A review of linezolid: the first oxazolidinone antibiotic

Resistance of Gram-positive bacterial pathogens, such as Staphylococcus aureus and Enterococcus faecium, to existing antibiotics continues to increase, and new antibiotics with activity against these pathogens are in demand. Linezolid (Zyvox, Pharmacia and Upjohn) is the first agent of a new class of antibiotics called the oxazolidinones. Linezolid possesses excellent microbial activity against a wide variety of Gram-positive pathogens including those resistant to methicillin and vancomycin (Vancocin, Eli Lilly). Linezolid is available for intravenous and oral administration and possesses excellent bioavailability. It exhibits good penetration into pulmonary, as well as skin and related structure tissues, and does not require dosage adjustment in hepatic or renal dysfunction. Linezolid is generally well-tolerated, with the predominant adverse effect manifesting as a duration dependent, reversible thrombocytopenia. Linezolid possesses monoamine oxidase inhibitor activity and caution is warranted with coadministration of adrenergic or seritonergic medications. Clinical trials conducted with linezolid in skin and structure infections, lower respiratory tract infections and vancomycin-resistant enterococcal infections demonstrate that linezolid is an effective therapy. Recent data suggest that linezolid may be superior to vancomycin for the treatment of infections caused by methicillin-resistant S. aureus. Linezolid is an excellent and promising new antibiotic for the treatment of resistant Gram-positive pathogens.     

Efficacy of linezolid versus comparator therapies in Gram-positive infections

Treatment of Gram-positive bacterial infections is currently a therapeutic challenge because many of these pathogens are now resistant to standard antimicrobial agents. The emergence of multidrug-resistant, Gram-positive pathogens emphasizes the need for new antimicrobial therapy. Linezolid is an oxazolidinone antibiotic with a novel mechanism of action that works by inhibiting bacterial protein synthesis by blocking formation of the initiation complex. It is active against Gram-positive organisms resistant to other antibiotics, including methicillin-resistant Staphylococcus aureus (MRSA), penicillin-resistant Streptococcus pneumoniae and vancomycin-resistant enterococci (VRE). Results are encouraging from several large-scale, randomized, Phase III trials comparing the efficacy and safety of linezolid with standard comparator agents for the treatment of nosocomial pneumonia, community-acquired pneumonia, skin and skin structure infections, and infections due to MRSA and VRE. Intravenous/oral linezolid is a promising antimicrobial agent and provides the clinician with an additional treatment option, particularly among the limited therapies for resistant Gram-positive bacterial infections.     

Linezolid

Multiple antibiotic resistance is increasing worldwide in Gram-positive bacteria, especially in hospitals. Problem organisms include multiply resistant strains of pneumococci, Staphylococcus aureus and enterococci; for many of these the glycopeptide vancomycin has become the treatment of last resort. This situation has been made worse by the emergence of vancomycin-resistant enterococci and vancomycin-intermediate S. aureus. Fortunately, several compounds active against resistant Gram-positive bacteria are in active development. One of these is linezolid, the first of the oxazolidinones, a new class of antibacterial. Linezolid is a synthetic agent which is active against all the clinically important Gram-positive bacteria, including multiply resistant strains. It has good pharmacokinetics, with equal bioavailability by both oral and intravenous routes and no need for dose adjustment in patients with renal impairment. The drug has a good safety profile and clinical trials have given excellent results in a variety of skin and soft tissue, respiratory and bloodstream infections. Linezolid is a promising drug, which, together with prudent antibiotic use and the prevention and control of hospital infection, will help in the battle against multiply antibiotic resistant Gram-positive bacteria.     

Linezolid for the treatment of resistant gram-positive cocci

OBJECTIVE: To provide a comprehensive review of linezolid, the first of a new class of antibiotics, the oxazolidinones. Therapeutic issues regarding the emergence of multidrug-resistant bacteria and a brief history of the oxazolidinones are also discussed. DATA SOURCES: A MEDLINE search (1966-March 2001) was conducted to identify pertinent literature, including preclinical trials, clinical trials, and reviews. Unpublished clinical data, adverse effects, and dosing information were abstracted from product labeling. STUDY SELECTION: Clinical efficacy data were extracted from clinical trials, case reports, and abstracts that mentioned linezolid. Additional information concerning antibiotic resistance, the oxazolidinones, in vitro susceptibility and the pharmacokinetic profile of linezolid also was reviewed. DATA SYNTHESIS: Linezolid exhibits activity against many gram-positive organisms, including vancomycin-resistant Enterococcus faecium, methicillin-resistant Staphylococcus aureus, and penicillin-resistant Streptococcus pneumoniae. Linezolid inhibits bacterial protein synthesis at an early step in translation and is rapidly and completely absorbed from the gastrointestinal tract following oral administration. Efficacy has been demonstrated in a number of unpublished clinical trials in adults with pneumonia, skin and skin structure infections, and vancomycin-resistant E. faecium infections. The adverse effect profile is similar to that of comparator agents (beta-lactams, clarithrornycin, vancomycin). CONCLUSIONS: Linezolid is the first oral antimicrobial agent approved for the treatment of vancomycin-resistant enterococci. Since the oxazoildinones have a unique mechanism of action and expanded spectrum of activity against virulent and highly resistant gram positive pathogens, linezolid is a valuable alternative to currently available treatment options. Clinical trials evaluating linezolid and other oxazolidinones for antibiotic-resistant gram-positive infections, as well as comparator studies comparing linezolid with other candidate drugs, such as quinupristin/dalfopristin and choramphenicol, will further define the role of linezolid.     

Role of linezolid in the treatment of complicated skin and soft tissue infections

Staphylococcus aureus is the most common cause of complicated skin and soft tissue infections (cSSTIs). Antibiotic choices for these infections continue to evolve. History has seen penicillin progress to antistaphylococcal penicillins and cephalosporins, but these drugs are now giving way to drugs that are effective against methicillin-resistant S. aureus (MRSA). While vancomycin has been the gold standard to treat MRSA infections, newer therapeutic options have been developed over the last 5 years. These include quinupristin-dalfopristin, daptomycin, tigecycline and linezolid, which is the focus for this review. Linezolid is efficacious in the treatment of cSSTIs (including diabetic foot infections) caused by Gram-positive organisms (including MRSA), with a well-defined safety profile and straightforward dosing. It is also approved for nosocomial pneumonia, community-acquired pneumonia and uncomplicated skin and skin structure infections. Linezolid has an oral and parenteral formulation, which are equivalent. The oral formulation has the potential to offer economic benefits as compared with other therapies. Currently, there are only a few new antibiotics in development with MRSA activity. The proper use of all antibiotics, including these newer agents, is increasingly important if we are to slow the evolution of microbial resistance.     

Dalbavancin and telavancin: novel lipoglycopeptides for the treatment of Gram-positive infections

Two glycopeptide analogues of vancomycin and teicoplanin have been developed with improved pharmacokinetic/pharmacodynamic parameters. Dalbavancin was derived from teicoplanin, and telavancin is a derivative of vancomycin. The half-life of dalbavancin in humans is 147-258 h (6-11 days) allowing for weekly administration. Dalbavancin possesses more potent in vitro activity than vancomycin or teicoplanin. Dalbavancin has been investigated in uncomplicated and complicated skin and skin structure infections (SSSIs) in clinical trials and has demonstrated equivalent or superior (versus vancomycin only) efficacy versus comparators. Telavancin exhibits a dual mechanism of action, low potential for resistance development and is active against resistant pathogens, including methicillin-resistant Staphylococcus aureus (MRSA). Clinical trials involving SSSIs have demonstrated equivalent or superior (versus vancomycin for MRSA) efficacy compared with a standard therapy. Both telavancin and dalbavancin show promise as alternative treatments for patients with serious infections caused by resistant Gram-positive pathogens.     

Safety and efficacy of linezolid in 16 infants and children in Japan

Linezolid, an oxazolidinone antibiotic, exhibits a broad spectrum of activity against Gram-positive bacteria. It has been licensed for adult use in Japan since 2006 for MRSA infections, and has also been used off-label for pediatric patients. At our university hospital, a total of 16 infants and children (including one non-Japanese Asian) were administered linezolid owing to infection with multidrug-resistant Gram-positive bacteria, after consent had been provided. All patients had severe underlying diseases or indications for surgery. Eighty-eight percent of the causal microorganisms were methicillin-resistant Staphylococcus aureus (MRSA) or methicillin-resistant coagulase-negative Staphylococcus and all were sensitive to linezolid. Linezolid was administered because the antecedent anti-MRSA medications were ineffective or contraindicated, or intravenous-to-oral switch therapy was requested owing to cardiac or orthopedic surgical-site infections. The median duration of administration was 13 days (range 3-31 days). The overall efficacy was 91 % (10/11) in those for whom efficacy could be evaluated. Only two patients (both teen-aged) encountered linezolid-related adverse effects (13 %, 2/16). One patient showed elevation of liver enzymes (aspartate aminotransferase [AST] and alanine aminotransferase [ALT]), requiring that administration be withdrawn, but enzyme levels returned to normal after the patient had been switched to vancomycin. The other patient showed transiently decreased platelet counts. Linezolid is considered generally safe and effective for children in Japan, especially for those who cannot use other anti-MRSA medications or those who require oral antibiotics for infections with multidrug-resistant Gram-positive bacteria.     

Role of linezolid in the treatment of orthopedic infections

Gram-positive organisms, particularly staphylococci and streptococci, are responsible for the majority of bone and joint infections. The rising incidence of antimicrobial resistance among Staphylococcus aureus, coagulase-negative staphylococci and enterococci means that novel antibiotics with unique mechanisms of antimicrobial activity are needed, especially in orthopedic infections. Linezolid is the first of the oxazolidinones, a new class of antibacterial agents particularly effective against Gram-positive infections including methicillin- and vancomycin-resistant strains. With an excellent oral bioavailability and acceptable safety profile, linezolid offers a valuable alternative to more traditional therapies, such as glycopeptides. No large randomized trials have been published on its use in patients with orthopedic infections, but early results are encouraging. Reported adverse events, especially bone marrow suppression and optic neuropathy seen with prolonged administration, mean that treatment of such patients must be undertaken with careful follow-up of laboratory tests. Until now, little resistance has been reported.     

Impact of methicillin-resistant Staphylococcus aureus infections on key health economic outcomes: does reducing the length of hospital stay matter?

The clinical and economic impact of methicillin-resistant staphylococcal infections on the patient, the hospital and the community is significant and continues to increase. Methicillin-resistant Staphylococcus aureus infections, in particular, represent a substantial burden of resistant infections in the hospital. As such, it is important to analyse the cost parameters associated with an episode of infection. Key determinants of the total cost of an episode of infection include fixed costs and hotel costs; the contribution of antimicrobial therapy, including drug acquisition and delivery costs, is comparatively marginal. Therefore, decreasing the hospital length of stay by promoting earlier hospital discharge will significantly reduce overall costs and, in effect, increase the efficiency and cost-effectiveness of the hospital. Linezolid, the first marketed agent of a new class of oxazolidinone antibiotics, is effective in the treatment of serious Gram-positive infections. Its availability in both intravenous (iv) and oral formulations (the latter of which is 100% bioavailable) facilitates early discharge from hospital. Hospitalized patients with methicillin-resistant staphylococcal infections who were treated with either linezolid or vancomycin demonstrated no significant differences in clinical outcome or mortality. However, compared with vancomycin, administration of linezolid reduced the duration of iv therapy and increased the chance of being discharged during the first week of hospitalization. These effects were most pronounced in the subset of patients with skin and soft tissue infections. Knowledge of these data may influence prescribing practices at the individual and organizational levels and, in turn, reduce the economic burden associated with MRSS infections.     

万古霉素及利奈唑胺对体内不同部位金黄色葡萄球菌感染的抑菌效果研究

[目的]比较万古霉素及利奈唑胺在体内不同部位金黄色葡萄球菌（SA）感染取样的最小抑菌浓度（MIC）及抑菌效果.[方法]检测万古霉素和利奈唑胺对115株不同部位（痰、血及伤口渗出液）分离出的SA包括耐甲氧西林金黄色葡萄球菌（MRSA）的MIC值.[结果]MRSA检出率为64.3%.万古霉素及利奈唑胺对所有SA敏感.万古霉素在痰、血及伤口渗出液中的SA的MIC ≤0.5 μg/mL的比例分别为77.8%、51.5%及40.5%.利奈唑胺在痰、血及伤口渗出液中的SAMIC ≤2 μg/mL的比例分别为82.2%、87.9%及91.9%.[结论]万古霉素在治疗呼吸道SA感染较血行及皮肤软组织SA感染的效果好,利奈唑胺组在治疗皮肤软组织SA感染优于呼吸道及血行SA感染,在皮肤软组织SA感染中利奈唑胺可作为临床的首选.     

In vitro activities of linezolid against important gram-positive bacterial pathogens including vancomycin-resistant enterococci.

The emergence of resistance in gram-positive bacteria has necessitated a search for new antimicrobial agents. Linezolid is an oxazolidinone, a new class of antibacterial agents with enhanced activity against pathogens. We compared the activity of linezolid to those of other antimicrobial agents against 3,945 clinical isolates. Linezolid demonstrated potent activity against all isolates tested. For all vancomycin-susceptible enterococci, staphylococci, and streptococci, the activity of linezolid was comparable to that of vancomycin. Against oxacillin-resistant staphylococci and vancomycin-resistant enterococci, linezolid was the most active agent tested. In summary, linezolid appears to be a promising new antimicrobial agent for the treatment of gram-positive infections.     

Efficacy and safety of linezolid for Gram-positive orthopedic infections: a prospective case series

Linezolid is an attractive alternative for orthopedic infections because of oral bioavailability and activity against methicillin-resistant staphylococci and vancomycin-resistant enterococci. To determine efficacy and safety, we prospectively monitored 51 consecutive adults who were not vancomycin candidates and who received linezolid for 53 Gram-positive orthopedic infections, usually chronic osteomyelitis (n = 25) or prosthetic joint infection (n = 23). Pathogens were usually Staphylococcus aureus (n = 27) or coagulase-negative staphylococci (n = 19); 38 were methicillin resistant. After remission, 17 infections required long-term suppression, usually because of retained hardware. Clinical and microbiologic failure occurred in only one patient. The most common adverse events were thrombocytopenia (n = 5) and anemia (n = 5), necessitating treatment discontinuation in 3 patients. One patient developed reversible optic and irreversible peripheral neuropathy after 24 months of linezolid. Linezolid, with surgery, may be a reasonable alternative for Gram-positive orthopedic infections. We recommend weekly hematologic monitoring, and, if therapy lasts >2 months, periodic ophthalmologic monitoring.     

In vivo efficacy of continuous infusion versus intermittent dosing of linezolid compared to vancomycin in a methicillin-resistant Staphylococcus aureus rabbit endocarditis model

Linezolid is the first drug issued from the oxazolidinones, a novel class of antimicrobial agents with potent activity against gram-positive pathogens. A rabbit endocarditis model was used to compare the in vivo activities of different linezolid regimens mimicking intermittent dosing of 10 mg/kg of body weight every 12 h for 5 days or continuous (constant-rate) infusion of a daily dose of 20 mg/kg (for 5 days) or 40 mg/kg (for 3 and 5 days) and the activities of intermittent dosing and continuous infusion of vancomycin (for 5 days). The in vivo activities of these regimens were tested against three strains of methicillin-resistant Staphylococcus aureus. A human-like pharmacokinetic simulation was used for linezolid in order to improve the extrapolation of the results to human therapy. All linezolid regimens significantly reduced the numbers of S. aureus cells in aortic valve vegetations compared to the numbers in the control groups. Linezolid intermittent dosing had an in vivo bacteriostatic effect. Switching from intermittent dosing to continuous infusion (at the same dose) led to in vivo bactericidal activity, with a decrease of at least 3 log(10) CFU/g of vegetation compared to the counts for the controls. After 5 days of treatment, continuous infusion of linezolid (corresponding to a daily dose of 40 mg/kg in humans) seemed to be at least as effective as vancomycin against the three strains. No resistant variant was isolated from the vegetations during any of the treatments. These data suggest that continuous infusion of linezolid could be an appropriate alternative to the use of glycopeptides for the treatment of severe methicillin-resistant S. aureus infections.     

Daptomycin for treating infected diabetic foot ulcers: evidence from a randomized, controlled trial comparing daptomycin with vancomycin or semi-synthetic penicillins for complicated skin and skin-structure infections

OBJECTIVES: The predominant pathogens causing diabetic foot infections are Gram-positive cocci, many of which are now resistant to commonly prescribed antibiotics. Daptomycin is a new agent that is active against most Gram-positive pathogens. To compare the effectiveness of daptomycin against semi-synthetic penicillins or vancomycin, we analysed the subset of diabetic patients with an infected ulcer enrolled in two randomized, controlled investigator-blind trials of patients with complicated skin and soft-tissue infections presumptively caused by Gram-positive organisms. PATIENTS AND METHODS: Patients with a diabetic ulcer infection were prospectively stratified to ensure they were equally represented in the treatment groups, then randomized to either daptomycin [4 mg/kg every 24 h intravenously (iv)] or a pre-selected comparator (vancomycin or a semi-synthetic penicillin) for 7-14 days. RESULTS: Among 133 patients with a diabetic ulcer infection, 103 were clinically evaluable; 47 received daptomycin and 56 received a comparator. Most infections were monomicrobial, and Staphylococcus aureus was the predominant pathogen. Success rates for patients treated with daptomycin or the comparators were not statistically different for clinical (66% versus 70%, respectively; 95% CI, -14.4, 21.8) or microbiological (overall or by pathogen) outcomes. Both treatments were generally well tolerated, with most adverse events of mild to moderate severity. CONCLUSIONS: The clinical and microbiological efficacy and safety of daptomycin were similar to those of commonly used comparator antibiotics for treating infected diabetic foot ulcers caused by Gram-positive pathogens. Daptomycin should be considered for treating these infections, especially those caused by resistant Gram-positive pathogens.     

Comparative activity of linezolid and other new agents against methicillin-resistant Staphylococcus aureus and teicoplanin-intermediate coagulase-negative staphylococci.

The activity of linezolid was determined against 225 recently isolated methicillin-resistant Staphylococcus aureus (MRSA) and 20 methicillin-resistant coagulase-negative staphylococci (CoNS) with decreased levels of susceptibility to teicoplanin. Linezolid activity was compared with other new agents (quinupristin-dalfopristin, trovafloxacin, moxifloxacin, levofloxacin and telithromycin) and six other antimicrobials (erythromycin, clindamycin, gentamicin, vancomycin, teicoplanin and rifampicin). The in vitro activity of linezolid was similar to that of vancomycin. Linezolid inhibited all MRSA strains at between 0.1 and 2 mg/L and all CoNS strains tested at between 0.2 and 0.5 mg/L. These results suggest that linezolid would be useful for the treatment of infections involving these organisms.     

Linezolid for the treatment of patients with endocarditis: a systematic review of the published evidence

BACKGROUND: Linezolid is a bacteriostatic oxazolidinone antibiotic that has been proven to be effective for the treatment of patients with pneumonia, skin and soft tissue infections, and possibly bacteraemia, due to Gram-positive cocci. However, the drug is sometimes used for the treatment of patients with endocarditis due to Gram-positive cocci resistant to other antibiotics. METHODS: We carried out a review of the available literature to evaluate whether linezolid is also effective for the treatment of patients with infective endocarditis. RESULTS: We identified 23 case reports and 3 case series reporting the experience with 56 patients with endocarditis treated with linezolid. Evaluable data for 33 patients who received linezolid and for whom individual patient data were reported were further analysed. Prosthetic valve infective endocarditis accounted for 25% of the reviewed cases. Methicillin-resistant Staphylococcus aureus and vancomycin-intermediate S. aureus were the most commonly isolated cocci (24.2% and 30.3% of cases, respectively). Linezolid alone was administered to 66.7% of patients while the rest received the antibiotic in combination with rifampicin, gentamicin, fusidic acid or amikacin. A total of 63.6% (21/33) of patients with endocarditis were cured after linezolid administration. The overall and endocarditis-related mortality was 33.3% (11/33) and 12.1% (4/33), respectively. Thrombocytopenia developed in 30.8% (8/26) of patients for whom relevant data were available. CONCLUSIONS: The limited available evidence suggests that linezolid may be considered as a therapeutic option for the treatment of patients with endocarditis due to multidrug-resistant Gram-positive cocci. However, further published experience is needed to answer the question of whether a bacteriostatic antibiotic could be proven beneficial for patients with an infection for which bactericidal antibiotics have been traditionally used.     

Pathogenesis and prevention of immune reconstitution disease during antiretroviral therapy

A long-held doctrine is that bactericidal antibiotics are required for infections in neutropenic patients. We evaluated the available published evidence regarding the role of linezolid, a bacteriostatic antibiotic, in neutropenic patients with Gram-positive infection. We retrieved two prospective comparative studies (one of them a double-blind, randomized, controlled trial), a prospective cohort study, two retrospective studies and eight case reports that focused on the use of linezolid for Gram-positive bacterial infections in neutropenic patients. Linezolid was administered to 438 neutropenic patients, mainly on a compassionate-use basis, as other antibiotics failed to cure the infection or were associated with significant adverse events. The clinical cure rate ranged between 57 and 87.3% in the intention-to-treat population of the prospective studies. In total, 56 out of 438 (12.7%) neutropenic patients that received linezolid died during therapy. In the only randomized controlled trial that compared linezolid with vancomycin in the treatment of Gram-positive infections in neutropenic patients, mortality was 5.6 versus 7.6%, respectively (p = 0.4). In conclusion, the available evidence suggests that linezolid may be successful in a significant proportion of neutropenic patients with infection, despite the fact that it is a bacteriostatic agent. Such data seem to justify further studies regarding the role of linezolid in this patient population.     

Susceptibility of a variety of clinical isolates to linezolid: a European inter-country comparison

Using standardized in vitro susceptibility tests, 3382 bacteria recently isolated from skin, blood or respiratory tract infections were analysed for their susceptibility to linezolid, a new oxazolidinone, and a number of comparator antibacterial agents. Isolates originated in France, Italy, Germany, Spain, Sweden, The Netherlands and the UK. Laboratories in each country independently conducted broth microdilution susceptibility tests using NCCLS methods and epsilonometry (Etest). Isolates of Gram-positive cocci tested in each laboratory included methicillin-susceptible and -resistant Staphylococcus aureus, methicillin-susceptible and -resistant Staphylococcus epidermidis, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae and Enterococcus spp. Isolates of Moraxella catarrhalis and Haemophilus influenzae were also included. Where appropriate, comparator drugs (oxacillin, vancomycin, gentamicin, co-amoxiclav, ciprofloxacin, erythromycin, penicillin G, clindamycin and ampicillin) were also tested. Linezolid demonstrated excellent activity against all of the Gram-positive cocci with MIC50s ranging from 0.5 to 4 mg/L. The drug demonstrated only modest activity against M. catarrhalis and H. influenzae with MIC50s ranging from 4 to 16 mg/L.     

In vitro activity of linezolid alone and in combination with gentamicin,vancomycin or rifampicin against methicillin-resistant Staphylococcus aureus by time-kill curve methods

The in vitro activity of the oxazolidinone linezolid was studied alone and in combination with three antibiotics acting on different cellular targets. Oxazolidinones are bacterial protein synthesis inhibitors that act at a very early stage by preventing the formation of the initiation complex. Combinations of linezolid with gentamicin, vancomycin or rifampicin were evaluated against four methicillin-resistant Staphylococcus aureus strains, using killing curves in conjunction with scanning electron microscopy. Time-kill curves were performed over 24 h using an inoculum of 5 x 10(6)- 1 x 10(7) cfu/mL. Linezolid was studied at concentrations of 1 x, 4 x and 8 x MIC, with partner drugs at 8 x MIC. Addition of linezolid resulted in a decrease of antibacterial activity for gentamicin and vancomycin, and linezolid was antagonistic to the early bactericidal activity of gentamicin. Linezolid, in combination with rifampicin, showed an additive interaction for susceptible strains and inhibited rifampicin-resistant variants. Linezolid plus rifampicin appeared to be the most active combination against methicillin-resistant S. aureus strains in time-kill experiments.     

Dalbavancin: a novel antimicrobial

The increasing incidence of serious infections because of Gram-positive pathogens and the rising cost in parenteral administration of antimicrobials has inspired the development of a novel antibiotic. Dalbavancin is the first once a week antibiotic with activity against a broad range of Gram-positive pathogens. A large multicentre, pivotal, Phase III clinical trial, which included 854 patients with complicated skin and skin structure infections, compared 1-2 doses of dalbavancin vs. linezolid. The results demonstrated non-inferiority and a comparable safety profile. With its unique pharmacokinetic profile, ease of use and excellent safety profile, dalbavancin should provide a valuable addition to the armamentarium used to treat infections because of Gram-positive cocci.