Background

The increasing use of large amounts of toxic heavy metals in industrial processes, lack of proper disposal techniques, and their non-degradable nature is creating problems to the environment and in human health (Akhtar et al., 2013; Kumar et al., 2014). Some fungi have been isolated from anthropogenic contaminated sites and tolerate high amounts of toxic compounds (Rehman and Anjum, 2011). Furthermore, they can absorb, accumulate, and resist heavy metals (Fernández et al., 2017). Therefore, it is useful to describe protocols based on metallo-tolerant fungal capacity that serve as agents in the bioremediation process. One of the most common methodologies to determine heavy metal resistance is microdilution using flasks or polystyrene microtiter plates, broths, or synthetic culture media (RPMI 1640 medium), followed by spectrophotometry (Al-janabi, 2010). However, this method needs specific equipment and could be expensive for researchers from developing countries like Mexico (Calvillo-Medina et al., 2020). The agar dilution method proposed here works better for filamentous fungi and is an easy option to evaluate heavy metal fungal resistance (Iram et al., 2012). Based on this evidence, we reported a method to evaluate heavy metal resistance and to determine fungal tolerance index (TI), which could be calculated to identify the most tolerant microorganisms even with fastidious isolates. TI calculations can be used without agar dilution methods, e.g., when fungal isolates grow according to the microdilution method or the growth of filamentous bacteria in the presence of toxic compounds.