Unique features of L. elongisporus as compared to other species within the CTG clade

Candida species that translate the CUG codons as serine rather than leucine belong to the commonly called CTG clade [12]. This clade is notable because most of its members can cause diseases in humans. However, species within the CTG clade can vary significantly in several features, including their relative prevalence in humans, stress tolerance, and virulence properties [13]. For example, C. albicans, C. tropicalis, and C. parapsilosis are common agents of fungemia in humans while L. elongisporus and several others are infrequent pathogens. Second, compared to C. albicans and C. tropicalis, L. elongisporus is generally more sensitive to stressors such as salinity, H₂O₂, and pH fluctuation. Third, when phagocytosed, C. albicans and Candida dubliniensis often exhibit filamentous forms, while L. elongisporus remains in the yeast form. Fourth, both L. elongisporus and its close relative C. parapsilosis produce less biofilm than C. albicans, C. tropicalis, and C. dubliniensis on polystyrene surface [13].

Multiple species in the CTG clade, including C. albicans, Candida guilliermondii, Candida famata, and Candida lusitaniae, are capable of mating and/or undergoing sexual reproduction. Although L. elongisporus was previously suggested to be homothallic [8], its sexual reproductive structure, including multiple ascospores within each ascus, was not described until recently. Similarly, early comparative genomic analysis revealed that all 4 mating-type genes (MTLa1p, MTLa2p, MTLα1p, and MTLα2p) found in most other species in the CTG clade were apparently missing in L. elongisporus [11]. However, recent research on strains from India identified that all 19 Indian L. elongisporus isolates had sequences that partially match to the MTLa1p of C. parapsilosis and MTLα2p of C. albicans, while no match was found in L. elongisporus genomes to either MTLa2p or MTLα1p [6]. Surprisingly, despite not containing the complete mating loci, all bloodstream isolates and several environmental isolates from India developed asci containing multiple ascospores on acetate ascospore agar, suggesting L. elongisporus is capable of sexual reproduction [6]. In addition, loss of heterozygosity and signatures of recombination (including phylogenetic incompatibility and linkage equilibrium) were observed in the Indian population of this diploid yeast, consistent with secondary homothallism and/or frequent mitotic recombination for this species in the environment and in clinics [6].

Risk factors for L. elongisporus infections and treatment

To date, 39 cases with L. elongisporus infections have been reported (Table 1), among which 17 patients were males, 10 females, and 12 not specified. Among the 39 cases, 34 were systemic, including 33 bloodstream infections (of which 3 also had endocarditis) [14–16], and 1 meningitis [17]. Of the remaining five, 2 were oropharyngeal candidiasis and 3 were vaginitis [18,19]. Among the diseased patients, 9 were 50 years or older, 7 were between 10 and 49 years, 14 were neonates, and 9 unspecified, suggesting both elderly individuals and neonates are at risk of developing L. elongisporus infections. In addition, most patients with invasive infections had underlying conditions. For example, 4 of 7 patients over 60 years old had diabetes, and of the 14 neonates, 10 were born with low birth weight, 8 were premature, and 6 had thrombocytopenia (Table 1). Prior to fungal infections, 10 patients had catheters and 14 received antibiotic therapy.

Over the years, there has been a rising frequency of invasive candidiasis and along with it, an increasing mortality. Estimated crude in-hospital mortality related to invasive candidiasis is around 25% but the mortality rate differs among patient populations [20]. Among the reported invasive infections by L. elongisporus, 8 of 34 patients died, a rate (23.5%) similar to the overall mortality reported for invasive candidiasis. However, since most patients that developed invasive L. elongisporus infections also suffered from other comorbidities, the relative contribution of invasive L. elongisporus infection to the death of individual patients is often not clear.

So far, clinical isolates of L. elongisporus have shown an overall low level of minimal inhibitor concentrations (MICs) to most antifungal drugs. The reported MIC (μg/mL) ranges for clinical L. elongisporus isolates are: flucytosine: 0.06–1; amphotericin B: 0.012–0.5; fluconazole: 0.12–1; itraconazole: 0.008–0.25; voriconazole: 0.0017–0.12; posaconazole: 0.003–0.5; micafungin: 0.003–1; caspofungin: 0.008–0.5; and anidulafungin: 0.015–0.5. These MIC values are lower than most of those reported for C. albicans, C. tropicalis, and C. parapsilosis [21]. For example, despite the close evolutionary relationship between L. elongisporus and C. parapsilosis, the MICs of L. elongisporus to echinocandins are closer to C. albicans than to C. parapsilosis. This was largely due to the unique amino acid sequence of beta-1,3 glucan synthase, the target of echinocandins, in C. parapsilosis [22]. Indeed, echinocandins are highly effective for treating L. elongisporus infections but not for C. parapsilosis infections. Thus, it’s important to distinguish between L. elongisporus and C. parapsilosis isolates (as well as other pathogenic yeasts) in clinical microbiology labs and failure to do so can have significant treatment implications. However, 2- to 4-fold elevated fluconazole MICs have been observed in several environmental isolates [6], indicating that L. elongisporus can evolve resistance to antifungal drugs. Together, these results suggest continued monitoring of antifungal susceptibility patterns is required in order to develop appropriate treatment strategies.

Potential transmission routes of L. elongisporus

Lodderomyces elongisporus can survive in the hospital environment. Reports have shown patients developing L. elongisporus infections while receiving treatments for other comorbidities during their stay in hospitals [6,23,24], suggesting these infections were likely hospital-acquired. The ability to form ascospores and biofilms likely contributes to L. elongisporus survival in the clinical inanimate environment. Compared to vegetative cells, ascospores are usually more resistant to environmental pH fluctuation and extreme temperatures [25]. While phenotypic analysis based on a single strain revealed a limited ability of biofilm formation in L. elongisporus, the fact that strains were isolated from the catheter tip of patients suggests that biofilm formation ability is present in at least some clinical strains in vivo [9,23,24,26]. Moreover, elevated MICs against sodium hypochlorite, a common disinfecting agent, have been observed in multiple clinical-related isolates [6]. Together, these observations suggest that L. elongisporus possesses diverse features to enable its persistence in clinical settings.

Indeed, transmission of this species has been observed within hospital environments. For example, 10 neonates in the span of 6 months developed fungemia caused by L. elongisporus. These blood culture isolates were genetically very similar to each other [6]. Further investigation of the hospital environment revealed that the railing and the temperature panel of the open care warmer used by the infected neonates were also colonized by L. elongisporus. Genomic analysis confirmed the close relatedness of these hospital environmental strains and neonate strains. Interestingly, this study also reported that several L. elongisporus isolates from fruit surfaces were genetically very similar to the clinical isolates. Taken together, the results suggested that this yeast can be transmitted within and outside hospitals.

Aside from humans, L. elongisporus can also infect wild and domesticated animals. One report described a dog being quilled by a porcupine and subsequently developed pericarditis and endocarditis due to L. elongisporus infection [27]. Another study reported a porcupine with alopecia and dermatitis caused by L. elongisporus [28]. These studies suggest that porcupines could be a potential reservoir and/or carrier of L. elongisporus. The significance of animal and other sources of L. elongisporus in relation to human infections remains to be determined.

In-depth investigations of L. elongisporus are needed

Our literature search revealed that globally, 18 case studies have reported L. elongisporus infections. In addition, several retrospective analyses revealed that many clinical “Candida parapsilosis” isolates actually belonged to L. elongisporus. For example, 1 retrospective survey conducted in 2008 revealed that 10 of the 542 “Candida parapsilosis” isolates from blood samples from Mexico, Malaysia, and China were L. elongisporus [7]. In 2014, a study analyzing 389 “Candida” isolates from 244 patients in multiple intensive care units in China identified 2 L. elongisporus strains that caused invasive candidiasis [29]. In 2015, a Romanian multicenter study discovered 1 strain of this fungus from 551 clinical yeast isolates [30].

Candida parapsilosis is the second or third most frequent cause of candidemia in many geographic regions. Due to its high similarity to C. parapsilosis, many previously identified C. parapsilosis infections were likely caused by L. elongisporus. Thus, the prevalence of L. elongisporus infections is likely much higher than currently reported [31]. Accurate identification techniques, e.g., ITS sequencing and MALDI-TOF MS, are recommended to reveal the true incidence of L. elongisporus and other rare fungal pathogens. Broad ecological sampling and genotyping is needed to understand its environmental reservoirs, relationships among ecological samples, and potential threats ecological niche populations to humans and other animals [32]. Furthermore, its ability to adapt to growing at high concentrations of chemical disinfectants [6] calls for greater efforts to understand its genetic mechanisms of stress tolerance and how best to eliminate L. elongisporus from hospital environments.

Acknowledgments

We thank Heather Yoell for proofreading and Dr. Anu Chowdhary and her team for introducing us to this emerging fungal pathogen.