Dynamics of culturable mesophilic bacterial communities of three fresh herbs and their production environment

Abstract

Aim: Investigate dynamics of culturable mesophilic bacteria and selected food-contaminating bacteria from three herbs and their production environment. Methods and Results: Marjoram, basil and thyme were investigated during one growing season by sampling plants, organic fertilizers, soil, irrigation water and marketed products. Mesophilic bacteria and selected food-contaminating bacteria (Escherichia coli, Enterococcus spp., Bacillus cereus group) were cultured and identified by MALDI biotyping. Culturable mesophilic bacteria on marjoram and basil plants decreased over time by two orders of magnitude starting at above 106 colony forming units per gram (CFU per g), while they remained constant on thyme (~104 CFU per g). Compared to the last field sample, mesophilic bacteria were increased on all market-ready products by one order of magnitude. Marjoram and basil were dominated by B. cereus group, Enterobacter spp. and Pseudomonas spp., thyme by Bacillus spp. and Pseudomonas spp. All selected food-contaminating bacteria were detected in soil and reservoir-sourced irrigation water, whereas in municipal water, only B. cereus group and rarely Enterococcus spp. were found. Escherichia coli was detected only on young marjoram and basil plants (5 × 102 and 5 × 101 CFU per g, respectively), whereas Enterococcus spp. and B. cereus group were consistently detected on these two herbs. Thyme plants only contained B. cereus group consistently (above 103 CFU per g). Marketed marjoram and thyme contained Enterococcus spp. (5 × 102 and 104 CFU per g) and B. cereus group (~5 × 102 CFU per g), while no selected food-contaminating bacteria were found on marketed basil. Conclusions: Overall, culturable mesophilic bacteria were dominated by Pseudomonas spp. and Bacillus spp., with increased numbers on market-ready products. Selected food-contaminating bacteria were readily detectable, however, only the B. cereus group was found throughout in all systems. Significance and Impact of the Study: Insight into composition and development of mesophilic bacterial communities and selected food-contaminating bacteria of fresh herbs contributes to estimating consumer exposure.