NMR spectra were collected for cross-linked poly(N-isopropylacrylamide), poly(NIPAM), hydrogels in the presence of NaCl and CaCl2 aqueous solutions. Intensity variations in the 1H NMR signals of the polymer provide insight into the phase transition process. These data were used to observe a two-stage phase transition process. Thermodynamic quantities were obtained from a van't Hoff analysis of the temperature-dependent equilibrium constants, which were derived from the NMR data. The Delta H degrees and Delta S degrees values for the hydrogel in D2O are 3.4 kJ/mol and 11.2 J/mol.K for stage I, which is attributed to the formation of hydrophobic bonds between neighboring isopropyl groups. The formation of hydrogen bonds during stage II yielded Delta H degrees and Delta S degrees values of 14.8 kJ/mol and 48.4 J/mol.K in D2O. However, the corresponding Delta H degrees values in 150 mM NaCl and 150 mM CaCl2 are reduced to 1.5 and 1.8 kJ/mol for stage I of the dehydration process. This corresponds to the known effect of salts on hydrophobic bond energetics. The value of Delta S degrees also decreased to 4.9 and 5.9 J/mol.K in NaCl and CaCl2 solutions, respectively. However, the thermodynamic values during stage II were only slightly affected by the salts. The lower temperatures required to induce spontaneous precipitation implies that Delta G degrees of precipitation is reduced. With our measurement of equilibrium thermodynamics, we see that 150 mM NaCl and CaCl2 solutions have a greater effect on hydrophobic bond formation associated with the phase transition process. In this manner, these salts aid in solvent reorganization necessary to form the hydrophobic bond, and this suggests that the formation of hydrophobic bonds is a strong determining factor in the stability of poly(NIPAM) hydrogels in water.

Azole resistance among Aspergillus spp. is uncommon. Denning and colleagues have shown maximum rates of itraconazole resistance (MIC Ն 4 g/ml) of 4.2% for Aspergillus spp. and 2.1% for A. fumigatus (6, 15). These findings are supported by those of Verweij et al. (21) and Diekema et al. (8). Clinical and in vivo resistance to itraconazole (5, 6) and elevated voriconazole MICs (20) for A. fumigatus clinical isolates have been described previously, and clear advances in defining the mechanisms of azole resistance in this species have been made (1,2,6,9,(11)(12)(13)(14)(15)(16)22). However, no study has investigated azole cross-resistance in more than a handful of clinical Aspergillus isolates (15).As part of our global antifungal resistance surveillance programs, we collected hundreds of clinical isolates of Aspergillus spp. from medical centers worldwide between 2000 and 2006. We used this large collection of clinical Aspergillus isolates to describe the patterns of in vitro activity of four azoles (itraconazole, voriconazole, posaconazole, and ravuconazole) against Aspergillus spp. MATERIALS AND METHODS Organisms.A total of 771 unique patient clinical isolates of Aspergillus spp. were obtained from 62 different medical centers worldwide. The isolates were obtained from a variety of sources, including sputum, bronchoscopy, and tissue biopsy specimens. The collection of isolates included 553 A. fumigatus, 76 A. flavus, 59 A. niger, 35 A. terreus, and 24 A. versicolor isolates and 24 isolates of other Aspergillus species. All isolates were identified using standard microscopic morphology and were stored as spore suspensions in sterile distilled water at room temperature until they were used in the study. Before testing, each isolate was subcultured at least twice on potato dextrose agar (Remel, Lenexa, KS) to ensure viability and purity. As a screen for cryptic species within the A. fumigatus complex (e.g., A. lentulus), all A. fumigatus isolates for which the MIC of any azole was Ն2 g/ml were tested for growth at 50°C. All isolates screened grew at 50°C, confirmation that they were A. fumigatus rather than another species within the complex.Susceptibility testing. Posaconazole (Schering-Plough), voriconazole (Pfizer), ravuconazole (Bristol-Myers Squibb), and itraconazole (Janssen) were all obtained as reagent-grade powders from their respective manufacturers. The broth microdilution method was performed according to the CLSI M38-A standard (3). Trays containing a 0.1-ml aliquot of the appropriate drug solution (2ϫ final drug concentration) in each well were sealed and stored at Ϫ70°C until being used in the study. The stock conidial suspension (10 6 spores/ml) was diluted to a final inoculum concentration of 0.4 ϫ 10 4 to 5 ϫ 10 4 CFU/ml and dispensed into the microdilution wells. The final concentrations of drugs in the wells ranged from 0.008 to 8.0 g/ml. The inoculated microdilution trays were incubated at 35°C and read at 48 h.

Beta-lactam antibiotics kill Staphylococcus aureus bacteria by inhibiting the function of cell-wall penicillin binding proteins (PBPs) 1 and 3. However, β-lactams are ineffective against PBP2a, used by methicillin-resistant Staphylococcus aureus (MRSA) to perform essential cell wall crosslinking functions. PBP2a requires teichoic acid to properly locate and orient the enzyme, and thus MRSA is susceptible to antibiotics that prevent teichoic acid synthesis in the bacterial cytoplasm. As an alternative, we have used branched poly(ethylenimine), BPEI, to target teichoic acid in the bacterial cell wall. The result is restoration of MRSA susceptibility to the β-lactam antibiotic ampicillin with a MIC of 1 μg/mL, superior to that of vancomycin (MIC = 3.7 μg/mL). A checkerboard assay shows synergy of BPEI and ampicillin. Nuclear magnetic resonance (NMR) data show that BPEI alters the teichoic acid chemical environment. Laser scanning confocal microscopy (LSCM) images show BPEI residing on the bacterial cell wall where teichoic acids and PBPs are located.

Methicillin-resistant (MRSA) is a medical concern. Here, we show that branched polyethylenimine (BPEI), a nontoxic, cationic polymer, restores MRSA's susceptibility to β-lactam antibiotics. Checkerboard assays with MRSA demonstrated synergy between BPEI and β-lactam antibiotics. A time-killing curve showed BPEI to be bactericidal in combination with oxacillin. BPEI did not potentiate efficacy with vancomycin, chloramphenicol, or linezolid. When exposed to BPEI, MRSA increased in size and had difficulty forming septa. BPEI electrostatically binds to wall teichoic acid (WTA), a cell wall anionic polymer of Gram-positive bacteria that is important for localization of certain cell wall proteins. Lack of potentiation in a WTA knockout mutant supports the WTA-based mechanism. These data suggest that BPEI may prevent proper localization of cell wall machinery by binding to WTA; leading to cell death when administered in combination with β-lactam antibiotics. Negligible toxicity suggests the combination could be a viable treatment option.

Metals bind to the bacterial cell wall yet the binding mechanisms and affinity constants are not fully understood. The cell wall of gram positive bacteria is characterized by a thick layer of peptidoglycan and anionic teichoic acids anchored in the cytoplasmic membrane (lipoteichoic acid) or covalently bound to the cell wall (wall teichoic acid). The polyphosphate groups of teichoic acid provide one-half of the metal binding sites for calcium and magnesium, contradicting previous reports that calcium binding is 100% dependent on teichoic acid. The remaining binding sites are formed with the carboxyl units of peptidoglycan. In this work we report equilibrium association constants and total metal binding capacities for the interaction of calcium and magnesium ions with the bacterial cell wall. Metal binding is much stronger and previously reported. Curvature of Scatchard plots from the binding data and the resulting two regions of binding affinity suggest the presence of negative cooperative binding, meaning that the binding affinity decreases as more ions become bound to the sample. For Ca2+, Region I has a KA = (1.0 ± 0.2) × 106 M−1 and Region II has a KA = (0.075 ± 0.058) × 106 M−1. For Mg2+, KA1 = (1.5 ± 0.1) × 106 and KA2 = (0.17 ± 0.10) × 106. A binding capacity (η) is reported for both regions. However, since binding is still occurring in Region II, the total binding capacity is denoted by η2, which are 0.70 ± 0.04 µmol/mg and 0.67 ± 0.03 µmol/mg for Ca2+ and Mg2+ respectively. These data contradict the current paradigm of there being a single metal affinity value that is constant over a range of concentrations. We also find that measurement of equilibrium binding constants is highly sample dependent, suggesting a role for diffusion of metals through heterogeneous cell wall fragments. As a result, we are able to reconcile many contradictory theories that describe binding affinity and the binding mode of divalent metal cations.