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 world-
wide 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 wide-
spread consumption of unpasteurized milk and, in some areas, raw meat
products.
7.11Plesiomonas shigelloides
7.11.1 Introduction
Plesiomonas shigelloidesis the only species of the genus whose name is
derived from the Greek word for neighbour; an allusion to its similarity
toAeromonas. Its position as a causative agent of foodborne illness also
bears some similarity toAeromonas. It is not normally recovered from
human faeces, except in Thailand where a carriage rate of 5.5% has been
reported. The association with diarrhoea is largely based on its isolation
from patients suffering from diarrhoea in the absence of any other
known pathogens and the strongest of this evidence has come with
isolation from several patients in the same outbreak. However volunteer
feeding trials have failed to demonstrate a causal link.
7.11.2 The Organism and its Characteristics
A member of the family Enterobacteriaceae (previously classified in the
Vibrionceae),P. shigelloidesis a short, catalase-positive, oxidase-posi-
tive, Gram-negative rod. It is motile by polar, generally lophotrichous
flagella in contrast toAeromonasandVibriowhich are monotrichous. It
grows over a temperature range from 8–10 1 C to 40–45 1 C with an
optimum at around 37 1 C. It is not markedly heat resistant and is readily
eliminated by pasteurization treatments. Growth is possible down to pH
4.5 and the maximum salt concentration it will tolerate is between 3 and
5% depending on other conditions.
The organism is ubiquitous in surface waters and soil, more com-
monly in samples from warmer climates. Carriage in cold-blooded
animals such as frogs, snakes, turtles, and fish is common and it has
234 Bacterial Agents of Foodborne Illness