Production of staphylococcal enterotoxins in microbial broth and milk by Staphylococcus aureus strains harboring seh gene

• Most S. aureus express SEH at lower level in milk than in BHI + YE.
• All S. aureus decrease pH in BHI + YE, but only some S. aureus decrease milk pH.
• High SEH levels are secreted to milk only by S. aureus strains decreasing milk pH.
• Lactose metabolism genes are up-regulated in milk in strong SEH producers.
• hld, rot and sarA RNA are up-regulated in milk in strong SEH producers.

Twenty Staphylococcus aureus strains harboring seh gene, including one carrying also sec gene and 11 sea gene, were grown in BHI + YE broth and milk and were tested for SEA, SEC and SEH production. All strains decreased pH of BHI + YE broth at 24 h and increased them at 48 h. Seventeen S. aureus strains grown in milk changed pH for no > 0.3 unit until 48 h. Three other S. aureus strains significantly decreased pH during growth in milk. All S. aureus produced SEH in BHI + YE broth in amounts ranging from 95 to 1292 ng/ml, and from 170 to 4158 ng/ml at 24 and 48 h, respectively. SEH production in milk by 17 strains did not exceed 23 ng/ml at 24 h and 36 ng/ml at 48 h. Three S. aureus strains able to decrease milk pH produced 107–3029 ng/ml and 320–4246 ng/ml of SEH in milk at 24 and 48 h, respectively. These strains were grown in milk and BHI + YE broth with pH stabilized at values near neutral leading to a significant decrease of SEH production. Representative weak SEH producers were grown in milk at reduced pH resulting in moderate increase in SEH production. SEA was produced in milk by 10 S. aureus strains at 24–151 ng/ml at 24 h, and 31–303 ng/ml at 48 h. SEA production in milk was higher or comparable as in BHI + YE broth in 3 strains and lower for remaining strains. Production of SEC by sec-positive S. aureus strains was lower in milk than in BHI + YE broth, ranging from 131 to 2319 ng/ml at 24 and 48 h in milk and 296–30,087 ng/ml in BHI + YE at 24 and 48 h. Both lacE and lacG transcripts involved in lactose metabolism were significantly up-regulated in milk in strong SEH producers. In these strains hld, rot and sarA transcripts were up-regulated in milk as compared to weak SEH producers. Stabilization of milk pH at a value of raw milk significantly down-regulated hld, rot and sarA RNA in strong SEH producers. Milk was generally found unfavorable for enterotoxin production. However, certain S. aureus strains were not restricted in SEH and SEA expression in milk, unlike SEC which remained down-regulated in this environment. Therefore, low safety risk related to S. aureus producing SEC in milk, as suggested previously, may not pertain to certain SEA and SEH-producing strains.