Natural Gene Transfer and Antibiotic Resistance

Many pathogenic bacteria have developed resistance to antibiotics, particularly in environments where antibiotics are routinely used, such as hospitals and fish farms. (Massive amounts of antibiotics are often used in aquaculture to prevent infection in the fish and enhance their growth.) The continual presence of antibiotics in these environments selects for resistant bacteria, which reduces the effectiveness of antibiotic treatment for medically important infections.

Antibiotic resistance in bacteria frequently results from the action of genes located on R plasmids, small circular

M| Transfer always begins within F, and the orientation of F determines the direction of transfer.

^ In Hfr1, F is integrated between the leu and azi genes;.

thi his thr

F factor

-Chromosome leu leu

F factor

^ .so the genes are transferred beginning with leu. -^

gal pro lac leu thr thi his gal lac pro azi Genetic map

In Hfr5, F is integrated between thi and his.

thi his thr

F factor Chromosome

^ F has the opposite orientation in this chromosome; so the genes are transferred beginning with thi.

gal pro lac thi thr leu azi pro lac gal his Genetic map

8.18 The orientation of the F factor in an Hfr strain determines the direction of gene transfer.

Arrowheads indicate the origin and direction of transfer.

plasmids that can be transferred by conjugation. R plasmids have evolved in the past 50 years (since the beginning of widespread use of antibiotics), and some convey resistance to several antibiotics simultaneously. Ironic but plausible sources of some of the resistance genes found in R plasmids are the microbes that produce antibiotics in the first place.

The results of recent studies demonstrate that R plas-mids and their resistance genes are transferred among bacteria in a variety of natural environments. In one study, plasmids carrying genes for resistance to multiple antibiotics were transferred from a cow udder infected with E. coli to a human strain of E. coli on a hand towel: a farmer wiping his hands after milking an infected cow might unwittingly transfer antibiotic resistance from bovine- to human-inhabiting microbes. Conjugation taking place in minced meat on a cutting board allowed R plasmids to be passed from porcine

(a) Order of gene transfer (unaligned)

Hfr strain

thi thr leu azi pro lac gal his

(b) Order of gene transfer with genes aligned

Hfr strain

thr

leu

azi

pro

lac

gal

his

thi

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