematic review aimed to address the fitness cost of antibiotic resistance in S. aureus that emerged by mutations. Methods: A systematic review was conducted after searching in two databases (PubMed and Scopus) using specific keywords. We included peer-reviewed articles published only in English. All studies describing the fitness cost associated with antibiotic resistance in S. aureus were selected. For each article, the results of fitness testing, minimum inhibition concentrations of mutants, the position of mutation, and the appearance of compensatory mutations were recorded. Results: At all, 35 articles were recorded in the final analysis examining the fitness cost associated with antibiotic resistance in S. aureus that conferred by mutations. Analysis of the data showed that 26 studies reported that the emergence of antibiotic resistance was frequently associated with a fitness cost. Conclusion: This review summarized that the antibiotic resistance selection caused in the majority of cases a substantial fitness cost. Further in vivo experiments revealed that these mutations affected bacterial virulence and the ability to establish a successful infection.

Evolution of drug-resistant and virulent small colonies in phenotypically diverse populations of the human fungal pathogen Candida glabrata

Antimicrobial resistance frequently carries a fitness cost to a pathogen, measured as a reduction in growth rate compared to the sensitive wild-type, in the absence of antibiotics. Existing empirical evidence points to the following relationship between cost of resistance and virulence. If a resistant pathogen suffers a fitness cost in terms of reduced growth rate it commonly has lower virulence compared to the sensitive wild-type. If this cost is absent so is the reduction in virulence. Here we show, using experimental evolution of drug resistance in the fungal human pathogen Candida glabrata, that reduced growth rate of resistant strains need not result in reduced virulence. Phenotypically heterogeneous populations were evolved in parallel containing highly resistant sub-population small colony variants (SCVs) alongside sensitive sub-populations. Despite their low growth rate in the absence of an antifungal drug, the SCVs did not suffer a marked alteration in virulence compared with the wild-type ancestral strain, or their co-isolated sensitive strains. This contrasts with classical theory that assumes growth rate to positively correlate with virulence. Our work thus highlights the complexity of the relationship between resistance, basic life-history traits and virulence.

Epistasis analysis uncovers hidden antibiotic resistance-associated fitness costs hampering the evolution of MRSA

Background

Fitness costs imposed on bacteria by antibiotic resistance mechanisms are believed to hamper their dissemination. The scale of these costs is highly variable. Some, including resistance of Staphylococcus aureus to the clinically important antibiotic mupirocin, have been reported as being cost-free, which suggests that there are few barriers preventing their global spread. However, this is not supported by surveillance data in healthy communities, which indicate that this resistance mechanism is relatively unsuccessful.

Results

Epistasis analysis on two collections of MRSA provides an explanation for this discord, where the mupirocin resistance-conferring mutation of the ileS gene appears to affect the levels of toxins produced by S. aureus when combined with specific polymorphisms at other loci. Proteomic analysis demonstrates that the activity of the secretory apparatus of the PSM family of toxins is affected by mupirocin resistance. As an energetically costly activity, this reduction in toxicity masks the fitness costs associated with this resistance mutation, a cost that becomes apparent when toxin production becomes necessary. This hidden fitness cost provides a likely explanation for why this mupirocin-resistance mechanism is not more prevalent, given the widespread use of this antibiotic.

Conclusions

With dwindling pools of antibiotics available for use, information on the fitness consequences of the acquisition of resistance may need to be considered when designing antibiotic prescribing policies. However, this study suggests there are levels of depth that we do not understand, and that holistic, surveillance and functional genomics approaches are required to gain this crucial information.

Electronic supplementary material

The online version of this article (10.1186/s13059-018-1469-2) contains supplementary material, which is available to authorized users.

Antimicrobial discovery from natural and unusual sources

OBJECTIVES

Whether vertebrates/invertebrates living in polluted environments are an additional source of antimicrobials.

KEY FINDINGS

Majority of antimicrobials have been discovered from prokaryotes and those which are of eukaryotic origin are derived mainly from fungal and plant sources. With this in mind, it is important to note that pests, such as cockroaches come across pathogenic bacteria routinely, yet thrive in polluted environments. Other animals, such as snakes thrive from feeding on germ-infested rodents. Logically, such species must have developed an approach to protect themselves from these pathogens, yet they have largely been ignored as a potential source of antimicrobials despite their remarkable capability to fight disease-causing organisms.

SUMMARY

Animals living in polluted environments are an underutilized source for potential antimicrobials, hence it is believed that several novel bioactive molecule(s) will be identified from these sources to counter increasingly resistant bacterial infections. Further research will be necessary in the development of novel antimicrobial(s) from these unusual sources which will have huge clinical impact worldwide.

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.

Targeting Multiple Aminoacyl-tRNA Synthetases Overcomes the Resistance Liabilities Associated with Antibacterial Inhibitors Acting on a Single Such Enzyme

Bacterial aminoacyl-tRNA synthetases (aaRSs) represent promising antibacterial drug targets. Unfortunately, the aaRS inhibitors that have to date reached clinical trials are subject to rapid resistance development through mutation, a phenomenon that limits their potential clinical utility. Here, we confirm the intuitively correct idea that simultaneous targeting of two different aaRS enzymes prevents the emergence of spontaneous bacterial resistance at high frequency, a finding that supports the development of multitargeted anti-aaRS therapies.

Plasmidic qnrA3 enhances Escherichia coli fitness in absence of antibiotic exposure

The widespread presence of plasmid-mediated quinolone resistance determinants, particularly qnr genes, has become a current issue. By protecting DNA-gyrase from quinolones, Qnr proteins confer a low level quinolone resistance that is not sufficient to explain their emergence. Since Qnr proteins were hypothesized to act as DNA-binding protein regulators, qnr genes could have emerged by providing a selective advantage other than antibiotic resistance. We investigated host fitness of Escherichia coli isogenic strains after acquisition of the qnrA3 gene, inserted either alone onto a small plasmid (pBR322), or harbored on a large conjugative native plasmid, pHe96(qnrA3) found in a clinical isolate. The isogenic strains were derived from the susceptible E. coli CFT073, a virulent B2 group strain known to infect bladder and kidneys in a mouse model of pyelonephritis. In vitro experiments included growth analysis by automatic spectrophotometry and flow cytometry, and competitions with CFU enumeration. In vivo experiments included infection with each strain and pairwise competitions in absence of antimicrobial exposure. As controls for our experiments we used mutations known to reduce fitness (rpsL K42N mutation) or to enhance fitness (tetA deletion in pBR322). E. coli CFT073 transformed with pBRAM(PBR322-qnrA3) had significantly higher maximal OD than E. coli CFT073 transformed with pBR322 or pBR322ΔtetA, and in vivo competitions were more often won by the qnrA3 carrying strain (24 victories vs. 9 loss among 42 competitions, p = 0.001). In contrast, when pHe96(qnrA3) was introduced by conjugation in E. coli CFT073, it exerted a fitness cost shown by an impaired growth observed in vitro and in vivo and a majority of lost competitions (33/35, p<0.0001). In conclusion, qnrA3 acquisition enhanced bacterial fitness, which may explain qnr emergence and suggests a regulation role of qnr. However, fitness was reduced when qnrA3 was inserted onto multidrug-resistant plasmids and this can slow down its dissemination without antibiotic exposure.

Activity of and development of resistance to corallopyronin A, an inhibitor of RNA polymerase

We explored the properties of corallopyronin A (CorA), a poorly characterized inhibitor of bacterial RNA polymerase (RNAP). It displayed a 50% inhibitory concentration of 0.73 μM against RNAP, compared with 11.5 nM for rifampin. The antibacterial activity of CorA was also inferior to rifampin, and resistant mutants of Staphylococcus aureus were easily selected. The mutations conferring resistance resided in the rpoB and rpoC subunits of RNAP. We conclude that CorA is not a promising antibacterial drug candidate.

Antibiotic resistance and its cost: is it possible to reverse resistance?

Most antibiotic resistance mechanisms are associated with a fitness cost that is typically observed as a reduced bacterial growth rate. The magnitude of this cost is the main biological parameter that influences the rate of development of resistance, the stability of the resistance and the rate at which the resistance might decrease if antibiotic use were reduced. These findings suggest that the fitness costs of resistance will allow susceptible bacteria to outcompete resistant bacteria if the selective pressure from antibiotics is reduced. Unfortunately, the available data suggest that the rate of reversibility will be slow at the community level. Here, we review the factors that influence the fitness costs of antibiotic resistance, the ways by which bacteria can reduce these costs and the possibility of exploiting them.

Evolution of MRSA during hospital transmission and intercontinental spread

Current methods for differentiating isolates of predominant lineages of pathogenic bacteria often do not provide sufficient resolution to define precise relationships. Here, we describe a high-throughput genomics approach that provides a high-resolution view of the epidemiology and microevolution of a dominant strain of methicillin-resistant Staphylococcus aureus (MRSA). This approach reveals the global geographic structure within the lineage, its intercontinental transmission through four decades, and the potential to trace person-to-person transmission within a hospital environment. The ability to interrogate and resolve bacterial populations is applicable to a range of infectious diseases, as well as microbial ecology.

Genome sequence of a recently emerged, highly transmissible, multi-antibiotic- and antiseptic-resistant variant of methicillin-resistant Staphylococcus aureus, sequence type 239 (TW)

The 3.1-Mb genome of an outbreak methicillin-resistant Staphylococcus aureus (MRSA) strain (TW20) contains evidence of recently acquired DNA, including two large regions (635 kb and 127 kb). The strain is resistant to a wide range of antibiotics, antiseptics, and heavy metals due to resistance genes encoded on mobile genetic elements and also mutations in housekeeping genes.

[The cost of antibiotic resistance: analysis and consequences]

Antimicrobial resistance, either by mutation or acquisition of resistance determinants harbored by mobile genetic elements, may confer a biological cost for the bacteria. This biological cost can be evaluated by comparing the resistant mutant to the wild susceptible strain, in the absence of antibiotic selection. This fitness cost can affect the growth rate in vitro or the survival in the host or in the environment or the virulence capacity. Various studies have evidenced this cost, either in vitro or in vivo, in different analysis models. However, bacteria can evolve and adapt to reduce this cost, by compensatory mutations or fine regulation of resistance expression. This compensatory evolution allows resistant bacteria to persist even in the absence of antibiotic selection pressure.

Acquisition of high-level mupirocin resistance and its fitness cost among methicillin-resistant Staphylococcus aureus strains with low-level mupirocin resistance

We investigated whether methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates with low-level mupirocin resistance can serve as recipients of a pSK41-like plasmid conferring high-level mupirocin resistance without substantial fitness cost. Our results suggest that acquisition of the plasmid conferring high-level mupirocin resistance was not necessarily associated with fitness cost in some MRSA recipients with low-level mupirocin resistance.

Emergence and maintenance of resistance to fluoroquinolones and coumarins in Staphylococcus aureus: predictions from in vitro studies

OBJECTIVES

Fluoroquinolones and coumarins interfere with the activity of bacterial type II topoisomerase enzymes. We examined the development of resistance to these agents in Staphylococcus aureus and determined the effect of simultaneous topoisomerase IV and DNA gyrase mutations on the biological fitness of the organism. This work aimed to gain insight into how such mutants might arise and survive in the clinical environment.

METHODS

Spontaneous mutants resistant to fluoroquinolones and coumarins were selected in S. aureus. Resistance mutations were identified by DNA sequencing of PCR amplicons corresponding to the genes encoding topoisomerase IV and DNA gyrase. In vitro fitness of resistant mutants was compared with the antibiotic-susceptible progenitor strain using pair-wise competition assays.

RESULTS

Mutants simultaneously resistant to both a fluoroquinolone and either of the coumarins, novobiocin or coumermycin A1, could not be recovered following a single-step selection. However, mutants concurrently resistant to both classes of antimicrobial could be generated by step-wise selections. These mutants demonstrated reductions in competitive fitness of up to 36%.

CONCLUSIONS

Dual-targeting of topoisomerase IV and DNA gyrase enzymes, for example with the combination of a fluoroquinolone and a coumarin agent, could minimize the emergence of resistance to these drugs in S. aureus. However, resistance-associated fitness costs may not be sufficient to limit the survival of mutants with dual resistance, if they arose in the clinical setting.

Aminoacyl-tRNA synthetases: essential and still promising targets for new anti-infective agents

The emergence of resistance to existing antibiotics demands the development of novel antimicrobial agents directed against novel targets. Historically, bacterial cell wall synthesis, protein, and DNA and RNA synthesis have been major targets of very successful classes of antibiotics such as beta-lactams, glycopeptides, macrolides, aminoglycosides, tetracyclines, rifampicins and quinolones. Recently, efforts have been made to develop novel agents against validated targets in these pathways but also against new, previously unexploited targets. The era of genomics has provided insights into novel targets in microbial pathogens. Among the less exploited--but still promising--targets is the family of 20 aminoacyl-tRNA synthetases (aaRSs), which are essential for protein synthesis. These targets have been validated in nature as aaRS inhibition has been shown as the specific mode of action for many natural antimicrobial agents synthesized by bacteria and fungi. Therefore, aaRSs have the potential to be targeted by novel agents either from synthetic or natural sources to yield specific and selective anti-infectives. Numerous high-throughput screening programs aimed at identifying aaRS inhibitors have been performed over the last 20 years. A large number of promising lead compounds have been identified but only a few agents have moved forward into clinical development. This review provides an update on the present strategies to develop novel aaRS inhibitors as anti-infective drugs.

Genetic basis of resistance to fusidic acid in staphylococci

Resistance to fusidic acid in Staphylococcus aureus often results from acquisition of the fusB determinant or from mutations in the gene (fusA) that encodes the drug target (elongation factor G). We now report further studies on the genetic basis of resistance to this antibiotic in the staphylococci. Two staphylococcal genes that encode proteins exhibiting ca. 45% identity with FusB conferred resistance to fusidic acid in S. aureus. One of these genes (designated fusC) was subsequently detected in all fusidic acid-resistant clinical strains of S. aureus tested that did not carry fusB or mutations in fusA, and in strains of S. intermedius. The other gene (designated fusD) is carried by S. saprophyticus, explaining the inherent resistance of this species to fusidic acid. Fusidic acid-resistant strains of S. lugdunensis harbored fusB. Thus, resistance to fusidic acid in clinical isolates of S. aureus and other staphylococcal species frequently results from expression of FusB-type proteins.

The biological cost of mutational antibiotic resistance: any practical conclusions?

A key parameter influencing the rate and trajectory of the evolution of antibiotic resistance is the fitness cost of resistance. Recent studies have demonstrated that antibiotic resistance, whether caused by target alteration or by other mechanisms, generally confers a reduction in fitness expressed as reduced growth, virulence or transmission. These findings imply that resistance might be reversible, provided antibiotic use is reduced. However, several processes act to stabilize resistance, including compensatory evolution where the fitness cost is ameliorated by additional mutation without loss of resistance, the rare occurrence of cost-free resistance mechanisms and genetic linkage or co-selection between the resistance markers and other selected markers. Conceivably we can use this knowledge to rationally choose and design targets and drugs where the costs of resistance are the highest, and where the likelihood of compensation is the lowest.

Molecular genetic and structural modeling studies of Staphylococcus aureus RNA polymerase and the fitness of rifampin resistance genotypes in relation to clinical prevalence

The adaptive and further evolutionary responses of Staphylococcus aureus to selection pressure with the antibiotic rifampin have not been explored in detail. We now present a detailed analysis of these systems. The use of rifampin for the chemotherapy of infections caused by S. aureus has resulted in the selection of mutants with alterations within the beta subunit of the target enzyme, RNA polymerase. Using a new collection of strains, we have identified numerous novel mutations in the beta subunits of both clinical and in vitro-derived resistant strains and established that additional, undefined mechanisms contribute to expression of rifampin resistance in clinical isolates of S. aureus. The fitness costs associated with rifampin resistance genotypes were found to have a significant influence on their clinical prevalence, with the most common clinical genotype (H481N, S529L) exhibiting no fitness cost in vitro. Intragenic mutations which compensate for the fitness costs associated with rifampin resistance in clinical strains of S. aureus were identified for the first time. Structural explanations for rifampin resistance and the loss of fitness were obtained by molecular modeling of mutated RNA polymerase enzymes.

In vivo transfer of high-level mupirocin resistance from Staphylococcus epidermidis to methicillin-resistant Staphylococcus aureus associated with failure of mupirocin prophylaxis

OBJECTIVES

We examined the molecular basis of the emergence of mupirocin resistance in a methicillin-resistant Staphylococcus aureus (MRSA) strain colonizing a nursing home resident undergoing mupirocin prophylaxis.

PATIENT AND METHODS

A persistent carrier of mupirocin-susceptible MRSA participated in a trial of mupirocin for nasal decolonization among nursing home residents. During prophylaxis a high-level mupirocin-resistant MRSA emerged in the nasal isolates from this patient. S. aureus and coagulase-negative staphylococci were isolated prior to, during and after 14 days of mupirocin treatment. The staphylococcal isolates and their plasmids were examined by molecular genetic methods.

RESULTS

All mupirocin-susceptible and -resistant MRSA isolates possessed the same genotype. The patient was also colonized by a single mupirocin-resistant Staphylococcus epidermidis strain. The mupirocin-resistant MRSA and S. epidermidis strains harboured identical plasmids that carried the mupA determinant and genes for conjugative DNA transfer in staphylococci. These plasmids could be transferred in vitro from both clinical isolates to S. aureus RN2677.

CONCLUSIONS

The MRSA strain contained a conjugative plasmid expressing mupA that was identical with that found in the S. epidermidis strain which colonized the patient. These findings suggest that transfer of mupA from S. epidermidis to MRSA probably occurred during mupirocin prophylaxis.

In vitro and in vivo evaluations of the activities of lauric acid monoester formulations against Staphylococcus aureus

Due to increasing mupirocin resistance, alternatives for Staphylococcus aureus nasal decolonization are needed. Lauric acid monoesters combined with lactic, mandelic, malic, or benzoic acid are being evaluated as possible alternatives. We determined the in vitro activity of 13 lauric acid monoester (LAM) formulations and mupirocin against 30 methicillin-susceptible S. aureus (MSSA) isolates and 30 methicillin-resistant S. aureus (MRSA) isolates. We then used a murine model of MRSA nasopharyngeal colonization to compare the in vivo activity of mupirocin with three LAM formulations. MSSA and MRSA MIC(90) values were 0.25 microg/ml for mupirocin and </=4 microl/ml for all LAM formulations tested. Hsd:ICR mice were challenged with 10(8) CFU/naris MRSA. Five days later, S. aureus colonization was documented by culture. Treatment with bland, mupirocin, or one of three LAM ointments was then administered unblinded thrice daily for 2 days. Three days after treatment, both anterior nares were cultured for S. aureus. Administration of 128774-49E or 128774-53A was associated with greater eradication of MRSA carriage (24/34 [71%] or 33/40 [83%]) of animals, respectively) than bland ointment (12/38 [32%]) (P < 0.005). 128774-53A administration resulted in greater MRSA carriage eradication than mupirocin (19/38 [50%]) (P < 0.005) in this model. LAM formulations warrant evaluation for S. aureus nasal decolonization in humans.

Analysis of mupirocin resistance and fitness in Staphylococcus aureus by molecular genetic and structural modeling techniques

Chromosomal resistance to mupirocin in clinical isolates of Staphylococcus aureus arises from V(588)F or V(631)F mutations in isoleucyl-tRNA synthetase (IRS). Whether these are the only IRS mutations that confer mupirocin resistance or simply those that survive in the clinic is unknown. Mupirocin-resistant mutants of S. aureus 8325-4 were therefore generated to examine their ileS genotypes and the in vitro and in vivo fitness costs associated with them before and after compensatory evolution. Most spontaneous first-step mupirocin-resistant mutants carried V(588)F or V(631)F mutations in IRS, but a new mutation (G(593)V) was also identified. Second-step mutants carried combinations of previously identified IRS mutations (e.g., V(588)F/V(631)F and G(593)V/V(631)F), but additional combinations also occurred involving novel mutations (R(816)C, H(67)Q, and F(563)L). First-step mupirocin-resistant mutants were not associated with substantial fitness costs, a finding that is consistent with the occurrence of V(588)F or V(631)F mutations in the IRS of clinical strains. Second-step mutants were unfit, but fitness could be restored by subculture in the absence of mupirocin. In most cases, this was the result of compensatory mutations that also suppressed mupirocin resistance (e.g., A(196)V, E(190)K, and E(195)K), despite retention of the original mutations conferring resistance. Structural explanations for mupirocin resistance and loss of fitness were obtained by molecular modeling of mutated IRS enzymes, which provided data on mupirocin binding and interaction with the isoleucyl-AMP reactive intermediate.