Bacteria that cause food poisoning in animals and people survive longer than
expected in soil, says a British researcher. The finding calls into question
the British government鈥檚 rules for spreading sewage sludge on land where
livestock graze. Farmers who fertilise grassland with treated sewage sludge
are required by the Department of the Environment to wait three weeks before
allowing animals to graze. This is supposed to be sufficient time for
bacteria that could infect animals to die out.
Liz Wellington and her colleagues at the University of Warwick recently
reported that Salmonella typhimurium, a bacterium that causes diarrhoea,
survives in soil for longer than three weeks. They also report that the
bacteria cannot be detected by the usual methods because they enter a kind
of 鈥榙ormant鈥� state and so cannot be cultured in the laboratory (Journal of
Applied Bacteriology, vol 74, p 421) .
鈥業 think it鈥檚 a major concern because it indicates that most of the methods
we use to monitor bacteria don鈥檛 spot the actual populations present,鈥� says
Wellington. All previous investigations of soil using the conventional
method of growing cultures on agar suggested that S. typhimurium did die out
in three weeks. Wellington and her colleagues detected the dormant cells
with fluorescent-labelled antibodies which cling specifically to S.
typhimurium. The next step is to find out whether the dormant bacteria can
revert to the infectious state.
Advertisement
If the bacteria do become infectious again, says Wellington, the government
may have to rethink its rules for applying sewage sludge to agricultural
land 鈥� the fate of half the sludge produced in Britain. 鈥業t appears at the
moment that the three-week period is relatively safe, but if we increase
outputs of sludge to land in the future, and the amounts cover wider areas,
the problem might become more serious,鈥� she says.
A decade ago, Rita Colwell and colleagues at the University of Maryland
discovered that Vibrio cholerae, a waterborne bacterium that causes cholera,
also becomes dormant. She also found that it could revert back to its
virulent form, and showed that it infected human volunteers vaccinated
against cholera. 鈥榃e should be concerned about (Wellington鈥檚) findings,鈥�
says Colwell, 鈥榖ecause it shows that there鈥檚 an environmental source of some
of these pathogens that鈥檚 gone unrecognised.鈥�
A spokeswoman for the Department of the Environment said: 鈥業f it鈥檚 proved
that there鈥檚 a problem, I鈥檓 sure we鈥檒l be looking at it.鈥� According to
figures collected by the Public Health Laboratory Service, there were 3867
reported cases of poisoning with Salmonella bacteria in the first three
months of this year, of which 582 were caused by S. typhimurium.
