Pamela McGhee

Affinity of Ceftaroline and Other β-Lactams for Penicillin-Binding Proteins from Staphylococcus aureus and Streptococcus pneumoniae

ABSTRACT We compared the affinities of ceftaroline for all penicillin-binding proteins (PBPs) with those of ceftriaxone and cefotaxime in 6 Staphylococcus aureus and 7 Streptococcus pneumoniae isolates with various resistance phenotypes. Ceftaroline MICs were ≤1 μg/ml against all S. aureus isolates and were ≤0.25 μg/ml for 4 of 7 isolates of S. pneumoniae. Ceftaroline affinities for penicillin-susceptible S. pneumoniae strains were in the order PBP2X and -3 > PBP1A, -1B, and -2A > PBP2B, and ceftaroline had ≥4-fold higher 50% inhibitory concentrations (IC50s) (0.1 to 4 μg/ml) for PBP2X, -2A, -2B, and -3 than those for the other cephalosporins tested. Among 3 penicillin-resistant S. pneumoniae strains, ceftaroline had a high affinity for PBP2X (IC50, 0.1 to 1 μg/ml), a primary target for cephalosporin PBP binding activity, and high affinities for PBP2B (IC50, 0.5 to 4 μg/ml) and PBP1A (IC50, 0.125 to 0.25 μg/ml) as well, both of which are also known as major targets for PBP binding activity of cephalosporins. Ceftaroline PBP affinities in methicillin-susceptible S. aureus strains were greater than or equal to those of the 3 other β-lactams tested. Ceftaroline bound to PBP2a in methicillin-resistant S. aureus (IC50, 0.01 to 1 μg/ml) with up to 256-fold-higher affinity than those of other agents. Ceftaroline demonstrated very good PBP affinity against all S. aureus and S. pneumoniae strains tested, including resistant isolates.

Antipneumococcal Activity of Ceftobiprole, a Novel Broad-Spectrum Cephalosporin

ABSTRACT Ceftobiprole (previously known as BAL9141), an anti-methicillin-resistant Staphylococcus aureus cephalosporin, was very highly active against a panel of 299 drug-susceptible and -resistant pneumococci, with MIC50 and MIC90 values (μg/ml) of 0.016 and 0.016 (penicillin susceptible), 0.06 and 0.5 (penicillin intermediate), and 0.5 and 1.0 (penicillin resistant). Ceftobiprole, imipenem, and ertapenem had lower MICs against all pneumococcal strains than amoxicillin, cefepime, ceftriaxone, cefotaxime, cefuroxime, or cefdinir. Macrolide and penicillin G MICs generally varied in parallel, whereas fluoroquinolone MICs did not correlate with penicillin or macrolide susceptibility or resistance. All strains were susceptible to linezolid, quinupristin-dalfopristin, daptomycin, vancomycin, and teicoplanin. Time-kill analyses showed that at 1× and 2× the MIC, ceftobiprole was bactericidal against 10/12 and 11/12 strains, respectively. Levofloxacin, moxifloxacin, vancomycin, and teicoplanin were each bactericidal against 10 to 12 strains at 2× the MIC. Azithromycin and clarithromycin were slowly bactericidal, and telithromycin was bactericidal against only 5/12 strains at 2× the MIC. Linezolid was mainly bacteriostatic, whereas quinupristin-dalfopristin and daptomycin showed marked killing at early time periods. Prolonged serial passage in the presence of subinhibitory concentrations of ceftobiprole failed to yield mutants with high MICs towards this cephalosporin, and single-passage selection showed very low frequencies of spontaneous mutants with breakthrough MICs towards ceftobiprole.

Activity of Telavancin against Staphylococci and Enterococci Determined by MIC and Resistance Selection Studies

ABSTRACT This study used CLSI broth microdilution to test the activity of telavancin and comparator antimicrobial agents against 67 methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates. Twenty-six vancomycin-intermediate S. aureus (VISA) strains were among the isolates tested; all strains were susceptible to telavancin at ≤1 μg/ml, whereas 12/26 (46%) of these isolates were nonsusceptible to daptomycin at the same concentration. All strains were susceptible to quinupristin-dalfopristin, while resistance was found to all other drugs tested. Telavancin demonstrated potent activity against all vancomycin-susceptible isolates as well as against heterogeneously VISA and VISA resistance phenotypes. In multistep resistance selection studies, telavancin yielded one stable mutant after 43 days in one MRSA strain out of the 10 MRSA strains tested with the MIC rising eightfold from 0.25 μg/ml (parent) to 2 μg/ml. MICs for this clone did not increase further when passages were continued for the maximum 50 days. In contrast, daptomycin selected stable resistant clones (MIC increase of >4×) after 14 to 35 days in 4 of 10 MRSA strains with MICs increasing from 1 to 2 μg/ml (parents) to 4 to 8 μg/ml (resistant clones). Sequencing analysis of daptomycin resistance determinants revealed point mutations in the mprF genes of all four stable daptomycin-resistant clones. Teicoplanin gave rise to resistant clones after 14 to 21 days in 2 of 10 MRSA strains with MICs rising from 1 to 2 μg/ml (parents) to 4 to 16 μg/ml (stable resistant clones). Linezolid selected stable resistant clones after 22 to 48 days in 2 of 10 MRSA strains with MICs rising from 2 to 4 μg/ml (parents) to 32 μg/ml (resistant clones). Vancomycin yielded no resistant clones in 10 MRSA strains tested; however, MICs increased two- to fourfold from 1 to 8 μg/ml to 2 to 16 μg/ml after 50 days. No cross-resistance was found with any clone/antimicrobial combination. The two enterococci developed resistance to daptomycin, and one developed resistance to linezolid. Single-step mutation frequencies for telavancin (<4.0 × 10−11 to <2.9 × 10−10 at 2× MIC) were lower than the spontaneous mutation frequencies obtained with the comparators.

Single- and Multistep Resistance Selection Studies on the Activity of Retapamulin Compared to Other Agents against Staphylococcus aureus and Streptococcus pyogenes

ABSTRACT Retapamulin had the lowest rate of spontaneous mutations by single-step passaging and the lowest parent and selected mutant MICs by multistep passaging among all drugs tested for all Staphylococcus aureus strains and three Streptococcus pyogenes strains which yielded resistant clones. Retapamulin has a low potential for resistance selection in S. pyogenes, with a slow and gradual propensity for resistance development in S. aureus.

In Vitro Activity of CEM-101 against Streptococcus pneumoniae and Streptococcus pyogenes with Defined Macrolide Resistance Mechanisms

ABSTRACT CEM-101 had MIC ranges of 0.002 to 0.016 μg/ml against macrolide-susceptible pneumococci and 0.004 to 1 μg/ml against macrolide-resistant phenotypes. Only 3 strains with erm(B), with or without mef(A), had CEM-101 MICs of 1 μg/ml, and 218/221 strains had CEM-101 MICs of ≤0.5 μg/ml. CEM-101 MICs were as much as 4-fold lower than telithromycin MICs against all strains. For Streptococcus pyogenes, CEM-101 MICs ranged from 0.008 to 0.03 μg/ml against macrolide-susceptible strains and from 0.015 to 1 μg/ml against macrolide-resistant strains. Against erm(B) strains, erythromycin, azithromycin, and clarithromycin MICs were 32 to >64 μg/ml, while 17/19 strains had telithromycin MICs of 4 to 16 μg/ml; CEM-101 MICs were 0.015 to 1 μg/ml. By comparison, erm(A) and mef(A) strains had CEM-101 MICs of 0.015 to 0.5 μg/ml, clindamycin and telithromycin MICs of ≤1 μg/ml, and erythromycin, azithromycin, and clarithromycin MICs of 0.5 to >64 μg/ml. Pneumococcal multistep resistance studies showed that although CEM-101 yielded clones with higher MICs for all eight strains tested, seven of eight strains had clones with CEM-101 MICs that rose from 0.004 to 0.03 μg/ml (parental strains) to 0.06 to 0.5 μg/ml (resistant clones); for only one erm(B) mef(A) strain with a parental MIC of 1 μg/ml was there a resistant clone with a MIC of 32 μg/ml, with no detectable mutations in the L4, L22, or 23S rRNA sequence. Among two of five S. pyogenes strains tested, CEM-101 MICs rose from 0.03 to 0.25 μg/ml, and only for the one strain with erm(B) did CEM-101 MICs rise from 1 to 8 μg/ml, with no changes occurring in any macrolide resistance determinant. CEM-101 had low MICs as well as low potential for the selection of resistant mutants, independent of bacterial species or resistance phenotypes in pneumococci and S. pyogenes.

Multistep Resistance Development Studies of Ceftaroline in Gram-Positive and -Negative Bacteria

ABSTRACT Ceftaroline, the active component of the prodrug ceftaroline fosamil, is a novel broad-spectrum cephalosporin with bactericidal activity against Gram-positive and -negative isolates. This study evaluated the potential for ceftaroline and comparator antibiotics to select for clones of Streptococcus pneumoniae, Streptococcus pyogenes, Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae, Staphylococcus aureus, and Enterococcus faecalis with elevated MICs. S. pneumoniae and S. pyogenes isolates in the present study were highly susceptible to ceftaroline (MIC range, 0.004 to 0.25 μg/ml). No streptococcal strains yielded ceftaroline clones with increased MICs (defined as an increase in MIC of >4-fold) after 50 daily passages. Ceftaroline MICs for H. influenzae and M. catarrhalis were 0.06 to 2 μg/ml for four strains and 8 μg/ml for a β-lactamase-positive, efflux-positive H. influenzae with a mutation in L22. One H. influenzae clone with an increased ceftaroline MIC (quinolone-resistant, β-lactamase-positive) was recovered after 20 days. The ceftaroline MIC for this isolate increased 16-fold, from 0.06 to 1 μg/ml. MICs for S. aureus ranged from 0.25 to 1 μg/ml. No S. aureus isolates tested with ceftaroline had clones with increased MIC (>4-fold) after 50 passages. Two E. faecalis isolates tested had ceftaroline MICs increased from 1 to 8 μg/ml after 38 days and from 4 to 32 μg/ml after 41 days, respectively. The parental ceftaroline MIC for the one K. pneumoniae extended-spectrum β-lactamase-negative isolate tested was 0.5 μg/ml and did not change after 50 daily passages.

Incidence and Characteristics of Vancomycin Nonsusceptible Strains of Methicillin-Resistant Staphylococcus aureus at Hershey Medical Center

ABSTRACT All 982 methicillin-resistant Staphylococcus aureus strains collected from August 2006 to December 2007 were tested for vancomycin susceptibility by using 3-μg/ml vancomycin brain heart infusion screening plates, a vancomycin Etest, and a vancomycin/teicoplanin macro Etest. Three vancomycin-intermediate Staphylococcus aureus (VISA) (0.3%) and two heterogeneous VISA (0.2%) isolates were identified. The screening method yielded 895 cases of ≤1 colony and 87 positive results (with growth of >1 colony after 48 h); further Etests showed 82/87 isolates with growth on screening plates to be false positive. Repeat testing showed a false-positivity rate of only 15 of the original 87 isolates by plate screening.

Molecular and epidemiologic characteristics of linezolid-resistant coagulase-negative staphylococci at a tertiary care hospital.

We investigated emergence of linezolid resistance among coagulase-negative staphylococci at our tertiary care center in 2007. All 17 cases were healthcare associated, and prior administration of linezolid was documented <or=2 months before first isolation of linezolid-resistant coagulase-negative staphylococci for all but 1 patient. Pulse-field gel electrophoresis analysis of the 14 available strains demonstrated 1 predominant clonal type, suggesting nosocomial spread. In addition to mutations in 23S rRNA and L4 previously described, we observed novel alterations in the 23S rRNA gene (G(2215)A) and in the L3 protein (substitutions L(101)V, H(146)Q/R, F(147)I, V(154)L, M(156)T). The increase in linezolid-resistant coagulase-negative staphylococci correlated with nosocomial transmission of selected mutated strains in patients who had received linezolid.

Antipneumococcal Activity of DW-224a, a New Quinolone, Compared to Those of Eight Other Agents

ABSTRACT DW-224a is a new broad-spectrum quinolone with excellent antipneumococcal activity. Agar dilution MIC was used to test the activity of DW-224a compared to those of penicillin, ciprofloxacin, levofloxacin, gatifloxacin, moxifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin against 353 quinolone-susceptible pneumococci. The MICs of 29 quinolone-resistant pneumococci with defined quinolone resistance mechanisms against seven quinolones and an efflux mechanism were also tested. DW-224a was the most potent quinolone against quinolone-susceptible pneumococci (MIC50, 0.016 μg/ml; MIC90, 0.03 μg/ml), followed by gemifloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. β-Lactam MICs rose with those of penicillin G, and azithromycin resistance was seen mainly in strains with raised penicillin G MICs. Against the 29 quinolone-resistant strains, DW-224a had the lowest MICs (0.06 to 1 μg/ml) compared to those of gemifloxacin, clinafloxacin, moxifloxacin, gatifloxacin, levofloxacin, and ciprofloxacin. DW-224a at 2× MIC was bactericidal after 24 h against eight of nine strains tested. Other quinolones gave similar kill kinetics relative to higher MICs. Serial passages of nine strains in the presence of sub-MIC concentrations of DW-224a, moxifloxacin, levofloxacin, ciprofloxacin, gatifloxacin, gemifloxacin, amoxicillin-clavulanate, cefuroxime, and azithromycin were performed. DW-224a yielded resistant clones similar to moxifloxacin and gemifloxacin but also yielded lower MICs. Azithromycin selected resistant clones in three of the five parents tested. Amoxicillin-clavulanate and cefuroxime did not yield resistant clones after 50 days.

Antistaphylococcal Activity of CG400549, a New Experimental FabI Inhibitor, Compared with That of Other Agents

ABSTRACT Among 203 strains of Staphylococcus aureus, the MICs of CG400549 were 0.06 to 1.0 μg/ml, with MIC50 and MIC90 values of 0.25 μg/ml each. All strains were susceptible to linezolid and quinupristin-dalfopristin (MICs, 0.25 to 2.0 μg/ml). The daptomycin MICs were 0.25 to 2.0 μg/ml for methicillin-susceptible and 0.25 to 4.0 μg/ml against methicillin-resistant strains (including vancomycin-intermediate strains). Single-passage selection testing showed low resistance frequencies with CG400549, but multistep analysis showed that CG400549 yielded resistant mutants after 14 to 17 days in all strains tested.