Inactivation of Lactobacillus helveticus Bacteriophages by Thermal and Chemical Treatments

The effect of several biocides and thermal treatments on the viability of four Lactobacillus helveticus phages was investigated. Times to achieve 99% inactivation of phages at 63°C and 72°C in three suspension media were calculated. The three suspension media were tris magnesium gelatin buffer (10 mM Tris-HCl, 10 mM MgSO4, and 0.1% wt/vol gelatin), reconstituted skim milk sterile reconstituted commercial nonfat dry skim milk, and Man Rogosa Sharpe broth. The thermal resistance depended on the phage considered, but a treatment of 5 min at 90°C produced a total inactivation of high titer suspensions of all phages studied. The results obtained for the three tested media did not allow us to establish a clear difference among them, since some phages were more heat resistant in Man Rogosa Sharpe broth and others in tris magnesium gelatin buffer. From the investigation on biocides, we established that sodium hypochlorite at a concentration of 100 ppm was very effective in inactivating phages. The suitability of ethanol 75%, commonly used to disinfect utensils and laboratory equipment, was confirmed. Isopropanol turned out to be, in general, less effective than ethanol at the assayed concentrations. In contrast, peracetic acid (0.15%) was found to be an effective biocide for the complete inactivation of all phages studied after 5 min of exposure. The results allowed us to establish a basis for adopting the most effective thermal and chemical treatments for inactivating phages in dairy plant and laboratory environments.