Plasmids transmission

Plasmid transmission and the stability of plasmids in natural ecosystems have received considerable attention, but caution should be exercised in drawing general conclusions on the basis of the sometimes fragmentary evidence from laboratory experiments. Some important principles are illustrated by the following ... 

Resistance to a range of antibiotics is of increasing concern in clinical practice since the genes are often carried on transmissible plasmids. There are different types of mechanism that confer resistance, inclnding enzymatic covalent modification of the antibiotic, effective efflnx systems, and indnction of a cellnlar enzyme that is resistant to the antibiotic. Examples of these are used as illustration. 

It has been shown (Smith et al. 1978) that in enteric bacteria carrying thermosensitive plasmids coding for the utilization of citrate and for resistance to antibiotics, rates of transmission were negligible at 37°C but appreciable at 23°C—a temperature more closely approaching that which prevails in natural ecosystems. 

Rheinwald JG, AM Chakrabarty, C Gunsalus (1973) A transmissible plasmid controlling camphor oxidation in Pseudomonas putida. Proc Natl Acad Sci USA 70 885-889. 

Dunn NW, IC Gunsalus (1973) Transmissible plasmid coding early enzymes of naphthalene oxidation in Pseudomonas putida. J Bacteriol 114 974-979. 

Dabrock B, M Ke eler, B Averhoff, G Gottschalk (1994) Identification and characterization of a transmissible linear plasmid from Rhodococcus erythropolis BD2 that encodes isopropylbenzene and trichloro ethylene catabolism. Appl Environ Microbiol 60 853-860. 

To meet the infectivity requirement in the definition of a prion, the abnormal form of the protein must be transmissible to other cells and organisms of the same species. In yeast and filamentous fungi, infectious agents such as viruses and plasmids are naturally transmitted by cytoplasmic mixing during mating or heterokaryon formation (Wickner, 2001). 

Antibiotics are routinely added to animal feed in conventional agriculture. This can have various effects on humans. Direct transmission of antibiotic residues in animal products to people may cause direct toxicity, i.e. allergies, or lead to the emergence of resistant strains of bacteria. Another threat is antibiotic-resistant forms of bacteria harmful to mankind that might appear in animals and pass from them to humans (Smith 1974), or may impart resistance to other bacteria by plasmid or transposon interchange (Franco et al. 1990). The resulting dmg-resistant and harmful micro-organisms can then not be treated successfully (Silverstone 1993). 

Plasmid Degradative Pathway Size Transmissibility (Megadaltons) ... 

Mammalian cells (and some bacteria) lack the enzymes required for folate synthesis from PABA and depend on exogenous sources of folate therefore, they are not susceptible to sulfonamides. Sulfonamide resistance may occur as a result of mutations that (1) cause overproduction of PABA, (2) cause production of a folic acid-synthesizing enzyme that has low affinity for sulfonamides, or (3) impair permeability to the sulfonamide. Dihydropteroate synthase with low sulfonamide affinity is often encoded on a plasmid that is transmissible and can disseminate rapidly and widely. Sulfonamide-resistant dihydropteroate synthase mutants also can emerge under selective pressure. 

Several classes of (3-lactamases, often encoded in transmissible plasmids, have spread worldwide rapidly among bacteria, seriously decreasing the effectivenss of penicillins and other (3-lactam anti-biotics.t y Most (3-lactamases (classes A and C) contain an active site serine and are thought to have evolved from the dd transpeptidases, but the B typey has a catalytic Zn2+. The latter, as well as a recently discovered type A enzyme,2 hydrolyze imipenem, currently one of the antibiotics of last resort used to treat infections by penicillin-resistant bacteria. Some (3-lactam antibiotics are also powerful inhibitors of (3-lactamases.U/aa/bb These antibiotics may also have uses in inhibition of serine proteasesCC/dd such as elastase. Some antibiotic-resistant staphylococci produce an extra penicillin-binding protein that protects them from beta lactams.ee Because of antibiotic resistance the isolation of antibiotics from mixed populations of microbes from soil, swamps, and lakes continues. Renewed efforts are being... 

Of the many mechanisms of bacterial resistance to chloramphenicol and thiamphenicol, the plasmid-mediated transmissible resistance conferred by the presence in resistant bacteria of chloramphenicol-acetyltransferases (CAT) is the most important. 

The atzA, atzB, and atzC genes - encoding enzymes necessary for the first three steps in atrazine biodegradation by Pseudomonas strain ADP - reside on a large, self-transmissible plasmid (de Souza et al., 1998a, b). 

JA Heinemann, GF Sprague Jr. Transmission of plasmid DNA to yeast by conjugation with bacteria. Meth Enzymol 194 187-195, 1991. 

Continuing use of the third-generation cephalosporins and the introduction of p-lactamase inhibitor combinations (clavulanate with amoxycillin or ticarcillin, sulbactam with ampicillin, and tazobactam with piperacillin see section 4.2) resulted in the appearance of plasmids encoding class C P-lactamases. After several unconfirmed reports, the first proof that a class C P-lactamase had been captured on a plasmid came in 1990 when transmissible resistance to a-methoxy and oxyimino-P-lactams was shown to be mediated by an enzyme whose gene was 90% identical to the ampC gene of E. cloacae. They have subsequently been found worldwide. Strains with plasmid-mediated AmpC enzymes are typically resistant to aminopenicillins (ampicillin or amoxycillin), carboxypenicillins (carbenicillin or ticarcillin) and ureidopenicillins (piperacillin). The enzymes also provide resistance to the oxyimino cephalosporins (ceftazidime, cefo-... 

M. tuberculosis (the tubercle bacillus) and other mycobacteria are resistant to many bactericides. Resistance is either (a) intrinsic, mainly due to reduced cellular permeability or (b) acquired, due to mutation or the acquisition of plasmids. Tuberculosis remains an important public health hazard, and indeed the annual number of tuberculosis cases is rising in many countries. The greatest risk of acquiring infection is from the undiagnosed patient. Equipment used for respiratory investigations can become contaminated with mycobacteria if the patient is a carrier of this organism. It is important to be able to disinfect the equipment to a safe level to prevent transmission of infection to other patients (Table 17.2). 

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