Farida Siddiqui

Resistance to Linezolid: Characterization of Mutations in rRNA and Comparison of Their Occurrences in Vancomycin-Resistant Enterococci

ABSTRACT To assess the potential for emergence of resistance during the use of linezolid, we tested 10 clinical isolates of vancomycin-resistant enterococci (VRE) (four Enterococcus faecalis, fiveEnterococcus faecium, and one Enterococcus gallinarum) as well as a vancomycin-susceptible control (ATCC 29212) strain of E. faecalis. The enterococci were exposed to doubling dilutions of linezolid for 12 passes. After the final passage, the linezolid plate growing VRE contained a higher drug concentration with E. faecalis than with E. faecium. DNA sequencing of the 23S rRNA genes revealed that linezolid resistance in three E. faecalis isolates was associated with a guanine to uracil transversion at bp 2576, while the one E. faecium isolate for which the MIC was 16 μg/ml contained a guanine to adenine transition at bp 2505.

N‐Methyl‐d‐Aspartate Receptor Excitotoxicity Involves Activation of Polyamine Synthesis: Protection by α‐Difluoromethylornithine

Abstract: We investigated the role of polyamines and their regulatory enzyme ornithine decarboxylase in N‐Methyl‐D‐aspartate‐induced excitotoxicity in embryonic chick retina. N‐Methyl‐D‐aspartate (200 μM) produced an early increase in ornithine decarboxylase activity, putrescine concentration, and Ca2+ entry, leading to selective neuronal death by 30 min. This response was attenuated by the ornithine decarboxylase inhibitor α‐difluoromethylornithine and the N‐methyl‐D‐aspartate receptor antagonist 5‐aminophosphonovaleric acid. Exogenous putrescine increased intracellular putrescine and spermine levels and reversed neuroprotection by α‐difluoromethylornithine, but not by 5‐aminophosphonovaleric acid. N‐Methyl‐D‐aspartate‐receptor stimulation of putrescine/polyamine synthesis mediates abnormal Ca2+ entry and acute excitotoxic neuronal death. Postreceptor inhibition of the ornithine decar‐boxylase/polyamine cascade by α‐difluoromethylornithine may provide neuroprotection against N‐methyl‐D‐aspartate‐induced excitotoxicity.

Fluoroquinolone resistance is a poor surrogate marker for type II topoisomerase mutations in clinical isolates of Streptococcus pneumoniae.

ABSTRACT The association between fluoroquinolone susceptibility and DNA mutations coding for amino acid substitutions in the quinolone resistance-determining region was assessed with 44 clinical isolates ofStreptococcus pneumoniae. Twenty-three strains bore at least one amino acid substitution. Only seven strains with mutations were suggested by diminished susceptibility to ciprofloxacin (MIC, ≥2 μg/ml).

Regulation of N-Methyl-d-aspartate receptor-mediated calcium transport and norepinephrine release in rat hippocampus synaptosomes by polyamines

The role of polyamines (PA) synthesis in NMDA receptor-mediated45Ca2+ fluxes and norepinephrine release was studied in rat hippocampal synaptosomes. NMDA (50μM) caused a sharp (>2-fold) transient increase in PA synthesis regulating enzyme, ornithine decarboxylase (ODC) activity with concomitant elevation in PA levels in the order putrescine>spermidine>spermine. ODC inhibitor, α-difluoromethylornithine (DFMO), and NMDA antagonist, 2-amino-5-phosphonovaleric acid (D-AP5), both blocked increases in ODC activity and PA levels. Activation of NMDA receptors induced a sharp (3 to 4-fold) and quick (15 seconds) increase in45Ca2+ uptake by synaptosomes within 15 seconds of exposure at 37°C. The efflux of45Ca2+ and3H-norepinephrine (NE) release at 22°C from pre-loaded synaptosomes was also significantly (2 to 4-fold) enhanced by NMDA within 15 seconds. These NMDA receptor-mediated effects on calcium fluxes and NE release were blocked by NMDA receptor-antagonists (DAP-5 and MK-801) and PA synthesis inhibitor, DFMO and the DFMO inhibition nullified by exogenous putrescine. These observations establish that ODC/PA cascade play an important role in transduction of excitatory amino acid mediated signals at NMDA receptors.