Tdna

Chloroplast

Plant DNA

Plant cell nucleus

Plant DNA

Plant cell nucleus

Agrobacterium tumefaciens cell

Agrobacterium cells inoculated into wounded plant.

Crown gall tumor develops at wound site.

Bacteria-free tumor tissue is hormone independent and synthesizes opine.

Plant tumor cell contains T-DNA integrated into plant chromosome

Figure 1 Agrobacterium sp. Causes Crown Gall and Transforms Plant Cells mycin, chloramphenicol, and tetracycline, and a number of different heavy metals. Certain R plasmids confer resistance to some, but not all, of these antimicrobial substances; yet others confer resistance to more than these particular ones.

Perhaps the most important feature of many R plasmids is that they can be transferred to antimicrobial and heavy metal-sensitive bacteria, thereby conferring simultaneous resistance to several antimicrobials and heavy metals encoded by the R genes. Furthermore, many R plasmids have wide host ranges and can multiply in a wide variety of different Gramnegative genera. These include Shigella, Salmonella, Escherichia, Yersinia,, Klebsiella, Vibrio, and Pseudomonas. If R plasmids are present in one species in a mixed population of these organisms, other cells will receive the R plasmid and thus become resistant to a variety of antimicrobial agents. This transfer of R plasmids helps explain why so many different organisms in a hospital

212 Chapter 8 Bacterial Genetics environment are resistant to many different antimicrobials. ■ nosocomial infections, p. 499

Not surprisingly, antimicrobial-resistant organisms are most common in locations where these medications are in greatest use. The antimicrobials kill the sensitive cells and allow the few resistant cells in the population to grow. Soon, most of the cells in this environment are antimicrobial resistant. This process explains why hospitals harbor many antimicrobial-resistant bacteria. It is not surprising that antimicrobial-resistant organisms are more common in certain parts of the world, particularly in developing countries. This probably results from the overuse of antimicrobials in these countries, where antimicrobials can be sold without a doctor's prescription. Also, in areas that lack proper hygiene, bacteria are more readily transferred to people through contaminated food and water. Non-disease-causing bacteria such as E. coli that may be carried by healthy people can serve as a reservoir for R plasmids, which then can be transferred to disease-causing organisms.

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