Effect of the rdar Morphotype on Salmonella enterica Dispersal by Rain and Aerosols

Introduction: Salmonella has been associated with tomato outbreaks, and contamination has been traced to production areas. Mechanisms of Salmonelladispersal in the field are poorly understood.

Purpose: To assess the dispersal of Salmonellaby rain onto tomato plants as affected by the presence of the rdar morphotype, trichome density, and post-rain aerosol formation.

Methods: GFP- kanamycin-resistant Salmonella Typhimurium MAE 110 (with rdar morphotype) or MAE 119 (without rdar morphotype) were dispensed in Petri dishes at 10⁸ CFU/cm² in the center of a rain simulator. Rain intensities of 60 and 110 mm/h were applied for 5, 10, 20, and 30 min. For aerosol assessment, Salmonella was recovered from the air after rain at 0, 9.5, 28.5, 47.5, and 85.5 cm from the ground using an impinger containing lactose broth. Sixty-centimeter-tall tomato plants with fruits were placed inside the rain simulator immediately after the rain. For splash dispersal assessment, tomato plants varying in trichome densities were placed around the Salmonella source prior to the rain event. Salmonellacells were recovered from plants making leaf imprints on LB agar with kanamycin or by washing off the cells from leaf surfaces with peptone water.

Results: Salmonella recovery from the air after rain was mostly observed when cells expressing rdar (MAE 110) were used. Aerial Salmonella transferred to tomato fruits in plants following a distribution Beta (2.5950, 4.7393). Cells lacking rdar morphotype (MAE 119) showed a significantly higher dispersion than MAE 110 when the trichome density was below 200 per cm². Conversely, MAE 110 cells showed significantly higher dispersal at trichome densities above 300 per cm²when compared to MAE 119.

Significance: After rain, Salmonella may form aerosols and transfer to tomato fruits on plants. Salmonella dispersal by rain splash is affected by the density of leaf trichomes and bacterial rdar morphotype.