Characterization of nasal and blood culture isolates of methicillin-resistant Staphylococcus aureus from patients in United States Hospitals

A total of 299 nares and 194 blood isolates of methicillin-resistant Staphylococcus aureus (MRSA), each recovered from a unique patient, were collected from 23 U.S. hospitals from May 2009 to March 2010. All isolates underwent spa and staphylococcal cassette chromosome mec element (SCCmec) typing and antimicrobial susceptibility testing; a subset of 84 isolates was typed by pulsed-field gel electrophoresis (PFGE) using SmaI. Seventy-six spa types were observed among the isolates. Overall, for nasal isolates, spa type t002-SCCmec type II (USA100) was the most common strain type (37% of isolates), while among blood isolates, spa type t008-SCCmec type IV (USA300) was the most common (39%). However, the proportion of all USA100 and USA300 isolates varied by United States census region. Nasal isolates were more resistant to tobramycin and clindamycin than blood isolates (55.9% and 48.8% of isolates versus 36.6% and 39.7%, respectively; for both, P < 0.05). The USA300 isolates were largely resistant to fluoroquinolones. High-level mupirocin resistance was low among all spa types (<5%). SCCmec types III and VIII, which are rare in the United States, were observed along with several unusual PFGE types, including CMRSA9, EMRSA15, and the PFGE profile associated with sequence type 239 (ST239) isolates. Typing data from this convenience sample suggest that in U.S. hospitalized patients, USA100 isolates of multiple spa types, while still common in the nares, have been replaced by USA300 isolates as the predominant MRSA strain type in positive blood cultures.     

Antimicrobial activity of tigecycline against community-acquired methicillin-resistant Staphylococcus aureus isolates recovered from North American medical centers

A total of 1989 community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) were susceptibility tested by broth microdilution. Pulsed-field gel electrophoresis, SCCmec type, and polymerase chain reaction for Panton–Valentine leukocidin (PVL) genes were also performed. The overall tigecycline susceptibility rate was 98.2%. Glycopeptides, quinupristin/dalfopristin, linezolid, and chloramphenicol were also active against this collection (≤0.7% resistant). The vast majority (70.8%) of the CA-MRSA was SCCmec type IV, from which 88.4% belonged to the USA300-0114 clone and 94.7% were PVL positive. Tigecycline showed in vitro activity comparable with other highly active parenteral agents and represents an option for treating complicated infections caused by CA-MRSA.     

LEADER Program results for 2009: an activity and spectrum analysis of linezolid using 6,414 clinical isolates from 56 medical centers in the United States

The LEADER Program monitors the in vitro activity of linezolid in sampled U.S. medical centers using reference broth microdilution methods with supporting molecular investigations in a central laboratory design. This report summarizes data obtained in 2009, the 6th consecutive year of this longitudinal study. A total of 6,414 isolates from 56 medical centers in all nine Census regions across the United States participated in 2009. For the six leading species/groups, the following linezolid MIC(90) values were observed: Staphylococcus aureus, 2 μg/ml; coagulase-negative staphylococci (CoNS), 1 μg/ml; Enterococcus spp., 2 μg/ml; Streptococcus pneumoniae, 1 μg/ml; viridans group streptococci, 1 μg/ml; and beta-hemolytic streptococci, 1 μg/ml. Linezolid resistance was only 0.34% overall, with no evidence of significant increase in the LEADER Program since 2006. The predominant linezolid resistant mechanism found was a G2576T mutation in the 23S rRNA. L3/L4 riboprotein mutations were also found. The mobile multidrug-resistant cfr gene was found in four strains (two S. aureus strains and one strain each of S. epidermidis and S. capitis) from four different states, suggesting persistence but a lack of dissemination. Linezolid continues to exhibit excellent activity and spectrum, and this study documents the need for continued monitoring of emerging mechanisms of resistance over a wide geographic area.     

In vitro profiling of ceftaroline against a collection of recent bacterial clinical isolates from across the United States

This study evaluated the in vitro activity of ceftaroline, a novel cephalosporin with broad-spectrum activity against gram-negative and -positive pathogens, against 4,151 recent clinical isolates collected in the United States. Ceftaroline was very potent against bacteria found in community- and hospital-acquired infections, including methicillin-resistant Staphylococcus aureus, multidrug-resistant Streptococcus pneumoniae, and common Enterobacteriaceae spp.     

In vitro activity of ceftaroline against clinical isolates of Streptococcus pneumoniae recovered in 43 U.S. medical centers during 2010-2011

The in vitro activity of ceftaroline, a recently introduced parenteral cephalosporin, was assessed versus 1,750 isolates of Streptococcus pneumoniae recovered from patients with a variety of pneumococcal infections in 43 U.S. medical centers during 2010-2011. Using a breakpoint of ≤ 0.5 μg/ml for susceptibility, all of the isolates were found to be susceptible to ceftaroline. Ceftaroline MICs were consistently 16-fold lower than ceftriaxone MICs. Among the isolates characterized in this investigation, 38.9% were found to be nonsusceptible to penicillin (oral penicillin breakpoints) and 9.1% were nonsusceptible to ceftriaxone (nonmeningitis breakpoints).     

Benchmarking the in vitro activities of moxifloxacin and comparator agents against recent respiratory isolates from 377 medical centers throughout the United States

To benchmark the activity of moxifloxacin (a newer fluoroquinolone), a U.S. study comprising 16,141 contemporary isolates of Streptococcus pneumoniae (5,640), Haemophilus influenzae (6,583), and Moraxella catarrhalis (3,648) referred from 377 institutions during 1998 is described. For S. pneumoniae the modal MIC and MIC at which 90% of the isolates were inhibited (MIC(90)) for moxifloxacin were 0.12 and 0.25 microg/ml, respectively, independent of susceptibility to other drug classes, geography, or site of infection. Eleven isolates were intermediate or resistant to levofloxacin and grepafloxacin; of these isolates, 1 remained susceptible to sparfloxacin, 2 remained susceptible to moxifloxacin, and 4 remained susceptible to trovafloxacin. All 11 isolates possessed classic mutations in gyrA and/or parC known to confer reduced susceptibility to fluoroquinolones. Four isolates (originating from four separate states) belonging to a multidrug-resistant, fluoroquinolone-resistant clone were identified by pulsed-field gel electrophoresis. For moxifloxacin and trovafloxacin, at least 87% of isolates demonstrated MICs > or =3 twofold concentrations below the susceptibility breakpoints, in contrast to no more than 15% for levofloxacin, grepafloxacin, and sparfloxacin. Of the isolates that were multidrug resistant (7.4%), >98% remained susceptible to moxifloxacin. The modal MIC and MIC(90) for M. catarrhalis (both 0.06 microg/ml) and for H. influenzae (both 0.03 microg/ml) were independent of beta-lactamase production. These data demonstrate the in vitro activity of moxifloxacin and establish a baseline for future studies.     

Antimicrobial susceptibility and molecular characteristics of 857 methicillin-resistant Staphylococcus aureus isolates from 16 medical centers in Japan (2008-2009): nationwide survey of community-acquired and nosocomial MRSA

This study is a nationwide survey of all clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates, including community-acquired MRSA (CA-MRSA), in Japan. A total of 857 MRSA clinical isolates were collected from the 16 institutions throughout Japan that participated in the survey (2008-2009). The drug susceptibility and staphylococcal cassette chromosome mec (SCCmec) typing and the presence of specific pathogenic genes were evaluated. The isolates comprised SCCmec type II (73.6%), type IV (20%), and type I (6%). The percentage of SCCmec type IV isolates was significantly higher in outpatients than in inpatients. Most of the isolated strains were sensitive to vancomycin (VCM, MIC ≤2 μg/mL), linezolid (MIC ≤4 μg/mL), and teicoplanin (MIC ≤8 μg/mL). Although most strains were sensitive to VCM, the MIC value of VCM for SCCmec type II strains was higher than that for SCCmec type IV strains. Only 4 (2.3%) of 171 SCCmec type IV strains were Panton-Valentine leukocidin (lukS/F-PV)-positive. Thus, this result indicates a unique feature of SCCmec type IV strains in Japan. The information in this study not only is important in terms of local public health but will also contribute to an understanding of epidemic clones of CA-MRSA.     

Characterization of Erythromycin-Resistant Isolates of Staphylococcus aureus Recovered in the United States from 1958 through 1969

We tested 16 erythromycin-resistant clinical isolates of S. aureus, recovered from patients hospitalized in the United States from 1958 to 1969, for the presence of ermA, ermB, and ermC by using PCR. Fifteen of 16 isolates contained at least one copy of ermA; the remaining isolate, which was also clindamycin resistant, contained ermB. Eight of the 15 isolates harboring ermA, all of which were inducible, contained a single copy of the gene in the chromosome, while the remaining seven isolates had two copies of the gene. ermB was plasmid encoded and mediated constitutive resistance to erythromycin.     

In vitro activity of tedizolid against clinical isolates of Staphylococcus lugdunensis and Staphylococcus haemolyticus from Europe and the United States

Staphylococcus lugdunensis and Staphylococcus haemolyticus are unique among CoNS in that the former often causes aggressive disease, while the latter consistently exhibits high rates of multidrug resistance. We evaluated the in vitro susceptibility of contemporary (2012–2013) isolates from both pathogens to tedizolid and comparators, using standard methodology. Results were interpreted using CLSI and EUCAST breakpoints. Overall, 106 S. lugdunensis and 103 S. haemolyticus isolates were collected from 51 medical centers in the United States and 30 centers in 18 European countries. Tedizolid showed good activity against S. lugdunensis (MIC₅₀/MIC₉₀: 0.12/0.12?mg/L) and S. haemolyticus (MIC₅₀/MIC₉₀: 0.12/0.12?mg/L), inhibiting all isolates at MIC ≤0.25?mg/L. Based on the EUCAST breakpoint for staphylococci and when substituting the CLSI breakpoint for Staphylococcus aureus, all isolates were tedizolid susceptible. All isolates were also susceptible to linezolid, but the in vitro potency of tedizolid was 4-fold greater than that of linezolid against both S. lugdunensis and S. haemolyticus, based on MIC₉₀ values. S. lugdunensis exhibited ≥99% susceptibility to vancomycin, teicoplanin, gentamicin, levofloxacin, and trimethoprim-sulfamethoxazole; 7% of isolates were resistant to tetracycline, 11% to clindamycin, and 2% were methicillin-resistant. S. haemolyticus exhibited high rates of resistance to commonly used anti-staphylococcal agents: 71% of isolates were resistant to methicillin, 36%–37% to clindamycin, and 30%–50% to gentamicin. These in vitro findings suggest that tedizolid could be an alternative treatment option for infections due to these medically important CoNS pathogens. Additional clinical evaluation and continued surveillance of tedizolid in vitro activity against S. lugdunensis and S. haemolyticus are warranted.     

Telavancin In Vitro Activity against a Collection of Methicillin-Resistant Staphylococcus aureus Isolates,Including Resistant Subsets,from the United States

Telavancin had MIC₅₀, MIC₉₀, and MIC₁₀₀ values of 0.03, 0.06, and 0.12 μg/ml, respectively, against methicillin-susceptible Staphylococcus aureus, methicillin-resistant S. aureus (MRSA), and non-multidrug-resistant (non-MDR) and MDR subsets. MRSA with elevated MIC values for vancomycin (2 to 4 μg/ml) or daptomycin (1 to 2 μg/ml) had telavancin MIC₅₀ (0.06 μg/ml) values 2-fold higher than those of isolates with lower MIC results (MIC₅₀, 0.03 μg/ml). However, telavancin had MIC₉₀ and MIC₁₀₀ results of 0.06 and 0.12 μg/ml (100% susceptible), respectively, regardless of the MRSA subset.     

Comparative surveillance study of telavancin activity against recently collected gram-positive clinical isolates from across the United States

Telavancin is an investigational, rapidly bactericidal lipoglycopeptide antibiotic that is being developed to treat serious infections caused by gram-positive bacteria. A baseline prospective surveillance study was conducted to assess telavancin activity, in comparison with other agents, against contemporary clinical isolates collected from 2004 to 2005 from across the United States. Nearly 4,000 isolates were collected, including staphylococci, enterococci, and streptococci (pneumococci, beta-hemolytic, and viridans). Telavancin had potent activity against Staphylococcus aureus and coagulase-negative staphylococci (MIC range, 0.03 to 1.0 microg/ml), independent of resistance to methicillin or to multiple agents. Telavancin activity was particularly potent against all streptococcal groups (MIC(90)s, 0.03 to 0.12 microg/ml). Telavancin had excellent activity against vancomycin-susceptible enterococci (MIC(90), 1 microg/ml) and was active against VanB strains of vancomycin-resistant enterococci (MIC(90), 2 microg/ml) but less active against VanA strains (MIC(90), 8 to 16 microg/ml). Telavancin also demonstrated activity against vancomycin-intermediate S. aureus and vancomycin-resistant S. aureus strains (MICs, 0.5 microg/ml to 1.0 microg/ml and 1.0 microg/ml to 4.0 microg/ml, respectively). These data may support the efficacy of telavancin for treatment of serious infections with a wide range of gram-positive organisms.     

Quantitative susceptibility test methods in major United States medical centers.

To assess the current trend toward performance of routine quantitative susceptibility test methods, we surveyed microbiology laboratories in 41 major U.S. medical centers. Agar dilution and microdilution tests were used primarily by those laboratories routinely performing numerous minimal inhibitory concentration tests and few or no minimal bactericidal concentration tests. Macrotube dilution tests were used by laboratories performing relatively few minimal inhibitory concentration tests, but routinely performing minimal bactericidal concentration tests. Major methodological variations were reported by the surveyed laboratories and including: (i) preparation and storage of antimicrobial solutions, (ii) standardization and concentration of the test inoculum, (iii) interpretation of minimal inhibitory concentration endpoints, and (iv) determination of the antibiotic concentration required to be bactericidal. The results of this survey indicate that significant variations in test methods exist, even though quantitative susceptibility tests are commonly used.     

Activity of ceftobiprole against community-associated methicillin-resistant Staphylococcus aureus isolates recently recovered from US military trainees

Ceftobiprole MICs at which 50% and 90% of isolates were inhibited (MIC50 and MIC90), determined by the Clinical and Laboratory Standards Institute broth microdilution method, were both 1 microg/mL (range, 0.5-1 microg/mL) against 143 community-associated methicillin-resistant Staphylococcus aureus isolates and 0.5 microg/mL (range, 0.25-0.5 microg/mL) with 29 methicillin-susceptible isolates recovered from military trainees during 2 prospective investigations.     

Inducible clindamycin resistance and molecular epidemiologic trends of pediatric community-acquired methicillin-resistant Staphylococcus aureus in Dallas, Texas

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) infection occurs commonly in children. Clindamycin resistance may be inducible or constitutive, and the rates of inducible resistance in CA-MRSA that could produce clindamycin treatment failures vary worldwide. The double-disk test was performed in 197 erythromycin-resistant and clindamycin-susceptible CA-MRSA strains from children in Dallas, Texas, from 1999 to 2002 to determine inducible clindamycin resistance. Resistance mechanisms were studied by PCR; epidemiologic trends were studied by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST). Inducible resistance was demonstrated in 28 (93%+/-6%) of 30 tested isolates in 1999, 21 (64%, +/-11%) of 33 in 2000, 12 (23%+/-7%) of 52 in 2001, and 6 (7%+/-3%) of 82 in 2002. All noninducible strains had the msr(A) gene. Among inducible resistant strains, 31 had erm(B), 24 had erm(C), and 12 had erm(A) genes. Two distinct pulsed types were the most prevalent; one of them was the most common pulsed type in 1999, whereas in 2002 a different pulsed type was prevalent. MLST analyses determined that ST-8 was the most common type, with 76%+/-5% found in 2002. All but one of these clindamycin-susceptible, erythromycin-resistant ST-8 strains showed no induction of clindamycin resistance. We conclude that, among erythromycin-resistant, clindamycin-susceptible CA-MRSA strains isolated from children in Dallas, inducible methylase resistance became less common from 1999 to 2002 (P<0.001). The phenotype of strains was associated with their sequence type. Our results demonstrate a clonal shift in CA-MRSA in Dallas children from 1999 to 2002.     

Efficacy of human simulated exposures of ceftaroline administered at 600 milligrams every 12 hours against phenotypically diverse Staphylococcus aureus isolates

Ceftaroline exhibits bactericidal activity against Gram-positive pathogens, including methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) Staphylococcus aureus, as well as common Gram-negative pathogens. This study evaluated the efficacy of human simulated exposures of ceftaroline against S. aureus in both the neutropenic and immunocompetent mouse thigh infection models. Twenty-six S. aureus isolates (4 MSSA, 22 MRSA) with ceftaroline MICs ranging from 0.125 to 4 μg/ml were collected. All isolates were tested in the neutropenic model and a subset of 13 MRSA isolates were tested in the immunocompetent model. Two hours after inoculation, a ceftaroline regimen that simulated the percentage of the dosing interval that free-drug concentrations remained above the MIC of the infecting organism (fT>MIC) of humans administered ceftaroline at 600 mg every 12 h (q12h) infused over 1 h was given. The change in log(10) CFU/ml after 24 h of treatment was analyzed relative to the 0- and 24-h controls for neutropenic and immunocompetent mice, respectively. The human simulated regimen resulted in efficacy against all isolates tested in both infection models. In the neutropenic model, a 0.95 to 3.28 log(10) CFU/ml reduction was observed when compared with the 0-h control, whereas for the immunocompetent model, all isolates obtained a >1 log(10) CFU/ml reduction (log(10) CFU/ml reduction range: 1.06 to 2.43) in bacterial density. Irrespective of immune competency, a reduction in bacterial density was observed at the highest MIC of 4 μg/ml (fT>MIC of 27.5%). Human simulated exposures of ceftaroline 600 mg q12h provided predictable efficacy against all tested S. aureus isolates in the mouse thigh model independent of immune status. These data support the clinical utility of ceftaroline against S. aureus, including MRSA, with MICs of ≤4 μg/ml.     

qnr prevalence in ceftazidime-resistant Enterobacteriaceae isolates from the United States

We screened 313 ceftazidime-resistant Enterobacteriaceae isolates obtained in the United States from 1999 to 2004 for all three known qnr genes. A qnr gene was present in 20% of Klebsiella pneumoniae isolates, 31% of Enterobacter sp. isolates, and 4% of Escherichia coli isolates. qnrA and qnrB occurred with equivalent frequencies and, except for qnrB in enterobacters, were stable over time. qnrS was absent.     

达托霉素、万古霉素和替考拉宁对1985-2007年收集的甲氧西林耐药金葡菌的抑菌和杀菌活性测定

采用标准的最低抑菌浓度（MIC）和最低杀菌浓度（MBC）方法测定达托霉素、万古霉素和替考拉宁对1985-2007年间美国和欧洲医院收集的479株MRsA的MIC和MBC,并观察受试菌株对该3种抗菌药物的耐受性水平。479株MRSA均分离自血液和脓肿标本,曾用MIC方法测定证实对苯唑西林耐药,对达托霉素、万古霉素和替考拉宁敏感。     

Molecular characterization of methicillin-resistant Staphylococcus aureus bloodstream isolates from Croatia

OBJECTIVES: The objectives of this study were (i) to investigate the genetic background of methicillin-resistant Staphylococcus aureus (MRSA) bloodstream isolates from Croatia and (ii) to monitor the prevalence of Panton-Valentine leucocidin (PVL) and toxic shock syndrome toxin-1 (TSST-1) among these isolates. METHODS: Eighty-two hospital-acquired MRSA bloodstream isolates, collected in 2001 and 2002 in Croatia, were characterized by PFGE, staphylococcal cassette chromosome mec (SCCmec) typing and multilocus sequence typing (MLST). The presence of genes encoding PVL and TSST-1 was investigated by real-time PCR. RESULTS: All strains were multiresistant and were distributed among 16 different similarity groups as determined by PFGE. Two of the groups, groups H and K, harboured the majority of the MRSA strains with 52 and 12%, respectively. The predominant SCCmec type found among the isolates was type I (89%). Eleven per cent of the strains harboured a modified SCCmec type III, which contained, in contrast to the regular type III, an additional dcs region. One strain harboured a novel SCCmec type, containing the ccrC gene in combination with the mecI gene, the dcs region, the locus between pI258 and Tn554 (locus E) and the locus between Tn554 and orfX (locus F). MLST showed the presence of ST111-MRSA-I and ST247-MRSA-I among Croatian MRSA isolates. All isolates were negative for both PVL and TSST-1. CONCLUSIONS: These results indicate the emergence of ST111-MRSA-I and ST247-MRSA-I in Croatia among MRSA bloodstream isolates. The virulence factors PVL and TSST-1 were not present among these isolates.     

LEADER surveillance program results for 2006: an activity and spectrum analysis of linezolid using clinical isolates from the United States (50 medical centers)

Surveillance for emerging linezolid resistance among commonly occurring Gram-positive pathogens in the United States began with the 2002 ZAAPS program and more recently (2004) expanded as the LEADER program. The 2006 LEADER program processed 5374 strains from 50 medical centers (100 per site) located within the 9 US census regions; species and number tested by broth microdilution (% linezolid susceptible) included Staphylococcus aureus (2913, >99.9), coagulase-negative staphylococci (CoNSs) (808, 98.4), enterococci (547, 97.4), Streptococcus pneumoniae (546, 100.0), viridans group streptococci (189, 100.0), and beta-hemolytic streptococci (371, 100.0). In addition to 1 linezolid-nonsusceptible S. aureus, 3 strains were daptomycin-nonsusceptible, 4 were quinupristin/dalfopristin-intermediate, 2 were vancomycin-intermediate (vancomycin MIC values, 4 mug/mL), and all were methicillin-resistant S. aureus. Among the linezolid-resistant isolates (1 S. aureus, 13 CoNSs, 3 Enterococcus faecalis, and 10 Enterococcus faecium isolates), all but 3 Staphylococcus epidermidis isolates had the G2567T mutation. Overall, 99.55% of the tested 2006 LEADER program isolates remained susceptible to linezolid at current Clinical and Laboratory Standards Institute breakpoints.