Short communication: growth characteristics of Streptococcus uberis in UHT-treated milk

Streptococcus uberis is an important environmental pathogen associated with bovine mastitis as well as with high total bacterial numbers in bulk tank milk. This study was conducted to determine whether S. uberis reproduction is likely to contribute to high bacterial numbers in bulk tank milk. Four S. uberis raw milk isolates were individually inoculated into UHT-treated milk and incubated at 4.4 or 7 degrees C for up to 5 d to simulate appropriate cooling; at 10 degrees C for 5 d to simulate marginally inadequate cooling; at 21 or 25 degrees C for 7 h to simulate ambient temperatures; or at 32 degrees C for 7 h to simulate elevated temperature conditions. None of the S. uberis isolates grew at either 4.4 or 7 degrees C. Streptococcus uberis growth at 10 degrees C appeared to be ribotype-specific. Although ribotype 116-520-S-1 isolates did not grow at 10 degrees C, ribotype 116-520-S-2 isolate numbers increased up to 3.5 log10 cfu/mL within 5 d. Generation times were calculated as 2.7 +/- 0.1 h, 2.1 +/- 0.1 h, and 1.0 +/- 0.1 h for 116-520-S-1 isolates and 1.8 +/- 0.4 h, 1.3 +/- 0.3 h, and 0.8 +/- 0.1 h for 116-520-S-2 isolates at 21, 25, and 32 degrees C, respectively. Our results suggest that high numbers of S. uberis in bulk tank milk are more likely to reflect high numbers of S. uberis shed by mastitic cows, rather than multiplication of these organisms under cooling conditions required for production of Grade A milk.