Further, cell walls were boiled in 4% sodium dodecyl sulfate (SDS) for 30 min and recovered by centrifugation (30 000 g, 30 min, 20 °C), and the pellet was washed five times with water to remove residual SDS. The resulting preparation was lyophilized and used for the determination of total cell wall phosphate content. To measure total cell wall phosphate content, samples were assayed as published earlier (Eugster & Loessner, 2011). A 10-μL sample of a 10 mg mL−1 purified cell wall suspension was SP600125 in vivo first digested oxidatively using a NANOCOLOR® NanOx Metal (Macherey-Nagel) according to the manufacturer’s
protocol. Then, total phosphorus was determined photometrically by the use of a phosphate test kit (Spectroquant® Phosphate Test; Merck) as described by the manufacturer. To assure the accuracy and reliability of the results, a calibration check details curve was obtained with aqueous dilutions of a 1000 mg L−1 phosphate standard solution (VWR). All samples were decomposed and measured in triplicate. Wheat germ agglutinin (WGA)-Alexa Fluor 594® conjugate (Invitrogen) was used for the detection of N-acetylglucosamine (GlcNAc) in wall teichoic acids (WTA)
of Listeria cells. This lectin recognizes terminal GlcNAc substituents in cell wall polymers, such as WTA on the surface of L. monocytogenes Adenosine (Wright, 1984; Loessner et al., 2002; Eugster & Loessner, 2011). Binding assays with labeled WGA were performed as described elsewhere (Loessner et al., 2002; Eugster & Loessner, 2011). Bacterial cells were harvested in late log phase by centrifugation and resuspended in 1/10th volume of PBST buffer (120 mM NaCl, 50 mM phosphate, and 0.1% Tween 20, pH 8.0); 100 μL cells and 50 μL of Alexa Fluor 594® WGA solution (0.1 mg mL−1) were mixed and incubated for 10 min at 25 °C. Cells were removed from labeling solution by centrifugation (12 000 g, 1 min) and washed twice in PBST buffer. After washing, the cells were examined by fluorescence microscopy (Leica
TCS SPE; Leica, Heerbrugg, Switzerland). Additionally, the presence of GlcNAc was tested using GFP-labeled cell wall-binding domain (CBD) of bacteriophage endolysin PlyP35 (HGFP-CBDP35), which specifically recognizes GlcNAc residues in Listeria WTA (Eugster et al., 2011). Binding assays with HGFP-CBDP35 were performed as described earlier (Loessner et al., 2002; Schmelcher et al., 2010; Eugster et al., 2011). All experiments were repeated at least twice to confirm reproducibility. Categorical data were compared using the chi-square test or the Fisher’s exact test when appropriate. Continuous variables were compared using the Mann–Whitney U-test or Student’s t-test if number of repetitions was < 5.