Inoculum preparation conditions influence adherence of Salmonella enterica serovars on red leaf lettuce (Lactuca sativa).

Abstract

Salmonella enterica has been increasingly linked to outbreaks involving consumption of fresh produce. While several studies have identified genes whose products are involved in mediating S. enterica -plant interactions, the use of varying experimental approaches, serovars and plant types have generated variable and conflicting data. The purpose of this study was to determine whether conditions under which inocula are prepared for in vitro plant interaction studies influence the outcome of these studies. Seven S. enterica serovars were grown in media of differing salinity and physical state, and incubated at either 25 or 37°C. These cultures were then used to inoculate red leaf lettuce and adherent microbes were subsequently recovered. While all serovars were influenced by inoculum preparation conditions, some showed greater variation than others. Analysis of pooled serovar data revealed that inocula prepared from either plates or biphasic cultures showed higher levels of interaction with red leaf lettuce than those prepared from broth cultures. Furthermore, incubation at 37°C enhanced adherence after 30s or 5d of contact time, while adherence levels after 1h of contact time were increased in low-salt media. \xa0Broth-grown inocula were highly influenced by medium salinity and incubation temperature, while plate- and biphasic-grown inocula were only minimally affected. Therefore, inocula prepared from bacteria grown on plates or in biphasic culture would be most suitable for studies aiming to evaluate strategies to interfere with plant- S. enterica interactions. However, pooled data mask serovar-specific responses, and care should be taken to extrapolate these findings to individual serovars. The previous association of a serovar with outbreaks involving leafy greens did not correlate with levels of interaction with red leaf lettuce, suggesting that the occurrence of these serovars in/on these commodities does not reflect their fitness in the plant environment.