Short communication: Clostridial spore counts in vat milk of Alpine dairies

Specificity of the AMP-6000 Method for Enumerating Clostridium Endospores in Milk

Enumeration of endospores of butyric acid-forming clostridia in cheese milk is an essential part of milk quality monitoring for cheese producers to avoid late blowing, severe spoilage caused by clostridia during ripening. However, due to the lack of an internationally standardized method, different methods are used and it is important to consider how the choice of method affects the results. This is particularly relevant when clostridial spore counts in milk are considered for quality payments. The aim of this study was to evaluate the specificity of the AMP-6000 method for the enumeration of endospores of cheese spoiling clostridia in milk. First, to assess the prevalence of Clostridium diversity and to determine potential non-target species, we identified isolates from positive reactions of the AMP-6000 method used to quantify clostridial endospores in raw milk and teat skin samples by MALDI-TOF MS. Based on these results, a strain library was designed to evaluate method inclusivity and exclusivity using pure cultures of target and non-target strains according to ISO 16140-2:2016. Most target Clostridium tyrobutyricum strains, as well as all tested C. butyricum and C. sporogenes strains were inclusive. However, C. beijerinckii may be underestimated as only some strains gave positive results. All non-target strains of bacilli and lysinibacilli, but not all paenibacilli, were confirmed to be exclusive. This study provides performance data to better understand the results of microbiological enumeration of butyric acid-forming clostridia in milk and serves as a basis for future methodological considerations and improvements.

Low-Level Clostridial Spores' Milk to Limit the Onset of Late Blowing Defect in Lysozyme-Free, Grana-Type Cheese

The growth of clostridial spores during ripening leads to late blowing (LB), which is the main cause of spoilage in Grana Padano Protected Designation of Origin (PDO) cheese and other hard, long-ripened cheeses such as Provolone, Comté, and similar cheeses. This study aimed to verify the cause-effect relationship between the level of clostridial butyric spores (BCS) in milk and the onset of the LB defect. To this end, experimental Grana-type cheeses were produced without lysozyme, using bulk milk with different average BCS content. The vat milk from the so-called "virtuous" farms (L1) contained average levels of BCS of 1.93 ± 0.61 log most probable number (MPN) L^-1, while the vat milk from farms with the highest load of spores (L2), were in the order of 2.99 ± 0.69 log MPN L^-1. Cheeses after seven months of ripening evidenced a strong connection between BCS level in vat milk and the occurrence of LB defect. In L2 cheeses, which showed an average BCS content of 3.53 ± 1.44 log MPN g^-1 (range 1.36-5.04 log MPN g^-1), significantly higher than that found in L1 cheeses (p < 0.01), the defect of LB was always present, with Clostridium tyrobutyricum as the only clostridial species identified by species-specific PCR from MPN-positive samples. The L1 cheeses produced in the cold season (C-L1) were free of defects whereas those produced in the warm season (W-L1) showed textural defects, such as slits and cracks, rather than irregular eyes. A further analysis of the data, considering the subset of the cheesemaking trials (W-L1 and W-L2), carried out in the warm season, confirmed the presence of a climate effect that, often in addition to the BCS load in the respective bulk milks (L1 vs. L2), may contribute to explain the significant differences in the chemical composition and some technological parameters between the two series of cheeses. Metagenomic analysis showed that it is not the overall structure of the microbial community that differentiates L1 from L2 cheeses but rather the relative distribution of the species between them. The results of our trials on experimental cheeses suggest that a low-level BCS in vat milk (<200 L^-1) could prevent, or limit, the onset of LB in Grana-type and similar cheeses produced without lysozyme.

Late blowing defect in <em>Grottone</em> cheese: detection of clostridia and control strategies

“Grottone” is a pasta filata hard cheese produced in Campania region from cow’s milk and characterized by holes formation due to CO² development by Propionic Acid Bacteria. The contamination of raw milk with butyric acid-producing spore-forming clostridia represent a major concern for cheese producers since clostridia outgrowth may lead to the cheese late blowing defect during ripening. Detection of clostridial endospores in milk before processing and the use of antimicrobial compounds may represent an important control strategy. The present study is aimed to point out the most suitable procedure for the determination of clostridial spores in dairy samples, and to assess the inhibitory activity of several antimicrobial compounds against Cl. sporogenes. Based on results, MPN counts on Bryant and Burkey medium and CFU on RCM proved to be the most suitable protocols for routine testing. By using these procedures clostridial spores were detected in 10 out 13 milk samples and in all cheeses with late blowing defect. Within antimicrobial compounds, sodium nitrate is still the best choice for preventing late blowing, nevertheless a protective culture of Lacticaseibacillus casei proved to be a promising alternative. Nevertheless, the use of this protective culture in six Grottone cheese productions carried out at farm level, led to unsatisfactory results. Holes’ development was hampered likely for an inhibition of the PAB starter and the expected ‘Grouviera-type’ taste was not perceived by panellists. Based on results, the use of protective cultures needs to be contextualized and interactions with starters needs to be evaluated case by case.

Strain-Dependent Cheese Spoilage Potential of Clostridium tyrobutyricum

Clostridium tyrobutyricum, a Gram-positive, anaerobic, spore-forming bacterium, is considered as one of the main causative agents for spoilage of hard and semihard cheeses. Growth of C. tyrobutyricum in cheese is critically influenced by ripening temperature and time, pH, salt and lactic acid concentration, moisture and fat content, and the presence of other microorganisms. Previous studies revealed high intraspecies diversity of C. tyrobutyricum strains and variable tolerance toward pH, temperatures, and salt concentrations. These findings indicate that strain-dependent characteristics may be relevant to assess the risk for cheese spoilage if clostridial contamination occurs. In this study, we aimed to compare the phenotypes of 12 C. tyrobutyricum strains which were selected from 157 strains on the basis of genotypic and proteotypic variability. The phenotypic analysis comprised the assessment of gas production and organic acid concentrations in an experimental cheese broth incubated at different temperatures (37, 20, and 14 °C). For all tested strains, delayed gas production at lower incubation temperatures and a strong correlation between gas production and the change in organic acid concentrations were observed. However, considering the time until gas production was visible at different incubation temperatures, a high degree of heterogeneity was found among the tested strains. In addition, variation among replicates of the same strain and differences due to different inoculum levels became evident. This study shows, that, among other factors, strain-specific germination and growth characteristics should be considered to evaluate the risk of cheese spoilage by C. tyrobutyricum.