Involvement of a pasteurizer in the contamination of milk by Bacillus cereus in a commercial dairy plant

Bacillus cereus is a common contaminant in raw milk. The spores survive pasteurization and psychrotrophic strains of B. cereus often limit the keeping quality of pasteurized milk stored at > 6 °C (Griffiths, 1992). High numbers of B. cereus in pasteurized milk are most frequent when the cows are grazing (Slaghuis et al . 1997), mainly owing to increased levels of spores in raw milk resulting from teat contamination by soil (Christiansson et al . 1999). However, high numbers can also be found in pasteurized milk while the cows are housed indoors, and this is probably caused by additional contamination at the dairy plant (te Giffel et al . 1996; Larsen & Jorgensen, 1997; Lin et al . 1998). There is little information available about the sites of recontamination in the dairy. The use of typing techniques capable of discrimination below the species level, such as fatty acid profiles and random amplification of polymorphic DNA–polymerase chain reaction (RAPD–PCR), could be helpful in demonstrating contamination routes (Lin et al . 1998; Nilsson et al . 1998). Spores of B. cereus are very hydrophobic and readily adhere to surfaces of steel, glass and rubber (Ronner et al . 1990), and short cleaning-in-place programmes do not always eliminate all the spores (Ronner & Husmark, 1992). Spores adhering to surfaces are more difficult to eliminate by disinfectants than spores in solution (te Giffel et al . 1995). Many B. cereus spores germinate rapidly in milk upon heat activation and, if allowed to propagate undisturbed on surfaces, may form biofilms that are extremely difficult to eliminate (Mosteller & Bishop, 1993; Wirtanen et al . 1996; Kumar & Anand, 1998). This paper describes how we demonstrated the involvement of a pasteurizer in the contamination of pasteurized milk by B. cereus in a commercial dairy plant using a combination of classic microbiological analyses and typing of strains by RAPD–PCR.