title: Effect of chemico-physical parameters on the histidine decarboxylase (HdcA) enzymatic activity in Streptococcus thermophilus PRI60.
abstract: In this study the activity of the histidine decarboxylase (HdcA) of Streptococcus thermophilus PRI60 was determined during growth and in crude enzyme preparations to evaluate its hazardousness in dairy products. The effect of different pH values, lactose availability, NaCl concentration, and growth temperature on histamine production was evaluated in M17 medium during 168 h incubation. In each case, the production of histamine increased concomitantly with the cell number with a relatively small further rise during the stationary phase. In all cultures the maximum histamine levels were reached at the end of active growth. Histamine was detectable (10 to 55 mg/L) even when growth was strongly inhibited. The HdcA enzyme in crude cell-free extracts was mostly active at acidic pH values common in dairy products. NaCl concentrations lower than 5% did not affect its activity. The enzyme was quite resistant to heat treatments resembling low pasteurization, but was inactivated at 75 °C for 2 min. Given the features of the enzyme studied, efforts must be dedicated to a thorough risk analysis and development of strategies to contrast the presence of histaminogenic S. thermophilus strains in products from raw or mildly heat-treated milk.
  
  
  CONCLUSIONS
  During its growth Streptococcus thermophilus can produce histamine over a wide range of conditions encountered in cheesemaking and cheese ripening. The histidine-decarboxylase is even more active in cell-free extract and histamine can be accumulated independently of cell viability.
authors.affiliation: Dipartamento di Scienze degli Alimenti, Università degli Studi di Bologna, Cesena, Italy.
authors.forename: Giulia
authors.initials: G
authors.lastname: Tabanelli
authors.forename: Sandra
authors.initials: S
authors.lastname: Torriani
authors.forename: Franca
authors.initials: F
authors.lastname: Rossi
authors.forename: Lucia
authors.initials: L
authors.lastname: Rizzotti
authors.forename: Fausto
authors.initials: F
authors.lastname: Gardini
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D055598:Chemical Phenomena; D015169:Colony Count, Microbial; D003611:Dairy Products / Q000009:adverse effects / Q000032:analysis / Q000382:microbiology; D004795:Enzyme Stability; D005506:Food Contamination / Q000517:prevention & control; D006632:Histamine / Q000009:adverse effects / Q000378:metabolism; D006640:Histidine Decarboxylase / Q000737:chemistry / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D007785:Lactose / Q000378:metabolism; D050296:Microbial Viability; D009994:Osmolar Concentration; D061352:Pasteurization; D012965:Sodium Chloride / Q000378:metabolism; D048249:Streptococcus thermophilus / Q000201:enzymology* / Q000254:growth & development / Q000378:metabolism; D013696:Temperature
chemical_list: D001426:Bacterial Proteins; D012965:Sodium Chloride; D006632:Histamine; D006640:Histidine Decarboxylase; D007785:Lactose