Production of characteristic volatile markers and their relation to Staphylococcus aureus growth status in pork.

The 3-methyl-butanal and 3-methyl-butanoic acid are known as fingerprint compounds from Staphylococcus aureus. In this study, we investigated production of these two volatile biomarkers and their correlation to S. aureus growth in pork. Both 3-methyl-butanal and 3-methyl-butanoic acid presented high specificity for S. aureus in either media or pork. In sterile minced pork and pork broth, production of volatile biomarkers and the growth of S. aureus were significantly correlated for most single cultures. However, for mixed cultures, only 3-methyl-butanoic acid indicated correlations with growth of S. aureus. Similar trending was also discovered in raw pork, where production of 3-methyl-butanoic acid was significantly correlated with the growth of S. aureus, but not for 3-methyl-butanal. In summary, 3-methyl-butanoic acid was a more stable metabolic marker than 3-methyl-butanal which could be used as an indicator for the presence of S. aureus in pork. This rapid, convenient and cost-effective detection approach could be applied in meat industry to achieve specific detection of S. aureus.

Identification of a glutathione S-transferase gene of Physarum polycephalum as a biomarker for nanosized TiO2 exposure under dark conditions.

In this study, a glutathione S-transferase gene (gst) from sensitive Physarum polycephalum was selected for its ability to detect nanosized TiO2 (nTiO2 ) exposure under dark conditions. The concentration of nTiO2 (25, 40 and 60 nm) for subsequent assays was first determined (5-18 mg ml-1 ) and total GST enzyme activity of P. polycephalum was confirmed to be increased 6-44 fold in groups treated with nTiO2 . Second, an RNA-seq study was performed to identify candidate gst genes before isolation of an optimum gst gene of P. polycephalum (Ppgst), which encoded 223 amino acids. Third, the transcriptional level of the Ppgst gene was further confirmed to be positively correlated with nTiO2 exposure within the concentration range of (5-15 mg ml-1 ) by qPCR. In conclusion, these results indicated that the transcriptional level of Ppgst can reflect nTiO2 exposure, suggesting that it may be employed as a new biomarker for nTiO2 pollution under dark conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: This study identifies a new gst gene for indicating nanosized TiO2 under dark conditions and provides a new option for detection of nanosized TiO2 pollution under dark conditions.

The production characteristics of volatile organic compounds and their relation to growth status of Staphylococcus aureus in milk environment.

In our previous research, 3-methyl-butanal and 3-methyl-butanoic acid were identified as representative and specific volatile organic compounds released by Staphylococcus aureus in broth. In this study, we explored the production of the 2 volatiles and their correlation to Staph. aureus growth in milk under different conditions. We found significant correlations between the production of 3-methyl-butanoic acid and cell counts of 5 Staph. aureus strains in sterile milk, and there were no obvious differences for its production among 5 tested strains. The intensities of the 2 volatiles were similar and positively correlated with bacterial counts in cultures at 25°C and 37°C despite delayed production of volatiles at 25°C; however, neither compound could be detected at 4°C. The production of 3-methyl-butanoic acid was strongly correlated with growth of Staph. aureus mixed with Streptococcus agalactiae, Escherichia coli O157:H7, and Shigella flexneri, whereas correlations for 3-methyl-butanal were not statistically significant. Compared with the monoculture of Staph. aureus, in mixed cultures, production of 3-methyl-butanal was decreased and that of 3-methyl-butanoic acid was comparatively higher. In pasteurized and raw milks, production of 3-methyl-butanoic acid was correlated with growth of Staph. aureus, and 3-methyl-butanoic acid could be detected when Staph. aureus populations reached 106 to 107 cfu/mL in pasteurized milk and 105 to 106 cfu/mL in raw milk; the correlations for 3-methyl-butanal were not statistically significant. Our results suggest that 3-methyl-butanoic acid is a more suitable marker for high counts of Staph. aureus in milk, whereas 3-methyl-butanal is a transient metabolite and easily depressed by environmental factors.