Coliform bacteria in fresh vegetables: from cultivated lands to consumers

Fresh vegetables normally carry natural non-pathogenic epiphytic microorganisms, but during growth, harvest, transportation and further handling the produce can be contaminated with pathogens from animal and human sources. As most of these produce are eating without further processing, their microbial content may represent a risk factor for the consumer’s health and therefore a food safety problem. The consumption of fresh vegetables has been increasing in recent years, and since the early 1990s the reported outbreaks associated with consumption of fresh vegetables have grown steadily. Most of the reported outbreaks of gastrointestinal disease linked to the fresh produce have been associated with bacterial contamination, particularly with members of the Enterobacteriaceae family. In addition, the presence of antibiotic resistances both in epiphytic and pathogenic microorganisms in fresh vegetables may contribute to horizontal spreading of resistances among bacterial populations. In this study we have determined the presence of coliform bacteria as well as their antibiotic susceptibilities in fresh vegetables, as an indicator of their microbiological quality and their potential as a risk factor for consumer’s health. Samples of several fresh vegetables (n: 116) (i) collected directly from cultivated lands, (ii) from supermarkets and greengrocer’s shops in Valencia city (Spain) (including samples of ready-toeat four range vegetables), (iii) as well as ready-to-eat salads (n: 16) served in dinning halls of a nursery and a primary school (including fresh vegetables used as ingredients to prepare the salads) were analyzed. Coliforms and other enterobacterial species were isolated in a significant proportion of individual vegetable samples (average >50%), whereas this proportion increased in ready-to-eat salads (100%); the identified isolates included mainly species belonging to Klebsiella, Enterobacter, and other genera (Serratia, Citrobacter, Kluyvera, Pantoea, Flavimonas, Hafnia, and others), as well as four identified as Escherichia coli. Susceptibility of isolates to eleven common chemotherapeutic agents was tested. Most isolates were resistant to ampicillin, and to amoxicillin/clavulanic acid; although resistances to other chemotherapeutic agents were rare, some isolates showed multiresistance to 3-5 agents. Therefore, microbial contamination of fresh vegetables with opportunistic pathogens can be considered as a food safety concern, as consumption of these produce may represent a potential risk for the consumer’s health, particularly in debilitated or immunocompromised individuals. Since bacteria serving as a reservoir for resistance determinants may have great influence on resistance gene transfer in natural habitats, such as the human colon, the presence of antibiotic-resistant bacteria in fresh vegetables may constitute an additional food safety concern.