Advanced Techniques in Diagnostic Microbiology

456 S. R. Frye and M. Healy
(Stemper et al., 2004; Hanssen et al., 2005). Interestingly, for A. baumannii the
absence of multidrug-resistant strains in the community compared with 36.8%
prevalence among hospital isolates suggesting that the reservoir for epidemic
strains resides in the hospital environment itself, and the community is an unlikely
reservoir for hospital strains of A. baumannii (Zeana et al., 2003). Further,
candidemia must be considered as a potential cause of sepsis in the community
(Pasqualotto et al., 2005).
The etiology of infectious diseases includes the potential infection not only from
community sources but also from colonization of patients or health care workers.
Because MRSA can colonize healthy individuals, screening of employee strains
may indicate routes of transmission. One study using PFGE found that of seven
staff members, three carried the same strain as found in patients (Macfarlane et al.,
1999). Using CARE-2 probe-based DNA fingerprinting, Candida infection isolates
from burn patients, results indicated that strains collected from different patients
were different. It is noteworthy that patients with disseminated candidiasis had
a similar but unique strain isolated from all body locations, suggesting infection
from patient colonization (Gupta et al., 2004).
Molecular subtyping aids in further understanding the pathogenesis of infections
and their potential to relapse or cause new infections. Between 15% and 20%
of patients have recurrences of Clostridium difficile–associated diarrhea, which
is the most common hospital-acquired infectious diarrhea, after discontinuation
of therapy (Wilcox and Spencer, 1992). Two studies using ribotyping indicated
that the recurrence of infection due to the same strain as the original outbreak is
high, ranging from 79% to 90% (Svenungsson et al., 2003; Noren et al., 2004).
However, arbitrarily primed PCR showed a lower incidence of same-strain reinfection
at approximately 66% (Tang-Feldman et al., 2003). This discrepancy may
be due to the lower discrimination seen with ribotyping; rep-PCR and PFGE are
more discriminating than ribotyping when performing fingerprinting of C. difficile
(Spigaglia and Mastrantonio, 2003; Rahmati et al., 2005).
The rate of subsequent MRSA infections may be as high as 29% in 18 months,
and the recurrent infection is often severe (Huang and Platt, 2003). PFGE determined
that patients with long-term MRSA colonization often have several different
strains of MRSA that change over time (Maslow et al., 1995). Additionally, the
severe recurrent infections of Staphylococcus aureus, certain Gram-negative rods,
Nocardia species, and fungi seen with chronic granulomatous disease (CGD) often
present the same bacterial species but different strain type, as determined by PFGE
(Guide et al., 2003). In recurrent listeriosis, molecular subtyping by automated ribotyping
and PFGE confirmed either relapses of infection or reinfection due to a
common source by indication of same-strain isolates (Sauders et al., 2001).
It is generally accepted that tuberculosis results from a single infection with a
single Mycobacterium tuberculosis strain. Specific PCR-based results suggest that
multiple infections are frequent, implying high reinfection rates and the absence of
efficient protective immunity conferred by the initial infection. This finding could
influence our understanding of the epidemiology of disease in high-incidence
regions and our understanding for vaccine development (Warren et al., 2004).