Streptococcosis causes massive tilapia kills, which results in heavy economic losses of tilapia farming
industry. Out of the Streptococcosis, Streptococcus agalactiae is the major pathogen. The bacterium causes
higher mortality of tilapias in higher than lower temperatures. However, effect of temperature on
metabolic regulation which is related to the mortality is largely unknown. The present study showed 50%
and 70% mortality of tilapias cultured in 25 C and 30 C, respectively, in comparison with no death in
20 C following infection caused by S. agalactiae. Then, GC/MS based metabolomics was used to inves-
tigate a global metabolic response of tilapia liver to the two higher water temperatures compared to
20 C. Thirty-six and forty-five varied abundance of metabolites were identified in livers of tilapias
cultured at 25 C and 30 C, respectively. More decreasing abundance of amino acids and increasing
abundance of carbohydrates were detected in 30 C than 25 C groups. On the other hand, out of the
pathways enriched, the first five biggest impact pathways belong to amino acid metabolism. Decreasing
abundance of L-proline was identified as a crucial biomarker for indexing higher water temperature and a
potential modulator to reduce the high death. This was validated by engineering injection or oral
addition of L-proline. Exogenous L-proline led to elevated amino acid metabolism, which contributes to
the elevated survivals. Our findings provide a potential metabolic modulator for controlling the disease,
and shed some light on host metabolic prevention to infectious diseases.