Levofloxacin versus ciprofloxacin, flucloxacillin, or vancomycin for treatment of experimental endocarditis due to methicillin-susceptible or -resistant Staphylococcus aureus

Comparing the Synergistic and Antagonistic Interactions of Ciprofloxacin and Levofloxacin Combined with Rifampin against Drug-Resistant Staphylococcus aureus: A Time-Kill Assay

BACKGROUND

Treatment of device-related infections by drug-resistant Staphylococcus aureus can be challenging, and combination therapy has been proposed as a potential solution. We compared the effectiveness of levofloxacin-rifampin and ciprofloxacin-rifampin combinations in killing methicillin-resistant S. aureus (MRSA) using a time-kill assay.

METHODS

We randomly selected 15 vancomycin-susceptible S. aureus (VSSA) strains, 3 vancomycin-intermediate S. aureus (VISA) strains, and 12 heterogeneous VISA (hVISA) strains from the Asian Bacterial Bank. Time-kill experiments were performed in duplicate for each isolate. Viable bacterial counts were determined at 0 h, 4 h, 8 h, and 24 h for the ciprofloxacin- and levofloxacin-rifampin combinations at 1× MIC and 0.5× MIC. We compared synergistic and antagonistic interactions between the two combinations.

RESULTS

The viable bacterial count significantly decreased after 24 h of exposure to ciprofloxacin-rifampin and levofloxacin-rifampin combinations, with synergy observed more frequently in isolates exposed to ciprofloxacin-rifampin (43.3%) than levofloxacin-rifampin (20.0%) (p = 0.0082). The synergistic interactions of both combinations were more frequently observed in resistant strains with high MICs of ciprofloxacin (≥16 mg/L) and levofloxacin (≥8 mg/L). Levofloxacin tended to exhibit more frequent antagonistic interactions with rifampin than ciprofloxacin, although there was no statistical difference in antagonism between the two combinations.

CONCLUSIONS

Our study demonstrated that ciprofloxacin exhibits superior synergistic activity against MRSA strains, including VISA/hVISA, when combined with rifampin compared with levofloxacin. High MICs of fluoroquinolones were found to predict synergism. Our results suggest that ciprofloxacin may be a more effective choice than levofloxacin for combination therapy with rifampin in the treatment of MRSA infections.

Pseudomonas aeruginosa: Infections, Animal Modeling, and Therapeutics

Pseudomonas aeruginosa is an important Gram-negative opportunistic pathogen which causes many severe acute and chronic infections with high morbidity, and mortality rates as high as 40%. What makes P. aeruginosa a particularly challenging pathogen is its high intrinsic and acquired resistance to many of the available antibiotics. In this review, we review the important acute and chronic infections caused by this pathogen. We next discuss various animal models which have been developed to evaluate P. aeruginosa pathogenesis and assess therapeutics against this pathogen. Next, we review current treatments (antibiotics and vaccines) and provide an overview of their efficacies and their limitations. Finally, we highlight exciting literature on novel antibiotic-free strategies to control P. aeruginosa infections.

Addition of daptomycin to levofloxacin increased the efficacy of levofloxacin monotherapy against a methicillin-susceptible Staphylococcus aureus strain in experimental meningitis and prevented development of resistance in vitro

Introduction. Daptomycin and levofloxacin were tested as monotherapies and in combination against the antibiotic-susceptible S. aureus strain MSSA 1112 in a rabbit meningitis model and the effect of the combination on induction of resistance was determined in vitro.

Hypothesis/Gap Statement. Treatment combination might be more efficacious than the monotherapies regarding antibacterial efficacy in vivo and development of resistance.

Aim. To demonstrate a synergy of the antibiotic combination daptomycin and levofloxacin in experimental meningitis, and also its ability to reduce the emergence of resistance against the antibiotics in vitro.

Methodology. Changes of the susceptibility to fluoroquinolones and daptomycin were determined by the measurement of the MIC and mutations were detected by whole genome sequence comparison of the mutants with the parent strain MSSA 1112. Meningitis was induced by intracisternal inoculation of 105 c.f.u. (colony forming unit) of MSSA 1112 and treatment was started 10 h later by injection of daptomycin (15 mg kg−1) and levofloxacin (10 mg kg−1) standard doses. Cerebrospinal fluid (CSF) samples were repeatedly collected during therapy in order to determine killing rates and results of bactericidal activity were expressed in Δlog10c.f.u. ml-1 over 8 h.

Results. The combination of daptomycin with levofloxacin was significantly (P<0.001) superior to levofloxacin monotherapy and increased the antibacterial activity of daptomycin. In vitro, MSSA 1112 was cycled over 6 days with either increasing concentrations of levofloxacin or daptomycin or with a combination of levofloxacin with half of the MIC of daptomycin or daptomycin with half of the MIC of levofloxacin leading to mutations in target genes as identified by whole genome sequence analysis. Addition of low concentration of daptomycin (0.25 mg l−1) reduced levofloxacin-induced resistance in vitro. Addition of levofloxacin in low concentration (0.125 mg l−1) did not influence daptomycin-induced resistance.

Conclusion. These findings highlight the lack of reciprocal interference of antibiotics in combination with regard to the development of resistance.

Solar photo-Fenton disinfection of 11 antibiotic-resistant bacteria (ARB) and elimination of representative AR genes. Evidence that antibiotic resistance does not imply resistance to oxidative treatment

The emergence of antibiotic resistance represents a major threat to human health. In this work we investigated the elimination of antibiotic resistant bacteria (ARB) by solar light and solar photo-Fenton processes. As such, we have designed an experimental plan in which several bacterial strains (Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae) possessing different drug-susceptible and -resistant patterns and structures (Gram-positive and Gram-negative) were subjected to solar light and the photo-Fenton oxidative treatment in water. We showed that both solar light and solar photo-Fenton processes were effective in the elimination of ARB in water and that the time necessary for solar light disinfection and solar photo-Fenton disinfection were similar for antibiotic-susceptible and antibiotic-resistant strains (mostly 180-240 and 90-120 min, respectively). Moreover, the bacterial structure did not significantly affect the effectiveness of the treatment. Similar regrowth pattern was observed (compared to the susceptible strain) and no development of bacteria with higher drug-resistance values was found in waters after any treatment. Finally, both processes were effective to reduce AR genes (ARGs), although solar photo-Fenton was more rapid than solar light. In conclusion, the solar photo-Fenton process ensured effective disinfection of ARB and elimination of ARGs in water (or wastewater) and is a potential mean to ensure limitation of ARB and ARG spread in nature.

Flucloxacillin: A Review of Characteristics, Properties and Analytical Methods

Bacterial resistance is a growing and worrying factor. The high reproducibility of these resistant microorganisms tends to influence the development of new drugs and research related to product quality control. Among the existing antimicrobials, flucloxacillin (FLU) was designed for oral and injectable administration with bactericidal activity. FLU sodium is the form used in pharmaceutical formulations. It is an antimicrobial resistant against penicillinase, an enzyme responsible for cleaving the beta-lactam ring of penicilins, which leads to inactivity of the drug. Qualitative and quantitative analyzes are essential to ensure quality of pharmaceuticals and health of the population. It is important that quality control is effective and appropriate, only then we can win the battle against microbial resistance. In this work, we want to highlight tthe characteristics of FLU as an important antibiotic and methods for the determination of FLU in pharmaceutical products and biological matrices. Among the analytical methods described in the literature for the determination of FLU, high performance liquid chromatography (HPLC) stands out. Anyway, this method uses toxic solvents (e.g. acetonitrile) long columns, which provide long runs, as well as produces large amounts of waste. Currently, the priority changed to develop ecologically correct, conscious and sustainable methods. This new view on analytical methods should be applied to FLU analyzes and used to develop and improve existing methods.

Synergistic Interaction Between Phage Therapy and Antibiotics Clears Pseudomonas Aeruginosa Infection in Endocarditis and Reduces Virulence

Background.

Increasing antibiotic resistance warrants therapeutic alternatives. Here we investigated the efficacy of bacteriophage-therapy (phage) alone or combined with antibiotics against experimental endocarditis (EE) due to Pseudomonas aeruginosa, an archetype of difficult-to-treat infection.

Methods.

In vitro fibrin clots and rats with aortic EE were treated with an antipseudomonas phage cocktail alone or combined with ciprofloxacin. Phage pharmacology, therapeutic efficacy, and resistance were determined.

Results.

In vitro, single-dose phage therapy killed 7 log colony-forming units (CFUs)/g of fibrin clots in 6 hours. Phage-resistant mutants regrew after 24 hours but were prevented by combination with ciprofloxacin (2.5 × minimum inhibitory concentration). In vivo, single-dose phage therapy killed 2.5 log CFUs/g of vegetations in 6 hours (P < .001 vs untreated controls) and was comparable with ciprofloxacin monotherapy. Moreover, phage/ciprofloxacin combinations were highly synergistic, killing >6 log CFUs/g of vegetations in 6 hours and successfully treating 64% (n = 7/11) of rats. Phage-resistant mutants emerged in vitro but not in vivo, most likely because resistant mutations affected bacterial surface determinants important for infectivity (eg, the pilT and galU genes involved in pilus motility and LPS formation).

Conclusions.

Single-dose phage therapy was active against P. aeruginosa EE and highly synergistic with ciprofloxacin. Phage-resistant mutants had impaired infectivity. Phage-therapy alone or combined with antibiotics merits further clinical consideration.

The importance of 2D aggregates on the antimicrobial resistance of Staphylococcus aureus sessile bacteria

Biofilms formed on implanted devices are difficult to eradicate. Adhesion mechanism, high bacterial density, aggregation, induction of persisters and stressed bacteria are some of the factors considered when the antimicrobial resistance of these biofilms is analyzed. The aim of this work was to provide an alternative approach to the understanding of this issue by using a specially designed experimental set up that includes the use of microstructured (MS) surfaces (potential inhibitors of bacterial aggregation) in combination with antimicrobial agents (streptomycin and levofloxacin) against Staphylococcusaureus attached cells. Biofilms formed on smooth surfaces were used as plain controls (biofilmed-PC) characterized by the formation of dense 2D bacterial aggregates. Results showed bacterial persistence when streptomycin or levofloxacin were applied to PC-biofilms. The antimicrobial activity of both antibiotics was enhanced when bacteria were attached on MS, where single cells or small aggregates were observed. Thus, dense 2D aggregates of bacteria seem to be crucial as a required previous stage to develop the antimicrobial resistance.

Elucidation of DnaE as the Antibacterial Target of the Natural Product, Nargenicin

Resistance to existing classes of antibiotics drives the need for discovery of novel compounds with unique mechanisms of action. Nargenicin A1, a natural product with limited antibacterial spectrum, was rediscovered in a whole-cell antisense assay. Macromolecular labeling in both Staphylococcus aureus and an Escherichia coli tolC efflux mutant revealed selective inhibition of DNA replication not due to gyrase or topoisomerase IV inhibition. S. aureus nargenicin-resistant mutants were selected at a frequency of ∼1 × 10(-9), and whole-genome resequencing found a single base-pair change in the dnaE gene, a homolog of the E. coli holoenzyme α subunit. A DnaE single-enzyme assay was exquisitely sensitive to inhibition by nargenicin, and other in vitro characterization studies corroborated DnaE as the target. Medicinal chemistry efforts may expand the spectrum of this novel mechanism antibiotic.

Low efficacy of tobramycin in experimental Staphylococcus aureus endocarditis

The empiric treatment of infective endocarditis (IE) varies widely and, in some places, a regimen of penicillin in combination with an aminoglycoside is administered. The increasing incidence of Staphylococcus aureus IE, poor tissue penetration by aminoglycosides and low frequency of penicillin-susceptible S. aureus may potentially lead to functional tobramycin monotherapy. Therefore, this study aimed to evaluate tobramycin monotherapy in an experimental S. aureus IE rat model. Catheter-induced IE at the aortic valves were established with S. aureus (NCTC 8325-4) and rats were randomised into untreated (n = 22) or tobramycin-treated (n = 13) groups. The treatment group received tobramycin once-daily. Animals were evaluated at 1 day post infection (DPI), 2 DPI or 3 DPI. Quantitative bacteriology and cytokine expression were measured for valves, myocardium and serum. A decrease of bacterial load was observed in valves and the spleens of the treated (n = 6) compared to the untreated group at 2 DPI (n = 8) (p ≤ 0.02 and p ≤ 0.01, respectively), but not at 3 DPI (n = 7). Quantitative bacteriology in the myocardium was not different between the groups. Keratinocyte-derived chemokine (KC) in the aortic valves was significantly reduced at 2 DPI in the tobramycin-treated group (p ≤ 0.03). However, the expression of interleukin (IL)-1b, IL-6 and granulocyte-colony stimulating factor (G-CSF) in the valves was not different between the two groups. In the myocardium, a significant reduction in IL-1b was observed at 2 DPI (p ≤ 0.001) but not at 3 DPI. Tobramycin as functional monotherapy only reduced bacterial load and inflammation transiently, and was insufficient in most cases of S. aureus IE.

In vivo effect of flucloxacillin in experimental endocarditis caused by mecC-positive staphylococcus aureus showing temperature-dependent susceptibility in vitro

Methicillin-resistant Staphylococcus aureus (MRSA) carrying the mecC gene (mecC-MRSA) exhibited at 37°C MICs of oxacillin close to those of methicillin-susceptible S. aureus (MSSA). We investigated whether at this temperature, mecC-MRSA strains respond to flucloxacillin treatment like MSSA strains, using a rat model of endocarditis. Flucloxacillin (human-like kinetics of 2 g intravenously every 6 h) cured 80 to 100% of aortic vegetations infected with five different mecC-MRSA strains. These results suggest that mecC-MRSA infections may successfully respond to treatment with β-lactams.

Oral antibiotic therapy for the treatment of infective endocarditis: a systematic review

Background

The role of oral antibiotic therapy in treating infective endocarditis (IE) is not well established.

Methods

We searched MEDLINE, EMBASE and Scopus for studies in which oral antibiotic therapy was used for the treatment of IE.

Results

Seven observational studies evaluating the use oral beta-lactams (five), oral ciprofloxacin in combination with rifampin (one), and linezolid (one) for the treatment of IE caused by susceptible bacteria reported cure rates between 77% and 100%. Two other observational studies using aureomycin or sulfonamide, however, had failure rates >75%. One clinical trial comparing oral amoxicillin versus intravenous ceftriaxone for streptococcal IE reported 100% cure in both arms but its reporting had serious methodological limitations. One small clinical trial (n = 85) comparing oral ciprofloxacin and rifampin versus conventional intravenous antibiotic therapy for uncomplicated right-sided S. aureus IE in intravenous drug users (IVDUs) reported cure rates of 89% and 90% in each arm, respectively (P =0.9); however, drug toxicities were more common in the latter group (62% versus 3%; P <0.01). Major limitations of this trial were lack of allocation concealment and blinding at the delivery of the study drug(s) and assessment of outcomes.

Conclusion

Reported cure rates for IE treated with oral antibiotic regimens vary widely. The use of oral ciprofloxacin in combination with rifampin for uncomplicated right-sided S. aureus IE in IVDUs is supported by one small clinical trial of relatively good quality and could be considered when conventional IV antibiotic therapy is not possible.

Staphylococcus aureus mutants lacking the LytR-CpsA-Psr family of enzymes release cell wall teichoic acids into the extracellular medium

The LytR-CpsA-Psr (LCP) proteins are thought to transfer bactoprenol-linked biosynthetic intermediates of wall teichoic acid (WTA) to the peptidoglycan of Gram-positive bacteria. In Bacillus subtilis, mutants lacking all three LCP enzymes do not deposit WTA in the envelope, while Staphylococcus aureus Δlcp mutants display impaired growth and reduced levels of envelope phosphate. We show here that the S. aureus Δlcp mutant synthesized WTA yet released ribitol phosphate polymers into the extracellular medium. Further, Δlcp mutant staphylococci no longer restricted the deposition of LysM-type murein hydrolases to cell division sites, which was associated with defects in cell shape and increased autolysis. Mutations in S. aureus WTA synthesis genes (tagB, tarF, or tarJ2) inhibit growth, which is attributed to the depletion of bactoprenol, an essential component of peptidoglycan synthesis (lipid II). The growth defect of S. aureus tagB and tarFJ mutants was alleviated by inhibition of WTA synthesis with tunicamycin, whereas the growth defect of the Δlcp mutant was not relieved by tunicamycin treatment or by mutation of tagO, whose product catalyzes the first committed step of WTA synthesis. Further, sortase A-mediated anchoring of proteins to peptidoglycan, which also involves bactoprenol and lipid II, was not impaired in the Δlcp mutant. We propose a model whereby the S. aureus Δlcp mutant, defective in tethering WTA to the cell wall, cleaves WTA synthesis intermediates, releasing ribitol phosphate into the medium and recycling bactoprenol for peptidoglycan synthesis.

Streptococcus tigurinus is highly virulent in a rat model of experimental endocarditis

Streptococcus tigurinus is responsible for systemic infections in humans including infective endocarditis. We investigated whether the invasive trait of S. tigurinus in humans correlated with an increased ability to induce IE in rats. Rats with catheter-induced aortic vegetations were inoculated with 10⁴ CFU/ml of either of four S. tigurinus strains AZ_3a(T), AZ_4a, AZ_8 and AZ_14, isolated from patients with infective endocarditis or with the well known IE pathogen Streptococcus gordonii (Challis). Aortic infection was assessed after 24 h. S. tigurinus AZ_3a(T), AZ_4a and AZ_14 produced endocarditis in ≥80% of rats whereas S. gordonii produced endocarditis in only 33% of animals (P<0.05). S. tigurinus AZ_8 caused vegetation infection in 56% of the animals. The capacity of S. tigurinus to induce aortic infection was not related to their ability to bind extracellular matrix proteins (fibrinogen, fibronectin or collagen) or to trigger platelet aggregation. However, all S. tigurinus isolates showed an enhanced resistance to phagocytosis by macrophages and two of them had an increased ability to enter endothelial cells, key attributes of invasive streptococcal species.

The Staphylococcus aureus extracellular adherence protein promotes bacterial internalization by keratinocytes independent of fibronectin-binding proteins

Staphylococcus aureus, the leading causal pathogen of skin infections, is strongly associated with skin atopy, and a number of bacterial adhesins allow the microbe to adhere to and invade eukaryotic cells. One of these adhesive molecules is the multifunctional extracellular adherence protein (Eap), which is overexpressed in situ in authentic human wounds and was shown to delay wound healing in experimental models. Yet, its role during invasion of keratinocytes is not clearly defined. By using a gentamicin/lysostaphin protection assay we demonstrate here that preincubation of HaCaT cells or primary keratinocytes with Eap results in a concentration-dependent significant increase in staphylococcal adhesion, followed by an even more pronounced internalization of bacteria by eukaryotic cells. Flow cytometric analysis revealed that Eap increased both the number of infected eukaryotic cells and the bacterial load per infected cell. Moreover, treatment of keratinocytes with Eap strongly enhanced the internalization of coagulase-negative staphylococci, as well as of E. coli, and markedly promoted staphylococcal invasion into extended-culture keratinocytes, displaying expression of keratin 10 and involucrin as differentiation markers. Thus, wound-related staphylococcal Eap may provide a major cellular invasin function, thereby enhancing the pathogen's ability to hide from the host immune system during acute and chronic skin infection.

Haemin represses the haemolytic activity of Staphylococcus aureus in an Sae-dependent manner

Staphylococcus aureus is a major human pathogen and a common cause of nosocomial infections. This facultative pathogen produces a large arsenal of virulence factors, including the haemolysins, which allow the bacterium to lyse erythrocytes and thereby release large amounts of the haem-containing haemoglobin. The released haem is thought to be the main iron source of this organism during the course of infection, and is considered to be crucial for bacterial proliferation in vivo. High concentrations of haem and its degradation products, on the other hand, are known to be toxic for S. aureus, making it essential for the pathogen to tightly control haem release from red blood cells. Here we show that S. aureus responds to haemin by downregulating the expression of haemolysins. Subinhibitory concentrations of haemin were found to significantly reduce transcription of the haemolysin genes hlb (encoding β-haemolysin) and hlgA (encoding the S-class component of γ-haemolysin), while hla (encoding α-haemolysin) and RNAIII (encoding δ-haemolysin) transcription did not appear to be affected. The presence of haemin also reduced the haemolytic potential of the supernatants of S. aureus LS1 cultures. Inactivation of the sae locus in LS1 abolished the haemin effect on the transcription of haemolysin genes, indicating that the two-component regulatory system is required for this regulatory effect. Iron limitation, on the other hand, was found to induce the expression of haemolysins, and this effect was again abolished in the sae mutant, indicating that S. aureus modulates its haemolysin production in response to iron and haem availability in an Sae-dependent manner.

LytR-CpsA-Psr proteins in Staphylococcus aureus display partial functional redundancy and the deletion of all three severely impairs septum placement and cell separation

Staphylococcus aureus contains three members of the LytR-CpsA-Psr (LCP) family of membrane proteins: MsrR, SA0908 and SA2103. The characterization of single-, double- and triple-deletion mutants revealed distinct phenotypes for each of the three proteins. MsrR was involved in cell separation and septum formation and influenced β-lactam resistance; SA0908 protected cells from autolysis; and SA2103, although displaying no apparent phenotype by itself, enhanced the properties of msrR and sa0908 mutants when deleted. The deletion of sa0908 and sa2103 also further attenuated the virulence of msrR mutants in a nematode-killing assay. The severely defective growth phenotype of the triple mutant revealed that LytR-CpsA-Psr proteins are essential for optimal cell division in S. aureus. Growth could be rescued to varying degrees by any one of the three proteins, indicating some functional redundancy within members of this protein family. However, differing phenotypic characteristics of all single and double mutants and complemented triple mutants indicated that each protein played a distinct role(s) and contributed differently to phenotypes influencing cell separation, autolysis, cell surface properties and virulence.

Natural variability of in vitro adherence to fibrinogen and fibronectin does not correlate with in vivo infectivity of Staphylococcus aureus

Adherence to fibrinogen and fibronectin plays a crucial role in Staphylococcus aureus experimental endocarditis. Previous genetic studies have shown that infection and carriage isolates do not systematically differ in their virulence-related genes, including genes conferring adherence, such as clfA and fnbA. We set out to determine the range of adherence phenotypes in carriage isolates of S. aureus, to compare the adherence of these isolates to the adherence of infection isolates, and to determine the relationship between adherence and infectivity in a rat model of experimental endocarditis. A total of 133 healthy carriage isolates were screened for in vitro adherence to fibrinogen and fibronectin, and 30 isolates were randomly chosen for further investigation. These 30 isolates were compared to 30 infective endocarditis isolates and 30 blood culture isolates. The infectivities of the carriage isolates, which displayed either extremely low or high adherence to fibrinogen and fibronectin, were tested using a rat model of experimental endocarditis. The levels of adherence to both fibrinogen and fibronectin were very similar for isolates from healthy carriers and members of the two groups of infection isolates. All three groups of isolates showed a wide range of adherence to fibrinogen and fibronectin. Moreover, the carriage isolates that showed minimal adherence and the carriage isolates that showed strong adherence had the same infectivity in experimental endocarditis. Adherence was proven to be important for pathogenesis in experimental endocarditis, but even the least adherent carriage strains had the ability to induce infection. We discuss the roles of differential gene expression, human host factors, and gene redundancy in resolving this apparent paradox.

Penicillin-binding protein gene alterations in Streptococcus uberis isolates presenting decreased susceptibility to penicillin

Streptococcus uberis is an environmental pathogen commonly causing bovine mastitis, an infection that is generally treated with penicillin G. No field case of true penicillin-resistant S. uberis (MIC > 16 mg/liter) has been described yet, but isolates presenting decreased susceptibility (MIC of 0.25 to 0.5 mg/liter) to this drug are regularly reported to our laboratory. In this study, we demonstrated that S. uberis can readily develop penicillin resistance in laboratory-evolved mutants. The molecular mechanism of resistance (acquisition of mutations in penicillin-binding protein 1A [PBP1A], PBP2B, and PBP2X) was generally similar to that of all other penicillin-resistant streptococci described so far. In addition, it was also specific to S. uberis in that independent resistant mutants carried a unique set of seven consensus mutations, of which only one (Q(554)E in PBP2X) was commonly found in other streptococci. In parallel, independent isolates from bovine mastitis with different geographical origins (France, Holland, and Switzerland) and presenting a decreased susceptibility to penicillin were characterized. No mosaic PBPs were detected, but they all presented mutations identical to the one found in the laboratory-evolved mutants. This indicates that penicillin resistance development in S. uberis might follow a stringent pathway that would explain, in addition to the ecological niche of this pathogen, why naturally occurring resistances are still rare. In addition, this study shows that there is a reservoir of mutated PBPs in animals, which might be exchanged with other streptococci, such as Streptococcus agalactiae, that could potentially be transmitted to humans.

MsrR contributes to cell surface characteristics and virulence in Staphylococcus aureus

MsrR, a factor contributing to methicillin resistance in Staphylococcus aureus, belongs to the LytR-CpsA-Psr family of cell envelope-associated proteins. Deletion of msrR increased cell size and aggregation, and altered envelope properties, leading to a temporary reduction in cell surface hydrophobicity, diminished colony-spreading ability, and an increased susceptibility to Congo red. The reduced phosphorus content of purified cell walls of the msrR mutant suggested a reduction in wall teichoic acids, which may explain some of the observed phenotypes. Microarray analysis of the msrR deletion mutant revealed only minor changes in the global transcriptome, suggesting that MsrR has structural rather than regulatory functions. Importantly, virulence of the msrR mutant was decreased in a nematode-killing assay as well as in rat experimental endocarditis. MsrR is therefore likely to play a role in cell envelope maintenance, cell separation, and pathogenicity of S. aureus.

De novo design and in vivo activity of conformationally restrained antimicrobial arylamide foldamers

The emergence of drug-resistant bacteria has compromised the use of many conventional antibiotics, leading to heightened interest in a variety of antimicrobial peptides. Although these peptides have attractive potential as antibiotics, their size, stability, tissue distribution, and toxicity have hampered attempts to harness these capabilities. To address such issues, we have developed small (molecular mass <1,000 Da) arylamide foldamers that mimic antimicrobial peptides. Hydrogen-bonded restraints in the arylamide template rigidify the conformation via hydrogen bond formation and increase activity toward Staphylococcus aureus and Escherichia coli. The designed foldamers are highly active against S. aureus in an animal model. These results demonstrate the application of foldamer templates as therapeutics.

De novo designed synthetic mimics of antimicrobial peptides

Antimicrobial peptides are small cationic amphiphiles that play an important role in the innate immune system. Given their broad specificity, they appear to be ideal therapeutic agents. As a result, over the last decade, there has been considerable interest in developing them as intravenously administered antibiotics. However, it has proven difficult to accomplish this goal with peptide-based structures. Although it has been possible to solve some relatively simple problems such as susceptibility to proteolysis, more severe problems have included the expense of the materials, toxicity, limited efficacy, and limited tissue distribution. In an effort to overcome these problems, we developed small synthetic oligomers designed to adopt amphiphilic conformations and exhibit potent antimicrobial activity while being nontoxic to host cells. One class of these synthetic mimics of antimicrobial peptides (SMAMPs) is being developed as intravenous antibiotics.

Single-dose oral amoxicillin or linezolid for prophylaxis of experimental endocarditis due to vancomycin-susceptible and vancomycin-resistant Enterococcus faecalis

Endocarditis prophylaxis following genitourinary or gastrointestinal procedures targets Enterococcus faecalis. Prophylaxis recommendations advocate oral amoxicillin (2 g in the United States and 3 g in the United Kingdom) in moderate-risk patients and intravenous amoxicillin (2 g) or vancomycin (1 g) plus gentamicin in high-risk patients. While ampicillin-resistant (or amoxicillin-resistant) E. faecalis is still rare, there is a concern that these regimens might fail against vancomycin-resistant and/or aminoglycoside-resistant isolates. The present study tested oral linezolid as an alternative. Rats with catheter-induced aortic vegetations were given prophylaxis simulating human pharmacokinetics of oral amoxicillin (2- to 3-g single dose), oral linezolid (600 mg, single or multiple oral doses every 12 h), or intravenous vancomycin (1-g single dose). Rats were then inoculated with the minimum inoculum infecting 90% of the animals (90% infective dose [ID(90)]) or with 10 times the ID(90) of the vancomycin-susceptible E. faecalis strain JH2-2 or the vancomycin-resistant (VanA phenotype) E. faecalis strain UCN41. Amoxicillin was also tested with two additional vancomycin-susceptible E. faecalis strains, 309 and 1209. Animals were sacrificed 3 days later. All the tested bacteria were susceptible to amoxicillin and gentamicin. Single-dose amoxicillin provided 100% protection against all four isolates at both the ID(90) and 10 times the ID(90). In contrast, linezolid required up to four consecutive doses to provide full protection against the vancomycin-resistant isolate. Vancomycin protected only against the vancomycin-susceptible strain. The high efficacy of single-dose oral amoxicillin suggests that this regimen could be used for prophylaxis in both moderate-risk and high-risk patients without additional aminoglycosides. Linezolid appears to be less reliable, at least against the vancomycin-resistant strain.

New agents for Staphylococcus aureus endocarditis

PURPOSE OF REVIEW

The increasing prevalence of methicillin-resistant Staphylococcus aureus (MRSA) as well as newly discovered S. aureus strains with reduced susceptibility to vancomycin mandates development of new antistaphylococcal agents. This review summarizes currently available and forthcoming antimicrobials for treatment of S. aureus endocarditis.

RECENT FINDINGS

No new antimicrobial has been proven superior to antistaphylococcal penicillins for treatment of methicillin-sensitive S. aureus (MSSA) endocarditis. Vancomycin has become standard treatment for MRSA but poor outcomes have been reported, both with susceptible and intermediately resistant S. aureus strains (VISA). Linezolid has successfully treated individual cases of MRSA endocarditis, but limitations include long-term safety. Daptomycin has recently been proven effective and well tolerated for MSSA and MRSA bacteremia, including right-sided endocarditis. New glycopeptides, including dalbavancin and telavancin, as well as the new cephalosporin ceftobiprole, have not yet been studied for treatment of endocarditis but appear active against MRSA and potentially VISA.

SUMMARY

Antistaphylococcal penicillins remain the treatment of choice for MSSA. Of the currently available newer agents, daptomycin appears to have the most rapid bactericidal activity and provides a much-needed alternative to vancomycin for treatment of MRSA or MSSA bacteremia and right-sided endocarditis.

Mutations in penicillin-binding protein (PBP) genes and in non-PBP genes during selection of penicillin-resistant Streptococcus gordonii

Penicillin resistance in Streptococcus spp. involves multiple mutations in both penicillin-binding proteins (PBPs) and non-PBP genes. Here, we studied the development of penicillin resistance in the oral commensal Streptococcus gordonii. Cyclic exposure of bacteria to twofold-increasing penicillin concentrations selected for a progressive 250- to 500-fold MIC increase (from 0.008 to between 2 and 4 microg/ml). The major MIC increase (> or = 35-fold) was related to non-PBP mutations, whereas PBP mutations accounted only for a 4- to 8-fold additional increase. PBP mutations occurred in class B PBPs 2X and 2B, which carry a transpeptidase domain, but not in class A PBP 1A, 1B, or 2A, which carry an additional transglycosylase domain. Therefore, we tested whether inactivation of class A PBPs affected resistance development in spite of the absence of mutations. Deletion of PBP 1A or 2A profoundly slowed down resistance development but only moderately affected resistance in already highly resistant mutants (MIC = 2 to 4 microg/ml). Thus, class A PBPs might facilitate early development of resistance by stabilizing penicillin-altered peptidoglycan via transglycosylation, whereas they might be less indispensable in highly resistant mutants which have reestablished a penicillin-insensitive cell wall-building machinery. The contribution of PBP and non-PBP mutations alone could be individualized in DNA transformation. Both PBP and non-PBP mutations conferred some level of intrinsic resistance, but combining the mutations synergized them to ensure high-level resistance (> or = 2 microg/ml). The results underline the complexity of penicillin resistance development and suggest that inhibition of transglycosylase might be an as yet underestimated way to interfere with early resistance development.

Garenoxacin treatment of experimental endocarditis caused by viridans group streptococci

The activity of garenoxacin was compared to that of levofloxacin or penicillin in a rabbit model of Streptococcus mitis group (penicillin MIC, 0.125 microg/ml) and Streptococcus sanguinis group (penicillin MIC, 0.25 microg/ml) endocarditis. Garenoxacin and levofloxacin had MICs of 0.125 and 0.5 microg/ml, respectively, for both study isolates. Rabbits with catheter-induced aortic valve endocarditis were given no treatment, penicillin at 1.2x10(6) IU/8 h intramuscularly, garenoxacin at 20 mg/kg of body weight/12 h intravenously, or levofloxacin at 40 mg/kg/12 h intravenously. For both isolates tested, garenoxacin area under the curve (AUC)/MIC and maximum concentration of drug in serum (Cmax)/MIC ratios were 368 and 91, respectively. Rabbits were sacrificed after 3 days of treatment; cardiac valve vegetations were aseptically removed and quantitatively cultured. For S. mitis group experimental endocarditis, all studied antimicrobial agents were more active than no treatment (P<0.001), whereas for S. sanguinis group endocarditis, no studied antimicrobial agents were more active than no treatment. We conclude that AUC/MIC and Cmax/MIC ratios may not predict activity of some quinolones in experimental viridans group endocarditis and that garenoxacin and levofloxacin may not be ideal choices for serious infections caused by some quinolone-susceptible viridans group streptococci.

Efficacy of telavancin against penicillin-resistant pneumococci and Staphylococcus aureus in a rabbit meningitis model and determination of kinetic parameters

The penetration of telavancin was 2% into inflamed meninges and ca. 1 per thousand into noninflamed meninges after two intravenous injections (30 mg/kg of body weight). In experimental meningitis, telavancin was significantly superior to vancomycin combined with ceftriaxone against a penicillin-resistant pneumococcal strain. Against a methicillin-sensitive staphylococcal strain, telavancin was slightly but not significantly superior to vancomycin.

Daptomycin is more efficacious than vancomycin against a methicillin-susceptible Staphylococcus aureus in experimental meningitis

OBJECTIVES

To test the efficacy of daptomycin, a cyclic lipopeptide antibiotic, against a methicillin-susceptible Staphylococcus aureus strain in experimental rabbit meningitis and to determine its penetration into non-inflamed and inflamed meninges

RESULTS

Over a treatment period of 8 h, daptomycin (15 mg/kg) was significantly superior to the comparator regimen vancomycin (-4.54 +/- 1.12 log(10)/mL for daptomycin versus -3.43 +/- 1.17 log(10)/mL for vancomycin). Daptomycin managed to sterilize 6 out of 10 CSFs compared with 4 out of 10 for vancomycin. The penetration of daptomycin into inflamed meninges was approximately 5% and approximately 2% into non-inflamed meninges.

CONCLUSIONS

The superior bactericidal activity of daptomycin was confirmed in vivo and in time-killing assays in vitro.

LB11058, a new cephalosporin with high penicillin-binding protein 2a affinity and activity in experimental endocarditis due to homogeneously methicillin-resistant Staphylococcus aureus

LB11058 is a new synthetic cephalosporin with good affinity for staphylococcal penicillin-binding protein 2a (PBP2a). LB11058 was tested in vitro and in rats with experimental aortic endocarditis against three methicillin-resistant Staphylococcus aureus (MRSA) strains, one penicillinase-negative strain (strain COL), and two penicillinase-producing strains (COL-Bla+ and P8-Hom). The MICs of LB11058 for the organisms were 1 mg/liter. The MICs of vancomycin and ceftriaxone were 1 and >/=64 mg/liter, respectively. In population analysis profiles, none of the MRSA strains grew at >/=2 mg of LB11058/liter. Rats with endocarditis were treated for 5 days. LB11058 was highly bound to serum proteins in rats (>/=98%). However, binding was saturable above a threshold of 250 mg/liter. Therefore, continuous concentrations of 250 mg/liter in serum were infused to ensure a free fraction (>/=5 mg/liter) above the drug's MIC for the entire infusion period. Control treatments included simulation of human serum kinetics produced by intravenous vancomycin (1 g twice daily, free drug concentration above MIC, >/=90% of infusion period) or ceftriaxone (2 g/24 h, free drug concentrations above the MIC, 0% of infusion period). LB11058 successfully treated 10 of 10 (100%) and 13 of 14 (93%) of rats infected with COL-Bla+ and P8-Hom, respectively. This was comparable to vancomycin (sterilization of 8 of 12 [66%] and 6 of 8 [75%] rats, respectively). Ceftriaxone was inactive. Low concentrations of LB11058 (5 and 10 mg/liter, continuously infused) in serum were ineffective, as predicted by the pharmacodynamic parameters. At appropriate doses, LB11058 was highly effective both in vitro and in vivo. This finding supports the development of this beta-lactam with high PBP2a affinity for the treatment of MRSA infections.

Efficacy of garenoxacin in treatment of experimental endocarditis due to Staphylococcus aureus or viridans group streptococci

The activity of garenoxacin was investigated in rats with experimental endocarditis due to staphylococci and viridans group streptococci (VGS). The staphylococci tested comprised one ciprofloxacin-susceptible and methicillin-susceptible Staphylococcus aureus (MSSA) isolate (isolate 1112), one ciprofloxacin-susceptible but methicillin-resistant S. aureus (MRSA) isolate (isolate P8), and one ciprofloxacin-resistant mutant (grlA) of P8 (isolate P8-4). The VGS tested comprised one penicillin-susceptible isolate and one penicillin-resistant isolate (Streptococcus oralis 226 and Streptococcus mitis 531, respectively). To simulate the kinetics of drugs in humans, rats were infused intravenously with garenoxacin every 24 h (peak and trough levels in serum, 6.1 and 1.0 mg/liter, respectively; area under the concentration-time curve [AUC], 63.4 mg. h/liter) or levofloxacin every 12 h (peak and trough levels in serum, 7.3 and 1.5 mg/liter, respectively; AUC, 55.6 mg. h/liter) for 3 or 5 days. Flucloxacillin, vancomycin, and ceftriaxone were used as control drugs. Garenoxacin, levofloxacin, flucloxacillin, and vancomycin sterilized >/=70% of the vegetations infected with both ciprofloxacin-susceptible staphylococcal isolates (P < 0.05 versus the results for the controls). Garenoxacin and vancomycin also sterilized 70% of the vegetations infected with ciprofloxacin-resistant MRSA isolate P8-4, whereas treatment with levofloxacin failed against this organism (cure rate, 0%; P < 0.05 versus the results obtained with the comparator drugs). Garenoxacin did not select for resistant derivatives in vivo. In contrast, levofloxacin selected for resistant variants in four of six rats infected with MRSA isolate P8-4. Garenoxacin sterilized 90% of the vegetations infected with both penicillin-susceptible and penicillin-resistant isolates of VGS. Levofloxacin sterilized only 22 and 40% of the vegetations infected with penicillin-susceptible S. oralis 226 and penicillin-resistant S. mitis 531, respectively. Ceftriaxone sterilized only 40% of those infected with penicillin-resistant S. mitis 531 (P < 0.05 versus the results obtained with garenoxacin). No quinolone-resistant VGS were detected. In all the experiments successful quinolone treatment was predicted by specific pharmacodynamic criteria (D. R. Andes and W. A. Craig, Clin. Infect. Dis. 27:47-50, 1998). The fact that the activity of garenoxacin was equal or superior to those of the standard comparators against staphylococci and VGS indicates that it is a potential alternative for the treatment of infections caused by such bacteria.

Meropenem prevents levofloxacin-induced resistance in penicillin-resistant pneumococci and acts synergistically with levofloxacin in experimental meningitis

The aim of the present study was to investigate the potential synergy between meropenem and levofloxacin in vitro and in experimental meningitis and to determine the effect of meropenem on levofloxacin-induced resistance in vitro. Meropenem increased the efficacy of levofloxacin against the penicillin-resistant pneumococcal strain KR4 in time-killing assays in vitro and acted synergistically against a second penicillin-resistant strain WB4. In the checkerboard, only an additive effect (FIC indices: 1.0) was observed for both strains. In cycling experiments in vitro, levofloxacin alone led to a 64-fold increase in the MIC for both strains after 12 cycles. Addition of meropenem in sub-MIC concentrations (0.25 x MIC) completely inhibited the selection of levofloxacin-resistant mutants in WB4 after 12 cycles. In KR4, the addition of meropenem led to just a twofold increase in the MIC for levofloxacin after 12 cycles. Mutations detected in the genes encoding for topoisomerase IV (parC) and gyrase (gyrA) confirmed the levofloxacin-induced resistance in both strains. Addition of meropenem was able to completely suppress levofloxacin-induced mutations in WB4 and led to only one mutation in parE in KR4. In experimental meningitis, meropenem, given in two doses (2 x 125 mg/kg), produced a good bactericidal activity (-0.45 Deltalog10 cfu/ml.h) comparable to one dose (1 x 10 mg/kg) of levofloxacin (-0.44 Deltalog10 cfu/ml.h) against the penicillin-resistant strain WB4. Meropenem combined with levofloxacin acted synergistically (-0.93 Deltalog10 cfu/ml.h), sterilizing the CSF of all rabbits.

MsrR, a putative cell envelope-associated element involved in Staphylococcus aureus sarA attenuation

A novel membrane-associated protein, MsrR, was identified in Staphylococcus aureus which affects resistance to methicillin and teicoplanin, as well as the synthesis of virulence factors. MsrR belongs to the LytR-CpsA-Psr family of cell envelope-related transcriptional attenuators and was shown to be inducible by cell wall-active agents, such as beta-lactams, glycopeptides, and lysostaphin. The expression of msrR peaked in the early exponential growth phase and decreased sharply thereafter. msrR mutants showed increased sarA transcription and an earlier and higher expression of RNAIII, resulting in altered expression of virulence factors such as alpha-toxin and protein A. These observations suggest that MsrR is a new component involved in sarA attenuation and the regulatory network controlling virulence gene expression in S. aureus.

Cefotaxime acts synergistically with levofloxacin in experimental meningitis due to penicillin-resistant pneumococci and prevents selection of levofloxacin-resistant mutants in vitro

Cefotaxime, given in two doses (each 100 mg/kg of body weight), produced a good bactericidal activity (-0.47 Deltalog(10) CFU/ml. h) which was comparable to that of levofloxacin (-0.49 Deltalog(10) CFU/ml. h) against a penicillin-resistant pneumococcal strain WB4 in experimental meningitis. Cefotaxime combined with levofloxacin acted synergistically (-1.04 Deltalog(10) CFU/ml. h). Synergy between cefotaxime and levofloxacin was also demonstrated in vitro in time killing assays and with the checkerboard method for two penicillin-resistant strains (WB4 and KR4). Using in vitro cycling experiments, the addition of cefotaxime in sub-MIC concentrations (one-eighth of the MIC) drastically reduced levofloxacin-induced resistance in the same two strains (64-fold increase of the MIC of levofloxacin after 12 cycles versus 2-fold increase of the MIC of levofloxacin combined with cefotaxime). Mutations detected in the genes encoding topoisomerase IV (parC and parE) and gyrase (gyrA and gyrB) confirmed the levofloxacin-induced resistance in both strains. Addition of cefotaxime in low doses was able to suppress levofloxacin-induced resistance.

Comparison of levofloxacin, alatrofloxacin, and vancomycin for prophylaxis and treatment of experimental foreign-body-associated infection by methicillin-resistant Staphylococcus aureus

The prophylactic and therapeutic activities of two fluoroquinolones, levofloxacin and alatrofloxacin (the L-Ala-L-Ala prodrug of trovafloxacin), were compared to those of vancomycin in two different experimental models of foreign-body-associated infections caused by methicillin-resistant but quinolone-susceptible Staphylococcus aureus (MRSA) isolates. In a guinea pig model of prophylaxis, subcutaneously implanted tissue cages were infected with 10(3) CFU of MRSA, which was a 100% infectious dose in control animals. A single dose of 50 mg of levofloxacin per kg of body weight, administered intraperitoneally 3 h before bacterial challenge, was more efficient than vancomycin for the prevention of infections in tissue cages with MRSA inocula of 10(4) and 10(5) CFU. In a rat model used to evaluate therapy of chronic tissue cage infection caused by MRSA, the efficacies of 7-day high-dose regimens of levofloxacin (100 mg/kg once a day [q.d.] or 50 mg/kg twice a day [b.i.d.]) or alatrofloxacin (50 mg/kg q.d.) were compared to the efficacy of vancomycin (50 mg/kg b.i.d.). Active levels of levofloxacin, trovafloxacin, and vancomycin were continuously present in tissue cage fluid, with the levels exceeding the minimal bactericidal concentrations for MRSA during therapy. The q.d. and b.i.d. regimens of levofloxacin had equivalent activities and were significantly (P < 0.05) more active than alatrofloxacin or vancomycin in decreasing the viable counts of MRSA in tissue cage fluids. No quinolone-resistant mutants emerged during therapy with either fluoroquinolone. The mechanisms explaining the inferior activity of alatrofloxacin compared to the activity of levofloxacin against chronic foreign-body-associated infections by MRSA are unknown.

Efficacies of moxifloxacin, ciprofloxacin, and vancomycin against experimental endocarditis due to methicillin-resistant Staphylococcus aureus expressing various degrees of ciprofloxacin resistance

The new 8-methoxyquinolone moxifloxacin was tested against two ciprofloxacin-susceptible Staphylococcus aureus strains (strains P8 and COL) and two ciprofloxacin-resistant derivatives of strain P8 carrying a single grlA mutation (strain P8-4) and double grlA and gyrA mutations (strain P8-128). All strains were resistant to methicillin. The MICs of ciprofloxacin and moxifloxacin were 0.5 and 0.125 mg/liter, respectively, for P8; 0.25 and 0.125 mg/liter, respectively, for COL; 8 and 0.25 mg/liter, respectively, for P8-4; and >or=128 and 2 mg/liter, respectively, for P8-128. In vitro, the rate of spontaneous resistance of P8 and COL was 10(-7) on agar plates containing ciprofloxacin at two times the MIC, whereas it was <or=10(-10) on agar plates containing moxifloxacin at two times the MIC. Rats with experimental aortic endocarditis were treated with doses of drugs that simulate the kinetics in humans: moxifloxacin, 400 mg orally once a day; ciprofloxacin, 750 mg orally twice a day; or vancomycin, 1 g intravenously twice a day. Treatment was started either 12 or 24 h after infection and lasted for 3 days. Moxifloxacin treatment resulted in culture-negative vegetations in a total of 20 of 21 (95%) rats infected with P8, 10 of 11 (91%) rats infected with COL, and 19 of 24 (79%) rats infected with P8-4 (P < 0.05 compared to the results for the controls). In contrast, ciprofloxacin treatment sterilized zero of nine (0%) vegetations infected with first-level resistant mutant P8-4. Vancomycin sterilized only 8 of 15 (53%), 6 of 11 (54%), and 12 of 23 (52%) of the vegetations, respectively. No moxifloxacin-resistant derivative emerged among these organisms. However, moxifloxacin treatment of highly ciprofloxacin-resistant mutant P8-128 failed and selected for variants for which the MIC increased two times in 2 of 10 animals. Thus, while oral moxifloxacin might deserve consideration as treatment for staphylococcal infections in humans, caution related to its use against strains for which MICs are borderline is warranted.

Teicoplanin stress-selected mutations increasing sigma(B) activity in Staphylococcus aureus

A natural rsbU mutant of Staphylococcus aureus, unable to activate the alternative transcription factor sigma(B) via the RsbU pathway and therefore forming unpigmented colonies, produced first-step teicoplanin-resistant mutants upon selection for growth in the presence of teicoplanin, of which the majority were of an intense orange color. By using an asp23 promoter-luciferase fusion as an indicator, the pigmented mutants were shown to express increased sigma(B) activity. Increased sigma(B) activity was associated with point mutations in rsbW, releasing sigma(B) from sequestration by the anti-sigma factor RsbW, or to promoter mutations increasing the sigma(B)/RsbW ratio. Genetic manipulations involving the sigB operon suggested that the mutations within the operon were associated with the increase in teicoplanin resistance. The upregulation of sigma(B) suggests that a sigma(B)-controlled gene(s) is directly or indirectly involved in the development of teicoplanin resistance in S. aureus. Carotenoids do not contribute to teicoplanin resistance, since inactivation of the dehydrosqualene synthase gene crtM abolished pigment formation without affecting teicoplanin resistance. The relevant sigma(B)-controlled target genes involved in teicoplanin resistance remain to be identified.

Influence of a functional sigB operon on the global regulators sar and agr in Staphylococcus aureus

The growth phase-dependent activity profile of the alternate transcription factor sigma(B) and its effects on the expression of sar and agr were examined in three different Staphylococcus aureus strains by Northern blot analyses and by the use of reporter gene fusion experiments. Significant sigma(B) activity was detectable only in the clinical isolates MSSA1112 and Newman, carrying the wild-type rsbU allele, but not in the NCTC8325 derivative BB255, which is defective in rsbU. sigma(B) activity peaked in the late exponential phase and diminished towards the stationary phase when bacteria were grown in Luria-Bertani medium. Transcriptional analysis and a sarP1-sarP2-sarP3 (sarP1-P2-P3)-driven firefly luciferase (luc+) reporter gene fusion demonstrated a strong sigma(B) activity- and growth phase-dependent increase in sar expression that was totally absent in either rsbU or Delta rsbUVWsigB mutants. In contrast, expression of the agr locus, as measured by RNAIII levels and by an hldp::luc+ fusion, was found to be higher in the absence of sigma(B) activity, such as in rsbU or Delta rsbUVWsigB mutants, than in wild-type strains. Overexpression of sigma(B) in BB255 derivatives resulted in a clear increase in sarP1-P2-P3::luc+ expression as well as a strong decrease in hldp::luc+ expression. The data presented here suggest that sigma(B) increases sar expression while simultaneously reducing the RNAIII level in a growth phase-dependent manner.

Sub-inhibitory concentrations of vancomycin prevent quinolone-resistance in a penicillin-resistant isolate of Streptococcus pneumoniae

BACKGROUND

The continuous spread of penicillin-resistant pneumococci represents a permanent threat in the treatment of pneumococcal infections, especially when strains show additional resistance to quinolones. The main objective of this study was to determine a treatment modality impeding the emergence of quinolone resistance.

RESULTS

Exposure of a penicillin-resistant pneumococcus to increasing concentrations of trovafloxacin or ciprofloxacin selected for mutants resistant to these drugs. In the presence of sub-inhibitory concentrations of vancomycin, development of trovafloxacin-resistance and high-level ciprofloxacin-resistance were prevented.

CONCLUSIONS

Considering the risk of quinolone-resistance in pneumococci, the observation might be of clinical importance.

Response of complicated methicillin-resistant Staphylococcus aureus endocarditis to the addition of trovafloxacin

The newer fluoroquinolones have many properties such as safety, bioavailability, and tissue penetration that make them attractive in the therapy of complicated infections. Unfortunately, the rapid development of resistance by Staphylococcus aureus to ciprofloxacin has dampened interest in these agents for serious staphylococcal infections. A patient with right-sided methicillin-resistant Staphylococcus aureus (MRSA) endocarditis with a complicated clinical course received trovafloxacin in addition to vancomycin and rifampin. He was initially treated with vancomycin, gentamicin, and rifampin for serious MRSA infection, but because of complications, including septic central nervous system emboli, persistent fever, and leukocytosis, gentamicin was stopped and trovafloxacin begun. After this addition the patient improved and completely recovered. In vitro and animal model data show that many newer fluoroquinolones have excellent activity against S. aureus, including MRSA, and are also less likely to induce resistance. Animal models of endocarditis support their efficacy in serious staphylococcal infections.

Efficacy of levofloxacin in the treatment of experimental endocarditis caused by viridans group streptococci

Levofloxacin was investigated against viridans group streptococci in vitro and in rats with experimental aortic endocarditis. The MIC(90)s of levofloxacin and ciprofloxacin for 20 independent isolates of such bacteria were 1 and 8 mg/L, respectively. Rats were infected with two types of organism: either fully susceptible to levofloxacin MIC < or = 0.5 mg/L) or borderline susceptible (MIC 1-2 mg/L). Fully levofloxacin-susceptible bacteria comprised one penicillin-susceptible (MIC 0.004 mg/L) Streptococcus gordonii, and one penicillin-tolerant as well as one intermediate penicillin-resistant (MIC 0.125 mg/L) isogenic strains. Borderline levofloxacin-susceptible bacteria comprised one penicillin-susceptible Streptococcus sanguis and one highly penicillin-resistant Streptococcus mitis (MIC 2 mg/L). Rats were treated for 5 days with drug dosages simulating the following treatments in humans: (i) levofloxacin 500 mg orally once a day (q24 h), (ii) levofloxacin 500 mg orally twice a day (q12 h), (iii) levofloxacin 1 g orally q24 h, (iv) ciprofloxacin 750 mg orally q12 h, and (v) ceftriaxone 2 g iv q24 h. Levofloxacin was equivalent or superior to ceftriaxone, and was successful in treating experimental endocarditis irrespective of penicillin resistance. Nevertheless, standard levofloxacin treatment equivalent to 500 mg q24 h in human was less effective than twice daily 500 mg or once daily 1 g doses against borderline-susceptible organisms. Ciprofloxacin, used as a negative control, was ineffective and selected for resistant isolates. This underlines the importance of MIC determinations when treating severe streptococcal infection with quinolones. In the case of borderline-susceptible pathogens, total daily doses of 1 g of levofloxacin should be considered.

Levofloxacin in the empirical treatment of patients with suspected bacteraemia/sepsis: comparison with imipenem/cilastatin in an open, randomized trial

An open, randomized, multinational, multicentre study was conducted to compare the efficacy, safety and tolerability of levofloxacin 500 mg twice daily with imipenem/cilastatin 1 g iv three-times daily in the treatment of hospitalized adult patients with clinically suspected bacteraemia/ sepsis. Levofloxacin patients could change from iv to oral administration after a minimum of 48 h iv treatment if clinical signs and symptoms of sepsis had improved. The primary efficacy analysis was based on the clinical and bacteriological response at clinical endpoint. A total of 503 patients were randomized and 499 included in the intent-to-treat population. The per-protocol population comprised 287 patients with bacteriologically proven infection. Clinical cure rates at clinical endpoint in the intent-to-treat population and per-protocol population were 77% (184/239) and 89% (125/140), respectively, for levofloxacin and 68% (178/260) and 85% (125/147), respectively, for imipenem/cilastatin. At follow-up, the cure rates in the per-protocol population were 84% for levofloxacin and 69% for imipenem/cilastatin. The 95% confidence interval for both populations showed that levofloxacin was as effective as imipenem/cilastatin. A satisfactory bacteriological response was obtained in 87% (96/110) of levofloxacin patients and 84% (97/116) of imipenem/cilastatin patients at clinical endpoint. Adverse events possibly related to the study drug were reported in 74 (31%) levofloxacin patients and 79 (30%) imipenem/cilastatin patients. There were no clinically appreciable differences between the treatment groups. Levofloxacin 500 mg twice daily, either iv or as sequential iv/oral therapy, was as effective and well tolerated as imipenem/cilastatin 1 g iv three-times daily in the treatment of hospitalized patients with suspected bacteraemia/sepsis.

Efficacy of levofloxacin for experimental aortic-valve endocarditis in rabbits infected with viridans group streptococcus or Staphylococcus aureus

Levofloxacin is among the more active fluoroquinolones against streptococci and staphylococci. It is effective against moderately severe infections caused by these organisms, but its efficacy in the treatment of bacteremia and serious infections such as endocarditis is not well defined. We compared the efficacy of levofloxacin to those of standard agents in the rabbit model of aortic-valve endocarditis caused by fluoroquinolone-susceptible strains including a penicillin-susceptible strain of Streptococcus sanguis, a penicillin-resistant strain of Streptococcus mitis, a methicillin-resistant strain of Staphylococcus aureus, and a methicillin-susceptible strain of S. aureus. Levofloxacin administered intramuscularly at dosages of 20 to 40 mg/kg of body weight twice daily (b.i.d.) was completely ineffective against the penicillin-susceptible strain, with mean vegetation titers after 3 days of therapy not statistically significantly different from those for controls. Levofloxacin was no more effective than penicillin against the penicillin-resistant strain. Levofloxacin administered for 4 days at a dosage of 20 mg/kg b.i.d. was at least as effective as vancomycin administered intravenously at a dosage of 25 mg/kg b.i. d. against the methicillin-resistant S. aureus strain and was as effective as nafcillin administered intramuscularly at 100 mg three times daily against the methicillin-susceptible strain. Emergence of resistance to levofloxacin in vitro was less likely to occur than resistance to ciprofloxacin, and resistance to levofloxacin was not observed in vivo. Levofloxacin-rifampin combinations were antagonistic in vitro and in vivo. Levofloxacin was highly effective as a single agent against experimental staphylococcal endocarditis but was surprisingly ineffective against streptococcal endocarditis, suggesting that it has a potential role as treatment for serious S. aureus but not viridans group streptococcal infections in humans.

Levofloxacin

Levofloxacin (DR-3355, Daiichi) is a new fluoroquinolone antibiotic which is the active isomer of ofloxacin (DL-8280, Daiichi). By removing the inactive isomer, the in vitro activity of levofloxacin is 8-128 times higher than that of ofloxacin. This means that bacterial species, which have borderline susceptibility to ofloxacin and other first generation fluoroquinolones (e.g., pneumococci and organisms causing atypical pneumonia), are considerably more sensitive to levofloxacin. The pharmacokinetics of levofloxacin, which is available for both oral and i.v. administration, are characterised by a very high bioavailability, low (30-40%) protein binding, high tissue concentrations and elimination via the kidneys with minimal liver metabolism. As a consequence of the low degree of metabolism, levofloxacin does not interact with other drugs to any major extent. The safety and efficacy of levofloxacin are well documented in lower respiratory tract infections, skin and soft tissue infections, and urinary tract infections. The safety profile seems advantageous and the risks of phototoxicity, CNS toxicity and cardiac reactions (prolongation of QT-time) are low. Serious liver toxicity, leading to the recent withdrawal of trovafloxacin, has not been a problem in levofloxacin studies. Levofloxacin is a valuable addition to the group of fluoroquinolone antibiotics.