Acquired resistance

Plasmids have the ability to transfer within and between species and can therefore be acquired from other bacteria as well as a consequence of cell division. This property makes plasmid-acquired resistance much more threatening in terms ofthe spread of antibiotic resistance than resistance acquired due to chromosomal mutation. Plasmids also harbour transposons (section 2.1.3), which enhances their ability to transfer antibiotic resistance genes. 

Relative microbial responses to 5.1 Resistance acquired by mutation... 

Distinguishing feature Intrinsic resistance Acquired resistance... 

Tresochin/Bayer Imagon/Astra (it was until recently the best antimalarial drug, now rendered useless in most areas drug resistance acquired by the parasite)... 

Sadun, E.H. and Lin, S.S. (1959) Studies on the host-parasite relationships to Schistosoma japonicum. IV. Resistance acquired by infection, by vaccination and by the injection of immune serum, in monkeys, rabbits and mice. Journal of Parasitology 45, 543-548. 

Hawking F (1941) Drug resistance acquired during the treatment of sleeping sickness with tryparsamide and with Bayer 205. Am J Trop Med 21 469-475... 

DNA transfer of drug resistance Of particular clinical concern is resistance acquired due to DNA transfer from one organism to another. Resistance properties are usually encoded in extrachro-mosomal R factors (plasmids). These may enter cells by processes such as transduction (phage-mediated), transformation or, most importantly, bacterial conjugation. 

Knowledge of the mechanisms of action of antimicrobial agents is required for understanding resistance acquired through chromosomal mutation and selection, and forms the basis of selecting antimicrobials for concurrent use, either as combination preparations or separately. 

At normal levels of iron intake, absorption requires uptake from the intestinal lumen by the mucosa and transfer from the mucosa to the portal blood. Both events are inversely affected by the state of body iron stores. In iron deficiency states, nonferrous metals such as cobalt and manganese, which have an ionic radius similar to that of iron and form octahedral complexes with six-coordinate covalent bonds, also are absorbed at an increased rate. Oral administration of a large dose of iron reduces (or temporarily inhibits) the absorption of a second dose of iron by the absorptive enterocytes even in the presence of systemic iron deficiency. The mechanism of mucosal block, which resists acquiring additional iron by the en-teroeytes with high amounts of intracellular iron, is not yet understood. It probably involves set points established in the enterocytes for iron recently consumed in the diet (dietary regulator). 

Is resistance caused by one allele in one gene locus, or is resistance acquired when sufficient resistance alleles from many genes have been accumulated in several individuals in a population ... 

Vurma-Rapp, U., Kayser, F.Fl., Fladom, K., and Wiederkehr, F. 1990, Mechanism of imipenem resistance acquired by three Pseudomonas aeruginosa strains during imipenem therapy. Eur J Clin Microbiol Infect Dis 9 580-587. 

Drinker P, Thomson RM, Finn JL. 1927a. Metal fume fever II. Resistance acquired by inhalation of zinc oxide on two successive days. J Ind Hyg 9 98-105. 

A useful distinction can be made between resistance acquired (a) in the patient and (b) apart from him. Thus, in patients undergoing treatment for tuberculosis with streptomycin, isoniazid, or / -aminosalicylic acid, the causative organism Mycobacterium tuberculosis) often becomes resistant to one or more of these drugs. This has been found to take place, stepwise, by a selection of resistant strains within the patient s body. Resistant staphylococci, on the other hand, are very often acquired from other infected people (Knox, 1962). 

The fact that good electrical contact is made between the conductive polymer and the superconductor is noted from decreases in the four-point sample resistance with an onset temperature near 110 K and zero resistance close to 85 K for Pbo.3Bii.7Sri.6Ca2.4Cu30io samples measured with poly(3-hexylthiophene) contacts, as shown in Fig. 37.14A. Almost identical four-point sample resistance results are acquired with the use of silver contacts on the same specimen. Importantly, these results demonstrate that conductive polymer components can be used to prepare superconductor circuits that operate at temperatures both above and below 7c. Moreover, the values of the contact resistance acquired for the poly(3-hexylthiophene)/YBa3Cu307 5 structure are comparable to values acquired for systems in which the superconductor component is replaced with a noble metal material such as platinum (see Table 37.3). Al-... 

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