Longitudinal Metatranscriptomic Analysis of a Meat Spoilage Microbiome Detects Abundant Continued Fermentation and Environmental Stress Responses during Shelf Life and Beyond.

Microbial food spoilage is a complex phenomenon associated with the succession of the specific spoilage organisms (SSO) over the course of time. We performed a longitudinal metatranscriptomic study on one modified-atmosphere-packaged (MAP) beef product to increase understanding of the longitudinal behavior of a spoilage microbiome during shelf life and beyond. Based on the annotation of the mRNA reads, we recognized three stages related to the active microbiome that were descriptive of the sensory quality of the beef: acceptable product (AP), early spoilage (ES), and late spoilage (LS). Both the 16S RNA taxonomic assignments from the total RNA and functional annotations of the active genes showed that these stages were significantly different from each other. However, the functional gene annotations showed more pronounced differences than the taxonomy assignments. Psychrotrophic lactic acid bacteria (LAB) formed the core of the SSO, according to the transcribed reads. species were the most abundant active LAB throughout the study period, whereas the transcription activity of (mainly ) increased after the product had spoiled. In the beginning of the experiment, the community managed environmental stress by cold-shock responses, which were followed by expression of the genes involved in managing oxidative stress. Glycolysis, the pentose phosphate pathway, and pyruvate metabolism were active throughout the study at a relatively stable level. However, the proportional transcription activities of the enzymes in these pathways changed over time. It is generally known which organisms are the typical SSO in foods, whereas the actively transcribed genes and pathways during microbial succession are poorly understood. This knowledge is important, since better approaches to food quality evaluation and shelf life determination are needed. Therefore, we conducted this study to find longitudinal markers that are connected to quality deterioration in a MAP beef product. This kind of RNA marker could be used to develop novel types of rapid quality analysis tools in the future. New tools are needed, since even though SSO can be detected and their concentrations determined using the current microbiological methods, results from these analyses cannot predict how close in time a spoilage community is to the production of clear sensory defects. The main reason for this is that the species composition of a spoilage community does not change dramatically during late shelf life, whereas the ongoing metabolic activities lead to the development of notable sensory deterioration.