Purpureocillium lilacinum: A minireview

Abstract

Purpureocillium lilacinum (formerly Paecilomyces lilacinus) is a hyaline hyphomycete with a ubiquitous distribution. In the last decade this fungus has been increasingly found as the causal agent of infections in humans and other vertebrates. It is an emerging opportunistic pathogen and is increasingly reported, and can cause a wide spectrum of clinical manifestations, especially among immunocompromised patients or following surgical procedures. The pathogenic mechanisms are not yet fully understood. Ocular and cutaneous/subcutaneous infections are the most familiar clinical presentations, and these can also cause disseminated infections. Early and accurate species identification and susceptibility testing are vital. In general, surgical debridement combined with antifungal drug therapy, or the correction of predisposing factors, are usually required to obtain improvement. Infections present a therapeutic challenge, as they have intrinsic resistance to many antifungal agents, but voriconazole and posaconazole are good in vitro activity. The overall mortality was 22% and death was attributed to the infection in 46% of cases. Accurate diagnoses can be achieved through newer molecular biological techniques, and these can lead to appropriate management of infections due to this organism. Future studies should ideally aim to elucidate pathogenesis and determine more effective diagnoses and effective antifungal treatment.

Key Words: Purpureocillium lilacinum; Hyalohyphomycosis; Immunocompromised status; Identification; Treatment; Minireview

Core Tip: Purpureocillium lilacinum (formerly Paecilomyces lilacinus) is a hyaline hyphomycete with a ubiquitous distribution. In the last decade this fungus has been increasingly found as the causal agent of an emerging opportunistic pathogen, which can cause a wide spectrum of clinical manifestations, especially among immuno-compromised patients or following surgical procedures. The pathogenic mechanisms are not yet fully understood. Early and accurate species identification and susceptibility testing are vital. In general, surgical debridement combined with antifungal drug therapy, or the correction of predisposing factors, are usually required to obtain improvement.

INTRODUCTION

Species of Purpureocillium are some of the most commonly encountered saprophytes; a recent proposal suggests accommodating into the genus Purpureocillium lilacinum (P. lilacinum) (formerly Paecilomyces lilacinus), a highly studied species with biotechnological properties and the capability to cause infections in humans[1,2]. P. lilacinum is a hyaline hyphomycete known to have a ubiquitous distribution[2,3]. Its first description came over 100 years ago, and it is found throughout various natural environments, including soil, air, and decaying organic matter such as vegetables as a saprobe; it serves a crucial role as an agricultural biocontrol fungus, as it offers effective biological control of nematodes that are parasites of plants, and has significant effects on Aphidoidea, Tetranychus cinnbarinus, and Aleyrodidae, as well[4-8]. Additionally, the secondary metabolites produced by P. lilacinum have various biological properties, such as anticancer, antimicrobial, and insecticidal effects[4].

The term “hyalohyphomycosis” refers to a rare type of infection due to one of a number of fungi, such as P. lilacinum, that have unpigmented cell walls[9]. Within the past ten years, this fungus has increasingly been identified as the causal agent of human and other vertebrate infections; this emerging opportunistic human pathogen has been increasingly reported, and it can cause a wide variety of clinical manifestations, especially among immunocompromised patients, or following surgical procedures[10-14]. More specifically, insidious fungal infections have been encountered as recurrent problems in patients who are undergoing chronic steroid therapy for organ transplants, patients who have had foreign-body-related or implant-device-related surgery, patients with hematologic malignancies, diabetic patients, patients undergoing dialysis, and other vulnerable populations[15]. An analysis of invasive P. lilacinum infection cases found that the primary predisposing factors were hematological and oncological diseases in 31% of cases, steroid treatment in 27% of cases, solid organ transplants in 26% of cases, and diabetes mellitus in 19% of cases[16]. Additionally, infections have been reported in seemingly immunocompetent patients[17].

We recently encountered a case of ventilator-associated pneumonia due to P. lilacinum in an immunocompromised patient; it was our first time encountering this pathogen. The pathogenesis of this emerging pathogen has yet to be fully understood, which in turn motivated us to undertake this study. The aim of this minireview is to create awareness of P. lilacinum as an emerging pathogen, and to share general information, pathogenesis, symptoms, examinations, diagnosis, treatment, and prognosis. We reviewed the medical records at our hospital, going back to its establishment. We also performed a literature review with the keywords “Paecilomyces lilacinus” and “Purpureocillium lilacinum” to identify relevant studies or case reports, based on PubMed databases.

PATHOGENESIS

P. lilacinum’s pathogenic mechanisms for infection remain poorly understood[18]. The conidia can infect and destroy both macrophages and dendritic cells; consequently, this pathogenic fungus is clearly able to invade the phagocytic cells of humans[18]. Many studies have reasserted the host’s immune response’s crucial role in a number of infections, and it come to light that lymphocytes offer a major form of defense against fungal infections[15]. Although no data on immunity against P. lilacinum infections is currently available, a handful of studies on closer species have shown what essentially seems to be host cell suppression, impairment, and poor stimulatory capacity[15]. On the other hand, a “breakthrough infection” refers to a Purpureocillium infection that occurred under exposure to a systemic antifungal agent[16]. An analysis of 101 cases of invasive P. lilacinum showed breakthrough infections in 10% of these[16].

SYMPTOMS

The entry portal of the fungus usually involves breakdowns of the skin barrier, indwelling catheters, or inhalation. However, there have been some eye structure or skin infections caused by fluid contamination of P. lilacinum[17]. Therefore, the most common clinical presentations are infections of the eyes and of the skin (cutaneous and subcutaneous); the fungus can also lead to such disseminated infections as fungemia, endocarditis, peritonitis, osteomyelitis, and deep-seated or systemic infections[3,5,16,19,20]. According to reports, most cases were oculomycosis (51.3%), with the next most common type being cutaneous and sub-cutaneous infections (35.3%), followed by a smaller group of miscellaneous infections (13.4%)[17]. There are also reports, though less commonly, of other types of infections[17]. The most commonly encountered predisposing factor for oculomycosis is lens implantation[17]. Cutaneous and sub-cutaneous infections have been found primarily in recipients of transplants of solid organs and bone marrow, though other noteworthy predisposing factors include primary or acquired immunodeficiency, and surgery[17]. Additionally, upper respiratory tract infections have been found to be implicated in invasive rhinitis and sinusitis etiology[3]. Pulmonary infections are rare; they have a clinical presentation of fever accompanied by cough, and a radiographic presentation of pleural effusion, single-lung consolidation, and cavitary pulmonary disease[3,16,20]. Another analysis of invasive infection cases found that the most common sites of infection were the skin (37%) and the lungs (26%), with dissemination occurring in 22% of cases[16]. In cases such as these, misidentification of fungal cellulitis as bacterial cellulitis can lead to greater morbidity and mortality[19]. The most frequently recorded symptoms were pain and fever, at 40% and 34%, respectively[16]. The clinical presentation of pulmonary infection is reportedly coughing and fever[20].

EXAMINATIONS AND DIAGNOSIS

While P. lilacinum can potentially be a laboratory contaminant, often it is the etiological agent[15]. The most critical diagnostic measure is clinical isolation, through direct specimens sampled from the site(s) affected[16]. As a result, diagnosis has historically primarily been confirmed through culturing and histological examinations[16]. Finally, even if medical staff are able to obtain clinical samples, there may still be a need for advanced testing modalities, and interpretation of the results must be handled with caution, and with consideration given to clinical context[19]. Consequently, diagnosis and management rely on properly recognizing clinical symptoms, and on trained personnel correctly identifying clinical isolates at the species level[15]. These could be factors that could lead to the classification of P. lilacinum as a potential laboratory contaminant. On the other hand, according to the Consensus Guidelines for the Diagnosis and Management of Invasive Fungal Disease due to Moulds Other than Aspergillus in Haematology/Oncology Setting, 2021, serological testing is marginally recommended for use, but only in the event of suspected fusariosis (marginal recommendation, level II evidence)[5]. Therefore, serological testing biomarkers are not recommended.

Early, accurate species identification and susceptibility testing are vital, due to intrinsic resistance[5,7,16,21]. Whether microbiological procedures find success depends on using correct practices to collect samples, as well as on lab methodology[7]. An analysis of invasive P. lilacinum infection cases found that in 97% of cases, diagnosis was established through culturing[16]. However, P. lilacinum infections present a number of challenges, both diagnostically and therapeutically, particularly because the tissue morphology is virtually identical to that of hyalohyphomycotic agents such as Aspergillus spp.[3,15,16]. On culture plates, P. lilacinum forms flat colonies that are a powdery or velvety pink, red, or purple[5]. It undergoes adventitial sporulation akin to some other non-Aspergillus molds (such as Fusarium), which could allow for presumptive diagnoses; blood cultures are strongly encouraged[5].

P. lilacinum was identified by sequencing the ribosomal deoxyribonucleic acid (DNA)’s internal transcribed spacer (ITS) regions, then using GenBank to align, and performing a Basic Local Alignment Search Tool analysis[5,10]. It was found that P. lilacinum forms a well-supported phylogenic clade, with low intraspecific variability, based on in-depth sequence analyses and morphological examinations of four loci: (1) The ribosomal ITS; (2) 28S ribosomal DNA domains D1 and D2; (3) EF-1a; and (4) The largest subunit of ribonucleic acid polymerase II[1].

Recently, matrix-assisted laser-desorption ionization time-of-flight mass spectrometry has been shown to be a fast, reliable alternative to multilocus sequencing[14]. However, reproducibly identifying this group of important human pathogens required a major augmentation of commercially-available database data[14].

Here, we show the sample from the case we encountered. An 81-year-old female with rheumatoid arthritis, under treatment with 20 mg/day predonisolone and tumor necrosis factor-alpha inhibitor, complained of exertional dyspnea and malaise. A chest computed tomography scan showed ground-glass opacity, and she was admitted to our hospital. Her pneumonia improved with treatment for Pneumocystis jirovecii pneumonia; however, on day 17 of hospitalization, her symptoms worsened. She was transferred to an intensive care unit, and treatment was started with antimicrobial agents, caspofungin, and ganciclovir, for suspected concomitant development of invasive pulmonary aspergillosis and cytomegalovirus infection. Gradual stabilization of her respiratory condition followed. However, her respiratory condition worsened on day 43. A chest X-ray showed rapidly worsening infiltrative lung shadow, and increased value of beta-D-glucan in serum along with numerous filamentous fungi in the sputum were confirmed. She died on day 47. A direct sputum smear examination on day 43 revealed numerous branching mycelia with septa. A slide sputum culture, using lactophenol cotton blue staining, showed broom-like conidiogenous cells associated with oval condia (Figure 1A). These light purple colonies with velvety surface grew on potato dextrose agar, suggesting Purpureocillium fungi (Figure 1B). The ribosomal ribonucleic acid gene sequence of the ITS region was consistent with P. lilacinum. The clinical course is shown in Figure 2.

Figure 1 Morphological findings of the direct sputum smear. A: Microscopic findings with lactophenol cotton blue staining (magnification: 400 ×), and numerous branching mycelia with septa, together with broom-like conidiogenous cells associated with oval condia, are confirmed; B: Light purple colonies with velvety surface are grown on potato dextrose agar.

Figure 2 The clinical course of our case. Ag: Antigen; β-DG: Beta-D-glucan; CMV: Cytomegalovirus; CPFG: Caspofungin; CRP: C-reactive protein; DNA: Deoxyribonucleic acid; DRPM: Doripenem; FLCZ: Fluconazole; GCV: Ganciclovir; LVFX: Levofloxacin; LZD: Linezolid; MEPM: Meropenem; PIPC/TAZ: Piperacillin/tazobactam; P. jirovecii: Pneumocystis jirovecii; PTM: Pentamidine; ST: Sulfamethoxazole and trimethoprim; WBC: White blood cell.

Following identification of this fungus, it is advisable to perform susceptibility testing as a guide to determine the appropriate therapeutic course[15]. Some articles have shown an in vitro antifungal susceptibility which was evaluated for clinical isolation using different methods. In these 17 isolates, the least in vitro activity in susceptibility testing against P. lilacinum, with any of the reported methods, were amphotericin B, fluconazole, flucytosine, and itraconazole[16]. The lowest minimum inhibitory concentration (MIC) levels were seen in posaconazole (PSCZ) and voriconazole (VRCZ)[16]. All echinocandins tested were found to have contrasting data with variable in vitro activity against P. lilacinum[16]. However, no susceptibility testing was performed for isavuconazole[16]. The details of antibiotic susceptibility test outcomes by susceptibility testing method are shown in Table 1. One proposal suggests that P. lilacinum may develop antifungal responses through raised expression levels of cytochrome P450 enzymes and efflux pumps[6]. This sort of modulation could provide high levels of target enzymes to P. lilacinum, and could lead to the constant antifungal withdrawals, which would in turn cause a need for greater antifungal medication administration for fungal morbidity or mortality[6].

Table 1 Susceptibility testing method.

Antifungal drug	CLSI microdilution (n = 6)	Concentration gradient diffusion assay (n = 5)	EUCAST microdilution (n = 2)	Macrodilution method (n = 2)	Sensititre™ Yeast One™ (n = 2)
Amphotericin B	16.0 (8.0–32.0)	32.0 (32.0–32.0)	36.0 (8.0–64.0)	32.0 (32.0–32.0)	16.0 (16.0–16.0)
Anidulafungin	0.03 (0.03–0.03)	N/A	64.0 (64.0–64.0)	N/A	16.0 (16.0–16.0)
Caspofungin	0.1 (0.03–0.1)	6.0 (2.3–20.0)	4.5 (1.0–8.0)	N/A	40.0 (16.0–64.0)
Micafungin	0.03 (0.03–16.0)	N/A	36.0 (8.0–64.0)	N/A	N/A
Fluconazole	24.0 (12.0–144.0)	N/A	256.0 (256.0–256.0)	128.0 (128.0–128.0)	128.0 (128.0–128.0)
Itraconazole	16.5 (1.0–32.0)	32.0 (8.0–32.0)	24.0 (16.0–32.0)	8.5 (1.0–16.0)	16.5 (1.0–32.0)
Posaconazole	0.1 (0.1–0.1)	0.4 (0.2–0.5)	0.6 (0.3–1.0)	N/A	0.5 (0.5–0.5)
Voriconazole	0.6 (0.3–1.0)	0.1 (0.05–0.2)	0.4 (0.3–0.5)	N/A	0.3 (0.2–0.5)
Ketoconazole	N/A	N/A	N/A	1.0 (1.0–1.0)	N/A
Miconazole	N/A	N/A	N/A	0.5 (0.5–0.5)	N/A
Flucytosine	128.0 (128.0–128.0)	N/A	128.0 (128.0–128.0)	128.0 (128.0–128.0)	32.0 (32.0–32.0)

N/A: Not applicable.

TREATMENT

Generally, improvement requires either surgical debridement alongside antifungal drug therapy, or correcting predisposing factors such as neutropenia[5,15,17]. Central line-associated fungaemia caused by P. lilacinum, for example, has been shown to be possible to resolve through catheter removal and antifungal therapy[5].

P. lilacinum infections present a therapeutic challenge, due to the fungus’s intrinsic resistance to amphotericin B, itraconazole, fluconazole, and many other antifungal agents; however, VRCZ and PSCZ have both been found to be effective in in vitro activity[13,16-18,22]. VRCZ comes moderately recommended as a first-line treatment, pending results regarding susceptibility, as it has found success in use[5,10]. However, combined therapeutic regimens will necessitate case-by-case assessments[15]. More specifically, VRCZ combined with terbinafine comes moderately recommended as a first-line treatment option, as this is also associated with successful outcomes[5]. Case reports have described successful PSCZ treatment, leading it to be at least somewhat recommended as a therapy, either first-line or salvage[5]. The new triazole ravuconazole could represent a promising therapeutic alternative, as it also shows good in vitro activity against P. lilacinum[17]. However, there was significant variance in echinocandin susceptibility[16]. In addition, very little information exists to date regarding how patients infected by P. lilacinum with a pulmonary presentation have been treated[20]. An analysis of 101 cases of invasive P. lilacinum found that 23% of cases described monotherapy followed by combination therapy was[16]. Additionally, for combination therapy, the most common approach was a combination of amphotericin B with an azole antifungal (10%), followed in descending order by amphotericin B + other (4%), azoles + echinocandins (8%), azoles + other (4%), and other + other (1%)[16].

Current cutting-edge knowledge regards ibrexafungerp, a novel glucan synthase inhibitor triterpenoid antifungal agent that is a semi-synthetic derivative of the natural product enfumafungin[5]. In animal models, it is available in both oral and intravenous formulations, and has an oral bioavailability ranging from 35% to 51%[5]. In addition, its modest activity against P. lilacinum (MIC 2-6) has been demonstrated with in vitro data[5]. At the time of this writing, a phase III trial is underway to evaluate ibrexafungerp’s efficacy in severe, refractory cases of fungal infections (emerging fungi) (NCT03059992)[5].

PROGNOSIS

There have been several reports regarding prognosis. An analysis of invasive P. lilacinum infection cases showed a significant association between amphotericin B treatment and high mortality rates (39%)[16]. In addition, the overall mortality rate was 22%, with death being attributed to P. lilacinum infection in 46% of cases[16]. In another report, it was noted that 16.6% of patients with severe graft vs host disease died of refractory invasive disease[5]. However, 58.3% of patients who recovered their bone marrow function also saw their P. lilacinum infections resolved[5]. Furthermore, very little information has been gathered regarding prognoses in cases of P. lilacinum infection with a pulmonary presentation[20]. Further epidemiological research, improved diagnostics, and newly developed antifungal agents will all help continue to improve the currently-poor outcomes generally associated with invasive fungal disease due to P. lilacinum[5].

CONCLUSION

The properties of P. lilacinum are summarized in Table 2. The infection may be present as breakthrough-invasive fungal infections, or primary with non-specific symptoms and imaging findings. Precise diagnosis is critical, and accurate diagnoses can be achieved through culture and newer molecular biological techniques. Vigilance is essential when it comes to the organism's inherent resistance mechanisms to various antifungal drugs. We underscore the necessity of providing suitable medication based on post-diagnosis antimicrobial susceptibility test outcomes, and they can lead to appropriate management, including treatment, of infections due to this organism. Future studies should ideally aim to elucidate pathogenesis, and determine more effective diagnoses and effective antifungal treatment.

Table 2 The properties of Purpureocillium lilacinum.

Category	Details
Scientific name	Purpureocillium lilacinum
Taxonomy	Kingdom: Fungi
	Phylum: Ascomycota
	Class: Sordariomycetes
	Order: Hypocreales
	Family: Ophiocordycipitaceae
Habitat	Soil, decaying organic matter, and some plants as a saprophyte
Morphology	White to pale lavender conidial head
	Purple-lilac-colored conidia
	Typically grows as a saprobe on decaying organic matter
Reproductive structures	Conidia (asexual spores) are the primary method of reproduction
Ecology	Saprotrophic; decomposes organic matter, may also act as a biocontrol agent
Biocontrol applications	Used as a natural insecticide, especially for controlling pests like root-knot nematodes and various soil-borne pests
Pathogenicity	Typically, not harmful to humans or animals
	Some strains have shown potential as biocontrol agents for nematodes
Medicinal potential	No significant known medicinal uses, but some species in this family have been explored for their bioactive compounds
Growth conditions	Temperature: Optimal growth occurs between 25-30 °C (77-86 °F)
	PH: Slightly acidic to neutral environments
Patient characteristics for invasive infections
Underlying conditions	Hematological/oncological disease-acute leukaemia, solid tumours, lymphoma, autoimmune hemolytic anemia, chronic granulomatous disease, hypogammaglobulinemia, immune thrombocytopenic purpura
	Hematopoietic stem cell transplantation-allogenic, autologous, graft-versus-host disease, solid organ transplant
	Heart
	Kidney, chronic kidney disease, dialysis (hemodialysis, peritoneal dialysis)
	Liver
	Lung, chronic lung disease
	Diabetes mellitus
	Human immunodeficiency virus
	Long-term immunosuppression
	Neutropenia
	Major surgery
	Steroid treatment
	Trauma
Indwelling devices	Bronchial stent, central venous catheter, prosthetic aortic valve
Organ involvement	Blood, bone and joints, central nervous system, deep tissue, heart, lung, peritoneum, sinuses, skin
Signs and symptoms of infection	Cough, dyspnoea, erythema, fever, gastrointestinal symptoms, nasal obstruction/sinus tenderness, neurological signs, pain, skin nodules, skin oedema/swelling, skin ulcerations, tachypnoea, weight loss
Imaging procedures	Computed tomography (head, paranasal sinuses, chest), magnetic resonance imaging (head), ultrasound (heart), X-ray (chest)
Mycological evidence	Culture, histology, microscopy, polymerase chain reaction
Treatment	Prophylactic treatment, systemic antifungal therapy, non-systemic antifungal therapy, no treatment, combinations, surgical treatment, removal of indwelling devices