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.
7.10.2 The Organism and its Characteristics
Mycobacteriumspecies are generally non-fastidious, Gram-positive, non-
sporeforming, pleomorphic aerobes 1–4mm 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
R^1 CHOH.CHR^2 .COOH (where R^1 and R^2 are very long aliphatic
chains), and, as a result, the wall is very hydrophobic and waxy. This
confers a number of important properties on the organisms. For exam-
ple, 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
fastness’. 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.
7.10.3 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
232 Bacterial Agents of Foodborne Illness