Jasna Djordjevic

Enterotoxin production and antimicrobial susceptibility in Staphylococci isolated from traditional raw milk cheeses in Serbia

ABSTRACT This study was undertaken to determine the prevalence of coagulase positive staphylococci (CPS) by examining a total of 71 raw milk cheeses. Additionally, enterotoxigenicity, antimicrobial susceptibility and the presence of mecA and mecC genes in the staphylococcal isolates were investigated. The isolation and enumeration procedure of CPS followed the International Organization for Standardization (ISO) standard. The presumptive staphylococci were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) using the VITEK MS system. VIDAS® Staph enterotoxin II assay was used for the detection of classical enterotoxins. Antimicrobial susceptibility testing (AST) was accomplished performing the disk diffusion method. All suspected methicillin resistant staphylococci were investigated for the presence of mecA and mecC genes by PCR assay. A high prevalence (87.32%) of CPS was detected in the cheeses at contamination levels up to 5.58 log CFU g−1. Among 47 staphylococcal isolates screened for enterotoxin production, only one isolate, identified as S. hyicus, was confirmed as being enterotoxigenic. Resistance to penicillin (63.70%) was the most common resistance among the tested Staphylococcus aureus isolates. The dominant phenotypic resistance patterns in coagulase negative staphylococci (CNS) were resistance to ofloxacin and fusidic acid. All CNS isolates were susceptible to the clinically important antibiotics clindamycin, chloramphenicol, gentamicin, linezolid, rifampicin and trimethoprim-sulfamethoxazole. The mecA and mecC genes were not detected. To the best of our knowledge, this is the first study concerning evaluation of the presence of methicillin resistant staphylococci (MRS) in dairy products in Serbia.

Inhibition of Salmonella by thyme essential oil and its effect on microbiological and sensory properties of minced pork meat packaged under vacuum and modified atmosphere

The antibacterial activity of thyme essential oil (TEO) was evaluated against four serovars of Salmonella (S. Enteritidis, S. Typhimurium, S. Montevideo and S. Infantis), experimentally inoculated (106CFU/g) in minced pork, which was treated with different concentrations of the TEO (0.3%, 0.6% and 0.9%) packaged under vacuum or MAP (30%O2/50%CO2/20% N2) and stored at 3±1°C for 15days. GC-MS analysis of the TEO was performed in order to determine composition, and the predominant constituent was thymol (50.48%), followed by p-cymene and linalool. The minimum inhibitory concentration was determined for each Salmonella serovar studied. Among the tested active compounds, thymol and carvacrol exhibited the greatest inhibitory effect followed by TEO, with minimum inhibitory concentrations of 320 to 640μg/ml. S. Enteritidis was the most sensitive serovar. During the storage period, Salmonella counts in pork were reduced by 1.69-4.05logCFU/g. The influence of TEO on Enterobacteriaceae, lactic acid bacteria and total viable count was determined in control mince with no added Salmonella. The most pronounced antibacterial effect was achieved by the combination MAP and 0.9% TEO. Although the antibacterial activities of all studied concentrations of TEO in pork were evident and significant (P<0.05), sensory analysis showed that 0.3% TEO was the most acceptable to trained panellists.

MALDI-TOF mass spectrometry for the identification of Serbian artisanal cheeses microbiota

The study was carried out on 110 bacterial isolates obtained from Baird-Parker agar plates (n = 47) and de Man, Rogosa, Sharpe agar plates with added sorbic acid (n = 63), which were analyzed by MALDI-TOF mass spectrometry. Among 47 non-staphylococcal Baird-Parker agar isolates, Macrococcus caseolyticus was the most abundant (n = 37), followed by Enterococcus faecalis (n = 6). Corynebacterium aurimucosum, Corynebacterium glutamicum, Cronobacter sakazakii, and Enterococcus faecium were also identified. Among 63 non-Lactobacillus isolates, 18 isolates were identified as Leuconostoc mesenteroides, 17 as Enterococcus faecalis, 14 as Lactococcus lactis, 5 as Enterococcus durans, 5 as Lactococcus garvieae, 2 as Leuconostoc pseudomesenteroides, and 2 as Pediococcus pentosaceus. Enterococcus faecalis was the only species obtained from both types of media. All isolates were successfully identified to the species level. According to the results of this study, MALDI-TOF mass spectrometry proved to be useful tool for the identification of selected lactic acid bacteria and Baird-Parker agar isolates from artisanal cheeses.

Survival of Salmonella spp. in minced meat packaged under vacuum and modified atmosphere

The effect of different modified atmosphere packaging regimes on the behavior of Salmonella spp. on minced meat was studied. Minced meat was experimentally contaminated with a Salmonella spp. cocktail (S. Enteritidis, S. Typhimurium, S. Infantis and S. Arizonae), packaged under vacuum or modified atmosphere with initial headspaces containing 20%O2/50%CO2/30%N2 and 20%O2/30%CO2/50%N2) and stored at 3 ± 1 °C for 12 days. Samples were analyzed for Salmonella spp., viable and lactic acid bacteria count every third day. Salmonella spp. counts decreased during storage in all packaging types, with reductions of about 1.5 log CFU/g. A significant difference (p < 0.01) was noted between Salmonella spp. counts in meat packaged in vacuum and modified atmospheres, although there was no significant difference in Salmonella spp. count between meat packaged in 50%CO2, and meat packaged in 30%CO2. At the end of the study, there were significant differences (p < 0.01; p < 0.05) in total viable and lactic acid bacterial counts between meat packaged in vacuum and modified atmosphere, and the lowest counts were noted in meat packaged in modified atmosphere with 50%CO2.

Effect of modified atmosphere and vacuum packaging conditions on selected chemical and physico-chemical parameters of marinated and salted Atlantic mackerel (Scomber scombrus)

Chemical and physico-chemical parameters of marinated and salted Atlantic mackerel (Scomber Scombrus), with emphasis on the quality and safety parameters in modified atmosphere packaging (MAP) and vacuum packaging (VP), were investigated. Quality assessment of mackerel stored in MAP (40% CO2+60% N2) and VP for up to 50 days at 4±1 °C was done by the monitoring of pH value, total volatile basic nitrogen (TVB-N), thiobarbituric acid (TBA) and histamine. The pH value of fish meat was significantly lower in the marinated samples. The highest concentration of TVB-N was recorded in the salted mackerel stored under VP whereas the lowest TVB-N in the marinated mackerel stored under MAP conditions. The formation of TBA increased with the time of storage and was the lowest in the marinated mackerel stored in MAP. The concentration of histamine increased during storage and its level reached over 10 mg/100 g for the salted mackerel stored under VP conditions. The marinated mackerel packed in MAP had extended shelf life at 4±1 °C compared to that packaged in VP according to physico-chemical analysis.