Advanced Techniques in Diagnostic Microbiology

452 S. R. Frye and M. Healy
Noncommercial typing systems may also be expensive if they require specialized
equipment, such as the specialized gel-electrophoresis unit required for PFGE.
The DiversiLab System has a lower capital equipment cost and competitive cost
per test.
One issue common to all fingerprinting methods, including the DiversiLab System,
is the difficulty of the data interpretation, mainly because the results are qualitative
and complex. For example, chromosomal RFLP and AFLP yields complex
DNA profiles that can be challenging to interpret (Vos et al., 1995; Olive
and Bean, 1999). Although standardized guidelines have been applied to PFGE
(Tenover et al., 1995), this method is often time-consuming and difficult because
it depends on visual analysis of the fingerprints. With the DiversiLab System software,
interpretation continues to be subjective. However, an interpretation guide
is provided and the software provides a variety of analysis tools to assist the
user. To aid the infection control team in producing specialized data reports, such
as a search on a particular isolate over a specified period, the DiversiLab software
offers 10 fields to incorporate sample demographics, which are linked to
the isolate and can be displayed on the report (Fig. 26.2). These demographics
may include time, date, location, or other particulars about the sample. By using
the DiversiLab software package to assist in monitoring over time and providing
useful trend reports, proactive measures can be taken to prevent contamination.
Applications for Strain Typing in the Clinical Laboratory
Source Tracking of Pathogens in Outbreak and
Clonal Spread
Rapid typing can significantly reduce costs associated with treatment, containment,
and decontamination. The Centers for Disease Control and Prevention (CDC) estimates
that 2 million patients acquire nosocomial infections each year, and 90,000
of those patients die as a result of their infections. Infection control programs
that include DNA fingerprinting allow outbreaks to be tracked and controlled in
real-time. As a laboratory tool, typing is used to assist in tracking sources of contamination
and the spread of hospital-associated infections (HAI) (Watterson and
Drobniewski, 2000; Taha, 2002; Wu and Della-Latta, 2002).
Methicillin-resistant Staphylococcus aureus is currently the most common
antibiotic-resistant pathogen reported associated with HAI in the United States
(CDC, 2004). Even in previously unaffected countries, MRSA has become a common
problem (Faria et al., 2005). Strain typing of MRSA is important for performing
outbreak investigations of nosocomial infections. Tracking the strains of
MRSA in a facility over time has previously demonstrated a shift in the type of
organism most commonly seen (Perez-Roth et al., 2004). Strain typing can also
reveal the spread of a clone not only through a hospital but to a different facility.
For example, strains in Hong Kong were linked to clones with United Kingdom