Antimicrobial Susceptibility and Staphylococcal Chromosomal CassettemecType in Community- and Hospital-Associated Methicillin-ResistantStaphylococcus aureus

Effects of gamma-irradiation on antibiotic resistance and diagnostic molecular markers of methicillin-resistant Staphylococcus aureus in Egyptian cancer patients

Purpose: This in-vitro study aimed to assess in 120 [40 community-acquired (CA-MRSA) & 80 hospital-acquired (HA-MRSA)] isolates from cancer patients whether the transmissible staphylococcal cassette chromosome mec (SCCmec) typing, and the Panton-Valentine leukocidin (PVL) virulence genes detection could be employed as tools for molecular diagnostic purposes to distinguish both methicillin-resistant Staphylococcus aureus (MRSA) categories in radiotherapy treated cancer patients.Materials and methods: SCCmec typing was determined by the combination of the type of the cassette chromosome recombinase genes (ccr) gene complex and the class of the methicillin resistance (mec) gene complex. Besides, a rapid slide latex agglutination test (LAT) and antibiotic resistance spectrum determination before and after irradiation were performed.Results: In the strict sense, with the effect of irradiation; the presence of SCCmec subtypes IVa (22.5% vs. 10.0%), b (47.5% vs. 25.0%), & d (7.5 vs. 2.5%) or type V (15.0% vs. 7.5%) genetic elements and PVL genes (p < .001) were not proved as a signature for CA-MRSA. While, the larger SCCmec types II, and III elements were not detected in 14, and 19 from the 38, and 36 typed HA-MRSA isolates (p < .001), respectively. Remarkable effects on class A & class B mec gene complex and type2, type 3 & type 5 ccr gene complex and an increase in agglutination reaction strength in response to gamma irradiation external stimulus were observed.Conclusions: Different heterogeneous genetic composition with upregulation mecA gene expression was detected after irradiation in the HA- MRSA studied population. CA-MRSA showed remarkable ability to acquire multi-antibiotic resistance after irradiation and propose a novel paradigm for future chemotherapy against the multi-resistant pathogens whose proliferation especially among immunocompromised cancer patients is on the increase.

Antibiotic susceptibility and genomic variations in Staphylococcus aureus associated with Skin and Soft Tissue Infection (SSTI) disease groups

Background

Staphylococcus aureus is associated with human skin and soft tissue infections (SSTIs); however, the involvement of virulence factors in different clinical presentations is unclear.

Methods

We analyzed methicillin-resistant S. aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) strains from Taiwan to determine correlations among the clinical characteristics of SSTIs, antimicrobial susceptibility and virulence factors of S. aureus with specific genetic backgrounds.

Results

We identified 177 MRSA isolates and 130 MSSA isolates among the 307 SSTI-associated S. aureus isolates. Hospital-acquired (HA)- and community-acquired (CA)-MRSA isolates accounted for 61.6 % and 38.4 % of the isolates, respectively. Clinical presentations in SSTI patients differed significantly for the disease groups. Deep-seated MRSA infections presented with higher amputation rate than MSSA infections. MRSA isolates were all susceptible to linezolid, teicoplanin, and vancomycin, and >94 % of isolates were erythromycin- and clindamycin-resistant. Staphylococcal cassette chromosome (SCCmec) types IV, V, and VII were the most frequent in the CA-MRSA group (n = 68); types III, IV and V were the most frequent in the HA-MRSA group (n = 109). Panton-Valentine leukocidin (PVL) genes were significantly more frequent in CA-MRSA strains (75.0 %) than in HA-MRSA (33.0 %) and MSSA (24.6 %) and were found in 66.7 % (74/111) strains isolated from the abscess group. Exfoliatin A genes were more common in catheter-related exit-site MSSA infections (37.5 %) compared with other MSSA disease groups (P < 0.05). Exfoliatin B and superantigen exotoxin genes were uncommon in all SSTI disease types. Pulsotypes A (ST239), C, and D (ST59) were the predominant MRSA genotypes in deep-seated infections.

Conclusions

If not treated appropriately, deep-seated MRSA-associated infections present with higher amputation rates than deep-seated MSSA-associated infections. PVL-positive MRSA strains caused more frequently pus-forming lesions and less bacteremia and invasive diseases. Methods for discriminating CA-MRSA from HA-MRSA strains are now unreliable due to circulation of both ST 239 and ST 59 strains in the community and nosocomial settings. Initial antibiotic treatments should consider MRSA for patients with SSTIs in areas where MRSA is prevalent.

In vivo bioluminescence imaging to evaluate systemic and topical antibiotics against community-acquired methicillin-resistant Staphylococcus aureus-infected skin wounds in mice

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) frequently causes skin and soft tissue infections, including impetigo, cellulitis, folliculitis, and infected wounds and ulcers. Uncomplicated CA-MRSA skin infections are typically managed in an outpatient setting with oral and topical antibiotics and/or incision and drainage, whereas complicated skin infections often require hospitalization, intravenous antibiotics, and sometimes surgery. The aim of this study was to develop a mouse model of CA-MRSA wound infection to compare the efficacy of commonly used systemic and topical antibiotics. A bioluminescent USA300 CA-MRSA strain was inoculated into full-thickness scalpel wounds on the backs of mice and digital photography/image analysis and in vivo bioluminescence imaging were used to measure wound healing and the bacterial burden. Subcutaneous vancomycin, daptomycin, and linezolid similarly reduced the lesion sizes and bacterial burden. Oral linezolid, clindamycin, and doxycycline all decreased the lesion sizes and bacterial burden. Oral trimethoprim-sulfamethoxazole decreased the bacterial burden but did not decrease the lesion size. Topical mupirocin and retapamulin ointments both reduced the bacterial burden. However, the petrolatum vehicle ointment for retapamulin, but not the polyethylene glycol vehicle ointment for mupirocin, promoted wound healing and initially increased the bacterial burden. Finally, in type 2 diabetic mice, subcutaneous linezolid and daptomycin had the most rapid therapeutic effect compared with vancomycin. Taken together, this mouse model of CA-MRSA wound infection, which utilizes in vivo bioluminescence imaging to monitor the bacterial burden, represents an alternative method to evaluate the preclinical in vivo efficacy of systemic and topical antimicrobial agents.

Management of drug and food interactions with azole antifungal agents in transplant recipients

Azole antifungal agents are frequently used in hematopoietic stem cell and solid organ transplant recipients for prevention or treatment of invasive fungal infections. However, because of metabolism by or substrate activity for various isoenzymes of the cytochrome P450 system and/or P-glycoprotein, azole antifungals have the potential to interact with many of the drugs commonly used in these patient populations. Thus, to identify drug interactions that may result between azole antifungals and other drugs, we conducted a literature search of the MEDLINE database (1966-December 2009) for English-language articles on drug interaction studies involving the azole antifungal agents fluconazole, itraconazole, voriconazole, and posaconazole. Another literature search between each of the azoles and the immunosuppressants cyclosporine, tacrolimus, and sirolimus, as well as the corticosteroids methylprednisolone, dexamethasone, prednisolone, and prednisone, was also conducted. Concomitant administration of azoles and immunosuppressive agents may cause clinically significant drug interactions resulting in extreme immunosuppression or toxicity. The magnitude and duration of an interaction between azoles and immunosuppressants are not class effects of the azoles, but differ between drug combinations and are subject to interpatient variability. Drug interactions in the transplant recipient receiving azole therapy may also occur with antibiotics, chemotherapeutic agents, and acid-suppressive therapies, among other drugs. Initiation of an azole antifungal in transplant recipients nearly ensures a drug-drug interaction, but often these drugs are required. Management of these interactions first involves knowledge of the potential drug interaction, appropriate dosage adjustments when necessary, and therapeutic or clinical monitoring at an appropriate point in therapy to assess the drug-drug interaction (e.g., immunosuppressive drug concentrations, signs and symptoms of toxicity). These aspects of drug interaction management are essential not only at the initiation of azole antifungal therapy, but also when these agents are removed from the regimen.

Retrospective cohort study of hospitalized adults treated with vancomycin or clindamycin for methicillin-resistant Staphylococcus aureus skin infections

BACKGROUND

Vancomycin alternatives, including clindamycin, have in vitro activity against current strains of methicillin-resistant Staphylococcus aureus (MRSA), but clinical evidence of their effectiveness is needed.

OBJECTIVE

The aim of this work was to compare health outcomes for hospitalized adult patients treated with vancomycin and clindamycin for skin and soft- tissue infections caused by MRSA.

METHODS

This was a retrospective chart review of patients admitted to University Hospital (San Antonio, Texas) with culture-proven MRSA skin or soft-tissue infections from July 1, 2006, to December 31, 2006. Patients were subdivided into groups according to antibiotics received on the first day of hospital admission. The primary outcome was composite failure, which was defined as having an additional positive MRSA culture 5 to 90 days after initial culture or requiring an additional intervention (eg, new course of antibiotics or additional incision and drainage within 90 days after initiation of therapy). Descriptive statistics were used to characterize each group; χ(2), Fisher exact, and Wilcoxon rank sum tests were used to assess differences between the vancomycin and clindamycin groups.

RESULTS

Ninety-one patients received vancomycin (n = 40) or clindamycin (n = 51) for a MRSA skin infection. Most vancomycin-treated patients received 1 g IV q12h (92.5% [37/40]), whereas most clindamycintreated patients received 600 mg IV q8h (51.0% [26/51]) or 900 mg IV q8h (27.5% [14/51]). The vancomycin and clindamycin groups had no significant differences with regard to median age (38 vs 37 years, respectively), male sex (62.5% [25/40] vs 74.5% [38/51]), or Hispanic ethnicity (77.5% [31/40] vs 78.4% [40/51]). All MRSA isolates were susceptible to vancomycin and trimethoprimsulfamethoxazole. Few patients who received clindamycin were resistant to clindamycin (3.9% [2/51]). No patients died in the hospital. There were no significant differences between the vancomycin (n = 40) and clindamycin (n = 51) groups with respect to composite failure (15.0% [6/40] vs 7.8% [4/51], respectively), microbiologic failure (2.5% [1/40] vs 3.9% [2/51]), additional inpatient interventions (5.0% [2/40] vs 3.9% [2/51]), or additional outpatient interventions (12.5% [5/40] vs 3.9% [2/51]). Most patients (93.4% [85/91]) received incision and drainage. When those who did not were excluded from the analyses, all trends remained unchanged.

CONCLUSIONS

In a single institution with a low rate of clindamycin resistance, there were no significant differences between vancomycin and clindamycin for the treatment of these hospitalized patients with MRSA skin infections, on the basis of clinical outcomes data. This finding warrants further investigation in a randomized controlled trial.

High prevalence of ST121 in community-associated methicillin-susceptible Staphylococcus aureus lineages responsible for skin and soft tissue infections in Portuguese children

In order to evaluate the incidence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in Portugal, we analyzed a collection of 38 S. aureus isolates recovered from 30 children attending the pediatric emergency department of a central hospital in Lisbon due to skin and soft tissue infections. Molecular characterization identified seven clonal lineages among the 35 methicillin-susceptible S. aureus (MSSA) isolates, of which the major lineage PFGE A/t159/ST121 included 63% of the isolates. The three MRSA isolates belonged to the Pediatric clone PFGE D/t535/ST5-IV (n = 2) and to the European CA-MRSA clone PFGE G/t044/ST80-IVc (n = 1). All isolates harbored several virulence factors, namely, leukocidins. Panton-Valentine leukocidin (PVL) was produced by isolates from five MSSA lineages and by the ST80 MRSA. Of interest, this is the first reported isolation of CA-MRSA ST80 in Portugal.

Community-associated methicillin-resistant Staphylococcus aureus: epidemiology and clinical consequences of an emerging epidemic

Staphylococcus aureus is an important cause of skin and soft-tissue infections (SSTIs), endovascular infections, pneumonia, septic arthritis, endocarditis, osteomyelitis, foreign-body infections, and sepsis. Methicillin-resistant S. aureus (MRSA) isolates were once confined largely to hospitals, other health care environments, and patients frequenting these facilities. Since the mid-1990s, however, there has been an explosion in the number of MRSA infections reported in populations lacking risk factors for exposure to the health care system. This increase in the incidence of MRSA infection has been associated with the recognition of new MRSA clones known as community-associated MRSA (CA-MRSA). CA-MRSA strains differ from the older, health care-associated MRSA strains; they infect a different group of patients, they cause different clinical syndromes, they differ in antimicrobial susceptibility patterns, they spread rapidly among healthy people in the community, and they frequently cause infections in health care environments as well. This review details what is known about the epidemiology of CA-MRSA strains and the clinical spectrum of infectious syndromes associated with them that ranges from a commensal state to severe, overwhelming infection. It also addresses the therapy of these infections and strategies for their prevention.

Identifying exposure targets for treatment of staphylococcal pneumonia with ceftobiprole

Ceftobiprole is a cephalosporin with potent activity against methicillin (meticillin)-resistant Staphylococcus aureus (MRSA). In order to treat patients with severe staphylococcal pneumonia, it is important to understand the drug exposure required to mediate the killing of multiple log(10) cells in a preclinical-infection model. We measured drug exposure in terms of the percentage of penetration of the drug into epithelial lining fluid (ELF) and in terms of the time for which the drug concentration was above the MIC (time>MIC) in plasma and ELF. In a murine model of staphylococcal pneumonia, we demonstrated that ceftobiprole penetrated into ELF from the plasma at a median level of nearly 69% (25th to 75th percentile range, 25 to 187%), as indexed to the ratio of values for the area under the concentration-time curve in ELF and plasma. The total-drug times>MIC in ELF that were required to kill 1 log(10) and 2 log(10) CFU/g of lung tissue were 15% and 25% of the dosing interval. We also examined the penetration of ELF by ceftobiprole in volunteers, demonstrating mean and median penetration percentages of 25.5% and 15.3%, respectively (25th to 75th percentile range, 8 to 30%). Attainment rates were calculated for kill targets of 1 log(10) and 2 log(10) CFU/g, taken from the murine model, but using the volunteer ceftobiprole ELF penetration data. The standard dose for ceftobiprole is 0.5 g every 8 h as a 2-h infusion. The attainment rates remained above 90% for 1-log(10) and 2-log(10) CFU/g kill targets at MICs of 1 and 0.5 mg/liter, respectively. Taking the expectation over the distribution of ceftobiprole MICs for 4,958 MRSA isolates showed an overall target attainment of 85.6% for a 1-log(10) CFU/g kill and 79.7% for a 2-log(10) CFU/g kill. It is important to derive exposure targets in preclinical-infection models of the infection site so that these targets can be explored in clinical trials in order to optimize the probability of a good clinical outcome.

Trimethoprim-sulfamethoxazole for methicillin-resistant Staphylococcus aureus: a forgotten alternative?

Methicillin-resistant Staphylococcus aureus (MRSA) is a growing infectious concern, mainly in the context of its rapid adaptation to novel antibiotic options for its treatment and the growing morbidity, mortality, and healthcare costs associated with its emergence. the authors sought to investigate whether an older antibiotic, such as trimethoprim-sulfamethoxazole (SXT), may have a role in treating MRSA-related infections, according to the available literature on the subject. The authors reviewed literature data on: resistance of MRSA to SXT worldwide in recent years, efficacy of SXT for MRSA decolonization or prophylaxis from MRSA infections, and clinical therapeutic efficacy of SXT in treating mild or severe community-acquired or hospital-acquired MRSA infections. Resistance varies worldwide, in general being low in the industrialized world and higher in developing countries. SXT is one of the numerous understudied options for MRSA decolonization and is growingly recognized as potentially effective in preventing MRSA infections in certain settings. Limited data on its therapeutic efficacy are encouraging, at least for mild, community-acquired infections. SXT may represent a cost-effective alternative weapon against MRSA. Its utility against this increasingly threatening pathogen need clarification through further clinical trials.

In vivo activity of the pyrrolopyrazolyl-substituted oxazolidinone RWJ-416457

RWJ-416457 is an investigational pyrrolopyrazolyl-substituted oxazolidinone with activity against antibiotic-susceptible and -resistant gram-positive pathogens. Efficacies of RWJ-416457, linezolid, and vancomycin against methicillin-susceptible Staphylococcus aureus (MSSA) and community-associated methicillin-resistant S. aureus (CA-MRSA) in murine skin and systemic infections were compared, as were efficacies against Streptococcus pneumoniae in a lower respiratory infection. In staphylococcal systemic infections, RWJ-416457 was equipotent with to twofold more potent than linezolid, with 50% effective dose values ranging from 1.5 to 5 mg/kg of body weight/day. RWJ-416457 was two- to fourfold less potent than vancomycin against MSSA but up to fourfold more potent than vancomycin against CA-MRSA. In MSSA and CA-MRSA skin infections, RWJ-416457 demonstrated an efficacy similar to that of linezolid, reducing CFU/g skin approximately 1.0 log(10) at all doses tested; vancomycin yielded greater reductions than the oxazolidinones, with decreases in CFU/g skin of 3 log(10) (MSSA) and 2 log(10) (CA-MRSA). In the pneumococcal model, RWJ-416457 was two- to fourfold more potent than linezolid. The free-drug area under the concentration-time curves at 24 h (fAUC(24)) were similar for RWJ-416457 and linezolid. The half-life of RWJ-416457 was up to threefold longer than that of linezolid for all routes of administration. The fAUC(24)/MIC ratio, the pharmacodynamic parameter considered predictive of oxazolidinone efficacy, was approximately twofold greater for RWJ-416457 than for linezolid. Since the fAUC values were similar for both compounds, the higher fAUC/MIC ratios of RWJ-416457 appear to result from its greater in vitro potency. These results demonstrate that RWJ-416457 is a promising new oxazolidinone with efficacy in S. aureus or S. pneumoniae mouse infection models.

Community-associated meticillin-resistant Staphylococcus aureus infections: epidemiology, recognition and management

Meticillin-resistant Staphylococcus aureus (MRSA) is an important cause of infection, particularly in hospitalized patients and those with significant healthcare exposure. In recent years, epidemic community-associated MRSA (CA-MRSA) infections occurring in patients without healthcare risk factors have become more frequent. The most common manifestation of CA-MRSA infection is skin and soft tissue infection, although necrotizing pneumonia, sepsis and osteoarticular infections can occur. CA-MRSA strains have become endemic in many communities and are genetically distinct from previously identified MRSA strains. CA-MRSA may be more capable colonizers of humans and more virulent than other S. aureus strains. Specific mechanisms of pathogenicity have not been elucidated, but several factors have been proposed as responsible for the virulence of CA-MRSA, including the Panton-Valentine leukocidin, phenol-soluble modulins and type I arginine catabolic mobile element. The movement of CA-MRSA strains into the nosocomial setting limits the utility of using clinical risk factors alone to designate community- or healthcare-associated status. Identification of unique genetic characteristics and genotyping are valuable tools for MRSA epidemiological studies. Although the optimum pharmacological therapy for CA-MRSA infections has not been determined, many CA-MRSA strains remain broadly susceptible to several non-beta-lactam antibacterial agents. Empirical antibacterial therapy should include an MRSA-active agent, particularly in areas where CA-MRSA is endemic.

Pharmacodynamics of tigecycline against phenotypically diverse Staphylococcus aureus isolates in a murine thigh model

Tigecycline is a currently marketed antimicrobial agent with activity against resistant gram-positive cocci, including methicillin-resistant Staphylococcus aureus (MRSA). Despite the proven efficacy of tigecycline in the treatment of infections caused by these pathogens, questions remain as to the exposure-response relationship best associated with its efficacy. The purpose of this study was to define this relationship against seven distinct S. aureus isolates by using a neutropenic murine thigh model. Single-dose pharmacokinetics were evaluated, and free drug exposures were calculated after determination of protein binding. Doses of 1.56 to 400 mg/kg of body weight divided 1 to 8 times daily were administered against two methicillin-susceptible S. aureus isolates, two hospital-associated MRSA (HA-MRSA) isolates, and three community-associated (CA-MRSA) isolates. Tigecycline pharmacokinetics were best described by a two-compartment model, with a mean half-life of 9.9 h. Protein binding was dose dependent (range, 92.9 to 81.2%). MICs were 0.25 microg/ml for all isolates, except for HA-MRSA 56 (MIC, 0.5 microg/ml) and CA-MRSA 156 (MIC, 0.125 microg/ml). Tigecycline displayed efficacy against all isolates, producing maximum decreases in log(10) numbers of CFU/ml of 1.8 to 2.3 from 0-h controls. Mean correlation coefficients for free-drug (f) concentration exposures derived from the parameters fT>MIC (the percentage of time during which the concentration of f remains above the MIC), fC(max)/MIC (the ratio of the maximum concentration of f to the MIC), and fAUC/MIC (the ratio of the area under the concentration-time curve of f to the MIC) were 0.622, 0.812, and 0.958, respectively. Values for the mean effective exposure index at 80% (EI(80)) and 50% (EI(50)) for fAUC/MIC were 5.4 microg/ml (range, 2.8 to 13 microg/ml) and 2.6 microg/ml (range, 0.6 to 5.1 microg/ml), respectively. Experiments with nonneutropenic mice infected with CA-MRSA 156 resulted in maximum kill at all fAUC/MIC exposures tested (1.8 to 8.8 microg/ml). The fAUC/MIC ratio is the pharmacodynamic parameter most predictive of tigecycline efficacy. Furthermore, the presence of a functioning immune system markedly reduces the required exposure.

Management and Treatment Considerations for Infections Caused by Methicillin-Resistant Staphylococcus aureus

Infections caused by methicillin-resistant Staphylococcus aureus have been documented since the 1960s. Four decades later, this microorganism has evolved from a pathogen found primarily in the health care environment to a major pathogen of community-acquired infections. Interestingly, the community-acquired methicillin-resistant Staphylococcus aureus isolates tend to be genetically distinct from their health-care–acquired relatives, have a propensity to be more virulent, yet be susceptible to more antibiotics. Furthermore, infections caused in the pediatric population have a susceptibility pattern that differs from those found in adults. With an increase in infections caused by methicillin-resistant Staphylococcus aureus, empiric therapy should contain an agent with activity against this microorganism. Currently several options exist for the treatment of skin and skin structure infections, yet very few options are available for the treatment of more invasive diseases caused by methicillin-resistant Staphylococcus aureus. This article will review the history of methicillin-resistant Staphylococcus aureus, Staphylococcus aureus economic considerations, and the current plus future treatment options for these infections.

Community-Associated Methicillin-Resistant Staphylococcus aureus in the Pediatric Population

PURPOSE

To review the epidemiology and prevalence of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), define the differences between community-acquired and hospital-acquired strains, highlight the advantages and disadvantages of antibiotics commonly used to treat infections caused by this pathogen, and identify strategies to limit the spread of this organism and prevent future outbreaks.

DATA

Literature was accessed through MEDLINE using the search terms community-acquired methicillin-resistant Staphylococcus aureus, community-associated methicillin-resistant Staphylococcus aureus, CA-MRSA, pediatrics, and children. Articles evaluated were published in the English language and limited to human studies. References of literature identified by initial search techniques were reviewed for additional relevant articles.

DATA SYNTHESIS

Community-associated methicillin-resistant Staphylococcus aureus has become a prominent pathogen in pediatric patients in the last ten years. Its increasing prevalence has been reported throughout the United States, and it is the cause of over one half of all skin and soft tissue infections seen in many hospitals and emergency departments. The risk factors for infection with this pathogen differ from those associated with hospital-acquired strains. Mild to moderate infections can generally be treated with oral antibiotics, while more serious infections may require parenteral therapy. Sulfamethoxazole/trimethoprim and clindamycin are the preferred oral agents due to their efficacy, tolerability, well established side effect profiles, and cost. Vancomycin is the standard of care for parenteral therapy, although clindamycin is an acceptable parenteral alternative. More costly agents such as linezolid, daptomycin, and quinupristin/dalfopristin should be reserved for patients with severe infections, multiple allergies, or in strains with unusual resistance patterns. The best way to prevent and control outbreaks is to maintain standard infection control procedures including excellent hand hygiene.

CONCLUSIONS

CA-MRSA is a serious and frequently seen pathogen. Proper antibiotic selection that takes into account patient factors, disease severity, ease of administration, and cost is necessary to maximize favorable patient outcomes.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Pharmacodynamic characterization of ceftobiprole in experimental pneumonia caused by phenotypically diverse Staphylococcus aureus strains

Ceftobiprole (BPR) is an investigational cephalosporin with activity against Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) strains. The pharmacodynamic (PD) profile of BPR against S. aureus strains with a variety of susceptibility phenotypes in an immunocompromised murine pneumonia model was characterized. The BPR MICs of the test isolates ranged from 0.25 to 2 mug/ml. Pharmacokinetic (PK) studies were conducted with infected neutropenic BALB/c mice; and the BPR concentrations were measured in plasma, epithelial lining fluid (ELF), and lung tissue. PD studies with these mice were undertaken with eight S. aureus isolates (two methicillin-susceptible S. aureus strains, three hospital-acquired MRSA strains, and three community-acquired MRSA strains). Subcutaneous BPR doses of 2 to 125 mg/kg of body weight/day were administered, and the change in the number of log(10) CFU/ml in lungs was evaluated after 24 h of therapy. The PD profile was characterized by using the free drug exposures (f) determined from the following parameters: the percentage of time that the concentration was greater than the MIC (T > MIC), the maximum concentration in serum/MIC, and the area under the concentration-time curve/MIC. The BPR PK parameters were linear over the dose range studied in plasma, and the ELF concentrations ranged from 60 to 94% of the free plasma concentration. fT > MIC was the parameter that best correlated with efficacy against a diverse array of S. aureus isolates in this murine pneumonia model. The 80% effective dose (ED(80)), ED(50), and stasis exposures appeared to be similar among the isolates studied. BPR exerted maximal antibacterial effects when fT > MIC ranged from 6 to 22%, regardless of the phenotypic profile of resistance to beta-lactam, fluoroquinolone, erythromycin, clindamycin, or tetracycline antibiotics.

Activities of clindamycin, daptomycin, doxycycline, linezolid, trimethoprim-sulfamethoxazole, and vancomycin against community-associated methicillin-resistant Staphylococcus aureus with inducible clindamycin resistance in murine thigh infection and in vitro pharmacodynamic models

Controversy exists about the most effective treatment options for community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) and about the ability of these strains to develop inducible resistance to clindamycin during therapy. Using both in vitro pharmacodynamic and murine thigh infection models, we evaluated and compared several antimicrobial compounds against CA-MRSA. Strains with inducible macrolide lincosamide-streptogramin type B (iMLS(B)) resistance and strains in which resistance was noninducible were evaluated. Two levels of inocula (10(5) and 10(7)) were evaluated for clindamycin activity in the in vivo model. In both models, the antimicrobial evaluation was performed in triplicate, and bacterial quantification occurred over 72 h, with drug doses that were designed to simulate the free drug area-under-the-concentration-time curve values (fAUCs) obtained from human samples. When the activity of clindamycin against the iMLS(B) strains was evaluated, constitutive resistance was noted at 24 h (MIC of >256), and failure was noted at an inoculum of > or =10(6) in the in vivo models. However, at a low inoculum (10(5)) in the murine thigh-infection model, clindamycin demonstrated modest activity, reducing the CFU/thigh count for clindamycin resistance-inducible strains at 72 h (0.45 to 1.3 logs). Overall, administration of daptomycin followed by vancomycin demonstrated the most significant kill against all strains in both models. Against the clindamycin noninducible strain, clindamycin and doxycycline demonstrated significant kill. Doxycycline, linezolid, and trimethoprim-sulfamethoxazide (not run in the murine model) demonstrated bacteriostatic activity against clindamycin resistance-inducible isolates. This study demonstrates that clindamycin's activity against the iMLS(B) strains tested is partially impacted by inoculum size. At present, there are several alternatives that appear promising for treating clindamycin resistance-inducible strains of CA-MRSA.

Microbiology of antibiotic resistance in Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) isolates came into existence soon after the introduction of methicillin. Historically, MRSA isolates have been associated with nosocomial infections and rapidly developed resistance to multiple drug classes. However, in recent years, different strains with unique phenotypes have emerged in the community, and the reservoir of community-associated MRSA is rapidly expanding. Community-associated pathogens are likely to cause life-threatening systemic infections, especially in children and elderly individuals, and may also cause serious skin and soft-tissue infections in healthy individuals. Compared with nosocomial strains, community-associated MRSA isolates are associated with increased virulence and currently are more likely to be susceptible to a variety of antibiotics. The epidemiological and microbiological differences between community-associated and nosocomial MRSA infections necessitate different strategies to prevent and treat the 2 types of infections. Vancomycin nonsusceptibility in S. aureus is on the increase, further complicating therapy.