Figure 2 Extracellular DNA accumulates in the matrix of S. Typhimurium Vistusertib in vivo biofilms. Biofilms of strain 14028 were cultivated in flow chambers at 37°C for 2 days in LB medium and stained for extracellular DNA. Cells in the biofilm were stained with the membrane staining dye FM 4–64 (A). The middle panel depicts the accumulation of extracellular

DNA with CYT387 molecular weight TOTO-1 staining (B). The images are merged on the right (C). The large image shows the xy plane and the bottom panel shows the xz plane. The scale bar equals 15 μM. The wild-type 14028 strain carrying the pmrH-gfp construct forms aggregates on the surface of glass (D). The merged image of green fluorescence from pmr expression and red from propidium iodide staining, which stains both dead cells and extracellular DNA (E). DNA-enriched planktonic cultures show increased antibiotic resistance The presence of extracellular check details DNA may lead to

increased S. Typhimurium pmr expression, increased AP resistance and thus help to explain the antibiotic resistance phenotype that is characteristic of biofilms. To determine the influence of DNA on antibiotic resistance, we tested the antibiotic susceptibility of S. Typhimurium 14028 planktonic cultures in the presence and absence of exogenous DNA (pH 7.4). The addition of 0.5% DNA (5 mg/ml) led to a 16-fold increased resistance to polymyxin B and colistin, a 4-fold increased resistance to gentamicin and a >4 fold increase in resistance to ciprofloxacin (Table  1). Both phoPQ and pmrAB mutants did not demonstrate DNA-induced resistance to polymyxin B and colistin. However, both mutants had parental levels of DNA-induced resistance to gentamicin and ciprofloxacin, indicating that resistance to these antibiotics was independent

of the phoPQ and pmrAB systems (Table  1). Extracellular DNA is known to bind to aminoglycosides through electrostatic interactions [25], and it was recently shown that exogenous DNA shields P. aeruginosa from aminoglycoside killing, independent Tideglusib of the pmr resistance mechanism [26]. Table 1 Extracellular DNA induces antibiotic resistance in S. Typhimurium Strain Minimal inhibitory concentration (MIC) Polymyxin B Colistin Gentamicin Ciprofloxacin   – + DNAa – + DNAa – + DNAa – + DNAa 14028 1 16 1 16 0.125 0.5 0.125 >0.5 phoPQ 1 0.5 1 1 0.125 0.25 0.125 >0.5 ΔpmrAB 0.5 0.5 0.5 0.5 0.125 0.5 0.125 >0.5 a The minimal inhibitory concentration (MIC) values were determined in NM2 medium containing 1 mM Mg2+ (pH 7.4) with or without the addition of 0.5% fish sperm DNA-sodium salt (5 mg/ml).