Anomalous Blue Colouring of Mozzarella Cheese Intentionally Contaminated With Pigment Producing Strains of Pseudomonas Fluorescens

Abstract

In summer 2010 a large outbreak of anomalous blue coloration of mozzarella cheese was recorded in Italy and some northern European countries. Official laboratory analysis and health authorities linked the outbreak to the contamination of processing water with strains of Pseudomonas fluorescens, although several expert raised the question of how to unequivocally link the blue coloring to the presence of the micro-organism. In an attempt to set-up a method to determine whether a given Pseudomonas spp. strain is responsible of the defect, an in vitro system for the evaluation of blue colouring of mozzarella cheese intentionally contaminated with strains of Pseudomonas fluorescens. was developed The system is aimed to ascertain whether P. fluorescens strains, isolated from mozzarella cheese with anomalous blue coloration, are able to reproduce the blue coloration under controlled experimental condition. 96 trials of experimental inoculation of mozzarella cheese in different preservation liquids, were conducted using various suspension of Pseudomonas spp. (P. fluorescens ATCC 13525, P. fluorescens CFBP 3150, one P. fluorescens field strain isolated from blue-colored mozzarella cheese and P. aeruginosa ATCC 10145 as positive control) at different concentrations and incubated at different temperatures. Growth curve of all Pseudomonas spp. strains tested demonstrated that after three days of incubation the concentration was generally higher than 106 CFU/g of mozzarella cheese incubated in Tryptic Soy Broth (TSB), and higher than 105 CFU/g of mozzarella cheese incubated in preservation liquid. All mozzarella cheeses inoculated with the field strain of Pseudomonas fluorescens showed the characteristic anomalous blue coloration, which is often associated with Pseudomonas fluorescens contamination of water used during mozzarella cheesemaking. With the proposed system, which enabled a considerable amount of samples to be analysed under controlled experimental conditions and a large number of data to be generated in a short time, we demonstrated that the presence of Pseudomonas fluorescens in blue coloured mozzarella cheese is a necessary and sufficient condition

Setup of a rapid method to distinguish among dead, alive, and viable but not cultivable cells of Pseudomonas spp. in mozzarella cheese

“…population is close to 8 log cfu/g of cheese. This result is in accordance with the previous studies (Sechi et al, 2013;Cenci-Goga et al, 2014) where the authors correlated the appearance of the anomalous blue coloration of mozzarella cheese to a specific P. fluorescens cell number, thus confirming the reliability of the method here described.…”

“…Moreover, we observed the blue coloration in all contaminated samples when Pseudomonas spp. concentration reached about 8 log cfu/g in accordance with Sechi et al (2013). We subjected samples stored at 4°C and 12°C to qPCR and RT-qPCR after 17 d of incubation, even if the blue color appeared in a sample challenged with P. fluorescens DSM 50106 T and stored at 4°C (data not shown).…”

Blue pigment in fresh cheese produced by Pseudomonas fluorescens

“…5) have been described in cheese, mainly in Mozzarella cheese (Alberghini, Tallone, & Giaccone, 2010;Cantoni, Marchisio, & Galli, 2000;Cantoni, Milesi, & Iacumin, 2006;Cantoni, Stella, Cozzi, Iacumin, & Comi. 2003;Cantoni et al, 2001;Cibotti et al, 2011;Shannon, Olson, & von Elbe, 1968;Sechi et al, 2011), and in Latin-style fresh cheese (Martin et al, 2011). The causative agent in all these cases was identified as a specific P. fluorescens strain that could be associated with contaminated water in contact with food, or with raw milk via biofilms in milk tanks (Simoes et al, 2009).…”

A century of gray: A genomic locus found in 2 distinct Pseudomonas spp. is associated with historical and contemporary color defects in dairy products worldwide

“…Pseudomonas fluorescens biovar IV caused blue-pigmented spoilage of queso fresco cheese from a processor in the United States in 2006 (Martin et al, 2011), and in 2010, 70,000 units of mozzarella cheese produced at a facility in Germany and sold in Italy turned blue and were confiscated by Italian officials (Greenhalgh, 2010). Again, P. fluorescens was responsible, and indigoidine was suggested as the pigment causing the defect (Cantoni and Bersani, 2010;Sechi et al, 2011;Cenci-Goga et al, 2014). Multilocus sequence typing identified a "blue branch" of strains in the P. fluorescens phylogeny (Andreani et al, 2015b;Chierici et al, 2016).…”