Influence of Growth Temperature on Inactivation and Injury of Escherichia coli O157:H7 by Heat, Acid, and Freezing

Abstract

The influence of growth temperature on heat-, lactic acid-, and freeze-induced inactivation and injury of Escherichia coli O157:H7 in 0.1% peptone water was investigated. Three strains of E. coli O157:H7 isolated respectively from salami, apple cider, and ground beef were evaluated. Growth of strains at 10 degrees C compared with growth at 37 degrees C had a significant impact on reducing (P < 0.01) D values obtained for heating (DH value), acid exposure (DA value), with the exception of the cider strain stored in lactic acid solutions. When strains were cultivated at 10 and 37 degrees C and heated at 54 and 56 degrees C, the salami strain possessed the highest (P < 0.01) DH values (5.9 to 59.7 min). When grown at 10 degrees C, the beef strain had the lowest (P < 0.01) DH values after heating at 52, 54, and 56 degrees C (11.2, 4.1 and 2.5 min, respectively). The salami strain grown at 10 degrees C had the highest (P < 0.01) DA values in all concentrations of lactic acid. When grown at 37 degrees C, the salami strain had the highest (P < 0.01) DA values after storage in 0.1 and 0.25% lactic acid, while DA values for the salami and beef strains did not differ (P > 0.05) when stored in 0.5% lactic acid. Portions of strain populations were sublethally injured by heat and lactic acid treatments, as evidenced by the inability of injured organisms to form colonies on tryptone soy agar containing 2% NaCl. Strains cultured at 10 degrees C were more susceptible to sublethal heat injury than the strains cultured at 37 degrees C. Storage of test strains at -20 degrees C for 7 months resulted in a 4- to 6-log CFU/ml reduction in viable population, but induced only minimal sublethal injury. After 5 months at -20 degrees C, strains cultured at 10 degrees C were more sensitive to freeze inactivation than strains cultured at 37 degrees C. When grown at 10 and cultured at 37 degrees C. When grown at 10 and 37 degrees C and stored at -20 degrees C for 7 months, the cider strain possessed higher (P < 0.01) DF values than the beef and salami strains.

Does Enterohemorrhagic Escherichia coli O157:H7 Enter the Viable but Nonculturable State in Salted Salmon Roe?

“…However, this change was readily reversed by passage in N broth for 100 generations, suggesting that the original phenotype of O157 might be NaCl resistant. Under various stress conditions such as NaCl, heating, freezing (4,11,22,24), and animal passage, reported here, bacterial cells might enter the VNC state as a protective mechanism. Resuscitation might then occur under certain conditions (25), meaning that the NaCl sensitivity of O157 might be one protective mechanism against foreign stress.…”

“…Since all the causative foods were manufactured by the same company, the salmon roe was probably contaminated with O157 during the production process. Although the definitive source of O157 could not be identified because the roe was stored frozen for 9 months, it appeared that O157 could survive freezing and a high concentration of NaCl (22) and retain its pathogenicity for humans. In addition, it was proved by the most probable number method that about 0.75 to 1.5 viable cells of O157 could cause infection (1).…”

Fish: a potential source of bacterial pathogens for human beings

“…An outbreak caused by salted salmon roe contaminated, probably during the production process, with enterohaemorrhagic E. coli (EHEC) O157 occurred in Japan in 1998 (Asai et al, 1999). The roe was stored frozen for 9 months but it appears that O157 could survive freezing and a high concentration of NaCl and retained its pathogenicity for humans (Semanchek and Golden, 1998).…”

Thermal inactivation ofEscherichia coliO157:H7

“…Heat resistance was greater when cells were in the stationary rather than the logarithmic growth phase (Todd et al 1993;Jackson et al 1996;Kaur et al 1998), and when cells were grown at 37 or 40°C rather than 10, 23, 25 or 30°C (jackson et al 1996;Kaur et al 1998;Semanchek and Golden 1998;Fig. 3).…”

“…The highest z-value was 7·29°C for cells heated in chicken slurry (Betts et al 1993) and the lowest value for serotype 0157:H7 was Table 1 for parameters of these lines), and the broken line on (a) indicates the 'line of safety' corresponding to a ID reduction from the recommendations of the ACMSF (1995). Data are from: Doyle and Schoeni 1984;Line et al 1991;Pierson 1992, 1993;Abdul-Raouf et al 1993;Betts et al 1993;Nishikawa et al 1993;Todd et al 1993;Ahmed and Conner 1995;Ahmed et al 1995;Beuchat 1995, 1996;Czechowicz et al 1996;Jackson et al 1996;Splittstoesser et al 1996;Teo et al 1996;Blackburn et al 1997;Juneja et al 1997a;Kotrola and Conner 1997;Kotrola et al 1997;Orta-Ramirez et al 1997;Williams and Ingham 1997;Clavero et al 1998;Ingham and Uljas 1998;Juneja et al 1998;Kaur et al 1998;~intavalla et al 1998;Semanchek and Golden 1998;Williams and Ingham 1998;Veeramuthu et al 1998;Duffy et al 1999;George et al 1999;Stringer et al 2000 3·9°C for cells heated in ground beef and enumerated on MEMB (Clavero et al 1998). A z-value of 3·5°C was reported for a VTEC strain of serotype 0157:NM in beef (Todd et al 1993).…”