The scientists found that while heat treatment, like that used in pasteurisation, killed most of the bacteria, some did recover.
"Since more hygienic cheese-making practices have negligible effects on NSLAB contamination, survival during pasteurisation is the most probable entry route for the few NSLAB cells found in cheese during early ripening," said the researchers, who also tried to identify the mechanisms of heat resistance.
The scientists from the University of Bari, Italy, INRA in France and the University College, Cork, Ireland looked at strains of Lactobacillus plantarum, one of a group of bacteria known as nonstarter lactic acid bacteria (NSLAB) believed to be essential to the ripening process and flavour development in a variety of cheeses including cheddar, feta and pecorino.
"The results of this study show that the heat resistance of L. plantarum is a complex process which is also related to cold shock and general stress responses," they added.
Full findings of the study are published in the March issue of Applied and Environmental Microbiology, p. 1336-1346, Vol. 70, No. 3.
With increasingly complex industrialised food processes, methods, such as pasteurisation, to beat harmful foodborne pathogens are essential. According to the World Health Organisation, the global incidence of foodborne disease is difficult to estimate, but it has been reported that in 2000 alone 2.1 million people died from diarrhoeal diseases. A great proportion of these cases can be attributed to contamination of food and drinking water.
In industrialised countries, the percentage of people suffering from foodborne diseases each year has been reported to be up to 30 per cent. In the US, for example, around 76 million cases of foodborne diseases, resulting in 325,000 hospitalisations and 5,000 deaths, are estimated to occur each year.
While most foodborne diseases are sporadic and often not reported, foodborne disease outbreaks may take on massive proportions. For example, in 1994, an outbreak of salmonellosis due to contaminated ice cream occurred in the USA, affecting an estimated 224,000 persons. In 1988, an outbreak of hepatitis A, resulting from the consumption of contaminated clams, affected some 300,000 individuals in China, reports the UN-backed organisation.