It is widely assumed that the high level of intrinsic resistance to beta-lactam antibiotics exhibited by mycobacteria results from the combination of factors including permeability to the drugs, 1-lactamase production, and affinity for penicillin-binding proteins (PBPs). We conducted an evaluation of the second and third factors by isolating nitrosoguanidine-induced mutants from the 1B-lactamase-producing strain Mycobacterium fortuitum ATCC 19542 that displayed either elevated or reduced resistance to various beta-lactam antibiotics. The mutants studied included Dl (a "-lactamase producer with high penicillin resistance), 'y 27 (a low-level 13-lactamase producer with low penicillin resistance), and D316 (a high-level 13-lactamase producer with high penicillin resistance). In all strains examined, four major PBPs, named 1, 2a, 2b, and 3, with apparent molecular weights of 102,000, 90,000, 87,000, and 50,000, respectively, were found. The MICs of various beta-lactams toward ATCC 19542 and its mutants were considered in the context of f-lactamase production, the quantity of PBPs synthesized, and their affinities for beta-lactam antibiotics. The data obtained show that 13-lactamase production is likely to be an important factor in the expression of resistance by clinical isolates and that PBP alterations can contribute to resistance at least in laboratory-derived mutants.
The kinetics of both intracellular and extracellular beta-lactamase production and the relationship between extracellular enzyme and in vitro susceptibility of Mycobacterium fortuitum to beta-lactam antibiotics have been studied. To this end we used a panel of stable nitrosoguanidine-induced mutants of M. fortuitum derived from the parental strain ATCC 19542 and differing in beta-lactamase production from 0.0001 to 278 U/liter in Mueller-Hinton broth. For overproducers of beta-lactamase (mutants A188, B180, C207, D316, and E31), MICs of benzylpenicillin, amoxicillin, ampicillin, and cephaloridine progressively increased with the amount of enzyme released into the medium, whereas MICs of imipenem and cefoxitin did not. The resistance of the mutants to amoxicillin was reduced up to 32-fold by clavulanic acid, whereas that to ampicillin was reduced 8-fold by sulbactam. These data suggest that the enzyme participated in the mechanisms of resistance to the beta-lactam antibiotics. However, for a mutant of M. fortuitum (gamma 27) with virtually nonexistent beta-lactamase production, the antibiotics still had relatively high MICs (for instance, benzylpenicillin and cephaloridine had MICs of 64 and 32 micrograms/ml, respectively). This suggests that, aside from beta-lactamase production, other mechanisms such as cell wall permeability and/or affinity for penicillin-binding proteins could coexist in M. fortuitum and explain its natural resistance to beta-lactam antibiotics.
Soft ground with low bearing capacities is found to large areas in the world. In order to increase the bearing capacity of soft ground and to decreases settlement, some methods were developed for improving the mechanical properties of ground. As a stiffener of soil improvement, bamboo chips were used in this study. Bamboo is one of the fastest-growing plants on Earth and with a large distribution around the world. As a natural material, it is even better for environment and has excellently mechanical characteristic. The bamboo chips were mixed into the test specimen as a stiffener of soil improvement. Unconfined compression test were performed with test specimen, then mechanical characteristics of reinforced soil by cement and bamboo chips were investigated. From the test results, the strength and ductility characteristic of soil improvement are rather better when the bamboo chips are mixed.
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