Growth and survival of E. coli O157:H7 during the manufacture and ripening of a smear-ripened cheese produced from raw milk

Abstract

Aims: The behaviour of Escherichia coli O157:H7 was studied during the manufacture and ripening of a smear-ripened cheese produced from raw milk. Methods and Results: Cheese was manufactured on a laboratory scale using milk (20 l) inoculated with E. coli O157:H7, and enumeration was carried out using CT-SMAC. From an initial level of 1á52 0á03 log cfu ml ±1 in the milk (34 2 cfu ml ±1 ), the numbers increased to 3á4 0á05 log cfu g ±1 in the cheese at day 1. During ripening, the numbers decreased to <1 cfu g ±1 and <10 cfu g ±1 in the rind and core, respectively, after 21 days, although viable cells were detected by enrichment after 90 days. The presence of E. coli O157:H7 in the cheese was con®rmed by latex agglutination and by multiplex PCR. Conclusions:The results indicate that the manufacturing procedure encouraged substantial growth of E. coli O157:H7 to levels that permitted survival during ripening and extended storage. Signi®cance and Impact of the Study: The presence of low numbers of E. coli O157:H7 in milk, destined for raw milk cheese manufacture, could constitute a threat to the consumer. INTRODUCTIONSince its identi®cation as a human pathogen in 1982 (Riley et al. 1983), Escherichia coli O157:H7 has become a pathogen of major concern for the food and dairy industries because of it's ability to cause severe illness, in particular, haemorrhagic colitis, haemolytic uraemic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP). Several well documented food-borne outbreaks, such as that in Scotland in 1996±97, have heightened public awareness of food as a vehicle for transmission of this pathogen.Many outbreaks of E. coli O157:H7-related food-borne illness have been linked with the consumption of contaminated meat, since the intestinal tract of cattle is the primary reservoir for this pathogen (Willshaw et al. 1994;Zhao et al. 1995). Other foodstuffs, however, have been implicated in many outbreaks, including water, lettuce, alfalfa sprouts and apple juice (Buchanan and Doyle 1997). The consumption of unpasteurized milk and dairy products manufactured from unpasteurized milk has also been associated with transmission of E. coli O157:H7 (CDSC 1998; 1999a,b). In 1999, over 11% of the total number of reported cases of infection caused by E. coli O157:H7 in England and Wales were due to dairy products (CDSC 2000). In many parts of Europe, indigenous cheeses manufactured from unpasteurized milk are consumed, giving rise to concern that these products may be a threat to consumer safety as a result of the presence of pathogens such as E. coli O157:H7.The ability of verotoxigenic E. coli to grow and survive during the manufacture of fresh (Arocha et al. 1992), hard (Reitsma and Henning 1996) and Camembert (Ramsaran et al. 1998) cheese has been investigated. In fresh cheese, E. coli O157:H7 grew from an initial level of about 10 5 cfu ml ±1 to ®nal numbers of about 10 7 cfu g ±1 during the manufacture of cottage cheese. However, during the heating phase, total inactivation occurred. During the manufacture ...

behavior of Escherichia coli O157:H7 during the ripening of herby cheese manufactured from raw milk

“…Besides, increased salt amount might have inhibited the acidity in brined cheese. E. coli O157:H7 trend in brined herby cheese is similar to those reported by many researchers (Maher et al, 2001;Saad et al, 2001;Özer et al, 2004).…”

“…Generally, the number of the microorganism drop continuously during storage (Reitsma and Henning, 1996;Ramsaran et al, 1998;Küplülü et al, 1999;Maher et al, 2001;Özer et al, 2004).…”

“…Many factors, such as competitive flora, starter culture, heat, pH value, salt, a w value, inoculation level, cheese production method and storage conditions effect the growth of E. coli O157:H7 in cheese (Reitsma and Henning, 1996;Ramsaran et al, 1998;Glass et al, 1998;Küplülü et al, 1999;Maher et al, 2001;Spano et al, 2003;Özer et al, 2004). GC: Cheese milk was inoculated with E. coli O157:H7 at level of log 3 cfu/mL and ripened in embedded type SC: Cheese milk was inoculated with E. coli O157:H7 at level of log 3 cfu/mL and ripened in brined type GD: Cheese milk was inoculated with E. coli O157:H7 at level of log 5 cfu/mL and ripened in embedded type SD: Cheese milk was inoculated with E. coli O157:H7 at level of log 5 cfu/mL and ripened in brined type ND: Not detected, L.A.: Lactic acidity, a w : Water activity…”

“…Also, there is a synergistic effect among them (Guraya et al, 1998). Survival of E. coli O157:H7 has shown significant change in different cheese varieties (Reitsma and Henning, 1996;Hudson et al, 1997;Ramsaran et al, 1998;Saad et al, 2001;Maher et al, 2001). …”

“…In another study, Özer et al (2004) reported that the counts of E. coli O157:H7 decreased to zero in scalded Urfa cheese after 30 days, whereas this organism survived up to 90 days in unscalded Urfa cheese. Additionally, some investigators demonstrated that E. coli O157:H7 was capable of survival in Turkish white brined cheese (Küplülü et al, 1999) and smear-ripened cheese (Maher et al, 2001) for at least 90 days.…”