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ii.
Biological impurities
For nonbiological samples
The presence of biological substances normally does not cause much of a
problem in the analysis and interpretation of nonbiological evidence.
For biological samples
The impact of biological impurities on biological evidence depends on the
organism from which the material comes, whether the organism itself is
present or just material from it, and the nature of the test being conducted.
In some cases, a problem may be created by the presence of live organisms,
in other cases from genetic material deposited by the organism. We’ll examine
each situation in turn.
Microorganisms
: Microorganisms such as bacteria and fungi grow well in
warm, moist conditions — conditions found at many a crime scene. They
literally feed off physiological fluids that are a rich source of nutrients for
them. Microorganismal spores are ubiquitous in our environment. All they
need is the right conditions to grow and multiply. Microorganismal contam-
ination can destroy a sample almost overnight. This underscores the need for
proper storage of both evidentiary and reference samples as soon as possible.
Material such as soil contains large numbers of microorganisms, and may well
destroy a sample before it can be collected and preserved. There is nothing
we can do about the growth and action of microorganisms before the crime
scene is discovered. However, once the samples are collected, drying them as
quickly as possible, and then freezing, can limit the destructive potential of
any inherent microorganisms. Because microorganisms are ubiquitous in
nature, they can also contaminate a sample during analysis in the laboratory.
The analyst must adhere to good laboratory practice to avoid this circum-
stance. Because blood antigens or molecules similar to them are practically
ubiquitous throughout living organisms, false positives in, for example, ABO
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Good Field Practice — Processing a Crime Scene
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typing, might be produced by certain types of bacteria (Gaensslen, 1983), or
in one known case, by ants (Sidebar 8). This potential problem has abated as
DNA testing, which is generally much more specific, has replaced serological
typing systems. DNA typing systems are generally specific for at least primate
DNA, but occasionally a bacterial result may fall within the read region
(Fernandez-Rodriguez et al., 1996; Fregeau et al., 1999). However, the expe-
rienced analyst who is familiar with the specifics of a typing system should
be able to distinguish a microorganismal result using a variety of indicators.
Nonhuman physiological substances
. Similarly, addition of animal, but
nonhuman, physiological fluids generally has no effect on the successful and
correct DNA typing of a sample. Although higher organisms may share some
serological similarities with humans, this usually has no consequence because
the dog, cat, or even gorilla is unlikely to have committed the crime or even
contributed to the sample. But the analyst must be aware of any known cross-
reaction that might produce results from nonhuman DNA sources, thereby
complicating interpretation of the results. This provides yet another reason
for the analyst to know something about the history of the sample; in the
unusual circumstance where blood or other body fluids from higher organ-
isms other than human might be part of the sample, this information helps
the analyst to interpret correctly any anomalies that might be present in the
results.
Human physiological substances
. The presence of more than one source
of human tissue or fluid in a sample is not uncommon, and is a consequence
of ordinary living. Clothing will routinely acquire perspiration, saliva, urine,
or small spots of blood (Stoney, 1991). Some fluids, such as blood, are
obvious because they have color. Others, such as saliva or perspiration, are
less obvious. Some evidence, most notably vaginal samples, will always have
material from the vaginal donor in addition to the evidence fluid (with or
without sperm). A thorough analysis of the item is essential to detect these
impurities, and their presence must be incorporated into the interpretation
of the evidence. The type of contamination that sends serologists and DNA
analysts running for cover is physiological material from another human
being. To put this in perspective, however, it must be stressed that unless the
introduced material shows exactly the same marker pattern as one of the
principals in the crime, it would still not produce a false positive. That is,
the wrong guy is unlikely to be falsely implicated. One caveat to this involves
the laboratory contamination of an evidence sample with material from a
reference sample. This type of mistake is devastating.