Genotyping of Candida parapsilosis from three neonatal intensive care units (NICUs) using a panel of five multilocus microsatellite markers: broad genetic diversity and a cluster of related strains in one NICU.

Candida parapsilosis (CP) (n = 40) isolated from an unselected patient population in the neonatal intensive care units (NICUs) of three US hospitals were collected over periods of 3.5-9 years. Two previously published microsatellite markers and three additional trinucleotide markers were used to produce multiplex genotypes, which revealed broad strain diversity among the NICU isolates with a combined index of discrimination (D) = 0.997. A cluster of eight related CP strains from four infants in a single NICU was observed. An extended collection of 24 CP isolates from the general population of that hospital showed that the cluster of NICU isolates was related to three isolates from general hospital patients. This microsatellite marker set is suitable to investigate clusters of colonizing and infecting strains of CP.

Bloodstream and non-invasive isolates of Candida glabrata have similar population structures and fluconazole susceptibilities.

We have compared multilocus sequence typing (MLST) and fluconazole susceptibility profiles of Candida glabrata bloodstream isolates obtained during active, population-based surveillance to those obtained from non-sterile sites of individuals with no evidence of fungal disease (i.e., non-invasive isolates) in the same US city during an overlapping time period. In each of the two populations, different proportions of the same six major sequence types (STs) encompassed 82% of the isolates. One ST was more prevalent in the candidemia population and two other STs were more prevalent in the non-invasive population, but the overall allelic frequencies within the groups suggested little, if any, genotypic diversity between them. Fluconazole susceptibility profiles of isolates from the patients in the two groups were not significantly different and were not associated with a particular sequence type. Our results support the hypothesis that C. glabrata strains causing bloodstream infections are genetically indistinguishable from those normally residing in/on the host, suggesting that relative pathogenicity may be closely tied to commensalism.

Candidemia surveillance in Brazil: evidence for a geographical boundary defining an area exhibiting an abatement of infections by Candida albicans group 2 strains.

Prospective population surveillance has been conducted for candidemia in Brazil (A. L. Colombo, M. Nucci, B. J. Park, et al., J. Clin. Microbiol. 44:2816-2823, 2006). In the present study, a total of 63 isolates from 61 patients, representing 11 medical centers from nine geographic regions, were characterized by multilocus sequence typing (MLST). A total of 48 unique profiles or diploid sequence types (DSTs) were observed, with nine new sequence types (STs) and 32 new DSTs. There were no apparent correlations between center/region and DST patterns. Subtypes were compared to those in a known characterized reference set, including a large database of strains obtained worldwide. Significantly, only one C. albicans group 2 isolate was found in our collection, although isolates from this particular group are commonly found worldwide. These data, combined with information from other previously reported studies, establish a statistically significant diminishment of group 2 strains in Central and South America, including Mexico and portions of the Southwestern United States.

Multicenter collaborative study for standardization of Candida albicans genotyping using a polymorphic microsatellite marker.

Microsatellite-based genotyping for Candida albicans can give discrepant results between laboratories when expressed in fragment sizes, because their determination depends on electrophoretic conditions. The interlaboratory reproducibility was assessed in six laboratories provided with an allelic ladder. Despite variations in size determinations, alleles were correctly assigned, making data transportable between laboratories.

Multilocus sequence type analysis reveals both clonality and recombination in populations of Candida glabrata bloodstream isolates from U.S. surveillance studies.

The human commensal yeast Candida glabrata is becoming increasingly important as an agent of nosocomial bloodstream infection. However, relatively little is known concerning the genetics and population structure of this species. We have analyzed 230 incident bloodstream isolates from previous and current population-based surveillance studies by using multilocus sequence typing (MLST). Our results show that in the U.S. cities of Atlanta, GA; Baltimore, MD; and San Francisco, CA during three time periods spanning 1992 to 2009, five populations of C. glabrata bloodstream isolates are defined by a relatively small number of sequence types. There is little genetic differentiation in the different C. glabrata populations. We also show that there has been a significant temporal shift in the prevalence of one major subtype in Atlanta. Our results support the concept that both recombination and clonality play a role in the population structure of this species.

Multilocus sequence typing of sequential Candida albicans isolates from patients with persistent or recurrent fungemia.

Multilocus sequence typing (MLST) is a useful tool to explore the phylogenetics and epidemiology of Candida albicans isolates recovered from cases of invasive candidiasis. The goal of this study was to determine whether the same or different strains were responsible for persistent or recurrent fungemia through the use of MLST and ABC typing on sequential C. albicans isolates from the same patient. We applied both typing methods to 21 C. albicans strains recovered from 8 patients with persistent or recurrent candidemia. The isolates were collected during a multicenter surveillance study in four public tertiary care hospitals in Brazil. Persistent candidemia was defined as two or more blood cultures positive for C. albicans on 2 or more separate days. Recurrent candidemia was defined as an episode of candidemia occurring at least 1 month after the apparent complete resolution of an infectious episode caused by Candida species. We observed that, except for one patient, all strains from the first and second samples of the same patient showed the same MLST diploid sequence type (DST), ABC type and susceptibility profile to antifungals. Three distinct strains, well discriminated by MLST, were found in the seven samples collected sequentially over 10 days from one patient. The strains from the first four samples were indistinguishable, the fifth and sixth were also indistinguishable but different from the first four and seventh samples. Significantly, the seventh strain was the only C. albicans clade 2 isolate found in our total collection involving 61 patients, although clade 2 is commonly found worldwide. To the best of our knowledge, this is the first study describing the recovery of three distinct C. albicans strains in the same patient with a persistent blood stream infection within a short period of time.

Development of a MLST-biased SNP microarray for Candida albicans.

We have developed a single nucleotide polymorphism (SNP) detection microarray for the human pathogenic yeast, Candida albicans, consisting of synthetic oligonucleotides bound to microscope slides. The array consists of multiple replicates of 79 SNPs, derived from 19 discrete loci located on all eight chromosomes. These loci include seven genes consisting of 57 SNPs that comprise a multi-locus sequence typing (MLST) consensus scheme for the species. The remaining 22 SNPs are from 12 additional loci located at intervals on the remaining chromosomes. In order to include highly informative polymorphisms from the MLST set on the array we performed a linkage analysis of major genotypes between the two pairs of MLST-linked genes. In addition, we performed a matched-set analysis for each SNP located within individual MLST loci. This analysis resulted in the reduction of informatively redundant mutations in the array for a large percentage of strains. We believe that a SNP array will be helpful in extending our knowledge of the epidemiology and genetics of C. albicans as a supplement to MLST typing.

Evidence for a pseudo-outbreak of Candida guilliermondii fungemia in a university hospital in Brazil.

Fungal infections due to Candida species represent an important cause of nosocomial bloodstream infections. We report a large pseudo-outbreak of Candida guilliermondii fungemia that occurred in a university hospital in Brazil. C. guilliermondii was identified in 64 (43%) of the 149 blood samples drawn between June 2003 and July 2004. The samples were from patients in different wards of the hospital but concentrated in pediatric units. None of the patients had clinical signs of fungemia, and observational analysis revealed errors in the collection of blood samples. During the investigation of the pseudo-outbreak, C. guilliermondii was isolated from environmental surfaces and from the skin and nails of members of the nursing team. Through a subtyping analysis it was found that some of the nonpatient isolates were highly related to the patient isolates, and all the patient isolates were highly related. This is consistent with the hypothesis that the pseudo-outbreak was from a limited number of common sources. The adoption of intervention measures was effective in resolving the outbreak, supporting the hypothesis that the outbreak was due to poor techniques of drawing blood samples for culture.

Molecular genotyping of Candida parapsilosis group I clinical isolates by analysis of polymorphic microsatellite markers.

Candida parapsilosis, a pathogenic yeast, is composed of three newly designated genomic species that are physiologically and morphologically indistinguishable. Nosocomial infections caused by group I C. parapsilosis are often associated with the breakdown of infection control practices and the contamination of medical devices, solutions, and indwelling catheters. Due to the low levels of nucleotide sequence variation that are observed, an investigation of the size polymorphisms in loci harboring microsatellite repeat sequences was applied for the typing of C. parapsilosis group I isolates. PCR primer sets that flank the microsatellite repeats for seven loci were designed. Following amplification by PCR, the size of each amplification product was determined automatically by capillary electrophoresis. A total of 42 C. parapsilosis group I isolates were typed by microsatellite analysis, and their profiles were compared to the hybridization profiles obtained by use of the Cp3-13 DNA probe. A high degree of discrimination (discriminatory power = 0.971) was observed by microsatellite analysis. The number of different alleles per locus ranged from 14 for locus B to 5 for locus C. Microsatellite analysis detected 30 different microsatellite genotypes, with 24 genotypes represented by a single isolate. Comparison of the genotypes obtained by microsatellite analysis and those obtained by analysis of the Cp3-13 hybridization profiles showed that they were similar, and these methods were able to identify related and unrelated isolates. Some discrepancies were observed between the methods and may be due to higher mutation rates and/or homoplasy by microsatellite markers. Identical results were observed between microsatellite analysis and Cp3-13 DNA hybridization profile analysis for C. parapsilosis isolates obtained from two patients, demonstrating the reproducibilities of the methods in vivo. Identical microsatellite profiles were observed for isolates displaying different phenotypic switching morphologies. Indistinguishable Cp3-13 DNA hybridization profiles were observed for six epidemiologically related isolates; however, only three of six primary isolates had identical microsatellite profiles. Size variation at a single locus was observed for three of six isolates obtained either after the outbreak period or from a different body site, suggesting the potential of the method to detect microevolutionary events. Interestingly, for most loci a single allele per strain was observed; in contrast, two alleles per locus were observed for some strains, and consistent with the findings for natural isolates, some isolates may be aneuploid. Due to the potential for high throughput, reproducibility, and discrimination, microsatellite analysis may provide a robust and efficient method for the genotyping of large numbers of C. parapsilosis group I isolates.

Assessment of ribosomal large-subunit D1-D2, internal transcribed spacer 1, and internal transcribed spacer 2 regions as targets for molecular identification of medically important Aspergillus species.

Molecular approaches are now being developed to provide a more rapid and objective identification of fungi compared to traditional phenotypic methods. Ribosomal targets, especially the large-subunit RNA gene (D1-D2 region) and internal transcribed spacers 1 and 2 (ITS1 and ITS2 regions), have shown particular promise for the molecular identification of some fungi. We therefore conducted an assessment of these regions for the identification of 13 medically important Aspergillus species: Aspergillus candidus, Aspergillus (Eurotium) chevalieri, Aspergillus (Fennellia) flavipes, Aspergillus flavus, Aspergillus fumigatus, Aspergillus granulosus, Aspergillus (Emericella) nidulans, Aspergillus niger, Aspergillus restrictus, Aspergillus sydowii, Aspergillus terreus, Aspergillus ustus, and Aspergillus versicolor. The length of ribosomal regions could not be reliably used to differentiate among all Aspergillus species examined. DNA alignment and pairwise nucleotide comparisons demonstrated 91.9 to 99.6% interspecies sequence identities in the D1-D2 region, 57.4 to 98.1% in the ITS1 region, and 75.6 to 98.3% in the ITS2 region. Comparative analysis using GenBank reference data showed that 10 of the 13 species examined exhibited a < or = 1-nucleotide divergence in the D1-D2 region from closely related but different species. In contrast, only 5 of the species examined exhibited a < or = 1-nucleotide divergence from sibling species in their ITS1 or ITS2 sequences. Although the GenBank database currently lacks ITS sequence entries for some species, and major improvement in the quality and accuracy of GenBank entries is needed, current identification of medically important Aspergillus species using GenBank reference data seems more reliable using ITS query sequences than D1-D2 sequences, especially for the identification of closely related species.

The human commensal yeast, Candida albicans, has an ancient origin.

Candida albicans, the primary causative agent of candidiasis, is a ubiquitous member of the human flora and is capable of causing severe invasive disease. Despite its importance as a human pathogen, little is known concerning those factors creating and maintaining genetic diversity within the species and how extant strains reflect their evolutionary history. Based on nucleotide polymorphism frequencies, we estimated the time to a most recent common ancestor for the species to be about 3-16 million years, with variation due to molecular clock calibration. As C. albicans genotypes have broad geographic associations, this suggests that the origins of DNA sequence variation in extant populations coincided with early hominid evolution. This is consistent with an emerging view of a genetically complex organism that is able to survive under host immunity as an obligate commensal species.

Evidence for aneuploidy and recombination in the human commensal yeast Candida parapsilosis.

Isolates of Candida parapsilosis, including representatives of the three major sub-species groups, were screened for single nucleotide polymorphisms (SNPs) by sequencing five independent loci totaling 4kb per isolate. Group I isolates were highly conserved and in some cases, group I alleles were found in group II and III strains. Unique alleles were also associated with groups II and III, consistent with earlier observations of intergroup divergence. There was no heterozygosity in any strain, and a FACS analysis demonstrated that for all three groups nuclei are variant in size, ranging from 0.5 to 1.0 x the size of other diploid yeast genomes. This suggests that natural isolates of C. parapsilosis are aneuploid, with some isolates being essentially haploid. Taken collectively with the observation of group I alleles within group II and III strains, we propose that some form of recombination is occurring between groups.

Stability of allelic frequencies and distributions of Candida albicans microsatellite loci from U.S. population-based surveillance isolates.

Allelic distributions and frequencies of five Candida albicans microsatellite loci have been determined for strains isolated from the bloodstream and obtained through active population-based surveillance in two U.S. metropolitan areas between 1998 and 2000. These data were compared to data for isolates obtained from two other U.S. regions in 1992 to 1993. In a majority of pairwise combinations between sites, no evidence was seen for shifts in microsatellite allelic frequencies. One to three alleles were highly predominant and correlated with major genotypes. These data both support the concepts of allelic stability and genetic equilibria and suggest that, in the United States, strains of C. albicans isolated from the bloodstream may form a defined, genetically homogeneous population across geographical distance and time.

Population structure of Candida albicans, a member of the human flora, as determined by microsatellite loci.

This study examines the macrogeographic population structure of Candida albicans, a yeast commensal of humans, through a population genetic analysis of 5 microsatellite loci in 13 cities. The populations were predominantly clonal with some recombination. About 5% of the genetic variation is between populations and the overall pattern is one of intermediate differentiation. We did not find a single widespread genotype but instead found high, macrogeographic gene flow in these clinical populations; the most common genotype was limited to Atlanta and San Francisco. Homogeneity is evident within large geographic regions, such as Europe, Asia, and the USA, and isolation by distance accounted for 39% of the variation observed. Overall gene flow for a member of the human flora is variable but can be extensive, with an average of 4.5 migrants per generation (N(m)). Eastern hemisphere populations were less divergent than those of the Americas and Caribbean, consistent with the expansion of humans out of the eastern hemisphere.

Evidence for a more recently evolved clade within a Candida albicans North American population.

Candida albicans is diploid and displays a primarily clonal mode of reproduction. There is, however, evidence for meiosis and the degree to which this occurs in nature is unknown. Although random mating would act to obscure clonal lineages, previous studies have demonstrated that collections of North American isolates display three major partitions with no evidence of geographic clustering. To better understand the extent of sexuality and its role in the phylogeny of the species, a reference subset of 50 isolates representing this tripartite division was analysed using 1 minisatellite, 5 microsatellites (MSs) and 15 nuclear polymorphisms (NP). A total of 87 alleles were observed for 21 loci and 12/16 informative loci exhibited a departure from Hardy-Weinberg expectations (G(2)

Towards understanding the evolution of the human commensal yeast Candida albicans.

Allelic frequencies and relationships for one dimorphic locus and three unlinked polymorphic loci have been determined for 114 unrelated isolates of Candida albicans, including 14 laboratory reference strains and 50 strains from each of two geographic regions. Although there was no indication of geographical partitioning, there were significant correlations for specific allelic pairs among loci and little evidence that any alleles were in Hardy-Weinberg equilibrium. This gives additional support for the concept that the primary mode of genetic inheritance in this species is clonal, with other intracellular genetic events playing a lesser role in the creation of genomic diversity. Through inference of this and other known attributes of closely related Candida species, such as sequence analysis of IS1 and the ITS2 (internal transcribed spacer 2) region of the rDNA cistron, the deduced phylogeny suggests an evolutionarily recent origin for many frequently isolated strains. This finding will be of interest in the context of understanding pathogenicity and drug resistance in this human commensal yeast.