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Antimicrobial Agents and Chemotherapy, August 2000, p. 2182-2184, Vol. 44, No. 8
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Selection of Naturally Occurring Extended-Spectrum TEM beta -Lactamase Variants by Fluctuating beta -Lactam Pressure

Jesús Blazquez,* María-Isabel Morosini, María-Cristina Negri, and Fernando Baquero

Servicio de Microbiología, Hospital Ramón y Cajal, Madrid 28034, Spain

Received 22 October 1999/Returned for modification 27 February 2000/Accepted 21 April 2000

Despite the large number of in vitro mutations that increase resistance to extended-spectrum cephalosporins in TEM-type beta -lactamases, only a small number occur in naturally occurring enzymes. In nature, and particularly in the hospital, bacteria that contain beta -lactamases encounter simultaneous or consecutive selective pressure with different beta -lactam molecules. All variants obtained by submitting an Escherichia coli strain that contains a blaTEM-1 gene to fluctuating challenge with both ceftazidime and amoxicillin contained only mutations previously detected in naturally occurring beta -lactamases. Nevertheless, some variants obtained by ceftazidime challenge alone contained mutations never detected in naturally occurring TEM beta -lactamases, suggesting that extended-spectrum TEM variants in hospital isolates result from fluctuating selective pressure with several beta -lactams rather than selection with a single antibiotic.

* Corresponding author. Mailing address: Servicio de Microbiología, Hospital Ramón y Cajal, Carretera de Colmenar Km 9.100, Madrid 28034, Spain. Phone: (34)-91-336 83 30. Fax: (34)-91-336 88 09. E-mail: jblazquez{at}hrc.insalud.es.

Antimicrobial Agents and Chemotherapy, August 2000, p. 2182-2184, Vol. 44, No. 8
0066-4804/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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