Heat resistance in Salmonella enteritidis phage type 4: the influence of storage temperatures before heating

Storage of cultures of Salmonella enteritidis PT4 at either 4 degrees or 8 degrees C before heating significantly increased heat sensitivity. The differences between fresh and stored cultures, which became apparent after 4-7 h, were more pronounced with cultures stored at the lower temperature and in those heated at 60 degrees rather than 55 degrees C. Incubation of the stored cultures in either egg or Lemco broth for 30 min at 37 degrees C prior to heating enabled the organisms to recover heat resistance.     

Differential efficiency of mutagenesis at three genetic loci in CHO cells by a benzo[a]pyrene diol epoxide

The formation of DNA adducts by the ultimate carcinogen 7r,8t-dihydroxy-9t,10t-oxy-7,8,9,10-tetrahydrobenzo[alpha]pyrene (BPDE-I) has been implicated in the process of carcinogenesis. In a line of Chinese hamster ovary (CHO) cells designated AT3-2 and in two derivative mutant lines, UVL-1 and UVL-10, originally selected for hypersensitivity to UV-irradiation, we have measured the formation of BPDE-I: DNA adducts and the production of biological damage. The quantity and quality of BPDE-I: DNA adducts formed initially in the 3 cell lines are identical over a wide range of BPDE-I doses. However, the UVL lines are unable to remove adducts from their DNA, while the AT3-2 cells remove about 50% of the BPDE-I: DNA adducts in a 24-h incubation. Correlated with this, the UVL lines are more sensitive to the lethal effects of BPDE-I than are the AT3-2 cells. Mutant frequencies were measured at the aprt, hprt and oua loci and were found to increase linearly with BPDE-I: DNA adduct formation at doses which gave greater than 50% survival. At the hprt and oua loci, the efficiency of mutation induction was similar for AT3-2 and UVL-10 cells. UVL-1 cells showed slightly higher (within a factor of 2-3) mutant frequencies in response to BPDE-I compared to AT3-2 at these two loci. However, at the aprt locus the repair-deficient cells were much more highly mutable (9-15-fold) than the repair-proficient AT3-2 cells. Based on the measured average level of adduct formation, it is calculated that 15% of the BPDE-I: DNA adducts in the aprt gene are converted into mutations. However, the possibility exists that the aprt locus is subject to higher levels of modification by BPDE-I than is the bulk DNA, which would lead to an artifactually high apparent conversion frequency.     

Factors influencing alkali-induced heat resistance in Salmonella enteritidis phage type 4

The culture of cells of Salmonella enteritidis PT4 at either 25, 30 or 37°C in media at pH values between 8.0 and 9.75 resulted in significant increases in heat resistance. At 37°C, induction was rapid and was dependent on protein synthesis, being inhibited by chloramphenicol. Thermotolerance was stable when cells were transferred from pH 9.2 to pH 7.0 and cultures only became heat sensitive again following significant multiplication at the lower pH.