Heat-induced injury in Listeria monocytogenes

Abstract

Heating of Listeria monocytogenes (Scott A strain) in potassium phosphate buffer (0.1 M, pH 7.2) at 52°C for 1 h led to injury, with the heat-injured cells failing to produce colonies on agar medium containing 5% NaCl. The detection of injury was based on the use of differential media: plating on tryptose phosphate broth+2% agar and 1% sodium pyruvate (TPBA+P) and on tryptose phosphate broth+2% agar and 5% NaCl (TPBA+S). Only non-injured Listeria formed colonies on TPBA+S whereas both heat-injured and non-injured cells formed colonies on TPBA+P. The bacterial count on TPBA+P minus that on TPBA+S represents the extent of heat injury. A large number of selective agars were tested and compared to TPBA+P for their ability to support repair and colony formation of heat-injured L. monocytogenes. Media containing 0.025% phenylethanol, 0.0012-0.0025% acriflavin, 0.1-0.2% potassium tellurite, 0.001% polymyxin B sulfate, 5% NaCl or a combination of these ingredients were detrimental to the recovery of heat-injured L. monocytogenes. Media currently in use for L. monocytogenes are not satisfactory for the recovery of injured cells. © 1988 Society for Industrial Microbiology.