Investigation of mold and yeast contaminations in cheese samples

Abstract

Fungi are common contaminants of cheese. Although they are added during the cheesemaking as starter cultures, they might also contaminate the cheese ripening or storage environments. Candida, Penicillium and Aspergillus species can cause serious systemic mycosis in humans and animals. This study aimed to investigate the diversity of the fungal microbiota in 100 samples of various cheese types and screen the azole resistance of Aspergillus flavus isolated from these samples. Twenty aged kashar, 20 fresh kashar, 25 white pickled, 25 curd cheese, and 10 cream cheese were collected from different vendors over six months in Bursa Province. Potato Dextrose Agar (PDA) was used to isolate the fungi. Sabouraud Dextrose Agar (SDA) and PDA were used for conventional identification. After microscopic and macroscopic evaluations, isolates were identified species-wise using the Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) system. Seventy-six mold and 81 yeast isolates were isolated and selected from 94 out of 100 cheese samples. As a result of conventional evaluation, 156 isolates (excluding one yeast isolate) could be identified. Based on the MALDI-TOF MS analysis, 55.6% (79/142) of the isolates were yeasts assigned to 31 species across five genera. In comparison, 44.4% were molds assigned to 28 species across five genera. The predominant fungal genus detected was Candida (45.1%, 64/142), followed by Penicillium (32.4%, 46/142). The most frequently isolated fungal species, C. famata (n=37), was found in all cheese types. One strain of A. flavus complex was isolated from one curd cheese sample and was susceptible to azole. This study successfully assesses the fungal microbiota of various cheeses from Bursa, consisting of diverse groups of yeasts and molds. While most of the molds consisted of Penicillium spp, detecting azole-sensitive A. flavus complex underlines the need for regular monitoring of cheese microflora owing to the risk of resistance development.