PubMed · 8586159
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L B Rice, B E Murray. 1995. Beta-lactamase-producing enterococci.. https://pubmed.ncbi.nlm.nih.gov/8586159/
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Integrative and conjugative elements (ICEs) are typically found integrated in a bacterial host chromosome. They can excise, replicate, and transfer from cell to cell. Many contain genes that confer phenotypes to host cells, including antibiotic resistances, specialized metabolisms, phage defense, and symbiosis or pathogenesis determinants. Recent studies revealed that at least three ICEs (ICEclc, Tn916, and TnSmu1) cause growth arrest or death of host cells upon element activation. This review highlights the complex interactions between ICEs and their hosts, including the recent examples of the significant costs to host cells. We contrast two examples of killing, ICEclc and Tn916, in which killing, respectively, benefits or impairs conjugation and emphasize the importance of understanding the impacts of ICE-host relationships on conjugation. ICEs are typically only active in a small fraction of cells in a population, and we discuss how phenotypes normally occurring in a small subset of host cells can be uncovered.
Lac(+) revertants of Escherichia coli that occur after prolonged nonlethal selection display a high frequency of transposon loss when the transposon Tn10 and the reverting lacI33 allele are linked on an F'128 episome. As many as 20% of the Lac(+) revertants are sensitive to tetracycline, about half because of transposon loss, nearly all by precise excision, and the remainder because of amplification of both the transposon and the linked lac allele. Lethality of the amplified products in the presence of tetracycline is a peculiarity of the tetA gene at high gene dosage. The selective conditions on lactose medium result in 10% transposon-free revertants, whether or not a requirement for conjugal DNA transfer is imposed. In addition, a similar fraction, about 5% of Lac(-) unreverted colonies that are products of transfer between cells experiencing nonlethal selection are also tetracycline-sensitive, and all are attributable to loss of the Tn10 transposon. These results suggest the possibility that the high frequency of transposon loss is a consequence of conjugal transfer, making this loss a marker for that transfer. We suggest that conjugal DNA transfer may be a prominent feature in the mutability process that occurs during nonlethal selection and that the subset of bacteria displaying hypermutability are those that experience such transfer.
Several Pseudomonas aeruginosa strains, including one urinary isolate producing an extended-spectrum beta-lactamase TEM-24, were isolated from a long-term-hospitalized woman. Three TEM-24-producing enterobacterial species (Enterobacter aerogenes, Escherichia coli, and Proteus mirabilis) were isolated from the same patient. TEM-24 and the resistance markers for aminoglycosides, chloramphenicol, and sulfonamide were encoded by a 180-kb plasmid transferred by conjugation into E. coli HB101.