Panel A: A. baumannii cells resuspended from biofilm 10,000× magnification. The bundle-like fibers NVP-HSP990 mw embedding the bacterial cells are indicated by the arrow. Panel B: A. baumannii cells resuspended from biofilm and treated with 1 Unit cellulase for 30 minutes, 12,000× magnification. In find more addition to its role of adhesion factor, cellulose, as well as other EPS, can protect bacterial cells

from environmental stresses such as desiccation and oxidative stress [11, 29]. Thus, we tested the A. baumannii SMAL clone grown either in M9Glu/sup or in LB1/4 for resistance to desiccation and to challenge with H2O2. A. baumannii SMAL displayed high levels of resistance to both stresses, which was expected since this is a common feature for the Acinetobacter genus [1]; growth in different media did not significantly affect its resistance level (data not shown), suggesting that, in A. baumannii SMAL, cellulose production might be more related to surface ARRY-438162 adhesion than to resistance to environmental stresses. Exposure to subinhibitory

concentrations of imipenem affects biofilm formation The A. baumannii SMAL clone is sensitive to carbapenems such as imipenem (Table 1). However, in many cases, imipenem treatments failed to eradicate the A. baumannii SMAL clone from patients, often resulting in relapses. We investigated the possibility that, although sensitive to imipenem in standard Minimal Inhibitory Concentration (MIC) determination assays, the A. baumannii SMAL clone might possess mechanisms of resistance or tolerance to this antibiotic. Exposure to subinhibitory concentrations of antibiotics can result in the induction of adaptive responses and in biofilm stimulation [33], which appears to increase tolerance to antibiotics via different molecular mechanisms

(reviewed in [34]). Thus, we tested the effect of subinhibitory concentrations of imipenem on biofilm formation by A. baumannii SMAL: concentrations of imipenem BCKDHB ranging between 0.03 and 0.125 μg/ml, which correspond respectively to 1/16 and 1/4 of the MIC of imipenem in M9Glu/sup medium, resulted in biofilm stimulation by up to 3-fold, both at 30°C (Figure 4) and at 37°C (data not shown). Growth rate was not impaired by imipenem at any of the concentrations tested. In contrast, treatment of A. baumannii SMAL with subinhibitory concentrations of tetracycline did not result in any significant induction of biofilm formation (data not shown), suggesting that biofilm induction is a specific effect of imipenem. Since in M9Glu/sup medium surface adhesion by A. baumannii SMAL is mediated by cellulose production (Figure 2C), we tested whether imipenem-induced biofilm stimulation could be inhibited by treatment with cellulase. As shown in Figure 3, although cellulase did affect biofilm formation both in the presence and in the absence of imipenem, the extent of biofilm stimulation induced by the antibiotic is very similar (ca. 3-fold) regardless of the presence of cellulase.