Antimicrobial activity of Zataria multiflora essential oil against Heyndrickxia sporothermodurans isolated from UHT milk

Abstract Background Heyndrickxia sporothermodurans is a highly heat-resistant, spore-forming bacterium associated with spoilage of ultra-high-temperature (UHT) milk. The persistence of its spores after UHT processing necessitates effective strategies for controlling subsequent germination and outgrowth. Methods In this study, the antimicrobial activity of Zataria multiflora essential oil (EO) was evaluated against H. sporothermodurans strain Ab.359 TZ (NCBI accession number PX916884), isolated from local UHT milk in Fars Province, Iran. EO composition was determined by GC–MS, and its effects on vegetative growth, spore germination, and population dynamics were assessed in broth and UHT milk over five days. EO concentrations were expressed as volume/volume (v/v). Results GC–MS analysis revealed thymol (32.51%) and carvacrol (32.17%) as the major EO components. The minimum inhibitory concentration was 0.09% (v/v). EO treatment caused an initial reduction in vegetative-cell counts and delayed subsequent population recovery and sporulation, while its effects in the UHT milk model included suppression of spore outgrowth and overall bacterial population recovery during the experimental period. Complete inhibition of spore germination was not observed. Conclusion These findings demonstrate that Z. multiflora EO can suppress vegetative proliferation and spore-associated population dynamics of H. sporothermodurans strain Ab.359 TZ under the experimental conditions used. Further studies are required to evaluate its sensory acceptability, long-term efficacy under relevant storage conditions, and suitability for potential application in dairy products. Graphical Abstract Zataria multiflora essential oil (EO), rich in thymol and carvacrol, effectively suppresses vegetative growth and delays spore germination of Heyndrickzxia sporothermodurans Ab.359 TZ, a heat-resistant, spore-forming spoilage organism isolated from UHT milk and registered in NCBI. The EO markedly reduces bacterial population dynamics in UHT milk by inhibiting early vegetative proliferation, limiting sporulation, and partially restraining spore outgrowth over storage time. These findings highlight the potential of Zataria multiflora EO as a natural preservative strategy to control spoilage-associated microbial dynamics and extend the shelf life of UHT milk.