Evaluating the efficacy of turbimetric measurements as a rapid screening technique to assess fungal susceptibility to antimicrobial compounds as exemplified by the use of sodium metabisulfite

Abstract

The in vitro susceptibility to sodium metabisulphite (NaMBS) was investigated in 10 different food spoilage filamentous fungi, namely Aspergillus flavus, A. carbonarius, A. niger, A. ochraceus, A. tubingensis, A. westerdijkiae, Cladosporium cladosporioides, Fusarium oxysporum, Penicillium commune and P. expansum. The fungi were inoculated in sterile 96-well microtiter plates containing Yeast-extract Sucrose (YES) semi-solid agar supplemented with NaMBS in concentrations ranging from 2000 to 3.9 mg l^-1 and incubated at 25 °C. Growth was monitored by absorbance measurements at 600 nm using a multi-spectrophotometer. The surface areas under the optical density (OD) vs. time growth curves obtained were used to calculate the fractional area f(a), from which the non-inhibitory (NIC) and minimum inhibitory concentrations (MIC) of the antifungal agent were calculated for each fungus using the Gompertz model for decay. Most Aspergillus species showed remarkable resistance to NaMBS, presenting NIC and MIC values higher than 250 and 2500 mg l^-1, respectively. The most susceptible fungi were the two Penicillium species and A. carbonarius, which presented very low NIC (<100 mg l^-1) and MIC (<1300 mg l^-1) values, whereas C. cladosporioides and F. oxysporum presented intermediate values. The method has the advantage of good repeatability and accuracy, rapid results within 48-72 h, growth detection and susceptibility to the antifungal agent for several fungi at the same time, and optimal use of microbiological media by using small volumes of consumables.

Keywords: Fungi; MIC; Modelling; NIC; Optical density; Sodium metabisulfite.