Abstract
Hydroponic produce is gaining popularity due to its suitability for urban agriculture. The general public also considers that hydroponic produce is free from microbiological contamination. In this study, we compared the frequency and abundance of tetracycline-resistant and sulphadiazine-resistant bacteria and the minimal inhibitory concentration (MIC) of these isolates in conventional, organic, and hydroponic lettuce sold in retail. We also determined the frequency of samples carrying tetB, tetX, sul1, sul2, and int1 genes by PCR and further quantified the copy number of tetX, sul1, and int1 genes in samples positive for these genes using qPCR. As expected, the number of resistant bacteria and the MICs of these isolates were lowest in hydroponic lettuce and highest in organic lettuce. All tested resistant genes, except int1, were detected in samples of all three production methods, but no significant difference was observed between the three groups in the frequency of samples carrying the resistance genes examined or in their copy number. To the best of our knowledge, it is the first study directly reporting the existence of antibiotic-resistant bacteria and resistance genes in hydroponic vegetables sold in retail. The result highlights that the risk of antibiotic-resistant bacteria contamination in hydroponic produce should be further investigated.
Highlights
Hydroponic vegetables are grown in soilless systems and rely on the use of nutrient solutions to support their growth
Given that tetracycline and sulphonamide are the most used veterinary antibiotics and are present in the environment in high concentrations [18], in this study, we aimed to compare the frequency and abundance of tetracycline-resistant and sulphadiazine-resistant bacteria and the minimum inhibitory concentration of these bacteria from conventional, organic, and hydroponic lettuce sold in retail in Hong Kong
No significant difference in the frequency of samples harbouring tetracycline-resistant and sulphadiazine-resistant bacteria was observed between the lettuce of the three production methods (Table 1)
Summary
Hydroponic vegetables are grown in soilless systems and rely on the use of nutrient solutions to support their growth This form of agriculture is suitable for the vertical agriculture system used in industrialised and local urban farming systems [1,2,3] because of its efficiency in land and water usage [4] and increased yield and economic benefits [5]. A similar effect was observed in lettuce grown with wastewater supplemented with mineral fertiliser [7] Due to these unique features, it is argued that the hydroponic agriculture system is more sustainable than conventional agriculture system and is one of the means to strengthen urban food security and city resilience against the impact brought by climate change [8,9,10].
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