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Scientists have long assumed that the principal effect of antimicrobials in nature is inhibition of microbial growth. But might low concentrations of natural antibiotics also serve other functions in the bacterial community? To address this possibility, investigators in Spain assessed the effects of subinhibitory concentrations of tobramycin, tetracycline, and ciprofloxacin on gene expression in Pseudomonas aeruginosa.
Of the 555 P. aeruginosa genes studied, tobramycin induced expression of 9 and repressed expression of 31. Similarly, ciprofloxacin induced 8 and repressed 21 genes, and tetracycline induced 8 and repressed 18. Functional assays showed that all three antibiotics induced formation of static biofilm by the bacteria. Tetracycline…