Mitigation of Human-Pathogenic Fungi That Exhibit Resistance to Medical Agents: Can Clinical Antifungal Stewardship Help?

Claire M Hull; Nicola J Purdy; Suzy C Moody


Future Microbiol. 2014;9(3):307-325. 

In This Article

Human-Pathogenic Molds Present in the Environment

It is well acknowledged that the natural habitat of most opportunistic human-pathogenic fungi (hereafter focusing on molds) is the environment,[15] and concern has long-surrounded the scale and epidemiology of plant-pathogenic molds that exhibit resistance to the agricultural azoles applied to cash crops.[61] However, despite the fact that certain species (or related species) of plant-pathogenic fungi can cause opportunistic infections in humans (e.g., Fusarium solani and Fusarium oxysporum), it has been argued that, from a medical azole-resistance perspective, environmentally ubiquitous molds (Table 2 & Table 3) are not under significant selective pressure because they are not exposed to the antifungals used in medicine.[15] Contemporary research and epidemiological trends that have come to light over the last 5 years now suggest that this assumption should be considered with greater caution.

Azole Resistance in A. fumigatus: A Model

Environmental populations of the haploid filamentous saprotroph, A. fumigatus contribute considerably to global biogeochemical nutrient cycling and ecosystem balance. Paradoxically, the same A. fumigatus populations are also responsible for the production of airborne conidia (Figure 1) that cause invasive opportunistic infections in animals and humans. The isolation of clinical isolates of A. fumigatus from antifungal drug-naive patients that exhibit poor sensitivity to medical azoles has led to speculation that mechanisms governing this resistance might arise first in environmental A. fumigatus following their exposure to agricultural azoles and not as a result of classical drug selection pressures[49,62] in the clinic.[63–67] Today, the epidemiology and threat of multidrug resistance in A. fumigatus is a global concern,[67–76] and the specific impact of environmental azoles on the development and spread of resistance to medical azoles in Aspergillus species was the recent subject of a risk assessment undertaken by the European Centre for Disease Prevention and Control.[19] This report highlights that the problem of azole resistance in A. fumigatus varies across Europe, currently presenting a major issue in certain countries (e.g., The Netherlands[67]) and a lesser challenge in others (e.g., Spain[71]). Here, it is noteworthy that eolian transfer of airborne spores is an important dispersal route for plant-pathogenic molds (e.g., Mycosphaerella graminicola) and that prevailing wind patterns (from west to east) over Europe are proposed to explain patterns of azole resistance in this species;[77] parallels with the geographical spread of azole-resistance in A. fumigatus are possible and remain to be investigated. In the context of the present article, it is particularly significant that the risk assessment provides several lines of evidence that support an environmental origin for multiazole resistance in A. fumigatus, these include:

Figure 1.

Environmentally ubiquitous, spore-forming fungi. (A) Resistant strains can arise in patients through classical drug selection pressure exerted by medical antifungal agents [49,62]; nonantifungal clinical drugs that exhibit off-target effects might also be important [50]. (B) Resistance as an inherent feature of environmental (and subsequently clinical) strains subject to selection pressure from agricultural antifungals and/or their metabolites. The importance of other biocides and xenobiotics present in the natural environment is currently uncharted. Clinical antifungal stewardship cannot mitigate the problems of resistance associated with environmental factors. Dark-fill circles: Antifungal (medical and environmental) cross-resistance; Light circles: Antifungal sensitive.

  • The presence of azole-resistant Aspergillus in azole-naive patients;

  • The dominance of a single and stable resistance mechanism (TR34/L98H) in Aspergillus CYP51A (sterol 14α-demethylase) protein;

  • The 34 base-pair tandem repeat sequence (TR34) is not reported in A. fumigatus isolates that have acquired resistance to medical azoles during patient therapy; TR34 is however found in azole-resistant plant-pathogenic isolates;

  • Environmental A. fumigatus isolates genetically cluster to azole-resistant patient TR34/L98H isolates and are distinct from susceptible (wild-type) A. fumigatus;[76]

  • A. fumigatus (TR34/L98H) isolates exhibiting resistance to medical azoles are cross-resistant to certain agricultural azoles (e.g., propiconazole, tebuconazole, epoxiconazole, difenoconazole and bromuconazole) that were introduced between 1990 and 1996, prior to the first culture of TR34/L98H from a clinical specimen in 1998;

  • Triazole fungicides (examples above) are used extensively in agriculture for crop protection (e.g., cereals, fruit, vegetables, flowers and ornamentals), control of lawn diseases and preservation of materials across Europe;[61]

  • Molecule-alignment studies employing homology models of the A. fumigatus CYP51A azole protein target (sterol 14α-demethylase) indicate overlap and similarities between the docking characteristics of agricultural and medical azoles.[78]

The potential impact of agricultural azoles on the development and spread of resistance to medical azoles in A. fumigatus has important implications for the scope of clinical antifungal stewardship. Given the human health consequences of disease caused by azole-resistant Aspergillus, it is hardly surprising that research to further identify the causes underpinning the development of azole resistance in both clinical and environmental settings is now a major priority. Comparable investigations of other environmentally ubiquitous, human-pathogenic species and the importance of selection pressure from nonazole xenobiotics are now needed and provide a focus for the remainder of this review.

Xenobiotic Selection Pressures Outside the Clinic

The overall impact of selection pressure from agricultural azoles on other human-pathogenic molds is largely unknown (Figure 1) and for the prudent clinician, the problem does not stop there. Here, the spectrum of nonazole xenobiotics that could exert selection pressure for antifungal resistance – both inside and out of the clinic – warrants consideration. It has been shown that azole-based over-the-counter antifungal agents can promote and exacerbate azole resistance in Candida spp.[79] and the potential for off-target effects from non-antifungal medical drugs to select for stress adaptations (including pleiotropic resistance mechanisms) and/or drug-refractory human-commensal fungi has recently been discussed.[50] Aside from medicinal drugs, the effect of chronic selection pressure from chemical sterilization, biocides and cleaning agents used in medical, agricultural, industrial, leisure and domestic settings combined with that imposed by pharmaceutical waste and personal care products[80,81] on the evolution of resistance traits in fungi is undeniable but uncharted. Insecticides and pesticides are a further concern, not least because many persist in the environment or have unknown biological footprints. Finally, chemical defense molecules and natural compounds (e.g., mycotoxins, antibiotics and secondary metabolites) produced by microbes inhabiting the same ecological niche as environmental populations of human-pathogenic fungi add a further layer of complexity to our understanding of the selection pressure(s) outside the clinic that can influence the resistance traits of isolates encountered within.

Beyond A. fumigatus

Just as azoles are not the only chemical agents that exert selection pressure for antifungal resistance, A. fumigatus is not the only environmentally ubiquitous spore-forming mold. Recent estimates using high-throughput DNA sequencing indicate that there are over 13 million fungal species on earth;[82] the number of these that are – or could potentially become – human-pathogenic is unknown. Efforts to improve understanding regarding the diversity, ecology and biology of fungal agents of disease are steadily growing and several species including those responsible for invasive mucormycosis ( Table 2 & Table 3 ) are a concern. The need to increase knowledge and to translate our conceptual understanding about the threat of resistance from spore-forming, environmentally ubiquitous molds beyond Aspergillus is clear.