Biomedical Engineering Reference
In-Depth Information
(Richardson 1991). Fungi are microscopic elements encountered in the
environment, in domestic installations, in food and in humans and are
considered as normal inhabitants of skin, blood, the intestines and the
mouth. Although a variety of yeast species normally live in the oral cavity,
their presence does not in itself imply a diseased state, as the prevalence
of asymptomatic yeast carriers in healthy individuals ranges from 3
to 48% (Arendorf and Walker 1980) and is even higher (45 to 65%) in
healthy children (Odds 2010). The most commonly isolated yeast species
in the oral cavity is C.
albicans
(Pankhurst 2006). While a median carrier
rate of 38.1% was observed for
C. albicans
alone in community-dwelling
outpatients (Odds 2010), a rate exceeding 70% has been reported in elderly
hospitalized patients (Zomodian et al. 2011) and is even higher in HIV-
infected individuals (MacPhail et al. 2002). Other members of the genus
Candida
found in the oral cavity are potential pathogens yet rarely cause
disease (Saalwachter et al. 2006). Host factors such as reduced salivary fl ow
rate, low salivary pH, increased salivary glucose concentration, prior use
of antibiotics, smoking, the wearing of removable dentures and general
health status are reportedly associated with increased carrier rates for
oral cavity yeast (Kadir et al. 2002).
In humans,
C.
albicans
is found most often on the posterior dorsum of
the tongue, although there have been reports of secondary colonization
in other areas, (Sen et al. 1997). Longitudinal studies have shown that a
steady carrier state of
C. albicans
occurs in a signifi cant proportion of healthy
individuals and that this infection is remittent and therefore recurring in
most of the other subjects (Cutler 2005). In immunocompromised hosts,
Candida
infections are often caused by a resident strain and this same
strain can cause recurrent infections (Miceli et al. 2011); however, resident
strains may be supplanted by other strains to provoke the onset of a
Candida
infection. Epidemiological studies have shown that a large proportion of
the strains involved in infections at a particular site are often genetically
similar (Lasker et al. 2001), which suggests that these strains have specifi c
virulence factors that make them more effective at colonizing the host and/
or causing disease.
C.
albicans
fungus can assume various morphological forms from
blastospore through pseudohyphae to hyphae (Biswas et al. 2007). Indeed,
as shown in
Figure 1,
as early as three hours after contact with a normal
human gingival epithelial cell culture,
C. albicans
switched from blastospore
to pseudohyphal form. There is a common belief that the hyphal form is
more invasive and pathogenic compared to the blastospore form (Evans
1981).
C.
albicans
strains exhibit a high frequency of form switching when
nutritionally stressed. Frequent form switching can simultaneously affect
the expression of many potential virulence factors and may be a genetic
mechanism that enables
C
.
albicans
to adapt to environmental changes.
