About 80% of methicillin-and cefazolin-resistant strains of Staphylococcus aureus isolated clinically in Japan in 1982 retained their resistance even after elimination of penicillinase-encoding plasmids. The penicillinbinding proteins (PBPs) of the penicillinase-free, methicillin-and cephem-resistant subclones of Staphylococcus iureus (MRSA) were cotnpared with those of spontaneous susceptible revertants which had been obtained by the replica method after 10 subcultures in drug-free media. A new PBP fraction (PBP2') having a molecular weight of 78,000 and low binding affinities for various ,-lactam antibiotics was found in MRSA exclusively, The levels of resistance of MRSA strains were reduced markedly by culturing them at 43°C or at pH 5.2 or both.We found that the binding capacity of PBP2' for 14C-labeled penicillin G was decreased by preintubation of the membrane fractions of MRSA strains at 43°C for 60 min and that the amount of PBP2' in MRSA strains grown at pH 5.2 was less than that the amount of PBP2' in MRSA strains grown at pH 7.0. Temperature-and pH-dependent expression of resistance in MRSA is likely to reflect the temperature sensitivity and neutral pH-dependent production of the specific PBP fraction (PBP2'). We suggest that MRSA strains can grow in the presence of l-lactam antibiotics because of the low affinities of the specific PBP2' fraction for various ,I-lactam antibiotics.The isolation frequencies of strains of methicillin-and cephem-resistant Staphylococcus aureus (MRSA) from clinical specimens have been gradually increasing (3,7,8,21). These strains tend to be resistant to various ,B-lactam antibiotics, although the levels of resistance to each drug are different. In particular, these organisms are highly resistant to many of the expanded-spectrum cephem antibiotics with few exceptions. This fact may be due to the wide clinical use of semisynthetic penicillins and third-generation cephem antibiotics which possess rather weak anti-staphylococcal activity.Methicillin-resistant strains of Staphylococcus aureus were first reported in 1961 (15). These strains expressed cross resistance to other P-lactam antibiotics (22), and the resistance was intrinsic rather than due to penicillinase, as penicillinase-free mutants of the methicillin-resistant strains retained the resistance (9). Furthermore, the resistance was unusual in that the cell populations were heterogeneous (24) with respect to the levels of resistance in each cell. The levels of resistance in MRSA strains were affected mnarkedly by growth conditions; namely, the levels of resistance of MRSA strains were higher at 30°C and pH 7.0 than at 43°C and pH 5.2, respectively (1,(18)(19)(20). The levels of resistance were also higher in media containing 5% NaCl than in salt-free media (2).The mechanism of methicillin and cephem resistance in MRSA remained unanswered for a long time. Brown and Reynolds (4) and Hayes and colleagues (14) reported decreased affinity of penicillin-binding protein 3 (PBP3) no. 342, 1982) reported that in addition ...
Molecular cloning and nucleotide sequence analysis were performed for the identification of the regulator genes of mclhicillin resistance in the genome of a MRSA strain N3 15. Two open reading frames (orfs) were identified in the S'-Banking region of the rrrecA gene. Predicted amino acid sequences of these ork showed exlcnsive homology to the co-inducer and the repressor protein of the pcnicillinasc (P&se) production in Sruphyrylocaccws uttreus m well as in &~aci~lrrs h'cltattjlmtis. These orfs are considered to encode putative co-inducer and repressor proteins specific for the regulation of methicillin resistance in MRSA./nrrA; Repressor; Co-inducer; Methicillin rcsistancc; Stupltylococcus auretts
A total number of 125 methicillin-resistant (MIC, greater than or equal to 16) coagulase-negative Staphylococcus strains isolated in Japan were surveyed for the distribution of the mecA gene, the structural gene for penicillin-binding protein 2', which is the causative genetic element for the intrinsic resistance of methicillin-resistant Staphylococcus aureus. Screening with colony hybridization by using a cloned mecA gene probe revealed that 121 strains (96.8%) belonging to the nine coagulase-negative Staphylococcus species (S. epidermidis, S. haemolyticus, S. saprophyticus, S. sciuri, S. simulans, S. hominis, S. capitis, S. warneri, and S. caprae) carried mecA in their genome, indicating wide distribution of the gene among coagulase-negative Staphylococcus species. Most (93.4%) of the mecA-carrying strains were producers of penicillinase. Four strains, including two S. haemolyticus and two S. saprophyticus strains, did not carry mecA in spite of their resistance to methicillin. One of them was of low-level resistance (MIC, 16), but three of them had moderate- to high-level resistance to methicillin (MIC, 64). Analysis of gel electrophoretic banding patterns of penicillin-binding proteins of these strains showed absence of penicillin-binding protein 2' but some alterations in signal intensities of the other penicillin-binding proteins. The result indicated that about 3% of methicillin-resistant coagulase-negative staphylococci in these hospitals had a resistance mechanism different from that associated with the production of penicillin-binding protein 2', as has been reported in the case of a borderline methicillin-resistant strain of S. aureus.
In situ heating and electron-beam irradiation in the transmission electron microscope were performed to study melting of submicron Al-11.6 at. % Si particles supported on a C thin film. It was found that electron irradiation could be used to melt the particles, even when the hot-stage sample holder was kept at a much lower temperature ( approximately 125 degrees C) than the initial melting point of the particles. Comparison between the experimentally observed melting behavior and analytical calculations indicate that melting of the submicron Al-Si particles under electron-beam irradiation is caused by a temperature rise due to electron thermal spikes in the particles and poor thermal conduction away from the particles. These results have important implications in transmission electron microscopy studies of nanoparticles supported on thin films or poorly conducting substrates.
Bipolar resistance switching was investigated on sputtered Pr0.7Ca0.3MnO3 (PCMO) sandwiched by Pt- and Ti-electrodes. Based on electrical conductivity measurements and a combination of electron energy loss spectroscopy analysis and transmission electron microscopy observation, we found that the negative differential resistance observed in the forming process originates from the motion of oxygen ions at the Ti/PCMO interface. We propose that the observed resistance switching is caused by an oxidation/reduction reaction at the interface.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.