Colonization of Tomatoes by Salmonella Montevideo Is Affected by Relative Humidity and Storage Temperature

Abstract

The influences of the relative humidity (RH) and storage temperature on the colonization of tomato surfaces by Salmonella Montevideo were studied. Red, ripe tomatoes (Lycopersicon esculentum) were spot inoculated in three separate trials with 100μl (approximately 106 CFU) of Salmonella Montevideo and stored for 90 min at 22°C under 97% RH to facilitate attachment of cells to the blossom end of tomato surfaces. Following this attachment step, tomatoes were washed to remove loosely adhered cells and then stored at 22 or 30°C for up to 10 days under RH of 60, 75, 85, or 97%. At 0, 0.4, 1, 4, 7, and 10 days of storage, three tomatoes were individually hand massaged in 50 ml of 0.1% peptone water and the washes were separately analyzed to enumerate populations of Salmonella Montevideo. The number of Salmonella Montevideo cells attached after 90 min at 22°C was 3.8 log CFU per tomato; this level was determined to be the initial colonizing population. After 10 days of storage at 30°C, the Salmonella Montevideo population increased to 0.7, 1.0, 1.2, and 2.2 log CFU per tomato at 60, 75, 85, and 97% RH, respectively. A similar trend was observed at 22°C, although populations were lower than at 30°C. Scanning electron micrographs of tomato cuticles after storage revealed a well-defined biofilm containing bacteria. These findings reinforce the importance of maintaining stored tomatoes at temperatures that do not support growth of pathogenic bacteria and demonstrate the growth-promoting effects of high humidity.