Destruction, Injury, and Repair of Listeria Species Exposed to Sanitizing Compounds

The efficacy of four commonly used dairy plant sanitizers against Listeria coupled with an examination of ability of these sanitizers to induce injury was investigated. Listeria monocytogenes F 5069, F 5027, and Listeria innocua CWD 350 were tested against different concentrations of sanitizers (for periods of 30 s and 2 min) which included two quaternary ammonium compounds (QAC), an acid anionic sanitizer, and a chlorine-containing sanitizer. According to the Sanitizer Test of the Association of Analytical Chemists, all sanitizers showed efficacy of >99.999% against all test organisms, regardless of the type of sanitizer, its concentration, or exposure time. Two enrichment broths consisting of the University of Vermont (UVM), which is currently used for selective enrichment of Listeria , and Listeria repair broth (LRB), which is a nonselective highly nutritious medium, were used to determine the ability of standard enrichment procedures to recover injured Listeria . Use of these enrichment procedures suggests that sanitizers may induce injury rather than death as indicated by recovery of the organisms in LRB versus failure of growth in UVM, or through variation in the lag phase and viable cell count between both broths. The extent of cell death, injury, and repair was found to be affected by the type and concentration of sanitizer, exposure time, bacterial strain, and the enrichment procedure. QAC were the most effective while the acid anionic sanitizer was the least effective. The lethal effect of the sanitizer was found to increase by increasing its concentration or exposure time. L. monocytogenes F 5027 was the most resistant of the strains tested to QAC and the acid anionic sanitizer, while L. innocua was more resistant to the chlorine sanitizer. In all instances LRB proved to be superior to UVM in promoting repair and growth of sanitizer-injured cells.