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Downregulated gene in Pseudomonas biofilm increases tobramycin resistance

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WESTPORT, CT (Reuters Health) – Despite striking differences between free-living Pseudomonas aeruginosa cells and those that form biofilms, only about 1% of genes are differentially expressed in the two growth modes, according to a report in the October 25th issue of Nature. One of these genes, rpoS, has been found to affect both the morphology and the antibiotic sensitivity of biofilms.

Using DNA microarray technology, Dr. E. P. Greenberg, of the University of Iowa College of Medicine in Iowa City, and colleagues compared gene expression in planktonic and biofilm cells. They found that 34 genes were activated and 39 were repressed in biofilm populations.

The rpoS gene, which codes for an RNA polymerase sigma subunit and is involved in the transcription of other P. aeruginosa genes, was downregulated in the biofilm. According to the authors, previous work has shown that an rpoS-deletion mutant of the bacterium is hypervirulent in mice and is involved in biofilm development.

After growing a wild-type strain and an rpoS mutant as biofilms, the investigators found that within 4 hours, the mutant had covered much more of the vessel in which it was grown than the wild-type strain had. After 24 hours, it had larger structured groups of bacteria. The rpoS mutant was also more resistant to tobramycin than the wild type, which itself was more resistant than planktonic cultures.

These results indicate that the microarray analysis was capable of identifying genes important for biofilm formation and antibiotic susceptibility, the researchers note. "Further studies will reveal which of the biofilm-induced genes are activated under different environmental conditions," which could be "of great use in the development of rapid screens for agents that block biofilm maintenance," they conclude.

Nature 2001;413:860-864.

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