Occurrence and Characteristics of Staphylococcus aureus Strains along the Production Chain of Raw Milk Cheeses in Poland

Possibility of transfer and activation of 'silent' tetracycline resistance genes among Enterococcus faecalis under high-pressure processing

In this study, the tetracycline resistance of Enterococcus faecalis strains isolated from food was determined and molecular analyses of the resistance background were performed by determining the frequency of selected tetracycline resistance genes. In addition, the effect of high-pressure stress (400 and 500 MPa) on the expression of selected genes encoding tetracycline resistance was determined, as well as changes in the frequency of transfer of these genes in isolates showing sensitivity to tetracyclines. In our study, we observed an increase in the expression of genes encoding tetracyclines, especially the tet(L) gene, mainly under 400 MPa pressure. The study confirmed the possibility of transferring genes encoding tetracyclines such as tet(M), tet(L), tet(K), tet(W) and tet(O) by horizontal gene transfer in both control strains and exposed to high-pressure. Exposure of the strains to 400 MPa pressure had a greater effect on the possibility of gene transfer and expression than the application of a higher-pressure. To our knowledge, this study for the first time determined the effect of high-pressure stress on the expression of selected genes encoding tetracycline resistance, as well as the possibility and changes in the frequency of transfer of these genes in Enterococcus faecalis isolates showing sensitivity to tetracyclines and possessing silent genes. Due to the observed possibility of increased expression of some of the genes encoding tetracycline resistance and the possibility of their spread by horizontal gene transfer to other microorganisms in the food environment, under the influence of high-pressure processing in strains phenotypically susceptible to this antibiotic, it becomes necessary to monitor this ability in isolates derived from foods.

Growth of methicillin-resistant Staphylococcus aureus during raw milk soft cheese-production and the inhibitory effect of starter cultures

The consumption of raw milk or raw milk products might be a potential risk factor for the transmission of methicillin-resistant Staphylococcus aureus (MRSA). Therefore, we studied MRSA growth during raw milk soft cheese-production. Furthermore, we investigated the inhibitory effect of four starter cultures (Lactococcus lactis, Lacticaseibacillus rhamnosus, Lactiplantibacillus plantarum, Lactobacillus helveticus) on the growth of MRSA in a spot-agar-assay and in raw milk co-culture following a cheesemaking temperature profile. During the initial phases of raw milk cheese-production, MRSA counts increased by 2 log units. In the ripening phase, MRSA counts only dropped slightly and remained high up to the end of the storage. Comparable MRSA counts were found in the rind and core and strain-specific differences in survival were observed. In the spot-agar-assay, all four starter cultures showed strong or intermediate inhibition of MRSA growth. In contrast, in raw milk, only Lactococcus lactis strongly inhibited MRSA, whereas all other starter cultures only had minor inhibitory effects on MRSA growth. Our results indicate that MRSA follow a similar growth pattern as described for other S. aureus during raw milk soft cheese-production and illustrate the potential use of appropriate starter cultures to inhibit MRSA growth during the production of raw milk cheese.

Microbiological Risks of Traditional Raw Cow's Milk Cheese (Koryciński Cheeses)

Traditional and regional foods have been increasing in popularity among consumers in Poland for many years. The observed trend of searching for natural and authentic taste encourages many producers to craft products from raw milk, including Koryciński cheeses. The aim of this study was to assess the microbiological hazards resulting from the presence of pathogenic bacteria in Koryciński cheeses available in retail trade. The tests were carried out using accredited methods, including the detection of the presence of Salmonella spp., the enumeration of Listeria monocytogenes, the enumeration of coagulase-positive staphylococci, and the detection of staphylococcal enterotoxins in food when the number of coagulase-positive staphylococci in the sample exceeded the limit of 105 cfu/g. The research material consisted of 45 Koryciński cheeses. The tests conducted revealed that Salmonella spp. was not detected in any of the examined cheeses. However, coagulase-positive staphylococci were present in 68.9% of the samples. In as many as 15 tested cheeses, the level of S. aureus contamination was above 105 cfu/g; therefore, these samples were tested for the presence of staphylococcal enterotoxins. The presence of staphylococcal enterotoxins was found in one Koryciński cheese. In four cheeses, the number of L. monocytogenes exceeded the level of 102 cfu/g, the limit specified in Regulation 2073/2005 on microbiological criteria for foodstuffs. The obtained research results confirm the validity of monitoring the microbiological quality of Koryciński cheeses and the need to increase awareness of ensuring proper hygienic conditions of production, including the increased risk associated with unpasteurized milk products.

Comparison of the Properties of Pullulan-Based Active Edible Coatings Implemented for Improving Sliced Cheese Shelf Life

The development of active edible coatings with improved mechanical and barrier properties is a huge challenge. In this study, active edible coatings for sliced cheese have been developed using pullulan (Pull) in combination with two different biopolymers, chitosan (CS) and gelatine (Gel), and a combination of hydrolats as a source of active compounds with antimicrobial effects. In comparison to the monolayer coating, the bilayer coating system demonstrates improved barrier and mechanical properties. A preliminary assessment of the antimicrobial effect of lemongrass and curry plant hydrolats has revealed that both hydrolats exhibited antimicrobial activity against the targeted bacterium Staphylococcus aureus, albeit at different levels. The obtained results suggest that a mixture of 1.56% lemongrass and 12.5% curry plant hydrolats yielded a lower fractional inhibitory concentration (FIC) value. Bilayer coating systems (Pull/CS and Pull/Gel) with an incorporated mixture of hydrolats have demonstrated effectiveness in both cases: artificial contamination before application of the coating system and after application of the coating system. In both contamination scenarios, the coating systems consistently effectively limited bacterial proliferation, indicating the antimicrobial effect of the hydrolat mixture in the coating layers. In the case of artificial contamination before applying the coating system, both coatings demonstrated antimicrobial effectiveness, but the formulation with chitosan had a biocide effect, while the other, with gelatine, had only a bacteriostatic effect in a long-term setting. In the second case, both Pull/CS and Pull/Gel coatings demonstrated effectiveness in inhibiting bacterial growth regardless of the moment of contamination of the sample; the Pull/CS coating showed slightly better antimicrobial activity, achieving complete elimination of bacteria earlier compared with the Pull/Gel coating system.

High-Pressure Processing-Impacts on the Virulence and Antibiotic Resistance of Listeria monocytogenes Isolated from Food and Food Processing Environments

High-pressure processing (HPP) is one of the non-thermal methods of food preservation considered to be safe but may cause an increase/decrease in virulence potential and antibiotic resistance. The aim of the present study was to evaluate the survival of L. monocytogenes isolates after high-pressure processing (200 and 400 MPa for 5 min) and to determine changes in phenotypic and genotypic antibiotic resistance and virulence after this treatment. The 400 MPa treatment was shown to be effective in reducing pathogens to safe levels; however, the potential for cell recovery during storage was observed. In addition, studies on changes in virulence indicated possibilities related to a decrease in actA gene expression, overexpression of the hly and osfX gene, and an increase in biofilm-forming ability. The studies on changes in antibiotic resistance of isolates showed that all isolates showing initial susceptibility to lincomycin, fosfomycin, trimethoprim/sulfamethoxazole, and tetracycline became resistant to these antibiotics, which was associated with an increase in the values of minimum inhibitory concentrations. An increase in the expression of antibiotic resistance genes (mainly tetA_1, tetA_3, tetC) was also observed (mainly after the application of 200 MPa pressure), which was isolate dependent. However, it is noteworthy that the induced changes were permanent, i.e., they persisted even after the restoration of optimal environmental conditions. The results presented in our work indicate that the stress occurring during HPP can affect both phenotypic and genotypic changes in the virulence and antibiotic resistance potential of pathogens isolated from food and food processing environments. The potential associated with cell recovery and persistence of changes may influence the spread of virulent isolates of pathogens with increased antibiotic resistance in the food and food processing environment.

Impact of the Food-Related Stress Conditions on the Expression of Enterotoxin Genes among Staphylococcus aureus

Staphylococcus aureus is one of the most important foodborne pathogens. S. aureus has the capability to produce a variety of toxins, including staphylococcal enterotoxins (SEs). The aim of this study was to evaluate the survival rate of S. aureus cells and analyze enterotoxins gene expression after exposure to osmotic stress and acidic/alkaline stress. To determine survival rates, the traditional plate counting method and flow cytometry were used. The expression levels of the enterotoxin genes were performed by quantitative reverse transcription PCR (RT-qPCR). Expression changes differed depending on the stressors chosen. The obtained results in this study showed the effect of critical food-related stress conditions on SE gene expression in S. aureus. The study showed different expression levels of the tested enterotoxins genes depending on the stress. The most tested enterotoxin genes (seg, sei, and selo) after exposure to pH = 4.5 stress have similar expression as in the optimal condition. After alkaline treatment (pH = 9.6), a similar expression gene value as for the optimal condition was observed. The analysis of gene expression in response to stress caused by NaCl, showed that the expression of selp decreased, whereas selu, selm, and selo genes increased. A significantly decreased expression of the sea gene was observed.

Prevalence and Antibiotic Resistance of Bacillus sp. Isolated from Raw Milk

Milk, due to its diversity in terms of its nutritional content, is an important element of the human diet, as well as a good medium for the development of bacteria. The genus Bacillus contains ubiquitous aerobic, rod-shaped, endospore-producing gram-positive bacteria. Representatives of the Bacillus cereus group and the Bacillus subtilis group contribute to shortening the shelf life of milk and dairy products by degrading milk components and its additives. They also produce a number of heat-stable toxins and can cause a number of ailments, mainly in the digestive system. The aim of this research was to identify Bacillus sp. strains isolated from raw milk and to determine their antibiotic resistance. Strains isolated from raw milk samples (n = 45) were identified by MALDI-TOF MS. Ninety strains of Bacillus sp. were identified, for which the antibiotic resistance phenotype was determined. A total of 90 strains of Bacillus were classified in five groups (the Bacillus cereus group (n = 35), B. licheniformis (n = 7), the B. subtilis group (n = 29), B. pumilus (n = 16), and Bacillus sp. (n = 3). All isolates were susceptible to chloramphenicol and meropenem. The antibiotic resistance profiles of the tested groups of Bacillus spp. differed from each other, which is of particular concern in relation to multidrug-resistant representatives of the B. cereus group resistant to cefotaxime (94.29%), ampicillin (88.57%), rifampicin (80%), and norfloxacin (65.71%). Our study provides data on the prevalence and antibiotic sensitivity of Bacillus sp. In raw milk, suggesting a potential risk to health and the dairy industry.

In Vitro Antimicrobial Activity of Thymus vulgaris, Origanum vulgare, Satureja montana and Their Mixture against Clinical Isolates Responsible for Canine Otitis Externa

Otitis externa is a frequent inflammation among dogs, mainly caused by bacteria and yeasts that are often resistant to conventional drugs. The aim of the present study was to evaluate the in vitro antibacterial and antifungal activities of commercial essential oils (EOs) from Origanum vulgare, Satureja montana, and Thymus vulgaris, as well as a mixture of these three components, against 47 clinical bacterial strains (Staphylococcus sp., Streptococcus sp., Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens) and 5 Malassezia pachydermatis strains, previously cultured from the ears of dogs affected by otitis externa. The tested Gram-positive bacteria were sensible to the analysed EOs with MICs ranging from 1.25% (v/v) to <0.0195% (v/v); Gram-negative isolates, mainly P. aeruginosa, were less sensitive with MICs from >10% (v/v) to 0.039% (v/v). M. pachydermatis isolates were sensitive to all EOs with MICs from 4.25% (v/v) to 2% (v/v). However, the mixture was active against all bacterial (except one P. aeruginosa strain) and fungal tested isolates. The three EOs and their mixture seem to be an interesting alternative for treating canine otitis externa when conventional antimicrobials are not active.