The increasing incidence of inducible chromosomal AmpC β-lactamases within the clinic is a growing concern because these enzymes deactivate a broad range of even the most recently developed β-lactam antibiotics. As a result, new strategies are needed to block the action of this antibiotic resistance enzyme. Presented here is a strategy to combat the action of inducible AmpC by inhibiting the β-glucosaminidase NagZ, which is an enzyme involved in regulating the induction of AmpC expression. A divergent route facilitating the rapid synthesis of a series of N-acyl analogues of 2-acetamido-2-deoxynojirimycin is reported here. Among these compounds are potent NagZ inhibitors that are selective against functionally related human enzymes. These compounds reduce minimum inhibitory concentration values for β-lactams against a clinically relevant Gram-negative bacterium bearing inducible chromosomal AmpC β-lactamase, Pseudomonas aeruginosa. The structure of a NagZ–inhibitor complex provides insight into the molecular basis for inhibition by these compounds.

The effects of ice recrystallization are well-recognized throughout the literature. This phenomenon is the major cause for cellular damage during thawing of cells, ultimately reducing post-thaw viability and function. In this paper, we describe a method for quantifying the inhibitory effect on ice recrystallization of novel small molecules that are cryoprotectants for red blood cells. The method is ideally suited to the splat-cooling assay, where high-ice volume fractions are present. Using our method, we have derived first-order rate constants for the increase in the average crystal size based upon a "binning" approach of ice crystals as a function of size and time. Using this reliable metric, dose−response curves were constructed to obtain IC 50 values. Two very effective inhibitors of ice recrystallization, p-methoxyphenyl β-D-glucopyranoside (PMP-Glc) and pbromophenyl β-D-glucopyranoside (pBrPh-Glc), had IC 50 values of 16.3 and 14.8 mM, respectively. Interestingly, the Hill slopes from these dose−response curves were 5.12 ± 0.81 for PMP-Glc and 3.12 ± 0.62 for pBrPh-Glc, suggesting that an element of cooperativity may be involved in the mechanism by which these compounds inhibit ice recrystallization. This is particularly interesting, as unlike antifreeze (glyco)proteins, these small molecules do not bind to the ice surface. ■ INTRODUCTIONRecrystallization is an extensively studied phenomenon, and its importance in the field of geology and metallurgy has been thoroughly documented. 1,2 The phenomenon is defined as the nucleation and growth of large nondeformed grains in the presence of smaller ones. In deformed grains, the atom arrangement is not ideal, increasing the overall energy of the system. Thus, reorganization to nondeformed grains and an overall decrease in the energy of the system occur. 3−5 As such, recrystallization is a thermodynamically driven kinetic process resulting in an overall reduction in the free energy of the system.In ice, this growth of larger ice crystals at the expense of smaller ones is thought to occur through either grain boundary migration or Ostwald ripening. 6−12 Grain boundary migration occurs through the direct transfer of individual water molecules from smaller unstable ice grains or crystals to larger more favorable ones, resulting in an overall decrease in curvature in an ice crystal and a subsequent reduction in energy. However, grain boundary migration neglects the presence of bulk water or the quasi-liquid layer (QLL) between individual ice crystals. 8 In the Ostwald ripening model, water molecules transfer from the surface of small ice crystals through bulk water and the QLL to larger ice crystals. Smaller ice crystals have a higher surface area to volume ratio, which gives them a higher surface free energy; thus, the Ostwald ripening process results in an overall increase in average ice crystal size and also a reduction in free energy. 10−12 Ice recrystallization has been identified as a problem in the frozen food industry as well as during cryopreservation of bio...

Inducible AmpC β-lactamases deactivate a broad-spectrum of β-lactam antibiotics and afford antibiotic resistance in many Gram-negative bacteria. The disturbance of peptidoglycan recycling caused by β-lactam antibiotics leads to accumulation of GlcNAc-1,6-anhydroMurNAc-peptides, which are transported by AmpG to the cytoplasm where they are processed into AmpC inducers. AmpG transporters are poorly understood; however, their loss restores susceptibility toward β-lactam antibiotics, highlighting AmpG as a potential target for resistance-attenuating therapeutics. We prepare a GlcNAc-1,6-anhydroMurNAc-fluorophore conjugate and, using live E. coli spheroplasts, quantitatively analyze its transport by AmpG and inhibition of this process by a competing substrate. Further, we use this transport assay to evaluate the function of two AmpG homologues from Pseudomonas aeruginosa and show that P. aeruginosa AmpG (Pa-AmpG) but not AmpP (Pa-AmpP) transports this probe substrate. We corroborate these results by AmpC induction assays with Pa-AmpG and Pa-AmpP. This fluorescent AmpG probe and spheroplast-based transport assay will enable improved understanding of PG recycling and of permeases from the major facilitator superfamily of transport proteins and may aid in identification of AmpG antagonists that combat AmpC-mediated resistance toward β-lactam antibiotics.