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Although moist heat is known to damage many components of the bacterial cell, the cause of thermal death remains unknown (for reviews, see Allwood & Russell, 1970; Tomlins & Ordal, 1976; Welker, 1976). Single strand breaks in DNA as a result of exposure to moist heat have been observed in Escherichia coEi (Bridges et al., 1969 ; Sedgwick & Bridges, 1972 ; Woodcock & Grigg, 1972 ; Pauling & Beck, 1975 ; Grecz & Bhatarakamol, 1977) and Salmonella typhimurium (Gomez & Sinskey, 1973, 1975). These breaks are not considered to be the direct effect of heat on the DNA, since their formation is dependent on the recovery conditions (Gomez & Sinskey, 1973), the strain used and physiological state of the bacteria (Sedgwick & Bridges, 1972), and could not be demonstrated by heating the DNA in vitro (Woodcock & Grigg, 1972). Rather, the damage has been attributed to heat-activated nucleases (Sedgwick & Bridges, 1972; Gomez & Sinskey, 1973), the breaks being greatly reduced in an endonucleasedefective strain or if enzyme inhibitors are present during heating (Grecz & Bhatarakamol, 1977). The role of this damage in cell death is unclear. While Woodcock & Grigg (1972) reported that breakage and repair of the DNA in E. coli were accompanied by corresponding changes in viability, Bridges et al. (1969) were unable to show any repair of the breaks in their strains during recovery. However, Pauling & Beck (1975) found that a DNA ligase-defective mutant of E. coli was much more heat-sensitive than the wild-type strain and suggested that this sensitivity was due to an inability of the mutant to repair the thermally induced breaks. In S. typhimurium the damage was only detectable after many bacteria had lost their viability (Gomez & Sinskey, 1973), yet the lesions have to be repaired before the injured cells can resume growth on complex media (Gomez & Sinskey, 1975), and hydroxyurea, which depletes the bacteria of DNA precursors, interferes with recovery (Gomez et al., 1976). Clearly, DNA damage must be sought in other species, particularly Gram-positive bacteria, before its general significance in thermal death can be assessed. We have looked for evidence of DNA scissions in two thymine-requiring mutants of Streptococcus faecalis and we report here our preliminary findings that single strand breaks are produced when these bacteria are heated at 60 "C.

Production of Single Strand Breaks in the DNA of Streptococcus faecalis After Mild Heating