Physiology of dairy-associated Bacillus spp. over a wide pH range

Bacillus species isolated from alkaline wash solutions used fur cleaning in place in South African dairy factories have been suggested to contaminate product contact surfaces of dairy processing equipment and result in post-p asteurization spoilage of milk and milk products. Growth and attachment of such Bacillus isolates under alkaline and acidic conditions have not been p reviously described. Therefore, the in vitro growth temperature and pH rang es, attachment abilities and hydrophobicity, and enzyme production capabili ties of four Bacillus isolates (tentatively identified as B. subtilis 115, B. pumilus 122 B. licheniformis 137 and B. cereus 144) previously isolated from the alkaline wash solutions in a South African dairy were examined. Gr owth pH ranges were determined in buffered Standard One-like Nutrient Broth and in unbuffered 1% Milk Medium at pH values ranging fr om 3 to 12. Growt h and attachment to stainless steel surfaces and production of protease and lipase enzymes were determined in 1% Milk Medium at pH 4, 7 and 10. Colony hydrophobicity of each isolate by the Direction of Spreading Method (DOS) was also determined at pH 4, 7 and 10. In addition, Arrhenius plots were us ed to examine the growth temperature ranges of the isolates. All isolates g rew at pH values ranging from 4.5 to 9.5 in buffered Standard One-like Nutr ient Broth, and from pH 4 to 10 in 1% Milk Medium. All isolates also attach ed to stainless steel at pH 3, 7 and 10 in 1% Milk Medium. Generally the at tachment of B. subtilis 115, B. pumilus 122 and B. licheniformis 137 to sta inless steel surfaces was enhanced at pH 1 and 10, compared to pH 7. By con trast, the best attachment of B. cereus 144 cells to stainless steel surfac es was at pH 7. Planktonic and attached cells of all isolates produced prot eolytic enzymes at pH 7 and 10, but not at pH 4. Similarly, planktonic and attached cells of B. subtilis 115. B. pumilus 122 and B. licheniformis 137 produced lipolytic enzymes at pH 7 and 10, and weak lipolysis was observed at pH 4. The Bacillus cereus 144 isolate showed no lipolytic activity at pH 10. All isolates exhibited low hydrophobic properties at all pH values eve n though attachment to stainless steel at the same pH values occurred. None of the isolates flew below 11 degrees C or above 56 degrees C, and optimum growth temperatures were in the high mesophilic range (36-44 degrees C). ( C) 2000 Elsevier Science B.V. All rights reserved.