For example, if we wish to discern whether the biofilm is responding to iron limitation, we first identify a set of genes that are up-regulated in response to iron deprivation (e.g. the work of Ochsner [9]). The rank of each of these transcripts in the biofilm data set is then compared to transcript ranks for the same genes in data sets collected from both rapidly

growing and deliberately iron-starved cultures. In this way it becomes possible to evaluate physiological activities in the biofilm rather than just documenting differences between the biofilm and a reference state. In the experiments reported here, RNA was extracted from an entire, homogenized biofilm specimen. An obvious concern with this approach is that it neglects the inherent biological Vemurafenib heterogeneity of the biofilm [1]. We would like to address this concern upfront with two points. First, just because a population is heterogeneous

does not mean that measurements of population averages are invalid. Population averages are very widely and informatively used in biology. Second, we suggest that even the concept of an average may not be appropriate in this case. The current conceptual model of P. aeruginosa drip-flow biofilms is that they consist of two distinct populations: an aerobic, metabolically active upper layer and a lower, and larger, layer consisting of inactive Palbociclib cells containing very low levels of mRNA [10, 11]. Because the inactive cells contain so little RNA, this majority is expected to be essentially invisible on the microarray. From this perspective, the transcriptomes reported here may best be thought of as reflecting the properties of the transcriptionally-active subpopulation rather than the average behavior of the entire population. These concepts are elaborated on in the Results PAK5 and Discussion. Results and Discussion Three day old drip flow biofilms of P. aeruginosa were characterized with respect to antibiotic tolerance, oxygen availability, and microscale patterns of protein synthetic activity. These biofilms

contained 9.56 ± 0.31 cfu cm-2. Reduced antibiotic susceptibility of biofilm bacteria P. aeruginosa cells grown in biofilms were protected from killing by tobramycin and ciprofloxacin, in comparison to actively growing planktonic bacteria. Both antibiotics rapidly and effectively reduced viable cell numbers in an aerobic, planktonic culture. After 12 h of treatment with 10 μg ml-1 tobramycin or 1.0 μg ml-1 ciprofloxacin, planktonic log reductions measured were 3.18 ± 1.79 (n = 3, ± SD) and 4.84 ± 0.55 (n = 3, ± SD) for tobramycin and ciprofloxacin, respectively. In contrast, neither antibiotic was very effective against biofilms of P. aeruginosa. After 12 h exposure to antibiotic in continuously flowing medium, the log reductions in viable cell numbers were 0.72 ± 0.56 (n = 3, ± SD) and 1.37 ± 0.06 (n = 3, ± SD) for tobramycin and ciprofloxacin, respectively.