In a previous study using a different in vitro biofilm model, we reported that oxygen limitation could account for 70 percent or more of the protection from six antibiotics observed in P. aeruginosa colony biofilms [12]. A Bcl-2 inhibitor recent report showed that ciprofloxacin and tetracycline preferentially killed the metabolically active subpopulation in P. aeruginosa biofilms [65]. Oxygen limitation is known to occur in vivo in cystic fibrosis patients [86]. Further, molecular biological evidence suggests that P. aeruginosa in the cystic fibrosis lung experiences anaerobic conditions [87]. In an investigation

of in situ growth rates of P. aeruginosa obtained from chronic lung infections, approximately

11% of cells were determined to be in a non-growing stationary-phase based on their ribosome content [88]. The average specific growth rate of the growing bacterial cells was 0.31 h-1. This shows that a non-growing population may be relevant in vivo, though it suggests that the population of bacteria in the infected lung were overall more active than we describe here for learn more drip-flow check details biofilms. Heterogeneity within the biofilm Here we remark on the “”averaging”" that occurs when the entire biofilm is mashed up and extracted RNA is analyzed. This method mixes together the RNA from transcriptionally active cells in the aerobic upper layer of the biofilm with RNA from inactive bacteria in the lower layers of the biofilm. The result is not a simple average of the activities of the two layers because there is

so much less mRNA in the inactive bacteria. Indeed, the inactive bacteria may contribute little to the overall microarray signal. For this reason, the transcriptome that has been examined in this work may best be thought of as representing the transcriptionally-active supopulation of bacteria rather than an average of the entire biofilm population. A recently described laser capture microdissection technique provides Sulfite dehydrogenase a direct experimental approach for quantifying the amount of specific RNA sequences in distinct regions of the biofilm [10, 11]. This method begins with cryoembedding an intact biofilm and preparing frozen cross sections. Small user-defined areas of the cross section can be physically removed and amplified by PCR to detect specific transcripts. Application of this approach to drip-flow P. aeruginosa biofilms has revealed that the upper layer of the biofilm is enriched in mRNA compared to the lower layers [10, 11]. For example, in drip-flow biofilms the number of RNA copies of the housekeeping gene acpP was approximately 60 times smaller at the bottom of the biofilm compared to the top [10].