The canine gut microbiome is a complex ecosystem primarily dominated by five bacterial phyla: Firmicutes, Bacteroidetes, Fusobacteria, Proteobacteria, and Actinobacteria. When this ecosystem is balanced, a state known as 'eubiosis,' it supports healthy digestion, energy metabolism, and immune function. However, this balance can be disrupted by age, poor diet, chronic stress, or the use of antibiotics like metronidazole and tylosin, leading to a state of 'dysbiosis'. Dysbiosis doesn't just cause gastrointestinal upset; via the gut-brain axis, disruptions in the microbiome can alter stress hormone regulation and neurotransmitter production, contributing to behavioral changes such as fearfulness, anxiety, and aggression.