This association with soft cheeses appears to be due to the cheese
ripening process.L. monocytogenes survives poorly in unripened soft
cheeses such as cottage cheese but well in products such a Camembert
and Brie. During the ripening process, microbial utilization of lactate
and release of amines increase the surface pH allowing Listeria to
multiply to dangerous levels. There have also been two European out-
breaks of listeriosis in 1998 and 2003 and a major product recall in the
United States in 2004 associated with butter, hitherto considered a
relatively low risk food.
7.10Mycobacteriumspecies
7.10.1 Introduction
The genusMycobacteriumconsists largely of harmless environmental
organisms but is best known as the cause of two of the most feared and
ancient of human diseases, tuberculosis (TB) and leprosy. TB, described
by John Bunyan as ‘Captain of these men of death’, can sometimes be
foodborne and is therefore of more concern to us here.
There is archaeological evidence to suggest that TB was endemic in
much of the world from ancient times but with the rise in urbanisation
between the 18th and 20th centuries it became epidemic in many areas,
killing millions. Death rates in Europe and the United States peaked in
the 19th century when it has been estimated that 30% of all deaths under
the age of 50 in Europe were due to TB. By the late 20th century, a
combination of improved social conditions, childhood immunization,
screening and effective chemotherapy had reduced the incidence of TB in
the developed world to the point where public health officials talked
confidently of eliminating the disease altogether. This optimism proved
unfounded as we have seen increasing numbers of cases since the late
1980s in groups such as AIDS patients and the socially disadvantaged, as
well as the emergence of drug resistant strains. In the world’s poorer
countries, tuberculosis has always remained an important cause of
morbidity and mortality. In 1990 the WHO and International Union
against Tuberculosis and Lung Disease estimated that one-third of the
world’s population was infected with the tubercle bacillus and there were
7–8 million new cases each year.
Human illness is primarily associated withMycobacterium tuberculosis
which is thought to account for 98% of cases of pulmonary TB and 70%
of non-pulmonary forms. It is spread person to person by aerial trans-
mission of droplets produced by an infected person coughing, sneezing or
spitting.Mycobacterium bovisis very closely related toMyco. tuberculosis
but causes tuberculosis in cattle and other animals as well as in humans. It
too is spread by respiratory aerosols between animals, and from animals
Chapter 7 231