Isabelle Adt

Assessment of the mode of action of polyhexamethylene biguanide against Listeria innocua by Fourier transformed infrared spectroscopy and fluorescence anisotropy analysis

Polyhexamethylene biguanide (PHMB) is a cationic biocide. The antibacterial mode of action of PHMB (at concentrations not exceeding its minimal inhibitory concentration) upon Listeria innocua LRGIA 01 was investigated by Fourier transformed infrared spectroscopy and fluorescence anisotropy analysis. Fourier transformed infrared spectra of bacteria treated with or without PHMB presented some differences in the lipids region: the CH(2)/CH(3) (2924 cm(-1)/2960 cm(-1)) band areas ratio significantly increased in the presence of PHMB. Since this ratio generally reflects membrane phospholipids and membrane microenvironment of the cells, these results suggest that PHMB molecules interact with membrane phospholipids and, thus, affect membrane fluidity and conformation. To assess the hypothesis of PHMB interaction with L. innocua membrane phospholipids and to clarify the PHMB mode of action, we performed fluorescence anisotropy experiments. Two probes, 1,6-diphenyl-1,3,5-hexatriene (DPH) and its derivative 1-[4-(trimethyl-amino)-phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH), were used. DPH and TMA-DPH incorporate inside and at the surface of the cytoplasmic membrane, respectively. When PHMB was added, an increase of TMA-DPH fluorescence anisotropy was observed, but no changes of DPH fluorescence anisotropy occurred. These results are consistent with the hypothesis that PHMB molecules perturb L. innocua LRGIA 01 cytoplasmic membrane by interacting with the first layer of the membrane lipid bilayer.

Antimicrobial activity of camel milk casein and its hydrolysates

The aim of this study was to evaluate the antimicrobial activity of camel caseins and their hydrolysates by gastrointestinal proteolytic enzymes against 3 Gram-positive and 2 Gram-negative bacterial strains. Camel caseins (CN) were hydrolysed by successive action of pepsin and pancreatin. Hydrolysis of CN was checked by electrophoresis and gel filtration chromatography (GFC). Both techniques showed that CN was hydrolysed into peptides. Among the tested bacteria, a decrease of 19.3%±0.02 of E. coli XL1 blue cells growth was observed in the presence of undigested camel casein at a concentration of 20 mg ml−1. After successive hydrolyses by pepsin and pancreatin, camel milk casein hydrolysates still exhibited anti-bacterial activity against E. coli XL1 blue strain (19.73±0.01% growth inhibition under the same conditions). Gram-positive strain growth was not affected by intact camel CN, while, at the same concentration (20 mg ml–1), their hydrolysates slightly inhibited the growth of these bacteria. This sugg...