Both Cryptococcus neoformans and Cryptococcus gattii have been isolated from a variety of environmental sources in Colombia.
AimTo determine the viability of C. neoformans/C. gattii isolates in stored soil samples, filtrates and bird droppings from which these yeasts were previously recovered.
MethodsA total of 964 samples collected between 2003 and 2009, and kept at room temperature were processed. From them, 653 samples were from trees decaying wood, 274 from soil filtrates and 37 from bird droppings. When C. neoformans or C. gattii were recovered, the molecular type of each isolate was established by PCR fingerprinting using the single primer (GTG)5.
ResultsAmong the processed samples, 161 isolates were recovered. From those, 81 (50.3%) corresponded to C. gattii recovered from decaying wood of Eucalyptus spp., Corymbia ficifolia, Terminalia catappa and Ficus spp. trees, and 80 (49.7%) corresponded to C. neoformans recovered from Ficus spp. and eucalyptus trees, as well as from bird droppings. The most prevalent molecular type among the C. gattii and C. neoformans isolates was VGII and VNI, respectively.
ConclusionsThe re-isolation of C. neoformans/C. gattii from 10-year stored samples suggests that these yeasts are able to keep viable in naturally colonized samples.
Cryptococcus neoformans y Cryptococcus gattii han sido aislados de diversas fuentes ambientales en Colombia.
ObjetivosDeterminar la viabilidad de C. neoformans/C. gattii en muestras de suelo, filtrados y excrementos de aves en las que se habían aislado las levaduras previamente.
MétodosSe procesaron 964 muestras recogidas entre los años 2003 y 2009, y almacenadas a temperatura ambiente. De estas, 653 muestras provenían de detritos, 274 de filtrados de suelo y 37 de excrementos de aves. Una vez recuperados C. neoformans y C. gattii, se determinó el patrón molecular mediante la técnica de PCR huella digital con el iniciador (GTG)5.
ResultadosEntre las muestras procesadas, se recuperaron 161 aislamientos. De estos, 81 (50,3%) fueron C. gattii recuperados de detritos de Eucalyptus spp., Corymbia ficifolia, Terminalia catappa y Ficus spp., y 80 (49,7%) eran C. neoformans recuperados de Ficus spp. y eucaliptos, así como de excrementos de aves. El patrón molecular más prevalente entre C. gattii y C. neoformans fue VGII y VNI, respectivamente.
ConclusionesEl reaislamiento de C. neoformans/C. gattii de muestras almacenadas durante 10 años evidencia que estas levaduras son capaces de mantenerse viables en muestras colonizadas naturalmente.
Several studies carried out in Colombia have provided valuable data about the ecology of Cryptococcus neoformans and Cryptococcus gattii. The first environmental isolation of this pathogen was reported when C. neoformans serotype A was recovered from environmental samples in Bogota, Colombia, in the 1980s.3 Later, when Ellis and Pfeiffer demonstrated an association between C. gattii and Eucalyptus spp.,4 we conducted an environmental search in the city of Cúcuta, where an important number of clinical cases caused by this species had been reported in immunocompetent patients.10 At that time, more than 1500 samples were collected over a period of 11 months, and for the first time in Colombia and worldwide, C. gattii serotype C was recovered from environmental samples.1 Recently, we reported serotype B isolates recovered from soil samples associated with Ficus spp. in the same city.12 Additionally, serotype B was recovered from eucalyptus samples (Eucalyptus spp.) in a study undertaken in a forest area in the department of Cundinamarca.7,21 Furthermore, in a follow up study undertaken in 26 country areas, we analyzed data regarding samples recovered from pigeon droppings, eucalyptus and almond trees, and we found that nearly half of these sampling areas were positive for these two species of yeasts.14 More recently, C. gattii molecular type VGIII was recovered from Eucalyptus ficifolia (Corymbia ficifolia) detritus, which made it the first worldwide report on this association.9
In this study we aimed to determine the viability of C. neoformans and C. gattii isolates in naturally colonized samples associated to soil, trees decaying wood and bird droppings stored since 2003.
Materials and methodsA total of 964 samples were collected between 2003 and 2009 (Table 1) and stored at room temperature in three forms: trees decaying wood (n=653), filtrates from soil (n=274), and bird droppings (n=37). Using conventional techniques,24 5g of decaying wood or bird droppings were suspended in 25ml of phosphate buffer saline (PBS), and then mixed and filtered with sterile gauze. This filtrate was mixed with 100μl of chloramphenicol (40g/l), and 100μl of this suspension were plated onto modified Guizotia abyssinica agar plates with biphenyl, chloramphenicol and amikacin sulfate. The same procedure was applied to the stored filtrates. Plates were incubated at 27°C and observed daily during up to one month for growth and colony development. A maximum of 30 colonies suspected to be C. neoformans and/or C. gattii were streaked from each plate onto Sabouraud glucose agar plates and incubated for 48h at 27°C. Isolated colonies were tested using conventional techniques such as urease production, assimilation of nitrogen sources and growth on canavanine glycine bromothymol blue agar medium.15,16 Recovered isolates were kept on glycerol (10%) at −70°C.
Environmental recovery of isolates of the C. neoformans/C. gattii species complex in stored samples.
| Sampling year (reference) | Source | Species recovered | Previous study n (% positive) | This study n (% positive) | Total isolates recovered (this study) | CFU/g of sample (this study) | ||
|---|---|---|---|---|---|---|---|---|
| Trees decaying wood | Soil filtrates | Bird droppings | ||||||
| 2003 (21) | Eucalyptus spp. | C. gattii | 195 (23.6) | 146 (3.4) | 146 (4.1) | ND | 50 | 4.5×102–4.8×103 |
| 2004–2006 (7) | Almond trees | C. gattii | 40 (ND) | 40 (2.5) | ND | ND | 12 | 11.4×104 |
| 2005, 2007 (9,13) | Ficus spp. | C. neoformansC. gattii | 128 (11.7) | ND | 37 (5.4) | ND | 2 | 1.5×102 |
| Corymbia ficifolia | C. gattii | 91 (6.6) | ND | 6 | 0.5×102–1×102 | |||
| 2008–2009 (12) | Ficus spp. | C. neoformansC. gattii | 467 (0.8) | 467 (3) | ND | ND | 70 | 0.5×102–1.1×103 |
| Bird droppings | C. neoformans | 37 (0) | ND | ND | 37 (10.8) | 21 | ||
| Total | 867 | 653 | 274 | 37 | 161 | |||
ND: no data available.
Yeast concentration in each positive sample was estimated from the number of colonies observed in the Guizotia plates and expressed as colony forming units (CFU) per gram of sample.
For the molecular type determination, high molecular weight DNA was extracted as previously described.11 DNA concentration was determined through spectrophotometry at 260/280nm.
The following reference strains of the eight C. neoformans and C. gattii major molecular types were used: WM 148 (serotype A, VNI), WM 626 (serotype A, VNII), WM 628 (serotype AD, VNIII), WM 629 (serotype D, VNIV), WM 179 (serotype B, VGI), WM 178 (serotype B, VGII), WM 175 (serotype B, VGIII) and WM 779 (serotype C, VGIV).8,18
The molecular type of each isolate was determined by PCR fingerprinting using the microsatellite specific sequence (GTG)5 as single primer, according to a slightly modified method previously described.8,18 Amplification products were visualized in 1.4% agarose gels in 1X TBE buffer stained with 0.3mg/ml ethidium bromide at 80V for 3h. The bands were visualized under UV light. A 1kb molecular size marker (Promega) was loaded and the molecular types (VNI–VNIV and VGI–VGIV) were assigned by comparison to the reference strains of the eight major molecular types loaded in each gel.
ResultsAn overall positivity of 3.9% was obtained from the 964 samples processed; positive samples had been stored as decaying wood (52.6%), soil filtrates (36.9%), or bird droppings (10.5%). From them, 161 isolates were recovered and identified as C. neoformans (49.7%) and C. gattii (50.3%), based on the ability of C. gattii to assimilate canavanine in CGB medium, unlike C. neoformans.15,16C. gattii isolates were recovered from samples of Eucalyptus spp., C. ficifolia, Terminalia catappa (tropical almond tree) and Ficus spp. decaying wood. Sixteen (19.7%) isolates, recovered mostly from Ficus spp., were typed as VGI; 43 (53.1%) isolates recovered from eucalyptus trees, as VGII, and 22 (27.2%) recovered from C. ficifolia, tropical almond trees and Ficus spp., as VGIII isolates.
C. neoformans isolates were recovered from Ficus spp., Eucalyptus spp. and bird droppings. Molecular type VNI was observed in 78 (97.5%) isolates recovered from all the above-mentioned sources; the remaining 2 isolates recovered from Ficus spp. were VNII. In 2 samples recovered from Ficus spp. both C. neoformans and C. gattii were found. CFU were estimated between 0.5×102 and 11.4×104CFU/g of sample. Table 1 summarizes the results obtained.
DiscussionAs C. neoformans and C. gattii have been successfully isolated from avian habitats and from a significant variety of tree species,1,3,5–7,9,12,13,17,21,23 and given that during their saprobe growth stage blastoconidia may become airborne particles that are inhaled by humans or animals, which may result in pulmonary infection, it is very important to gain insight regarding their viability in the environment.
In this brief observation, it is clear that both C. neoformans and C. gattii are able to survive in environmental samples, that had been stored at room temperature, either as soil/plant material or as filtrates from the samples. Several studies have demonstrated the ability of cells to produce melanin under natural conditions, which protects them from adverse environmental conditions such as degradative enzymes, high temperatures and ultraviolet light, as well as from phagocytic environmental predators such as amoebae.19,20,22 As stated before 1999, C. gattii seemed to be restricted to eucalyptus trees in the tropics and subtropics, while C. neoformans was mostly associated to pigeon droppings with a rather worldwide distribution.5,6,19 However, when re-processing samples previously collected, it is possible to establish that both species can be isolated from the same sample and tree species; this shows the importance of isolating and characterizing as many colonies as possible from those suspected to be C. neoformans or C. gattii in the same sample.
Specific environmental conditions are necessary for these species to exist in a particular environment, obtain nutrients, elude predators and reproduce; therefore, their survival and abundance in a niche depend on a series of factors that are essential for their viability. In this study we found that C. neoformans and C. gattii can survive in stored samples for as long as 9–10 years. Further studies are needed to determine how virulence factors express in these isolates and compare their expression levels with those of the strains initially recovered.
Although conventional techniques for the recovery of this fungi from environmental samples are adequate, the standardization of a molecular test will be very useful for their detection in samples, especially in those with very low yeast concentration. A first attempt was made by our group in 2004, in which a technique for the molecular detection of the yeast in environmental samples was standardized; Cryptococcus spp. DNA was extracted from environmental samples and amplified with a specific PCR reaction.2 Our purpose, therefore, is to standardize a PCR using these naturally colonized samples for the detection of C. neoformans and C. gattii directly from the environment.
Conflict of interestsThe authors declare that no competing interests existed.
The authors are grateful to Marta Renza for her careful review of the English and to the reviewers and editor for their comments, which were constructive to improve the manuscript.
