First Isolation and Characterization of Serratia liquefaciens Associated with Rot Disease of Malus domestica (Apple) Fruit and Its Inhibition by Origanum vulgare (Oregano) Oil

Abstract

Microbial food spoilage has been a major source of concern because it widens the global food production-to-consumption gap. Thus, in the current study, we aimed to isolate, characterize and identify bacteria associated with spoiled apple fruit in Al-Ahsa city, Saudi Arabia. MDK33, a bacterial strain isolated from rotten apple fruit, was characterized at the phenotypic and genotypic levels. Furthermore, the antibacterial effect of oregano essential oil (OEO) against MDK33 was investigated. MDK33 formed circular colonies with entire margins, and the cells were Gram-negative rods with no endospores. Biochemical characterizations of MDK33, as revealed by the Biolog Gen III MicroPlate test system, indicated that the strain utilized 66 (~70%) of the 94 diverse metabolites and did not utilize 29 (~30%). MDK33 grew well on a variety of substrates, including pectin, gelatin, Tween 40 and dextrin, at pH 5 and 6; tolerated salts up to 4% NaCl; and was resistant to multiple antibiotics. Furthermore, based on the sequences of the 16S rRNA gene, MDK33 was identified as Serratia liquefaciens at a 99.73% identity level. Koch’s postulates were affirmed, confirming that S. liquefaciens MDK33 is the causative agent of apple rot disease in Al-Ahsa, Saudi Arabia. The minimum inhibitory concentration (MIC) of 0.23 mg/mL for oregano oil against S. liquefaciens MDK33 demonstrated remarkable antibacterial and antibiofilm activities. This is the first isolation of pathogenic S. liquefaciens as the causative agent of rot disease of apple fruit and its inhibition by oregano oil. Furthermore, the findings pave the way for oregano oil to be evaluated as a natural preservative to reduce post-harvest losses while maintaining high quality for sustainable food security in future studies.