Occurrence of MRSA endocarditis during linezolid treatment

Purine and pyrimidine synthesis differently affect the strength of the inoculum effect for aminoglycoside and β-lactam antibiotics

The inoculum effect has been observed for nearly all antibiotics and bacterial species. However, explanations accounting for its occurrence and strength are lacking. Previous work found that the relationship between [ATP] and growth rate can account for the strength and occurrence of the inoculum effect for bactericidal antibiotics. However, the molecular pathway(s) underlying this relationship, and therefore determining the inoculum effect, remain undiscovered. Using a combination of flux balance analysis and experimentation, we show that nucleotide synthesis can determine the relationship between [ATP] and growth and thus the strength of inoculum effect in an antibiotic class-dependent manner. If the [ATP]/growth rate is sufficiently high as determined by exogenously supplied nitrogenous bases, the inoculum effect does not occur. This is consistent for both Escherichia coli and Pseudomonas aeruginosa. Interestingly, and separate from activity through the tricarboxylic acid cycle, we find that transcriptional activity of genes involved in purine and pyrimidine synthesis can predict the strength of the inoculum effect for β-lactam and aminoglycosides antibiotics, respectively. Our work highlights the antibiotic class-specific effect of purine and pyrimidine synthesis on the severity of the inoculum effect, which may pave the way for intervention strategies to reduce the inoculum effect in the clinic.

IMPORTANCE

If a bacterial population can grow and reach a sufficiently high density, routine doses of antibiotics can be ineffective. This phenomenon, called the inoculum effect, has been observed for nearly all antibiotics and bacterial species. It has also been reported to result in antibiotic failure in the clinic. Understanding how to reduce the inoculum effect can make high-density infections easier to treat. Here, we show that purine and pyrimidine synthesis affect the strength of the inoculum effect; as the transcriptional activity of pyrimidine synthesis increases, the strength of the inoculum effect for aminoglycosides decreases. Conversely, as the transcriptional activity of purine synthesis increases, the strength of the inoculum effect for β-lactam antibiotics decreases. Our work highlights the importance of nucleotide synthesis in determining the strength of the inoculum effect, which may lead to the identification of new ways to treat high-density infections in the clinic.

Purine and pyrimidine synthesis differently affect the strength of the inoculum effect for aminoglycoside and β-lactam antibiotics

The inoculum effect has been observed for nearly all antibiotics and bacterial species. However, explanations accounting for its occurrence and strength are lacking. We previously found that growth productivity, which captures the relationship between [ATP] and growth, can account for the strength of the inoculum effect for bactericidal antibiotics. However, the molecular pathway(s) underlying this relationship, and therefore determining the inoculum effect, remain undiscovered. We show that nucleotide synthesis can determine the relationship between [ATP] and growth, and thus the strength of inoculum effect in an antibiotic class-dependent manner. Specifically, and separate from activity through the tricarboxylic acid cycle, we find that transcriptional activity of genes involved in purine and pyrimidine synthesis can predict the strength of the inoculum effect for β-lactam and aminoglycosides antibiotics, respectively. Our work highlights the antibiotic class-specific effect of purine and pyrimidine synthesis on the severity of the inoculum effect and paves the way for intervention strategies to reduce the inoculum effect in the clinic.

[Pharmacokinetic evaluation of linezolid in patients with major thermal injuries]

The aims of this multicentre open-label study was to evaluate the pharmacokinetics of linezolid in patients with burn injury above 20 % BSA and to compare them with healthy volunteers, matched in age, sex and weight. After a single 600 mg IV dose of linezolid, multiple blood and urine samples were taken from subjects, in order to determine linezolid concentrations, using a HPLC assay. C(max) and volume of distribution at steady state were not different between the two groups. Values describing clearance were altered in burns, leading to a reduction by half in AUC in these patients (42.5 versus 98.1 mghL(-1)). The enhancement of clearance was due to which of non renal clearance (323+/-191 versus 80.4+/-27.5 mLmin(-1)). We conclude that pharmacokinetics of linezolid are altered in burn patients, in a magnitude sufficient that linezolid concentration may be subtherapeutic in some patients and we suggest that the dosage interval may need to be decreased in this patient population.

Phosphorylation of MgrA and its effect on expression of the NorA and NorB efflux pumps of Staphylococcus aureus

MgrA is a global regulator in Staphylococcus aureus that controls the expression of diverse genes encoding virulence factors and multidrug resistance (MDR) efflux transporters. We identified pknB, which encodes the (Ser/Thr) kinase PknB, in the S. aureus genome. PknB was able to autophosphorylate as well as phosphorylate purified MgrA. We demonstrated that rsbU, which encodes a Ser/Thr phosphatase and is involved in the activation of the SigB regulon, was able to dephosphorylate MgrA-P but not PknB-P. Serines 110 and 113 of MgrA were found to be phosphorylated, and Ala substitutions at these positions resulted in reductions in the level of phosphorylation of MgrA. DNA gel shift binding assays using norA and norB promoters showed that MgrA-P was able to bind the norB promoter but not the norA promoter, a pattern which was the reverse of that for unphosphorylated MgrA. The double mutant MgrA(S110A-S113A) bound to the norA promoter but not the norB promoter. The double mutant led to a 2-fold decrease in norA transcripts and a 2-fold decrease in the MICs of norfloxacin and ciprofloxacin in strain RN6390. Thus, phosphorylation of MgrA results in loss of binding to the norA promoter, but with a gain of the ability to bind the norB promoter. Loss of the ability to phosphorylate MgrA by Ala substitution resulted in increased repression of norA expression and in reductions in susceptibilities to NorA substrates.

In vitro activity of daptomycin against Staphylococci isolated from bacteremia and community-onset skin and soft tissue infections in France: data from two nationwide studies

Staphylococci are a leading cause of skin and soft tissue infections (SSTIs) and bacteremia in France, a country with a high prevalence of oxacillin resistance. We evaluated the in vitro activity of daptomycin compared with reference compounds against 445 Staphylococcus aureus and 53 coagulase-negative Staphylococci (CNS) collected during two large nationwide studies performed in 2006 and 2007. The percentage of oxacillin resistance among S. aureus was 13.6% (SSTIs) and 30.7% (bacteremia). Daptomycin showed lower MIC(90) levels compared to vancomycin, teicoplanin, and linezolid (0.19 mg/L vs. 2, 1.5, and 1 mg/L, respectively), irrespective of oxacillin susceptibility. Amongst the CNS, 64.2% of the isolates originated from clinical bacteremia were resistant to oxacillin and 24.5% to teicoplanin; all but one Staphylococci were susceptible to daptomycin (MIC = 1.5 mg/l). As with linezolid, daptomycin seems to constitute an alternative option to treat some staphylococcal infections in the French context of high oxacillin resistance prevalence and high glycopeptides MIC.

Pharmacokinetic evaluation of linezolid in patients with major thermal injuries

AIMS

To evaluate the pharmacokinetics of linezolid following its administration in patients with major thermal injuries and in a group of healthy volunteers.

METHODS

In an open-label, multicentre design with two parallel groups, a group of patients with major thermal injuries (>20% body area) and a group of age-, sex- and weight-matched healthy volunteers, subjects received a single 600 mg intravenous dose of linezolid. Serial blood and urine collections were made and the concentrations of linezolid in these samples were determined by HPLC. Non-compartmental analyses were used to describe the pharmacokinetic disposition of linezolid.

RESULTS

C(max) concentrations and the volume of distribution at steady state (V(ss)) were not statistically different (P > 0.05) between the two groups of subjects. In contrast, values describing clearance [elimination rate constant (k(el)), t(1/2) and mean residence time (MRT)] were significantly different (P < 0.05) in patients with thermal injuries compared with volunteers, which lead to an approximate reduction by half in AUC(0-infinity) from 98.1 mg.h/L (volunteers) to 42.5 mg.h/L (patients). Although renal clearance was similar in the two groups (24.7 +/- 23 versus 30.6 +/- 14.3 mL/min; P = 0.156), non-renal clearance was substantially increased (323 +/- 191 versus 80.4 +/- 27.5 mL/min) in the patients with thermal injuries, though this difference did not achieve statistical significance (P = 0.063).

CONCLUSIONS

The pharmacokinetics of linezolid are altered in patients with major thermal injuries, mainly as a result of increased non-renal clearance. These changes are of sufficient magnitude that linezolid concentrations may be sub-therapeutic in some patients and we suggest that the dosage interval may need to be decreased in this patient population.

A new lipoglycopeptide: telavancin

The increase in infections caused by resistant Gram-positive organisms has led to an urgent need for new antibiotics. Telavancin is a rapidly bactericidal lipoglycopeptide with multiple mechanisms of action, including concentration-dependent inhibition of bacterial cell wall synthesis and disruption of the functional integrity of the cell membrane. Telavancin is active against a wide variety of Gram-positive organisms including Staphylococcus aureus with resistance to methicillin, reduced susceptibility to vancomycin, and full resistance to vancomycin. Telavacin is approximately 90% protein bound; it has a serum half-life of around 8 h and a prolonged postantibiotic effect, allowing once daily administration. Telavancin is eliminated principally through the urine, requiring dose adjustment in patients with renal impairment. The efficacy and safety of telavancin was demonstrated in a large program of patients with complicated skin and skin structure infections. Development of resistance has not been detected in clinical strains. Adverse events include taste disturbance, nausea and vomiting; a small proportion of patients experienced reversible increase in serum creatinine. Two large Phase III studies in patients with healthcare associated pneumonia were recently completed. Telavancin has the potential to become an important therapeutic option to treat serious infections produced by resistant Gram-positive cocci, particularly those caused by methicillin-resistant S. aureus.

In vitro selection of mutants of Streptococcus pneumoniae resistant to macrolides and linezolid: relationship with susceptibility to penicillin G or macrolides

OBJECTIVES

To evaluate the rate of acquisition of resistance to linezolid and macrolides in Streptococcus pneumoniae isolates with different levels of susceptibility to penicillin and erythromycin.

MATERIALS AND METHODS

Thirty strains of S. pneumoniae were tested by serial passages in subinhibitory concentrations of each antibiotic by the spiral method. The four copies of the 23S rRNA rrl gene of parent strains and linezolid-resistant mutants were amplified and sequenced.

RESULTS

The rate of acquisition of macrolide resistance did not differ when C-14 and C-16 macrolides were tested. Resistance to linezolid in strains susceptible to penicillin and erythromycin was difficult to produce. For mutants with low-level resistance to linezolid the cut-off value of the MIC was between 6 and 8 mg/L depending on the strain. All linezolid-resistant mutants displayed a mutation in 2-4 copies of the 23S rRNA rrl gene, mainly the G2576U mutation (27/30) with an additional C2610U mutation observed in certain mutants. Two new mutations were also noted, namely C2612A and C2571G. In three linezolid-resistant mutants no mutation was identified within the studied domain, suggesting another mechanism of resistance.

CONCLUSIONS

Linezolid resistance in pneumococcal strains susceptible to penicillin and macrolides was more difficult to obtain than with macrolides. Increased resistance to these agents may therefore influence the clinical use of linezolid.

Agents for the Treatment of Multidrug-resistant Gram-positive Endocarditis

Several newer agents with activity against multidrug- resistant gram-positive pathogens are available. These agents have in vitro and clinical data supporting their utility in the treatment of infections caused by pathogens such as methicillin-resistant staphylococci and vancomycin-resistant enterococci. Daptomycin appears to be rapidly bactericidal, and linezolid and quinupristin/dalfopristin also are cidal against staphylococci. Although the agents have several properties that are attractive for use in endocarditis, clinical data are limited. Further investigation with each agent and combination therapy are warranted before definitive recommendations can be made.

Trimethoprim-sulfamethoxazole as a viable treatment option for infections caused by methicillin-resistant Staphylococcus aureus

OBJECTIVE

To review available data regarding the efficacy of trimethoprim-sulfamethoxazole (TMP-SMX) for the treatment of infections caused by methicillin-resistant Staphylococcus aureus (MRSA).

DATA SOURCES

A MEDLINE search was performed (January 1966-December 2003) using the search terms Staphylococcus aureus , sulfamethoxazole, trimethoprim, co-trimoxazole, and methicillin resistance. Abstracts from infectious diseases meetings also were reviewed.

DATA SYNTHESIS

The reported rate of TMP-SMX resistance in S. aureus is highly variable. From a mechanistic standpoint, TMP-SMX resistance among MRSA appears to be distinct from multidrug resistance, although some anecdotal reports suggest otherwise. Clonal outbreaks of MRSA resistant to TMP-SMX have been described; of these, the Brazilian clone has more often been resistant to TMP-SMX than the Iberian clone. Rates of TMP-SMX resistance are particularly high in institutions serving large numbers of patients infected by the human immunodeficiency virus, due to increased exposure for Pneumocystis prophylaxis. Limited studies and case reports have found TMP-SMX useful against infections caused by MRSA.

CONCLUSIONS

A large body of anecdotal data, but only one randomized clinical trial, indicates the effectiveness of TMP-SMX as a treatment for MRSA infections. Double-blind, randomized controlled trials are needed to compare the two available oral agents-TMP-SMX and linezolid-against MRSA.

Impact of high-inoculum Staphylococcus aureus on the activities of nafcillin, vancomycin, linezolid, and daptomycin, alone and in combination with gentamicin, in an in vitro pharmacodynamic model

We evaluated the impact of high (9.5 log10 CFU/g) and moderate (5.5 log10 CFU/g) inocula of methicillin-susceptible and -resistant Staphylococcus aureus (MSSA and MRSA, respectively) on the activities of nafcillin, linezolid, vancomycin, and daptomycin, alone and in combination with gentamicin in an in vitro pharmacodynamic model with simulated endocardial vegetations over 72 h. Human therapeutic dosing regimens for nafcillin, daptomycin, vancomycin, linezolid, and gentamicin were simulated. At a moderate inoculum, nafcillin (MSSA only), vancomycin, and daptomycin demonstrated equivalent and significant (P < 0.01) bactericidal (99.9% kill) activities (decreases of 3.34 +/- 1.1, 3.28 +/- 0.4, and 3.34 +/- 0.8 log10 CFU/g, respectively). Bactericidal activity was demonstrated at 4 h for nafcillin and daptomycin and at 32 h for vancomycin. Linezolid demonstrated bacteriostatic activity over the course of the study period. At a high inoculum, daptomycin exhibited bactericidal activity against both MSSA and MRSA by 24 h (decrease of 5.51 to 6.31 +/- 0.10 log10 CFU/g). Nafcillin (versus MSSA), vancomycin, and linezolid (MSSA and MRSA) did not achieve bactericidal activity throughout the 72-h experiment. The addition of gentamicin increased the rate of 99.9% kill to 8 h for daptomycin (P < 0.01) and 48 h for nafcillin (MSSA only) (P = 0.01). The addition of gentamicin did not improve the activity of vancomycin or linezolid for either isolate for the 72-h period. Overall, high-inoculum Staphylococcus aureus had a significant impact on the activities of nafcillin and vancomycin. In contrast, daptomycin was affected minimally and linezolid was not affected by inoculum.