Modulation of the Drug Resistance by Platonia insignis Mart. Extract, Ethyl Acetate Fraction and Morelloflavone/Volkensiflavone (Biflavonoids) in Staphylococcus aureus Strains Overexpressing Efflux Pump Genes

BACKGROUND

Microbial resistance to antibiotics is a global public health problem, which requires urgent attention. Platonia insignis is a native species from the eastern Brazilian Amazon, used in the treatment of burns and wounds.

OBJECTIVES

To evaluate the antimicrobial activity of the hydroalcoholic extract of P. insignis (PIHA), the ethyl acetate fraction (PIAE), and its subfraction containing a mixture of biflavonoids (BF). Moreover, the effect of these natural products on the antibiotic activity against S. aureus strains overexpressing efflux pump genes was also evaluated.

METHODS

Minimal inhibitory concentrations were determined against different species of microorganisms. To evaluate the modulatory effect on the Norfloxacin-resistance, the MIC of this antibiotic was determined in the absence and presence of the natural products at subinhibitory concentrations. Inhibition of the EtBr efflux assays were conducted in the absence or presence of natural products.

RESULTS

PIHA showed a microbicidal effect against S. aureus and C. albicans, while PIAE was bacteriostatic for S. aureus. PIAE and BF at subinhibitory concentrations were able to reduce the MIC of Norfloxacin acting as modulating agents. BF was able to inhibit the efflux of EtBr efflux in S. aureus strains overexpressing specific efflux pump genes.

CONCLUSION

P. inignisis, a source of efflux pump inhibitors, including volkensiflavone and morelloflavone, which were able to potentiate the Norfloxacin activity by NorA inhibition, being also able to inhibit QacA/B, TetK and MsrA. Volkensiflavone and morelloflavone could be used as an adjuvant in the antibiotic therapy of multidrug resistant S. aureus strains overexpressing efflux pumps.

Antimicrobial activity of Mimosa caesalpiniifolia Benth and its interaction with antibiotics against Staphylococcus aureus strains overexpressing efflux pump genes

This study aimed to evaluate the antimicrobial activity of the dichloromethane fraction (DCMF) from the stem bark of Mimosa caesalpiniifolia and its effect on the activity of conventional antibiotics against Staphylococcus aureus strains overexpressing specific efflux pump genes. DCMF showed activity against S. aureus, Staphylococcus epidermidis and Candida albicans. Addition of DCMF at subinhibitory concentrations to the growth media enhanced the activity of norfloxacin, ciprofloxacin and ethidium bromide against S. aureus strains overexpressing norA suggesting the presence of efflux pump inhibitors in its composition. Similar results were verified for tetracycline against S. aureus overexpressing tetK, as well as, for ethidium bromide against S. aureus overexpressing qacC. These results indicate that M. caesalpiniifolia is a source of molecules able to modulate the fluoroquinolone- and tetracycline-resistance in S. aureus probably by inhibition of NorA, TetK and QacC respectively. SIGNIFICANCE AND IMPACT OF THE STUDY: Drug resistance is a common problem in patients with infectious diseases. Dichloromethane fraction from the stem bark of Mimosa caesalpiniifolia showed antimicrobial activity against Gram-positive bacterium Staphylococcus aureus and against Candida albicans, but did not show activity against Gram-negative specie Escherichia coli. Moreover, this fraction was able to potentiate the action of norfloxacin, ciprofloxacin and tetracycline against S. aureus strains overexpressing different efflux pump genes. Thus, Mimosa caesalpiniifolia is a source of efflux pump inhibitors which could be used in combination with fluoroquinolones or tetracycline in the treatment of infectious diseases caused by S. aureus strains overexpressing efflux pump genes.

2-Phenylquinoline S. aureus NorA Efflux Pump Inhibitors: Evaluation of the Importance of Methoxy Group Introduction

Antimicrobial resistance (AMR) represents a hot topic in drug discovery. Besides the identification of new antibiotics, the use of nonantibiotic molecules to block resistance mechanisms is a powerful alternative. Bacterial efflux pumps exert an early step in AMR development by allowing bacteria to grow at subinhibitorial drug concentrations. Thus, efflux pump inhibitors (EPIs) offer a great opportunity to fight AMR. Given our experience in developing Staphylococcus aureus NorA EPIs, in this work, starting from the 2-phenylquinoline hit 1, we planned the introduction of methoxy groups on the basis of their presence in known NorA EPIs. Among the 35 different synthesized derivatives, compounds 3b and 7d exhibited the best NorA inhibition activity by restoring at very low concentrations ciprofloxacin MICs against resistant S. aureus strains. Interestingly, both compounds displayed EPI activities at nontoxic concentrations for human cells as well as highlighted promising results by preliminary pharmacokinetic studies.

Studies on 2-phenylquinoline Staphylococcus aureus NorA efflux pump inhibitors: New insights on the C-6 position

The alarming and rapid spread of antimicrobial resistance among bacteria represents a high risk for global health. Targeting factors involved in resistance to restore the activity of failing antibiotics is a promising strategy to overcome this urgent medical need. Efflux pump inhibitors are able to increase antibiotic concentrations in bacteria, thus they can be considered true antimicrobial resistance breakers. In this work, continuing our studies on inhibitors of the Staphylococcus aureus NorA pump, we designed, synthesized and biologically evaluated novel 2-phenylquinoline derivatives starting from our hits 1 and 2. Two of the synthesized compounds (6 and 7) bearing a C-6 benzyloxy group showed the best NorA inhibition activity, thereby providing an excellent starting point to direct future chemical optimizations.

Searching for Novel Inhibitors of the S. aureus NorA Efflux Pump: Synthesis and Biological Evaluation of the 3-Phenyl-1,4-benzothiazine Analogues

Bacterial resistance to antimicrobial agents has become an increasingly serious health problem in recent years. Among the strategies by which resistance can be achieved, overexpression of efflux pumps such as NorA of Staphylococcus aureus leads to a sub-lethal concentration of the antibacterial agent at the active site that in turn may predispose the organism to the development of high-level target-based resistance. With an aim to improve both the chemical stability and potency of our previously reported 3-phenyl-1,4-benzothiazine NorA inhibitors, we replaced the benzothiazine core with different nuclei. None of the new synthesized compounds showed any appreciable intrinsic antibacterial activity, and, in particular, 2-(3,4-dimethoxyphenyl)quinoline (6 c) was able to decrease, in a concentration-dependent manner, the ciprofloxacin MIC against the norA-overexpressing strains S. aureus SA-K2378 (norA++) and SA-1199B (norA+/A116E GrlA).

Pharmacophore-Based Repositioning of Approved Drugs as Novel Staphylococcus aureus NorA Efflux Pump Inhibitors

An intriguing opportunity to address antimicrobial resistance is represented by the inhibition of efflux pumps. Focusing on NorA, the most important efflux pump of Staphylococcus aureus, an efflux pump inhibitors (EPIs) library was used for ligand-based pharmacophore modeling studies. By exploitation of the obtained models, an in silico drug repositioning approach allowed for the identification of novel and potent NorA EPIs.

Improved Potency of Indole-Based NorA Efflux Pump Inhibitors: From Serendipity toward Rational Design and Development

The NorA efflux pump is a potential drug target for reversal of resistance to selected antibacterial agents, and recently we described indole-based inhibitor candidates. Herein we report a second class of inhibitors derived from them but with significant differences in shape and size. In particular, compounds 13 and 14 are very potent inhibitors in that they demonstrated the lowest IC₅₀ values (2 μM) ever observed among all indole-based compounds we have evaluated.

Inhibition of the NorA efflux pump of Staphylococcus aureus by synthetic riparins

AIM

The goal of this study was to increase knowledge about the antimicrobial activity of some synthetic Riparin-derived compounds, alone or in combination with fluoroquinolone antibiotics, against a strain of Staphylococcus aureus resistant to fluoroquinolone by way of overexpression of the NorA efflux pump.

METHODS AND RESULTS

Microdilution tests showed that Riparins A and B did not show any significant antibacterial activity against Staph. aureus strains. On the other hand, the intrinsic antibacterial activity increased with increasing lipophilicity of the compounds, in the following order: Riparin-D (MIC 256 μg ml^-1 ; Log P 2·95) < Riparin-C (MIC 102 μg ml^-1 ; Log P 3·22) < Riparin-E (MIC 16 μg ml^-1 ; Log P 3·57). The addition of all riparins to growth media at subinhibitory concentrations caused an increase in the antibacterial activity of antibiotics against the NorA-overexpressing test strain. Riparin-B, which has two methoxyl groups at the phenethyl moiety, showed the best modulatory effect.

CONCLUSIONS

Riparin-E is a good anti-staphylococci agent, while Riparin-B functions as a NorA efflux pump inhibitor.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our data suggest the possibility of using Riparin-B in combination with norfloxacin or ciprofloxacin for therapy of infections caused by multi-drug resistant Staph. aureus.

Inhibition of the NorA multi-drug transporter by oxygenated monoterpenes

The aim of this study was to investigate intrinsic antimicrobial activity of three monoterpenes nerol, dimethyl octanol and estragole, against bacteria and yeast strains, as well as, investigate if these compounds are able to inhibit the NorA efflux pump related to fluoroquinolone resistance in Staphylococcus aureus. Minimal inhibitory concentrations (MICs) of the monoterpenes against Staphylococcus aureus, Escherichia coli and Candida albicans strains were determined by micro-dilution assay. MICs of the norfloxacin against a S. aureus strain overexpressing the NorA protein were determined in the absence or in the presence of the monoterpenes at subinhibitory concentrations, aiming to verify the ability of this compounds act as efflux pump inhibitors. The monoterpenes were inactive against S. aureus however the nerol was active against E. coli and C. albicans. The addition of the compounds to growth media at sub-inhibitory concentrations enhanced the activity of norfloxacin against S. aureus SA1199-B. This result shows that bioactives tested, especially the nerol, are able to inhibit NorA efflux pump indicating a potential use as adjuvants of norfloxacin for therapy of infections caused by multi-drug resistant S. aureus strains.

Characteristics of Patients With Healthcare-Associated Infection Due to SCCmecType IV Methicillin-ResistantStaphylococcus aureus

Objective.

Methicillin-resistantStaphylococcus aureus(MRSA) with the staphylococcal cassette chromosomemec(SCCmec) type IV allele is most commonly associated with community-acquired MRSA (CA-MRSA) infection; however, such organisms have also been identified in the healthcare setting. The objective of the present study was to characterize the epidemiology of and clinical outcomes associated with SCCmec-IV MRSA infection acquired in the healthcare setting, compared with infection caused by MRSA of other SCCmectypes.

Design.

We evaluated a cohort of 100 inpatients with MRSA infection that met the Centers for Disease Control and Prevention definition for healthcare-associated infection and compared the patients' demographic characteristics, the antimicrobial susceptibilities of the MRSA isolates, the infection types, and the associated clinical and microbiological outcomes. For each MRSA isolate, the SCCmectype and the presence of Panton-Valentine leukocidin (PVL) were determined by polymerase chain reaction methods.

Results.

SCCmec-IV MRSA isolates were isolated from 53 patients (42% of these isolates were positive for PVL), and SCCmec-II or SCCmec-III MRSA was isolated from 47 patients (3% of these isolates were positive for PVL). No differences were noted between the patients in the SCCmec-II/III group and the patients in the SCCmec-IV group with respect to age (median, 55 vs 50 years); sex (77% vs 64% of patients were male); medical service (surgical service, 60% in both groups; ICU admission, 55% vs 53%), Acute Physiology and Chronic Health Evaluation II score (median, 8 vs. 7); infection type; or underlying comorbidities, except for presence of a burn wound (13% vs 2%;P< .04). Patients in the SCCmec-II/III group were more likely to have multiple sites of infection (P= .006) and a longer length of stay (LOS) prior to detection of MRSA than were patients in the SCCmec-IV group (median, 4 vs 1 days;P< .001). Total LOS was significantly greater for patients in the SCCmec-II/III, compared with those in the SCCmec-IV group (P= .006). Multiple logistic regression identified liver disease and longer LOS prior to detection of MRSA as predictors of infection with SCCmec-II/III MRSA. Rates of susceptibility to clindamycin, gentamicin, ciprofloxacin, levofloxacin, and tetracycline was significantly greater among SCCmec-IV MRSA isolates, compared with type II/III isolates (P⩽ .05). Compared with SCCmec-IV isolates acquired in the community, the susceptibility rates among healthcare-associated SCCmec-IV isolates was significantly less for clindamycin, gentamicin, and levofloxacin, indicating that these organisms may quickly acquire resistance to non-β-lactam antibiotics, as do SCCmec-II/III strains.

Conclusions.

SCCmec-IV MRSA appears to have become established in hospitals. The onset of infection caused by SCCmec-IV strains is earlier than the onset of infection with SCCmec-II/III strains; however, associated types of infection are similar. Infection with SCCmec- II/III MRSA is currently associated with an adverse impact on outcome, compared with infection with SCCmec-IV MRSA. Further research is warranted to determine the impact of SCCmectype IV strains in hospital settings.

The "racemic approach" in the evaluation of the enantiomeric NorA efflux pump inhibition activity of 2-phenylquinoline derivatives

Among the mechanisms adopted by bacteria, efflux pumps (EPs) have been recognized as being significantly involved in contributing to resistance to commonly used antibacterial agents. However, little is known about their three-dimensional structures or the steric requirements for their inhibition. Lack of such knowledge includes NorA, one of the most studied Staphylococcus aureus EPs. In the present study, the use of two commercialized Cinchona alkaloid-based zwitterionic chiral stationary phases allowed the enantioseparation of four 2-((2-(4-propoxyphenyl)quinolin-4-yl)oxy)alkylamines 1-4 previously found to be potent S. aureus NorA efflux pump inhibitors when tested as racemates. In the identified optimal polar-ionic conditions (MeOH/THF/H2O-49/49/2 (v/v/v)+25mM formic acid, 12.5mM diethylamine), repeated consecutive injections of 1 allowed the isolation of sufficient amount of its enantiomers (2.6mg and 2.8mg, for (R)-1 and (S)-1, respectively) and then to evaluate their ability to inhibit the S. aureus NorA efflux pump. The biological evaluation highlighted the main contribution of the (R)-1 enantiomer to both the EtBr efflux inhibition and synergistic effect with against SA-1199B (norA+/A116E GrlA) respect to the racemate activity. The comparison between the experimental electronic circular dichroism and the time-dependent density functional theory calculations spectra of the two isolated enantiomeric fractions allowed for all compounds a clear and easy assignment of the enantiomeric elution order.

Indole Based Weapons to Fight Antibiotic Resistance: A Structure-Activity Relationship Study

Antibiotic resistance represents a worldwide concern, especially regarding the outbreak of methicillin-resistant Staphylococcus aureus, a common cause for serious skin and soft tissues infections. A major contributor to Staphylococcus aureus antibiotic resistance is the NorA efflux pump, which is able to extrude selected antibacterial drugs and biocides from the membrane, lowering their effective concentrations. Thus, the inhibition of NorA represents a promising and challenging strategy that would allow recycling of substrate antimicrobial agents. Among NorA inhibitors, the indole scaffold proved particularly effective and suitable for further optimization. In this study, some unexplored modifications on the indole scaffold are proposed. In particular, for the first time, substitutions at the C5 and N1 positions have been designed to give 48 compounds, which were synthesized and tested against norA-overexpressing S. aureus. Among them, 4 compounds have NorA IC50 values lower than 5.0 μM proving to be good efflux pump inhibitor (EPI) candidates. In addition, preliminary data on their ADME (absorption, distribution, metabolism, and excretion) profile is reported.

Benzocyclohexane oxide derivatives and neolignans from Piper betle inhibit efflux-related resistance in Staphylococcus aureus

Six active compounds were isolated fromPiper betleunder the guide of synergistic effects combined with antibiotic norfloxacin. The synergistic mechanism was explored and significant inhibition effects for EtBr efflux in SA1199B were found.

Clonal relatedness is a predictor of spontaneous multidrug efflux pump gene overexpression in Staphylococcus aureus

Increased expression of genes encoding multidrug resistance efflux pumps (MDR-EPs) contributes to antimicrobial agent and biocide resistance in Staphylococcus aureus. Previously identified associations between norA overexpression and spa type t002 meticillin-resistant S. aureus (MRSA), and a similar yet weaker association between mepA overexpression and type t008 meticillin-susceptible S. aureus (MSSA), in clinical isolates are suggestive of clonal dissemination. It is also possible that related strains are prone to mutations resulting in overexpression of specific MDR-EP genes. Exposure of non-MDR-EP-overexpressing clinical isolates to biocides and dyes can select for MDR-EP-overexpressing mutants. spa types t002 and t008 isolates are predominated by multilocus sequencing typing sequence types (STs) 5 and 8, respectively. In this study, non-MDR-EP gene-overexpressing clinical isolates (MRSA and MSSA) representing ST5 and ST8 were subjected to single exposures of ethidium bromide (EtBr) to select for EtBr-resistant mutants. Measurements of active EtBr transport among mutants were used to demonstrate an efflux-proficient phenotype. Using quantitative reverse-transcription PCR, it was found that EtBr-resistant mutants of ST5 and ST8 parental strains predominantly overexpressed mepA (100%) and mdeA (83%), respectively, regardless of meticillin sensitivity. Associations between clonal lineage and MDR-EP gene overexpression differed from those previously observed and suggest the latter is due to clonal spread of efflux-proficient strains. The predilection of in vitro-selected mutants of related strains to overexpress the same MDR-EP gene indicates the presence of a consistent mutational process.

Antimicrobial salvage therapy for persistent staphylococcal bacteremia using daptomycin plus ceftaroline

PURPOSE

Guidelines recommend daptomycin combination therapy as an option for methicillin-resistant Staphylococcus aureus (MRSA) bacteremia after vancomycin failure. Recent data suggest that combining daptomycin with a β-lactam may have unique benefits; however, there are very limited clinical data regarding the use of ceftaroline with daptomycin.

METHODS

All 26 cases from the 10 medical centers in which ceftaroline plus daptomycin was used for treatment of documented refractory staphylococcal bacteremia from March 2011 to November 2012 were included. In vitro (synergy studies, binding assays, cathelicidin LL-37 killing assays), and in vivo (virulence assays using a murine subcutaneous infection model) studies examining the effects of ceftaroline with daptomycin were also performed.

FINDINGS

Daptomycin plus ceftaroline was used in 26 cases of staphylococcal bacteremia (20 MRSA, 2 vancomycin-intermediate S aureus, 2 methicillin-susceptible S aureus [MSSA], 2 methicillin-resistant S epidermidis). Bacteremia persisted for a median of 10 days (range, 3-23 days) on previous antimicrobial therapy. After daptomycin plus ceftaroline was started, the median time to bacteremia clearance was 2 days (range, 1-6 days). In vitro studies showed ceftaroline synergy against MRSA and enhanced MRSA killing by cathelicidin LL-37 and neutrophils. Ceftaroline also induced daptomycin binding in MSSA and MRSA to a comparable degree as nafcillin. MRSA grown in subinhibitory concentrations of ceftaroline showed attenuated virulence in a murine subcutaneous infection model.

IMPLICATIONS

Ceftaroline plus daptomycin may be an option to hasten clearance of refractory staphylococcal bacteremia. Ceftaroline offers dual benefit via synergy with both daptomycin and sensitization to innate host defense peptide cathelicidin LL37, which could attenuate virulence of the pathogen.

A new plant-derived antibacterial is an inhibitor of efflux pumps in Staphylococcus aureus

An in-depth evaluation was undertaken of a new antibacterial natural product (1) recently isolated and characterised from the plant Hypericum olympicum L. cf. uniflorum. Minimum inhibitory concentrations (MICs) were determined for a panel of bacteria, including: meticillin-resistant and -susceptible strains of Staphylococcus aureus, Staphylococcus epidermidis and Staphylococcus haemolyticus; vancomycin-resistant and -susceptible Enterococcus faecalis and Enterococcus faecium; penicillin-resistant and -susceptible Streptococcus pneumoniae; group A streptococci (Streptococcus pyogenes); and Clostridium difficile. MICs were 2-8 mg/L for most staphylococci and all enterococci, but were ≥16 mg/L for S. haemolyticus and were >32 mg/L for all species in the presence of blood. Compound 1 was also tested against Gram-negative bacteria, including Escherichia coli, Pseudomonas aeruginosa and Salmonella enterica serovar Typhimurium but was inactive. The MIC for Mycobacterium bovis BCG was 60 mg/L, and compound 1 inhibited the ATP-dependent Mycobacterium tuberculosis MurE ligase [50% inhibitory concentration (IC(50)) = 75 μM]. In a radiometric accumulation assay with a strain of S. aureus overexpressing the NorA multidrug efflux pump, the presence of compound 1 increased accumulation of (14)C-enoxacin in a concentration-dependent manner, implying inhibition of efflux. Only moderate cytotoxicity was observed, with IC50 values of 12.5, 10.5 and 8.9 μM against human breast, lung and fibroblast cell lines, respectively, highlighting the potential value of this chemotype as a new antibacterial agent and efflux pump inhibitor.

Evaluation of Daptomycin Non-Susceptible Staphylococcus aureus for Stability, Population Profiles, mprF Mutations, and Daptomycin Activity

Introduction

Despite studies examining daptomycin non-susceptible (DNS) Staphylococcus aureus, examination of the stability and population profiles is limited. The objective was to evaluate the stability, population profiles, and daptomycin activity against DNS isolates.

Methods

The stability of 12 consecutive clinical DNS strains was evaluated by minimum inhibitory concentration (MICs) and population analysis profiles before and after 5 days of serial passage. Two pairs of DNS S. aureus having the same daptomycin MIC but different daptomycin population profiles were evaluated via an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model of simulated endocardial vegetations for 96 h against daptomycin 6 and 10 mg/kg/day. The sequence of mprF was determined for these isolates before and after 96 h of daptomycin exposure in the in vitro PK/PD model.

Results

Daptomycin MIC values were 2–4 mg/L (via Microscan) for the 12 clinical isolates; 9 were confirmed DNS and 3 were within 1 tube dilution of Microscan (daptomycin MIC 1 mg/L). All were stable to serial passage. There was variation in the isolates susceptibility to daptomycin on population analysis (daptomycin population AUC 14.01–26.85). The killing patterns of daptomycin 6 and 10 mg/kg/day differed between isolates with a left-shift and right-shift population profile to daptomycin. Two strains developed additional mprF mutations during daptomycin exposure in the in vitro PK/PD model resulting in P314L, L826F, S337L and a novel Q326Stop mutation.

Conclusions

The collection of DNS isolates was stable and displayed variation in susceptibility to daptomycin on population profile. Further research examining this clinical relevance is warranted.

Electronic supplementary material

The online version of this article (doi:10.1007/s40121-013-0021-7) contains supplementary material, which is available to authorized users.

Structural mechanism of transcription regulation of the Staphylococcus aureus multidrug efflux operon mepRA by the MarR family repressor MepR

The multidrug efflux pump MepA is a major contributor to multidrug resistance in Staphylococcus aureus. MepR, a member of the multiple antibiotic resistance regulator (MarR) family, represses mepA and its own gene. Here, we report the structure of a MepR–mepR operator complex. Structural comparison of DNA-bound MepR with ‘induced’ apoMepR reveals the large conformational changes needed to allow the DNA-binding winged helix-turn-helix motifs to interact with the consecutive major and minor grooves of the GTTAG signature sequence. Intriguingly, MepR makes no hydrogen bonds to major groove nucleobases. Rather, recognition-helix residues Thr60, Gly61, Pro62 and Thr63 make sequence-specifying van der Waals contacts with the TTAG bases. Removing these contacts dramatically affects MepR–DNA binding activity. The wings insert into the flanking minor grooves, whereby residue Arg87, buttressed by Asp85, interacts with the O2 of T₄ and O4′ ribosyl oxygens of A₂₃ and T₄. Mutating Asp85 and Arg87, both conserved throughout the MarR family, markedly affects MepR repressor activity. The His14′:Arg59 and Arg10′:His35:Phe108 interaction networks stabilize the DNA-binding conformation of MepR thereby contributing significantly to its high affinity binding. A structure-guided model of the MepR–mepA operator complex suggests that MepR dimers do not interact directly and cooperative binding is likely achieved by DNA-mediated allosteric effects.

Inhibition of drug efflux pumps in Staphylococcus aureus: current status of potentiating existing antibiotics

The emergence of multidrug-resistant Staphylococcus aureus coupled with a declining output of new antibiotic treatment options from the pharmaceutical industry is a growing worldwide healthcare problem. Multidrug efflux pumps are known to play a role in antibiotic and biocide resistance in S. aureus. These membrane transporters are capable of extruding drugs and other structurally unrelated compounds, hence decreasing intracellular concentration and increasing survival. Coadministration of efflux pump inhibitors (EPIs) with antibiotics that are pump substrates could increase intracellular drug levels, thus bringing renewed efficacy to existing antistaphylococcal agents. Numerous EPIs have been identified or synthesized over the past two decades; these include existing pharmacologic drugs, naturally occurring compounds, and synthetic derivatives thereof. This review describes the current progress in EPI development for use against S. aureus.

Evaluation of ceftaroline activity against heteroresistant vancomycin-intermediate Staphylococcus aureus and vancomycin-intermediate methicillin-resistant S. aureus strains in an in vitro pharmacokinetic/pharmacodynamic model: exploring the "seesaw effect"

A "seesaw effect" in methicillin-resistant Staphylococcus aureus (MRSA) has been demonstrated, whereby susceptibility to β-lactam antimicrobials increases as glyco- and lipopeptide susceptibility decreases. We investigated this effect by evaluating the activity of the anti-MRSA cephalosporin ceftaroline against isogenic pairs of MRSA strains with various susceptibilities to vancomycin in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model. The activities of ceftaroline at 600 mg every 12 h (q12h) (targeted free maximum concentration of drug in serum [fC(max)], 15.2 μg/ml; half-life [t(1/2)], 2.3 h) and vancomycin at 1 g q12h (targeted fC(max), 18 μg/ml; t(1/2), 6 h) were evaluated against 3 pairs of isogenic clinical strains of MRSA that developed increased MICs to vancomycin in patients while on therapy using a two-compartment hollow-fiber PK/PD model with a starting inoculum of ~10(7) CFU/ml over a 96-h period. Bacterial killing and development of resistance were evaluated. Expression of penicillin-binding proteins (PBPs) 2 and 4 was evaluated by reverse transcription (RT)-PCR. The achieved pharmacokinetic parameters were 98 to 119% of the targeted values. Ceftaroline and vancomycin were bactericidal against 5/6 and 1/6 strains, respectively, at 96 h. Ceftaroline was more active against the mutant strains than the parent strains, with this difference being statistically significant for 2/3 strain pairs at 96 h. The level of PBP2 expression was 4.4× higher in the vancomycin-intermediate S. aureus (VISA) strain in 1/3 pairs. The levels of PBP2 and PBP4 expression were otherwise similar between the parent and mutant strains. These data support the seesaw hypothesis that ceftaroline, like traditional β-lactams, is more active against strains that are less susceptible to vancomycin even when the ceftaroline MICs are identical. Further research to explore these unique findings is warranted.

Antibacterial sesquiterpenoid derivatives from Ferula ferulaeoides

Three new sesquiterpenoid derivatives 1, 2, and 3 were isolated from Ferula ferulaeoides. To confirm the structure, compound 2 was also synthesized via a condensation reaction between compound 1 and 2,2-dimethoxypropane. The structures of these three compounds were elucidated by means of spectroscopic and chemical methods. Their antibacterial activity against drug-resistant Staphylococcus aureus strains were evaluated with MIC values in the range of 0.5-128 µg/mL. Compounds 1 and 3 were capable of inhibiting efflux of ethidium bromide using an in vitro assay. The cytotoxicity of the compounds was evaluated on cultured HEK293 cells, and none of them showed toxicity to HEK293 cells at a concentration of 125 µg/mL.

Alternative mutational pathways to intermediate resistance to vancomycin in methicillin-resistant Staphylococcus aureus

BACKGROUND

 We used 2 in vitro experimental systems to compare phenotypic and genotypic changes that accompany selection of mutants of methicillin-resistant Staphylococcus aureus (MRSA) strain JH1 with low-level vancomycin resistance similar to the type found in vancomycin-intermediate S. aureus (VISA).

METHODS

 The previously described MRSA strain JH1 and its vancomycin-intermediate mutant derivative JH2, both of which were recovered from a patient undergoing vancomycin chemotherapy, were used in this study. Mutants of JH1 were selected in vitro by means of a pharmacokinetic/pharmacodynamic (PK/PD) model of simulated endocardial vegetations (SEVs) and by exposure to vancomycin in laboratory growth medium. Phenotypic abnormalities of JH1 mutants generated by each in vitro experimental system were compared to those of JH2, and whole genomes of 2 in vitro JH1 mutants were sequenced to identify mutations that may be associated with an increased vancomycin minimum inhibitory concentration.

RESULTS

 JH1R1 was selected from the PK/PD model, and JH1R2 was selected in laboratory growth medium. Both mutants displayed reduced vancomycin and daptomycin susceptibility and phenotypic alterations (eg, thicker cell walls and abnormal autolysis) that are typical of in vivo VISA mutants. Genome sequencing of JH1R1 identified point mutations in 4 genes, all of which were different from the mutations described in JH2, including 1 mutation in yycG, a component of the WalKR sensory regulatory system. Sequencing of the JH1R2 genome identified mutations in 7 genes, including 2 in rpoB.

CONCLUSION

 Our findings indicate that JH1 is able to develop VISA-type resistance through several alternative genetic pathways.

Mutagenesis and modeling to predict structural and functional characteristics of the Staphylococcus aureus MepA multidrug efflux pump

MepA is a multidrug and toxin extrusion (MATE) family protein and the only MATE protein encoded within the Staphylococcus aureus genome. Structural data for MATE proteins are limited to a single high-resolution example, NorM of Vibrio cholerae. Substitution mutations were created in MepA using gradient plates containing both a substrate and reserpine as an efflux pump inhibitor. Site-directed mutagenesis of plasmid-based mepA was used to reproduce these mutations, as well as unique or low-frequency mutations identified in mepA-overexpressing clinical strains, and to mutagenize conserved acidic residues. The effect of these changes on protein function was quantitated in a norA-disrupted host strain by susceptibility testing with and without inhibitors and by determining the proficiency of ethidium efflux. Up-function substitutions clustered in the carboxy half of MepA, near the cytoplasmic face of the protein. Repeated application of the same gradient plate conditions frequently reproduced identical substitution mutations, suggesting that individual residues are required for interaction with specific substrates. Acidic residues critical to protein function were identified in helices 4 and 5. In silico modeling revealed an outward-facing molecule, with helices 1, 2, 4, 7, 8, and 10 having contact with a central cavity that may represent a substrate translocation pathway. Functionally important residues within this cavity included S81, A161, M291, and A302. These data provide a critical starting point for understanding how MATE multidrug efflux proteins function and will be useful in refining crystallographic data when they are available.

Ligand Promiscuity between the Efflux Pumps Human P-Glycoprotein and S. aureus NorA

Thirty-two diverse compounds were evaluated for their ability to inhibit both Pgp-mediated efflux in mouse T-lymphoma L5178 MDR1 and NorA-mediated efflux in S. aureus SA-1199B. Only four compounds were strong inhibitors of both efflux pumps. Three compounds were found to inhibit Pgp exclusively and strongly, while seven compounds inhibited only NorA. These results demonstrate that Pgp and NorA inhibitors do not necessarily overlap, opening the way to safer therapeutic use of effective NorA inhibitors.

Mechanisms of in-vitro-selected daptomycin-non-susceptibility in Staphylococcus aureus

Daptomycin is highly active against Staphylococcus aureus, including multidrug-resistant strains and those with reduced susceptibility to vancomycin. However, daptomycin-non-susceptible (Dap(NS)) strains [minimum inhibitory concentration (MIC) >1mg/L] have been derived clinically and in vitro. The mechanism(s) by which this occurs is incompletely understood, but existing data indicate that it is multifactorial. Dap(NS) derivatives of one laboratory and three clinical strains of S. aureus produced using gradient plates were evaluated. The Dap(NS) phenotype included increases in glycopeptide and nisin MICs and in some instances defective autolysis and reduced susceptibility to lysostaphin lysis. Amino acid substitutions in MprF, YycG (WalK), or both, were identified in all Dap(NS) strains. Reduced cytochrome c binding and ability of daptomycin to depolarise whole cells correlated with the Dap(NS) phenotype, consistent with an increase in cell surface positivity. Gene expression data revealed increased expression of vraS, one member of a two-component system involved in the regulation of cell wall biosynthesis, in three of five Dap(NS) strains. The pathway to the Dap(NS) phenotype is not linear, as variable genetic and phenotypic changes may result in identical increases in MICs.

Goldenseal (Hydrastis canadensis L.) extracts synergistically enhance the antibacterial activity of berberine via efflux pump inhibition

Goldenseal (Hydrastis canadensis L.) is used to combat inflammation and infection. Its antibacterial activity in vitRO has been attributed to its alkaloids, the most abundant of which is berberine. The goal of these studies was to compare the composition, antibacterial activity, and efflux pump inhibitory activity of ethanolic extracts prepared from roots and aerial portions of H. canadensis. Ethanolic extracts were prepared separately from roots and aerial portions of six H. canadensis plants. Extracts were analyzed for alkaloid concentration using LC-MS and tested for antimicrobial activity against Staphylococcus aureus (NCTC 8325-4) and for inhibition of ethidium bromide efflux. Synergistic antibacterial activity was observed between the aerial extract (FIC 0.375) and to a lesser extent the root extract (FIC 0.750) and berberine. The aerial extract inhibited ethidium bromide efflux from wild-type S. aureus but had no effect on the expulsion of this compound from an isogenic derivative deleted for norA. Our studies indicate that the roots of H. canadensis contain higher levels of alkaloids than the aerial portions, but the aerial portions synergize with berberine more significantly than the roots. Furthermore, extracts from the aerial portions of H. canadensis contain efflux pump inhibitors, while efflux pump inhibitory activity was not observed for the root extract. The three most abundant H. canadensis alkaloids, berberine, hydrastine, and canadine, are not responsible for the efflux pump inhibitory activity of the extracts from H. canadensis aerial portions.

Discovery of novel inhibitors of the NorA multidrug transporter of Staphylococcus aureus

Four novel inhibitors of the NorA efflux pump of Staphylococcus aureus, discovered through a virtual screening process, are reported. The four compounds belong to different chemical classes and were tested for their in vitro ability to block the efflux of a well-known NorA substrate, as well as for their ability to potentiate the effect of ciprofloxacin (CPX) on several strains of S. aureus, including a NorA overexpressing strain. Additionally, the MIC values of each of the compounds individually are reported. A structure-activity relationship study was also performed on these novel chemotypes, revealing three new compounds that are also potent NorA inhibitors. The virtual screening procedure employed FLAP, a new methodology based on GRID force field descriptors.

In silico genetic correlations of multidrug efflux pump gene expression in Staphylococcus aureus

Regulatory mechanisms for chromosomal genes encoding multidrug resistance (MDR) efflux pumps (EPs) in Staphylococcus aureus are poorly defined. Microbiological, quantitative gene expression, mRNA half-life and genome data for 11 strains of S. aureus combined with bioinformatic analyses were used to identify correlates of increased MDR EP gene expression. The presence of qacA/B and/or increased expression of one to two MDR EP genes were identified in eight strains. Microbiological and gene expression data correlated in four instances, existing knowledge of the substrate specificity of NorC resulted in correlation in two others, and a transcriptional/translational disconnect is possible for the remaining two. In silico analyses and mRNA half-life determinations linked insertions of nucleotide repeats 3' to the -10 motif of the norA promoter with increased promoter activity. Mutations in the 5'-untranslated and/or coding regions were identified that may affect transcription efficiency. Substitutions of residues in the helix-turn-helix (HTH) motif of NorG may augment its positive regulation of norB. The correlations proposed provide a guide for further experimentation leading to a better understanding of MDR EP gene expression in this important pathogen.

From 6-aminoquinolone antibacterials to 6-amino-7-thiopyranopyridinylquinolone ethyl esters as inhibitors of Staphylococcus aureus multidrug efflux pumps

The thiopyranopyridine moiety was synthesized as a new heterocyclic base to be inserted at the C-7 position of selected quinolone nuclei followed by a determination of antibacterial activity against strains of Staphylococcus aureus. Selected thiopyranopyridinylquinolones showed significant antimicrobial activity, including strains having mutations in gyrA and grlA as well as other strains overexpressing the NorA multidrug (MDR) efflux pump. Most derivatives did not appear to be NorA substrates. The effect of the thiopyranopyridinyl substituent on making these quinolones poor substrates for NorA was investigated further. Several quinolone ester intermediates, devoid of any intrinsic antibacterial activity, were tested for their abilities to inhibit the activities of NorA (MFS family) and MepA (MATE family) S. aureus MDR efflux pumps. Selected quinolone esters were capable of inhibiting both MDR pumps more efficiently than the reference compound reserpine. Moreover, they also were able to restore, and even enhance, the activity of ciprofloxacin toward some genetically modified resistant S. aureus strains.

Treatment strategies for infective endocarditis

IMPORTANCE OF THE FIELD

Despite significant advances in medical, surgical, and critical care interventions, infective endocarditis (IE) remains a disease associated with considerable morbidity and mortality. Estimates from the American Heart Association place the incidence of IE in the US at 10,000 - 15,000 new cases each year. This may be due to the changing epidemiology of IE, including increasing antimicrobial resistance, increasing heart surgeries, prosthetic valve implantation, and widespread use of intravenous drugs. Furthermore, a new form of the disease, healthcare-associated IE, which is associated with new therapeutic modalities such as intravenous catheters, hyperalimentation lines, pacemakers, and dialysis shunts, has emerged.

AREAS COVERED IN THIS REVIEW

We present the latest therapeutic and preventive strategies for IE caused by a variety of bacterial and fungal pathogens. The general methods employed included an extensive literature search, confined to the last 10 years, using key words such as 'infective endocarditis', 'culture-negative endocarditis', 'treatment guidelines for IE', and 'prophylaxis for IE'.

WHAT THE READER WILL GAIN

Comprehensive information regarding the changing epidemiology of IE is provided. The latest guidelines with respect to therapy and prophylaxis of IE are reviewed.

TAKE HOME MESSAGE

Successful management of IE depends on maintaining a high index of suspicion for the disease and, when IE is diagnosed, close cooperation of medical and surgical disciplines is required. Further research is needed to better understand and provide optimal therapy for complex situations such as multidrug-resistant and polymicrobial IE.

Impact of inoculum size and heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) on vancomycin activity and emergence of VISA in an in vitro pharmacodynamic model

The activity of vancomycin against heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) and non-hVISA isolates, using an in vitro pharmacodynamic model, was reduced in the presence of a high inoculum amount (10(8) CFU/ml). A high bacterial load of >10(5) CFU/ml persisted for all strains with doses up to 5 g every 12 h against high inoculum amounts. No change in the vancomycin MIC was detected in any isolate at a moderate inoculum amount (10(6) CFU/ml), and bactericidal activity occurred only against the non-hVISA isolate (time to 99% kill, 7.5 h; P = 0.001).

Structural and biochemical characterization of MepR, a multidrug binding transcription regulator of the Staphylococcus aureus multidrug efflux pump MepA

MepR is a multidrug binding transcription regulator that represses expression of the Staphylococcus aureus multidrug efflux pump gene, mepA, as well as its own gene. MepR is induced by multiple cationic toxins, which are also substrates of MepA. In order to understand the gene regulatory and drug-binding mechanisms of MepR, we carried out biochemical, in vivo and structural studies. The 2.40 A resolution structure of drug-free MepR reveals the most open MarR family protein conformation to date, which will require a huge conformational change to bind cognate DNA. DNA-binding data show that MepR uses a dual regulatory binding mode as the repressor binds the mepA operator as a dimer of dimers, but binds the mepR operator as a single dimer. Alignment of the six half sites reveals the consensus MepR binding site, 5'-GTTAGAT-3'. 'Drug' binding studies show that MepR binds to ethidium and DAPI with comparable affinities (K(d) = 2.6 and 4.5 microM, respectively), but with significantly lower affinity to the larger rhodamine 6G (K(d) = 62.6 microM). Mapping clinically relevant or in vitro selected MepR mutants onto the MepR structure suggests that their defective repressor phenotypes are due to structural and allosteric defects.

Inability of a reserpine-based screen to identify strains overexpressing efflux pump genes in clinical isolates of Staphylococcus aureus

Overexpression of efflux pump genes conferring multidrug resistance (MDR) in Staphylococcus aureus results in reduced susceptibility to select biocides, dyes and fluoroquinolones. Reserpine is commonly used as an inhibitor of MDR efflux pumps and previous work from our laboratory using a reserpine-based screen to identify clinical isolates with an efflux phenotype revealed that nearly one-half overexpressed norA-B-C, mepA or mdeA. The accuracy of reserpine in predicting efflux pump gene overexpression in clinical strains was examined in detail. Bloodstream isolates of S. aureus previously classified as non-effluxing strains by the reserpine screen underwent gene expression analysis using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR). The reserpine screen failed to identify many strains shown by qRT-PCR to overexpress one or more MDR efflux pump genes. Microdilution susceptibility testing with and without reserpine also failed to predict efflux pump activity. Although gene expression does not always correlate with protein translation, our results indicate that in clinical S. aureus isolates the use of reserpine to predict the contribution of efflux to reduced susceptibility is not dependable. All strains used in studies designed to assess MDR efflux pump gene expression in clinical isolates should be evaluated by a method independent of in vitro susceptibility testing.

Multidrug efflux pump overexpression in Staphylococcus aureus after single and multiple in vitro exposures to biocides and dyes

Biocides and dyes are commonly employed in hospital and laboratory settings. Many of these agents are substrates for multiple-drug resistance (MDR)-conferring efflux pumps of both Gram-positive and Gram-negative organisms. Several such pumps have been identified in Staphylococcus aureus, and mutants overexpressing the NorA and MepA MDR pumps following exposure to fluoroquinolones have been identified. The effect of exposure to low concentrations of biocides and dyes on the expression of specific pump genes has not been evaluated. Using quantitative reverse-transcription PCR we found that exposure of clinical isolates to low concentrations of a variety of biocides and dyes in a single step, or to gradually increasing concentrations over several days, resulted in the appearance of mutants overexpressing mepA, mdeA, norA and norC, with mepA overexpression predominating. Overexpression was frequently associated with promoter-region or regulatory protein mutations. Mutants having significant increases in MICs of common pump substrates but no changes in expression of studied pump genes were also observed; in these cases changes in expression of as-yet-unidentified MDR pump genes may have occurred. Strains of S. aureus that exist in relatively protected environments and are repeatedly exposed to sublethal concentrations of biocides can develop efflux-related resistance to those agents, and acquisition of such strains poses a threat to patients treated with antimicrobial agents that are also substrates for those pumps, such as ciprofloxacin and moxifloxacin.

Synergy between gemifloxacin and trimethoprim/sulfamethoxazole against community-associated methicillin-resistant Staphylococcus aureus

OBJECTIVES

The rapid emergence of methicillin-resistant Staphylococcus aureus from the community (CA-MRSA) presents difficulties in making treatment choices. We evaluated whether combining another orally available agent commonly used to treat CA-MRSA with gemifloxacin would enhance gemifloxacin activity against CA-MRSA.

METHODS

Fifty strains of SCCmec IV, agr group 1, Panton-Valentine leucocidin-positive CA-MRSA were evaluated for susceptibilities to gemifloxacin, trimethoprim/sulfamethoxazole, doxycycline, levofloxacin, rifampicin, clindamycin and erythromycin. Twenty of these strains were evaluated for the potential for synergy between gemifloxacin and trimethoprim/sulfamethoxazole, clindamycin and rifampicin by time-kill analysis. Two strains were further evaluated in an in vitro pharmacokinetic/pharmacodynamic (PK/PD) model.

RESULTS

In time-kill analyses, gemifloxacin combined with trimethoprim/sulfamethoxazole produced additivity (6/20) or synergy (11/20) in 85% of the isolates tested. The addition of clindamycin to gemifloxacin showed additivity (3/20) or synergy (2/20) in 25% of the isolates. All isolates displayed indifference to the combination of gemifloxacin and rifampicin. In the PK/PD model, combining gemifloxacin and trimethoprim/sulfamethoxazole provided potent and sustained bactericidal activity to detection limits of 2 log(10) cfu/mL by 48 h; gemifloxacin combined with clindamycin or with rifampicin killed to detection limits by 56 h or later. One isolate developed efflux-mediated resistance to gemifloxacin at 96 h with gemifloxacin monotherapy. All combinations prevented the emergence of this resistance.

CONCLUSIONS

Synergy or additivity was demonstrated by time-kill analysis between gemifloxacin and trimethoprim/sulfamethoxazole in most isolates tested. In the PK/PD model, the addition of trimethoprim/sulfamethoxazole, clindamycin and rifampicin enhanced the activity of gemifloxacin against CA-MRSA and suppressed the emergence of resistance to gemifloxacin.

From phenothiazine to 3-phenyl-1,4-benzothiazine derivatives as inhibitors of the Staphylococcus aureus NorA multidrug efflux pump

Overexpression of efflux pumps is an important mechanism by which bacteria evade effects of substrate antimicrobial agents and inhibition of such pumps is a promising strategy to circumvent this resistance mechanism. NorA is a Staphylococcus aureus multidrug efflux pump, the activity of which confers decreased susceptibility to many structurally unrelated agents, including fluoroquinolones, resulting in a multidrug resistant (MDR) phenotype. In this work, a series of 1,4-benzothiazine derivatives were designed and synthesized as a minimized structural template of phenothiazine MDR efflux pump inhibitors (EPIs) in an effort to identify more potent S. aureus NorA EPIs. Almost all derivatives evaluated showed good activity in combination with ciprofloxacin against S. aureus ATCC 25923; some were capable of completely restoring ciprofloxacin activity in a norA-overexpressing strain (SA-K2378). Compounds 6k and 7j displayed good activity against SA-1199B, a strain that also overexpresses norA, in an ethidium bromide (EtBr) efflux inhibition assay.

Inhibitors of bacterial multidrug efflux pumps from the resin glycosides of Ipomoea murucoides

A reinvestigation of the CHCl 3-soluble extract from flowers of the Mexican medicinal arborescent morning glory, Ipomoea murucoides, through preparative-scale recycling HPLC, yielded six new pentasaccharides, murucoidins VI-XI (1- 6), as well as the known pescaprein III (7), stoloniferin I (8), and murucoidins I-V (9- 13). Their structures were characterized through the interpretation of their NMR spectroscopic and FABMS data. Compounds 1-6 were found to be macrolactones of three known glycosidic acids identified as simonic acids A and B, and operculinic acid A, with different fatty acids esterifying the same positions, C-2 on the second rhamnose unit and C-4 on the third rhamnose moiety. The lactonization site of the aglycone was placed at C-2 or C-3 of the second saccharide unit. The esterifying residues were composed of two short-chain fatty acids, 2-methylpropanoic and (2S)-methylbutyric acids, and two long-chain fatty acids, n-dodecanoic (lauric) acid and the new (8R)-(-)-8-hydroxydodecanoic acid. For the latter residue, its absolute configuration was determined by analysis of its Mosher ester derivatives. All members of the murucoidin series exerted a potentiation effect of norfloxacin against the NorA overexpressing Staphylococcus aureus strain SA-1199B by increasing the activity 4-fold (8 microg/mL from 32 microg/mL) at concentrations of 5-25 microg/mL. Stoloniferin I (8) enhanced norfloxacin activity 8-fold when incorporated at a concentration of 5 microg/mL. Therefore, this type of amphipathic oligosaccharide could be developed further to provide more potent inhibitors of this multidrug efflux pump.

Synthesis and evaluation of fluoroquinolone derivatives as substrate-based inhibitors of bacterial efflux pumps

Bacterial efflux pump systems contribute to antimicrobial resistance in pathogenic bacteria. The co-administration of bacterial efflux pump inhibitors with antibiotics is being pursued to overcome efflux-mediated resistance to antibiotics. In this study we investigated a strategy for converting broad-spectrum efflux pump substrates, fluoroquinolone antibacterial agents which are inherently recognized by most efflux transporters, into expanded spectrum efflux pump inhibitors. Employing this strategy against organisms expressing efflux pumps from the MFS, MATE and RND classes of pump systems, we report here the identification of an ofloxacin-based EPI that is a potent inhibitor of MFS (NorA) and MATE (MepA) efflux pumps in Staphylococcus aureus. The methods described here outline a process that we feel will be broadly applicable to the systematic identification of bacterial efflux pump inhibitors.

Teicoplanin pharmacodynamics in reference to the accessory gene regulator (agr) in Staphylococcus aureus using an in vitro pharmacodynamic model

OBJECTIVES

The accessory gene regulator (agr) has been identified as playing a role in the expression of reduced glycopeptide susceptibility in Staphylococcus aureus. Previous studies indicate that strains with agr dysfunctional group II polymorphism have a higher propensity for reduced vancomycin activity. We investigated this relationship in agr groups I-IV using another glycopeptide, teicoplanin, in an in vitro pharmacodynamic model.

METHODS

Teicoplanin doses ranging from 1.88 to 30 mg/kg daily (fAUC/MIC 26.1-380.7) were simulated and evaluated for activity and the development of reduced susceptibility over 72 h.

RESULTS

A dose-response relationship in activity was noted as doses escalated up to 30 mg/kg daily, but regrowth was identified with all doses. Teicoplanin doses of 3.75 and 1.88 mg/kg daily resulted in isolates with intermediate teicoplanin susceptibility (MIC = 16 mg/L) in agr groups II, IV (MIC = 16 mg/L) and III (MIC = 24 mg/L), regardless of function of the agr operon. Resistance to teicoplanin (> or = 32 mg/L) occurred in agr group I functional and dysfunctional isolates. Minimal changes in MIC were noted with 7.5 mg/kg daily doses in agr groups II-IV. However, this dose resulted in variable susceptibility (4-24 mg/L) in agr group I(+/-) isolates. Higher doses of 15 and 30 mg/kg daily did not produce changes in MIC in any isolate tested.

CONCLUSIONS

Agr function did not determine teicoplanin resistance proclivity and is consistent with the previously described higher mutation rate in S. aureus to teicoplanin. Further investigation of agr group and function is warranted for all glycopeptides and compounds with a similar mechanism of action.

The phenolic diterpene totarol inhibits multidrug efflux pump activity in Staphylococcus aureus

The phenolic diterpene totarol had good antimicrobial activity against effluxing strains of Staphylococcus aureus. Subinhibitory concentrations reduced the MICs of selected antibiotics, suggesting that it may also be an efflux pump inhibitor (EPI). A totarol-resistant mutant that overexpressed norA was created to separate antimicrobial from efflux inhibitory activity. Totarol reduced ethidium efflux from this strain by 50% at 15 microM (1/4x MIC), and combination studies revealed marked reductions in ethidium MICs. These data suggest that totarol is a NorA EPI as well as an antistaphylococcal antimicrobial agent.