Mycobacterium Species

 The genus Mycobacterium consists 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.

Mycobacterium Species

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 with Mycobacterium 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 transmission of droplets produced by an infected person coughing, sneezing or

Mycobacterium bovis is very closely related to Myco. 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 to humans, but can also be transmitted to humans by milk and, to a lesser
extent, by meat from tuberculous animals.

Mycobacterium paratuberculosis causes paratuberculosis, otherwise
known as Johne’s disease, in cattle and it has been suggested that it
may be implicated in the etiology of Crohn’s disease in humans. This
remains to be established, but if so, consumption of infected milk may be
a possible route of transmission.

The Organism and its Characteristics

Mycobacterium species are generally non-fastidious, Gram-positive, nonsporeforming, pleomorphic aerobes 1–4 mm in length. Myco. bovis is
mesophilic and is not heat-resistant, being readily killed by normal milk
pasteurization conditions.
A special feature of mycobacteria is the chemical composition of their
cell walls. These have a high lipid content made up of esterified mycolic
acids, complex branched-chain, hydroxy lipids with the general formula
.COOH (where R1
and R2
are very long aliphatic
chains), and, as a result, the wall is very hydrophobic and waxy. 
confers a number of important properties on the organisms. For example, uptake of nutrients from aqueous solution is impeded making them
very slow growing so that it often takes more than a week for growth to
be apparent on solid media. 
They are also very resistant to drying and
therefore can persist and remain infectious in the environment for long
periods. The cell wall is more resistant to degradation by lysosomal
enzymes in phagocytes enabling the pathogenic mycobacteria to survive
and grow in macrophages. Its hydrophobic nature also makes the cells
rather difficult to stain. However, once stained they are very resistant to
decolourization and have the characteristic diagnostic property of ‘acid
This was first noted by Ehrlich in 1882 and is detected using the
Ziehl–Neelsen staining procedure in which cells are stained with hot
carbol fuchsin, and mycobacteria, if present, will resist subsequent
decolourization with acid alcohol.

Pathogenesis and Clinical Features

Most forms of tuberculosis are chronic taking months or even years
before recovery or death. The commonest clinical signs include fever,
chills and weight loss, but other symptoms present depending on the
organs involved. The tissue damage produced is not a direct result of
microbial activity as the infecting organism does not produce toxins, but
is a consequence of the body’s immune response to the organism.
In foodborne tuberculosis, M. bovis enters the body through the
intestinal tract and the primary infection usually occurs at the mesenteric lymph nodes. The bacteria are engulfed by macrophages and are then
isolated in nodules called tubercles or granulomas which are mainly
composed of a dense accumulation of activated macrophages and
For many people this is as far as the infection proceeds,
the development of the tubercle is checked by surrounding it with a
fibrous wall and it then calcifies to a yellow gritty mass. In others,
however, illness ensues when the tubercle liquefies causing local tissue
necrosis and releasing the bacteria to spread infection around the body.

 Isolation and Identification

Tuberculous lesions can be identified in meat animals by post-mortem
inspection of carcasses but disease can also be identified in the live animal
(and humans) using the tuberculin test. In this, the animal exhibits
delayed hypersensitivity to injection of tuberculin, a protein preparation
from Myco. bovis. 
In clinical specimens mycobacteria can be identified
directly on the basis of their acid-fast reaction in the Ziehl–Neelson stain
when the organisms appear red and the surrounding tissue blue. The
organisms can be cultured on simple media but are very slow growing.

Association with Foods

In 1900 at the London Congress on Tuberculosis, Robert Koch caused
consternation when he concluded that the risk of transmission of bovine
tuberculosis to humans was so slight that he did not deem it advisable to
take any measures against it. 
The impact of this from the world’s leading
bacteriologist and discoverer of the tubercle bacillus can only be imagined
– especially on John McFadyean who was due to talk on the same subject
two days later! Nonetheless, when his turn came he felt compelled to
‘offer some criticism on the pronouncement of one, the latchet of whose
shoes I am not worthy to unloose’. 
He pointed out that in post-mortem
examinations of hundreds of children in London and Edinburgh, primary
infection appeared to have occurred through the intestines in approximately 28% of cases, and that 2% of all cows in Britain had tuberculosis
of the udder and were excreting the bacillus in their milk.
Pooling of milk increased the incidence of Myco. bovis so that in the
1920s and 1930s the organism could be isolated from 5–12% of milk
samples and the high rates of TB in children due to Myco. bovis were
attributed to the consumption of unpasteurized milk. 
In North America,
compulsory pasteurization regulations were introduced in a number of
the large cities from about 1910 and had a marked effect reducing the
incidence of bovine TB in children. In the UK there was considerable
resistance to the introduction of milk pasteurization, but the available
evidence suggests that its later and more gradual introduction had a similar effect. 
By 1944, all London’s milk was pasteurized and the death
rate from abdominal tuberculosis in children was 4% of what it had been
in 1921 when there was no pasteurization. In contrast, the death rate in
1944 in rural areas, where pasteurization was less extensively practised,
was 10 times the London rate.
Though there has been considerable recent concern about the worldwide resurgence of TB, the contribution of foodborne transmission is
probably insignificant in the developed world where it is effectively
controlled by the testing and elimination of infected cattle, rigorous
meat inspection and milk pasteurization. 
This may not be true in many
developing countries such as those of Africa where the extent of human
tuberculosis caused by Myc. bovis is not known and there is widespread consumption of unpasteurized milk and, in some areas, raw meat

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