§ Mr. HinchliffeTo ask the Secretary of State for Health what research has been carried out to investigate the proposition that listeria monocytogenes, damaged by heating and growing in chilled conditions, are more infectious and dangerous than normal listeria monocytogenes.
§ Mr. Dorrell[holding answer 12 November 1990]: Research at the university of Würzburg, Germany has indicated that levels of haemolysin (a haemolysin is a chemical produced by bacteria which damages or destroys red blood cells (and other mammalian cells)) in some listeria monocytogenes (Lm) have been increased 53W following recovery after heat shock. The production of haemolysin by some strains of Lm has been shown to be a marker for virulence in animals and tissue culture, and is believed to be relevant to the ability of the microorganisms to cause disease in humans. However, the observed higher incidence of haemolysin following heat shock was followed by further work with certain strains of Lm. This showed no increase in virulence for animals or tissue cultures, suggesting that an increased level of haemolysin produced by the micro-organism may not be important in producing disease.
Research by other groups has shown that Lm cultured at low temperatures appears to be more virulent in an animal model following intravenous inoculation but not more virulent following introduction into the alimentary tract, that is, oral route.
Research continues into the virulence of this micro-organism at Würzburg and at the Pasteur institute in Paris. In the United Kingdom the public health laboratory service is embarking on work to establish virulence markers and work in this field is also taking place at the Moredun research institute in Edinburgh.