darkening to deep yellow and eventually brick red, if exposed to light. In fluid media, growth is on the surface only, unless a wetting agent (e.g. Tween 80) is added to the medium. Cell structure and metabolism M. bovis is similar in structure and metabolism to M. tuberculosis. M. bovis is a Gram-positive, acid-fast, rod-shaped, aerobic bacterium. Unlike M. tuberculosis, M. bovis lacks pyruvate kinase activity, due to pykA containing a point mutation that affects binding of Mg2+ cofactor. Pyruvate kinase catalyses the final step of glycolysis, the dephosphorylation of phosphorenolpyruvate to pyruvate. Therefore, in M. bovis, glycolytic intermediates are unable to enter into oxidative metabolism. Although no specific studies have been performed, M. bovis seemingly must rely on amino acids or fatty acids as an alternative carbon source for energy metabolism. Pathogenesis During the first half of the 20th century, M. bovis is estimated to have been responsible for more losses among farm animals than all other infectious diseases combined. Infection occurs if the bacterium is ingested or inhaled. M. bovis is usually transmitted to humans by consuming raw milk from infected cows, although it can also spread via aerosol droplets. Actual infections in humans are nowadays rare in developed countries, mainly because pasteurisation kills M. bovis bacteria in infected milk. In the UK, cattle are tested for the disease as part of an eradication program and culled if they test positive. Such cattle can still enter the human food chain, but only after a meat inspector or a government veterinary surgeon has inspected the carcass and certified that it is fit for human consumption. However, in areas of the developing world where pasteurisation is not routine, M. bovis is a relatively common cause of human tuberculosis. Bovine tuberculosis is a chronic infectious disease which affects a broad range of mammalian hosts, including humans, cattle, deer, llamas, pigs, domestic cats, wild carnivores (foxes, coyotes) and omnivores (common brushtail possum, mustelids and rodents); it rarely affects equids or sheep. The disease can be transmitted in several ways; for example, it can be spread in exhaled air, sputum, urine, faeces, and pus, so the disease can be transmitted by direct contact, contact with the excreta of an infected animal, or inhalation of aerosols, depending on the species involved. Application to biotechnology M. bovis is the ancestor of the most widely used vaccine against tuberculosis, M. bovis bacillus Calmette-Guérin (BCG) which was