Paecilomyces lilacinus, a fungus widely distributed in soil, holds a unique position in agriculture and medicine. This organism, described over a century ago, was reclassified in 2011 to Purpureocillium lilacinum following molecular phylogenetic analysis. Despite this formal change, the older name Paecilomyces lilacinus continues to be frequently used in scientific literature and commercial products. The fungus is notable for its dual existence as a biological control agent in farming and an opportunistic pathogen in humans.
Agricultural Applications
This fungus is a recognized biological control agent, specifically as a nematicide. Nematodes are microscopic soil-dwelling worms that damage crops by hindering growth and causing significant losses. Purpureocillium lilacinum offers an alternative to chemical pesticides, aligning with sustainable agricultural practices.
The fungus parasitizes the eggs and female adults of plant-parasitic nematodes, including the highly destructive root-knot nematode (Meloidogyne incognita). Its thread-like hyphae attach to and penetrate nematode eggs. The fungus then grows inside, consuming the nematode and preventing hatching. This mechanism, coupled with the production of enzymes like chitinase and proteases, helps reduce nematode populations in the soil.
Role as a Human Pathogen
While beneficial in agriculture, Purpureocillium lilacinum is also an opportunistic human pathogen. It typically causes infections in individuals with weakened immune systems or those who have experienced trauma. The fungus is ubiquitous in the environment, found in soil, decaying matter, and even hospital water systems.
The fungus has an affinity for ocular structures, making eye infections, such as keratitis, the most frequently reported clinical manifestation. Such infections can occur in contact lens wearers, after intraocular lens implantation, or following ocular trauma. Beyond the eyes, Purpureocillium lilacinum can also cause cutaneous (skin) infections, presenting as erythematous papules, necrotic lesions, or cellulitis. Disseminated infections, where the fungus spreads throughout the body, are also reported, particularly in severely immunocompromised individuals.
Individuals most at risk for these infections include those with hematological and oncological diseases, organ transplant recipients, individuals undergoing steroid treatment, and those with diabetes mellitus. While most cases are seen in immunocompromised patients, infections can also occur in otherwise healthy individuals, often following direct inoculation through trauma or surgical procedures. The fungus’s ability to sporulate within infected tissue, known as “adventitious sporulation,” adds to its pathogenic potential.
Diagnosis and Treatment of Infections
Diagnosis of Purpureocillium lilacinum infections typically involves collecting a sample from the infected site for laboratory analysis. The sample is then cultured to grow the fungus, allowing identification through microscopic examination and molecular methods. Accurate species identification is important for guiding appropriate treatment due to the fungus’s specific resistance patterns.
Treating Purpureocillium lilacinum infections is challenging due to the fungus’s intrinsic resistance to several commonly used antifungal drugs. It demonstrates notable resistance to amphotericin B and fluconazole, which are often first-line treatments for many fungal infections. Treatment with amphotericin B has been associated with higher mortality rates in reported cases.
Newer antifungal agents, specifically triazoles like voriconazole and posaconazole, generally show better in vitro activity against Purpureocillium lilacinum. Voriconazole is frequently employed in treatment, and posaconazole has also shown promise, sometimes considered an alternative when other treatments fail. In many instances, particularly for localized infections or those involving foreign bodies, surgical removal of infected tissue or debridement is a necessary component of successful management.