permitting growth is 0.83. It readily produces enterotoxins, which are not
destroyedin heattreatment (Robertset al., 1996).
Staphylococcus aureusis a pathogenthat can also affectdairyproducts. Its
occurrencein sourmilkproducts suchas yoghurt is worthwhile investigating as
it is present in relativelyhighnumbersin raw milk(Benkerroumet al., 2002).
Accordingto studiesby Benkerroumet al. (2002), staphylococci grewrapidly
during the initialfermentation.Similar behaviour byS. aureushas previously
beenreported bothin yoghurt and cheese (Ahmedet al., 1983;Attaieet al.,
1987).Elverset al. (1999)isolatedS. aureusin a totalof 7% of foodcontact
sites and 8% of environmental sites from 10 small and medium sized
enterprisesproducing highrisk foodsin theirstudy, whichwas performedfor
the Ministryof Agriculture,Fisheries and Food (nowDEFRA) in the UK.The
source of S. aureusalmostalwaysoriginated fromfood handlersor from
utensils previouslycontaminatedby humans(Elverset al., 1999;Peterset al.,
1999).A surveyrevealedthatS. aureuswas involvedin 15%of the recorded
foodborne illnesses caused by dairyproducts in eightdeveloped countries
whereasL. monocytogeneswas involvedin 22%(Benkerroumet al., 2002).It is
resistant to dryingand mayalso colonise complexfood-processing equipment,
whichis left in wet conditions(Boltonet al., 1988).It can also be foundin the
dustin ventilation systems (Roberts et al., 1996).Resistance to oxidative
disinfectants has mainly beenassociatedwithbiofilm formation (Boltonet al.,
1988).Luppenset al.(2002)showedthatS. aureusbiofilmformedon stainless
steel, polystyrene and glass in a nutrient flow needed concentrations of
benzalkonium chloride that were 50 times higher and concentrations of
hypochloritethat were600 timeshigherto achieve4 log killingofS. aureus
compared with cells in suspensions. Supporting results were obtained by
M ̆retr ̆et al.(2003a).
3.2.6 Bacilluscereusbiofilms
Bacilluscereusis a Gram-positive, aerobic, spore-formingrod, normally present
in soil, dust, and water(Jay, 1996).It can also growwellanaerobically.Cells of
B. cereusare largeand motile.The growth temperaturefor this bacteriumis 4±
50 ÎC, withan optimumaround 28±35ÎC. Growth has beendemonstratedover
the pH range4.9±9.3(Jay,1996;Granum,2003;Shelef,2003;Svenssonet al.,
2004).The organism elaborates a number of toxinswithdistinct diarrhoealand
emetic syndromes (Shelef, 2003). B. cereus occurs extensively in the
environmentbut despite the fact that it is a commoncontaminantin raw milk,
foodpoisoningoutbreaks caused by dairyproductscontaminatedwithB. cereus
havebeenrare (Wirtanenet al., 2002;Svenssonet al., 2004).
In a dairyproduct survey,Wong(1998)showedthatB. cereuswas foundin
52%of ice-creams, 35%of soft ice-creams, 29%of milkpowders,17%of
fermented milks and 2% of pasteurised milks and fruit-flavoured milks.
Svenssonet al.(1999)foundindicationsof a prolongedcontaminationproblem
caused by mesophilicB. cereusstrains earlyin the productionchainof one
54 Handbookof hygiene controlin the foodindustry