Overview of Waterborne Pathogens - Waterborne Pathogens: Detection Methods and Applications - page 10

containing the nutrients and growth factors necessary for microorganism
growth; this is highly specific to different microorganisms, thus allowing selec-
tive growth of certain microorganisms from a mixed sample. However, many
microorganisms are nonculturable, meaning it is not possible to grow them in
the laboratory.
Self-limiting: This term is used to describe some of the diseases resulting from
infection with microorganisms. In self-limiting cases, the immune response of the
human body will eventually kick in, thus preventing further multiplication of the
pathogen and a return to health.
For more details, the reader is referred to standard microbiology textbooks.
Infectious Dose
The infectious dose does not refer to a minimum threshold value above which
infection occurs. Every single pathogen ingested has the possibility to initiate
infection. However, among the different pathogens, the probability that a single
organism will initiate infection can vary widely. The infectious dose attempts to
characterize this by providing a probability of infection. The infectious dose can
therefore be thought of as the dose above which the probability of infection
exceeds a certain value. The probability for illness to develop following infection
depends upon the degree of host damage and whether this is sufficient to result
in clinical symptoms.
Sometimes, clinical or epidemiological data are available from previous out-
breaks. Alternatively, in order to determine the infective dose, dose-response
experiments can be undertaken with healthy adult volunteers to determine the
probability of infection at different dose levels. These data are not available for all
pathogens and they do not indicate susceptibility of vulnerable subpopulations
to any of the studied pathogens. Additionally, experiments are often performed
with laboratory strains of pathogens, which may differ from the wild-type. How-
ever, these studies provide strong evidence that the infective dose of many
waterborne pathogens is very low.
In this chapter, and the WHO list, the relative infectivity is categorized as
low if the infective dose is greater than 10 4 pathogens, moderate for doses
between 10 2 and 10 4 and high for doses between 1 and 10 2 . There is no indica-
tion in the WHO list of what probability of infection this infective dose repre-
sents.