**http://www.scq.ubc.ca/?p=525**

**A PEACH OF A PATHOGEN: CRYPTOCOCCUS NEOFORMANS**

By Megan Simmer and David Secko

(August 2003)

When opportunity strikes you may just find Cryptococcus neoformans
making the most of it. Cryptococcus neoformans is pathogen. Cryptococcus
neoformans is also an opportunist. Over the past 10 years, infections of
this pathogen have been on the rise, mostly striking individuals with
compromised immune systems \[1\]. As an emergent disease it has taken
advantage of the recent rise in the rates of immune-suppressing
diseases, such as AIDS, as well as the use of drugs that suppress the
immune system \[1\]. This interesting organism is increasingly being
placed under a microscope to uncover how it is taking advantage of our
immune systems.

**A Shake of the Peach Tree**

In 1894, a little bit of fermented peach juice introduced the world to
an encapsulated yeast-like fungus that we now call C. neoformans \[2\]
(see Figure 1). It was isolated from fruit juice by the Italian
Sanfelice and shortly thereafter from the lesions of a woman \[3\],
immediately revealing its potential as a human pathogen. Over the years,
the organism Sanfelice first isolated has been re-isolated and given all
sorts of names, from Saccharomyces hominis, Cryptococcus hominis and
Torula histolytica, all the way to Debaryamyces hominis. This created a
little bit of confusion and the name was C. neoformans was finally
decided upon to describe all the isolates \[4\].

*Figure 2. The life cycle of C. neoformans. Notice how the growing cells
produce hyphae that fuse, making a basidium. Spores will ultimately be
produced on top of the basidium before being released into the
environment.*

Sexual reproduction in C. neoformans begins when two cells, each
carrying one complement of their genetic information (a state in which
they are called haploid), encounter one another and, as a result, fuse.
The potential to fuse is regulated partly by the two cells being
different mating types of which there are two flavors, "a" and "á". The
cycle of sexual reproduction, and thus of cell fusion, involves a series
of mitotic cell divisions that produce thread-like extensions called
hyphae (see Figure 2). At the end of the hypha the unique structure
mentioned above, the basidium, is formed. It is the basidium that will
bear the spores (sometimes called basidospores) that will eventually
form. For this to occur the two haploid nuclei residing in the basidium
must fuse, through an event called karyogamy, to form one diploid
nucleus. Meiosis and mitotic divisions will then go on to form spores.
As these haploid spores are released to produce more cells the life
cycle of C. neoformans will continue.

**The Four Varieties**

Four different types of C. neoformans occur throughout the world. Each
type differs slightly in its global distribution and pattern of
virulence. They are traditionally divided based on serotyping into
variations A to D \[5,6\]. Two of the four types (B & C) are called C.
neoformans var. **gattii**, while the others are known as C. neoformans
var. **grubii** (A) and C. neoformans var. **neoformans** (D). It is the
variations grubii and neoformans that are most commonly associated with
disease.

In the northern hemisphere the most common forms of C. neoformans are
var. grubbi and var. neoformans, which are both most commonly found in
pigeon poop \[1\]. Of the two varieties, it is C. neoformans var. grubbi
that makes up the vast majority of clinical strains isolated from
infected individuals, while var. neoformans is not normally pathogenic.
People who suffer from AIDS or who are receiving immunosuppressive drugs
after an organ transplant are at risk of contracting **cryptococcosis**,
the disease for which a C. neoformans infection is the cause \[1\].

In Australia and other sub-tropical areas of the southern hemisphere the
most common form of C. neoformans is var. gatti, making it ecologically
and epidemiologically different from C. neoformans var. neoformans and
var. grubbi. It is commonly found in association with eucalyptus trees
and infections of var. gatti are restricted to lung infections.
Interestingly, these infections occur in the immunocompetent population,
with an rate of infection is eight per million per year in Australia.
Table 1 summarizes the differences between C. neoformans var. neoformans
and C. neoformans var. gatti.

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**Pathogenic Qualities**

Cryptococcus as a genus contains approximately 37 species and out of all
of them only C. neoformans is pathogenic \[4\]. This has made it not
only a biological curiosity, but also a medical curiosity---especially
since there has been a jump in the global rate of C. neoformans
infections in the 1980s \[1\].

It is the occasional inhalation of the basidiospores that cause exposure
to the pathogen, although these spores are normally dealt with by the
immune system. But, when your immune system is not functioning properly
those inhaled cryptococcal cells can reproduce and spread throughout
your body. In such a case, cryptococcosis will result; commonly causing
meningitits, which is an inflammation of the membranes surrounding the
central nervous system (the brain and spinal cord). The incidence of
cryptococcal meningitis being about ten per million per year, usually
located in urban centers. Cryptococcosis has also been found to cause
pneumonia and skin lesions.

The rise in C. neoformans infection is commonly linked to a global
increase in the numbers of immunocompromised individuals, mostly closely
associated with the AIDS epidemic \[7\]. The incidence of HIV-infected
patients that develop cryptococcosis is around 10-15% in the Western
Countries, but the effect of C. neoformans on AIDS is crystallized in
Zimbabwe were the number jumps to 88% \[8\].

**A Changing Picture of where C. neoformans is found**

A traditional picture of the distribution of C. neoformans (see Table 1)
and the incidence of cryptococcosis is beginning to change. Case in
point, an outbreak of C. neoformans var. gatti infections on British
Columbia's Vancouver Island \[9\]. A surprise to everyone, since this is
the type of Cryptococcus is normally restricted to Australia. In 2001,
British Columbia also saw an increase in the cases of cryptococcosis in
pets, accompanied by a concomitant increase in the number of human
cases.

The British Columbia Centre for Disease Control (BCCDC) launched an
investigation into the disease. The detailed analysis that followed took
a close look at medical records, undertook clinical reviews of all
confirmed cases, environmental sampling, the identification of the
pathogenic strains by genetic fingerprint analysis, and geographical
mapping of the incidence of the disease.

As the results came in, it was found that since 1999 there have been
over 40 incidences of cryptococcosis on Vancouver Island---all caused by
C. neoformans var. gatti. The incidence of C. neoformans var. gatti,
both in clinical and environmental samples, was found to be the highest
on the central east coast of Vancouver Island. Visiting Rathtrevor
Beach, a popular park in the area, turned out to be a significant risk
factor for acquiring cryptococcosis. This was of interest since C.
neoformans var. gatti is commonly found in association with Douglas fir,
Garry Oak, Arbutus and Cedar trees. Activities like branch clean-up and
chopping wood were thus found to be protective against cryptococcal
illness. In 2001 the infection rate has peaked at 20 cases per million
per year.

The BCCDC analysis has answered many questions about the outbreak of
cryptococcal illness on Vancouver Island, but others remain. How did C.
neoformans var. gatti ended up on Vancouver Island? The fungus could
have "hitch-hiked" here via infected trees imported from Australia or
just naturally expanded its ecological niche due to climate changes. And
more importantly, what has changed to increase the virulence of this
fungus in comparison to its Australia counterpart?

**Researching the Virulence of C. neoformans**

Researchers have been working to determine how C. neoformans causes
disease in humans and the bulk of this effort has been directed at
understanding its virulence factors. Several have been identified and
two of the most heavily studies are the secretion of a polysaccharide
capsule and melanin synthesis \[1\].

*Polysaccharide capsule*---In response to low levels of iron in its
environment and physiological levels of CO~2~, C. neoformans can
synthesizes a thick polysaccharide capsule. The capsule interferes with
the action of special immune system cells called macrophages that devour
invading cells, thus protecting the C. neoformans cells from the immune
system. Additionally, if macrophages do manage to engulf the cryptococci
the capsule allows it survive and reproduce inside of the macrophage.

*Melanin*---This dark pigment is produced by the polymerization of
compounds called polyphenols. It is a compound similar to the one that
makes moles and freckles dark brown. For Cryptococcus, melanin offers
protection from a process called oxidative killing. In host immune cells
like macrophages, the killing of foreign cells is accomplished by
exposing the engulfed foreign cells to highly reactive molecules called
free radicals, which will react with molecules like DNA and proteins.
These uncontrolled reactions often prove fatal. Melanin serves to
protect cryptococci cells by reacting with free radicals to neutralize
them.

Each of these factors aids in the survival of C. neoformans within its
host, allowing it to evade the immune system and thus cause disease. It
is for this reason that they are labeled as virulence factors.

**Filling in the Gaps**

It was over a hundred years ago that C. neoformans was discovered and
since that time it has been scrutinized as a pathogen. But with
cryptococcosis emerging in the past twenty years as one of the most-life
threatening diseases in immune-compromised individuals, our
understanding of its biology and pathogenic qualities is in need of gap
filling. The medical research machine has been accelerating its focus on
C. neoformans and many new discoveries are springing free, certainly
making it a topic to keep your eye on.

**Additional Reading**

1\. Casadevall A, Perfect JR. (1998). Cryptococcus neoformans.
Washington DC: ASM Press. 541p.

2\. Segal E, Baum GL. (1994). Pathogenic Yeasts and Yeast Infections.
Boca Raton: CRC Press. 238p.

3\. Howard D. (2003). Pathogenic Fungi in Humans and Animals. New York:
Marcel Dekker

**References**

1\. Buchanan KL, Murphy JW. 1998. What Makes Cryptococcus neoformans a
Pathogen? Emerging Infectious Diseases 4(1): 71-83.

2\. Sanfelice F. 1895. Sull'azione patogena dei bastomiceti. Ann Inst
Igien Univ Roma 5: 239-262.

3\. Knoke M, Schwesinger G. 1994. One Hundred Years Ago: The History of
Cryptococcosis in Greifswald. Medical Mycology in the Nineteenth
Century. Mycoses 37: 229-233.

4\. Mitchell TG, Perfect JR. 1995. Cryptococcosis in the era of
AIDS---100 years after the discovery of Cryptococcus neoformans. Clin
Microbiol Rev 8:515-548.

5\. Levitz SM. 1991. The ecology of Cryptococcus neoformans and the
epidemiology of cryptococcosis. Rev Infect Dis 13: 1163-9.

6\. Franzot SP, et al. 1999. Cryptococcus neoformans var. grubii:
Separate varietal status for Cryptococcus neoformans serotype A
isolates. J Clin Microbiol 37: 838-840.

7\. Powderly WG. 1993. Cryptococcal meningitis and AIDS. Clin Infect Dis
17: 837-842.

8\. Hakim JG, et al. 2000. Impact of HIV infection on meningitis in
Harare, Zimbabwe: a prospective study of 406 predominantly adult
patients. AIDS 14: 1401-1407.

9\. [[BCCDC Health
Alert](http://www.bccdc.org/news.php?item=4)]{.underline}

*(Art by Jane Wang - note that high res versions of these image files
are available [[here](http://www.scq.ubc.ca/?page_id=247)]{.underline})*