Although triclosan is an antimicrobial agent, some fungi [387, 388] and bacteria
are able to degrade it co-metabolically or metabolically using it as sole carbon source
for their growth [89, 100, 104, 106, 389–393]. In addition, repeated exposure to
sublethal concentrations of triclosan may result in the development of resistant
colonies [394, 395]. The mechanisms of triclosan microbial resistance share some
similarities with those involved in antibiotic resistance [396, 397]. Several studies
have demonstrated the development of cross-resistance between triclosan and antibiotics [398–400]. Therefore, triclosan like other biocides is suspected to take part to
the selection pressure favorable to the emergence, spread, and maintenance of
antibiotic resistances among environmental microbial communities [395, 401–404].
In soils, triclosan was reported to degrade to variable extent, with various halflives depending on soil properties and conditions of incubation [51, 115, 125–
132]. Regarding its ecotoxicological impact on soil microorganisms, triclosan was
found to transiently inhibit microbial respiration, reduce microbial biomass [126],
and sulfatase activity [405]. These effects were positively related to the dose of
triclosan applied to the soil and inversely correlated with soil organic matter and clay
content, suggesting that soil characteristics control its bioavailability and induced
toxicity. Triclosan was also found to reduce the relative abundance of both Grampositive and negative bacteria and fungi [406]. Recently, studies performed in four
agricultural soils using shotgun sequencing observed an increase in Pseudomonas,
Sphingomonas, Methylobacillus, and Stenotrophomonas and identified the most
abundant functional genes associated with triclosan biodegradation [130].
3.7 Antifungals
Antifungals comprise a large and diverse group of drugs used to treat fungal diseases
in humans, animals and plants. Based on their action mode, antifungals can be
divided in three different classes: azoles, which inhibit the synthesis of ergosterol;
polyenes, which physicochemically interact with fungal membrane sterols; and
5-fluorocytosine, which inhibits macromolecular synthesis [407]. Among the different azoles, of particular interest is the case of the triazoles, which constitute a
synthetic group of heterocyclic compounds containing a five-membered ring of
two carbon atoms and three nitrogen atoms commonly used for the control of fungal
diseases in humans, animals, and plants. They include drugs such as fluconazole,
clotrimazole, and miconazole and plant protection products such as tebuconazole
and epoxiconazole. By inhibiting the activity of lanosterol 14α-demethylase (DMI),
a member of the cytochrome P450 catalytic activity, triazoles alter the bioconversion
of lanosterol to ergosterol, a fundamental component of the fungal cytoplasmic
membrane, preventing fungal growth [407, 408]. Therefore, triazoles are fungistatic
and not fungicidal, but although misleading, the term fungicide is commonly used in
agriculture for this type of pesticide.
Due to their efficacy and broad spectrum of activity, triazoles are among the most
common systemic fungicides used in the control of plant diseases [409]. Contrary to
Impact of PhACs on Soil Microorganisms
279
are able to degrade it co-metabolically or metabolically using it as sole carbon source
for their growth [89, 100, 104, 106, 389–393]. In addition, repeated exposure to
sublethal concentrations of triclosan may result in the development of resistant
colonies [394, 395]. The mechanisms of triclosan microbial resistance share some
similarities with those involved in antibiotic resistance [396, 397]. Several studies
have demonstrated the development of cross-resistance between triclosan and antibiotics [398–400]. Therefore, triclosan like other biocides is suspected to take part to
the selection pressure favorable to the emergence, spread, and maintenance of
antibiotic resistances among environmental microbial communities [395, 401–404].
In soils, triclosan was reported to degrade to variable extent, with various halflives depending on soil properties and conditions of incubation [51, 115, 125–
132]. Regarding its ecotoxicological impact on soil microorganisms, triclosan was
found to transiently inhibit microbial respiration, reduce microbial biomass [126],
and sulfatase activity [405]. These effects were positively related to the dose of
triclosan applied to the soil and inversely correlated with soil organic matter and clay
content, suggesting that soil characteristics control its bioavailability and induced
toxicity. Triclosan was also found to reduce the relative abundance of both Grampositive and negative bacteria and fungi [406]. Recently, studies performed in four
agricultural soils using shotgun sequencing observed an increase in Pseudomonas,
Sphingomonas, Methylobacillus, and Stenotrophomonas and identified the most
abundant functional genes associated with triclosan biodegradation [130].
3.7 Antifungals
Antifungals comprise a large and diverse group of drugs used to treat fungal diseases
in humans, animals and plants. Based on their action mode, antifungals can be
divided in three different classes: azoles, which inhibit the synthesis of ergosterol;
polyenes, which physicochemically interact with fungal membrane sterols; and
5-fluorocytosine, which inhibits macromolecular synthesis [407]. Among the different azoles, of particular interest is the case of the triazoles, which constitute a
synthetic group of heterocyclic compounds containing a five-membered ring of
two carbon atoms and three nitrogen atoms commonly used for the control of fungal
diseases in humans, animals, and plants. They include drugs such as fluconazole,
clotrimazole, and miconazole and plant protection products such as tebuconazole
and epoxiconazole. By inhibiting the activity of lanosterol 14α-demethylase (DMI),
a member of the cytochrome P450 catalytic activity, triazoles alter the bioconversion
of lanosterol to ergosterol, a fundamental component of the fungal cytoplasmic
membrane, preventing fungal growth [407, 408]. Therefore, triazoles are fungistatic
and not fungicidal, but although misleading, the term fungicide is commonly used in
agriculture for this type of pesticide.
Due to their efficacy and broad spectrum of activity, triazoles are among the most
common systemic fungicides used in the control of plant diseases [409]. Contrary to
Impact of PhACs on Soil Microorganisms
279