Microbiological quality and occurrence of antibiotic resistant bacteria in ready-to-eat salad

Abstract: An increase in demand for fresh vegetables resulted in an increased production of minimally processed, ready-to-eat salad in Sweden. That also brought new food safety challenges that are yet to be addressed. To assess whether ready-to-eat leafy green vegetables present a threat in terms food borne outbreaks, aerobic bacteria, as well as bacteria belonging to the Enterobacteriaceae family were recovered, using selective media, from 3 different sets (n=18) of ready-to-eat rocket salad. Bacterial investigation showed that most bacteria retrieved from ready-to-eat rocket salad belongs to the Pseudomonadaceae family, which are typical spoilage bacteria commonly associated with vegetables. No food-borne pathogens (e.g. Clostridium botulinum, Escherichia coli O157:H7, Salmonella spp., Shigella spp., Listeria monocytogenes) were isolated from any of three sets of rocket salad. Following the assumption that microbiological quality of the salad could change during the storage period, as well as a result of salad bags being opened, sealed, stored and opened again in the household, the total aerobic count as well as total amount of Enterobacteriaceae were assessed every day before the best-before date. Following the NSW food Authority guidance on the microbiological status of ready-to-eat food, it can be confirmed, that each of three sets of salad were from the first day after packaging, unsatisfactory in regards to total aerobic count, showing values higher than 5 log CFU/g. The bacterial investigation also concluded that opening the bags and storing them sealed in the household will produce little, if any effect on the microbiological quality and will not challenge the validity of the expiration dates. To asses weather ready-to-eat rocket salad available for sale in Sweden, could be a possible reservoir of antibiotic resistance, the isolated bacteria from each bag of salad were tested for susceptibility to eight antibiotics, representative of the six classes of antibiotic compounds commonly used in clinical settings in Sweden. The cases of possible acquired antibiotic resistance were rare, however, the bacteria suspected to be immune to drug action should be subject for further investigations aimed to discover the mechanism of such resistance.