Candida are non-encapsulated white yeasts that multiply
by budding. Candida albicans is the most frequently
encountered species in pathologies, but other species
such as Candida (Torulopsis) glabrata, C. krusei, and
C. tropicalis can also infect humans. Candida albicans is a
commensal species naturally present in the gut, mouth, and
vagina. This species may be involved in severe systemic
infections that can be fatal, but it is mainly involved in
infection of the mucous membranes and skin.
Cryptococcus neoformans is a yeast involved in cases
of cryptococcosis observed in individuals with acquired
immunodeficiency syndrome (AIDS). These yeasts are of
varying shapes and reproduce by budding. They have the
distinction of having a polysaccharide capsule around their
wall. They are found in soil and pigeon droppings. These
yeasts can enter the human body through the pulmonary
tree and spread throughout the body to the central nervous
system where they can cause subacute meningoencephalitis.
Dermatophytes, mainly the genera Epidermophyton,
Trichophyton, and Microsporum, are fungi involved in
skin infection and its appendages. These mycoses can be
associated with peeling or hair loss. They can cause circular
or annular lesions. In nutrient broth, these fungi can
produce a mycelium, conidiophores, and conidia. The
genus Microsporum forms both macroconidia (large asexual reproductive structures, fusiform with thick cell walls)
and microconidia (smaller asexual reproductive structures,
pyriform to clavate and smooth walled) on short conidiophores. The genus Trichophyton forms macroconidia
that are hyaline, multiseptate, variable in form, fusiform,
and spindle shaped to obovate with roughness cell walls.
Its microconidia are round or pyriform. The Epidermophyton produce macroconidia but do not seem to produce
microconidia. Several species are anthropophilic dermatophytes, and transmission to humans generally occurs
in wetlands or by contact with contaminated clothing.
15.2.5 Unicellular Eukaryotes
Even if some fungi are unicellular (see above), unicellular
eukaryotes that are discussed in this section belong to four
kingdoms: the discicristates (Heterolobosea: Naegleria),
excavates (Fornicata: Giardia), Amoebozoans (Entamoeba),
and alveolates (ciliates: Balantidium; Apicomplexa: Cryptosporidium). Their size varies between 10 and 300 μm. Most of
these microorganisms can transform into cysts when environmental conditions are unfavorable. The encystment is particularly common among parasitic forms and corresponds to a
state of dormancy. Cysts provide protection and allow the
transfer of an individual to another by passing through
an unfavorable environment, as, for example, the aquatic
environment. There are many cysts in river sediments.
The very thick envelope of cyst gives them excellent
resistance to disinfectants.
Giardia is a Fornicata (excavate) whose size varies
between 9 and 21 μm long and 2–4 μm wide. It has two
nuclei at the front end and five undulipodiums (flagella)
allowing rapid movement. The encysted form develops in
the colon. The reservoir and host of this pathogen are
humans, wild (bears, beavers), and domestic (cats and
dogs) animals. The ID 50 for humans is less than 10 ingested
cysts, and incubation can be long (3–25 days). The disease is
called giardiasis or lambliasis for the species Giardia
lamblia. It results in severe diarrhea and abdominal cramps,
malaise, and weight loss. Recovery is often spontaneous
and occurs within a few weeks in healthy individuals. Transmission is by fecal-oral ingestion of food or water
contaminated with human or animal feces. Survival in the
environment is very good, and cysts can remain infectious
for several months. This pathogen is very common and poses
serious problems of contamination in drinking water when
there is no filtration step in drinking water treatment because
it is very resistant to chlorine.
Cryptosporidium is an apicomplexan (alveolate) whose
size varies between 6 and 8 μm long and 2–4 μm wide.
They are unique in their method of invading host cells,
especially by the presence of the apical complex and
apicoplast. The sexual and asexual cycle is performed within
a single host. Sporozoites, trophozoites, and merozoites
remain attached to the intestinal epithelial cells. The mature
oocyte contains four sporozoites. The symptoms of cryptosporidiosis are similar to those of giardiasis with watery
diarrhea, abdominal cramps, and headaches. Immunocompromised people such as those with AIDS or cancer are
particularly vulnerable to these parasites but especially
Cryptosporidium. For these people, several deaths were
encountered. ID50 is about 130 organisms, and incubation
is between 1 and 12 days. Contamination is by fecal-oral
route, by ingesting contaminated food or water. Reservoirs
are man for the species C. parvum and animals (birds, fish,
reptiles, mammals).
Entamoeba histolytica is a pathogen whose reservoirs
are humans. This obligate parasite of the digestive tract
of humans can be in the form of trophozoite (12–50 μm in
diameter) or cyst (a form of resistance from 10 to 15 μm
in diameter). The ingestion of 1–10 cysts is enough to
cause amoebiasis or amoebic dysentery in humans and
primates. Ingestion is through water or food contaminated
with feces. Cysts can persist for several weeks in the
environment.
Naegleria fowleri is a discicristate pathogen. It is a naturally occurring amoeba in freshwater and soil. Trophozoites
are between 8 and 20 μm in diameter and produce lobopods
roughly rounded. Cysts are spherical and their size ranged
from 8 to 12 μm in diameter. They are coated with a single
15 Environmental and Human Pathogenic Microorganisms
629
by budding. Candida albicans is the most frequently
encountered species in pathologies, but other species
such as Candida (Torulopsis) glabrata, C. krusei, and
C. tropicalis can also infect humans. Candida albicans is a
commensal species naturally present in the gut, mouth, and
vagina. This species may be involved in severe systemic
infections that can be fatal, but it is mainly involved in
infection of the mucous membranes and skin.
Cryptococcus neoformans is a yeast involved in cases
of cryptococcosis observed in individuals with acquired
immunodeficiency syndrome (AIDS). These yeasts are of
varying shapes and reproduce by budding. They have the
distinction of having a polysaccharide capsule around their
wall. They are found in soil and pigeon droppings. These
yeasts can enter the human body through the pulmonary
tree and spread throughout the body to the central nervous
system where they can cause subacute meningoencephalitis.
Dermatophytes, mainly the genera Epidermophyton,
Trichophyton, and Microsporum, are fungi involved in
skin infection and its appendages. These mycoses can be
associated with peeling or hair loss. They can cause circular
or annular lesions. In nutrient broth, these fungi can
produce a mycelium, conidiophores, and conidia. The
genus Microsporum forms both macroconidia (large asexual reproductive structures, fusiform with thick cell walls)
and microconidia (smaller asexual reproductive structures,
pyriform to clavate and smooth walled) on short conidiophores. The genus Trichophyton forms macroconidia
that are hyaline, multiseptate, variable in form, fusiform,
and spindle shaped to obovate with roughness cell walls.
Its microconidia are round or pyriform. The Epidermophyton produce macroconidia but do not seem to produce
microconidia. Several species are anthropophilic dermatophytes, and transmission to humans generally occurs
in wetlands or by contact with contaminated clothing.
15.2.5 Unicellular Eukaryotes
Even if some fungi are unicellular (see above), unicellular
eukaryotes that are discussed in this section belong to four
kingdoms: the discicristates (Heterolobosea: Naegleria),
excavates (Fornicata: Giardia), Amoebozoans (Entamoeba),
and alveolates (ciliates: Balantidium; Apicomplexa: Cryptosporidium). Their size varies between 10 and 300 μm. Most of
these microorganisms can transform into cysts when environmental conditions are unfavorable. The encystment is particularly common among parasitic forms and corresponds to a
state of dormancy. Cysts provide protection and allow the
transfer of an individual to another by passing through
an unfavorable environment, as, for example, the aquatic
environment. There are many cysts in river sediments.
The very thick envelope of cyst gives them excellent
resistance to disinfectants.
Giardia is a Fornicata (excavate) whose size varies
between 9 and 21 μm long and 2–4 μm wide. It has two
nuclei at the front end and five undulipodiums (flagella)
allowing rapid movement. The encysted form develops in
the colon. The reservoir and host of this pathogen are
humans, wild (bears, beavers), and domestic (cats and
dogs) animals. The ID 50 for humans is less than 10 ingested
cysts, and incubation can be long (3–25 days). The disease is
called giardiasis or lambliasis for the species Giardia
lamblia. It results in severe diarrhea and abdominal cramps,
malaise, and weight loss. Recovery is often spontaneous
and occurs within a few weeks in healthy individuals. Transmission is by fecal-oral ingestion of food or water
contaminated with human or animal feces. Survival in the
environment is very good, and cysts can remain infectious
for several months. This pathogen is very common and poses
serious problems of contamination in drinking water when
there is no filtration step in drinking water treatment because
it is very resistant to chlorine.
Cryptosporidium is an apicomplexan (alveolate) whose
size varies between 6 and 8 μm long and 2–4 μm wide.
They are unique in their method of invading host cells,
especially by the presence of the apical complex and
apicoplast. The sexual and asexual cycle is performed within
a single host. Sporozoites, trophozoites, and merozoites
remain attached to the intestinal epithelial cells. The mature
oocyte contains four sporozoites. The symptoms of cryptosporidiosis are similar to those of giardiasis with watery
diarrhea, abdominal cramps, and headaches. Immunocompromised people such as those with AIDS or cancer are
particularly vulnerable to these parasites but especially
Cryptosporidium. For these people, several deaths were
encountered. ID50 is about 130 organisms, and incubation
is between 1 and 12 days. Contamination is by fecal-oral
route, by ingesting contaminated food or water. Reservoirs
are man for the species C. parvum and animals (birds, fish,
reptiles, mammals).
Entamoeba histolytica is a pathogen whose reservoirs
are humans. This obligate parasite of the digestive tract
of humans can be in the form of trophozoite (12–50 μm in
diameter) or cyst (a form of resistance from 10 to 15 μm
in diameter). The ingestion of 1–10 cysts is enough to
cause amoebiasis or amoebic dysentery in humans and
primates. Ingestion is through water or food contaminated
with feces. Cysts can persist for several weeks in the
environment.
Naegleria fowleri is a discicristate pathogen. It is a naturally occurring amoeba in freshwater and soil. Trophozoites
are between 8 and 20 μm in diameter and produce lobopods
roughly rounded. Cysts are spherical and their size ranged
from 8 to 12 μm in diameter. They are coated with a single
15 Environmental and Human Pathogenic Microorganisms
629