Carnobacterium: positive and negative effects in the environment and in foods

The genus Carnobacterium contains nine species, but only C. divergens and C. maltaromaticum are frequently isolated from natural environments and foods. They are tolerant to freezing/thawing and high pressure and able to grow at low temperatures, anaerobically and with increased CO2 concentrations. They metabolize arginine and various carbohydrates, including chitin, and this may improve their survival in the environment. Carnobacterium divergens and C. maltaromaticum have been extensively studied as protective cultures in order to inhibit growth of Listeria monocytogenes in fish and meat products. Several carnobacterial bacteriocins are known, and parameters that affect their production have been described. Currently, however, no isolates are commercially applied as protective cultures. Carnobacteria can spoil chilled foods, but spoilage activity shows intraspecies and interspecies variation. The responsible spoilage metabolites are not well characterized, but branched alcohols and aldehydes play a partial role. Their production of tyramine in foods is critical for susceptible individuals, but carnobacteria are not otherwise human pathogens. Carnobacterium maltaromaticum can be a fish pathogen, although carnobacteria are also suggested as probiotic cultures for use in aquaculture. Representative genome sequences are not yet available, but would be valuable to answer questions associated with fundamental and applied aspects of this important genus

Bacterial viruses enable their host to acquire antibiotic resistance genes from neighbouring cells

Prophages are quiescent viruses located in the chromosomes of bacteria. In the human pathogen, Staphylococcus aureus, prophages are omnipresent and are believed to be responsible for the spread of some antibiotic resistance genes. Here we demonstrate that release of phages from a subpopulation of S. aureus cells enables the intact, prophage-containing population to acquire beneficial genes from competing, phage-susceptible strains present in the same environment. Phage infection kills competitor cells and bits of their DNA are occasionally captured in viral transducing particles. Return of such particles to the prophagecontaining population can drive the transfer of genes encoding potentially useful traits such as antibiotic resistance. This process, which can be viewed as ‘auto-transduction’, allows S. aureus to efficiently acquire antibiotic resistance both in vitro and in an in vivo virulence model (wax moth larvae) and enables it to proliferate under strong antibiotic selection pressure. Our results may help to explain the rapid exchange of antibiotic resistance genes observed in S. aureus

Nisin Damages the Septal Membrane and Triggers DNA Condensation in Methicillin-Resistant <i>Staphylococcus aureus</i>

Nisin is applied as a food preservative in processed foods and has the potential to be used synergistically with antibiotics for treatment of patients infected by antibiotic-resistant bacteria, such as methicillin-resistant Staphylococcus aureus. The present study explores the antimicrobial effect of nisin on S. aureus viability and membrane integrity and, for the first time, used super-resolution microscopy to study morphological changes induced in S. aureus cells exposed to nisin. The exposure of S. aureus to nisin caused membrane depolarization and rapid killing. Super-resolution structured-illumination microscopy and transmission electron microscopy confirmed that nisin damages the cellular membrane and causes lysis of cells. Strikingly, condensation of chromosomal DNA was observed in all cells exposed to nisin, a phenotype not previously reported for this compound. Moreover, cells exposed to nisin were significantly smaller than non-exposed cells indicating the emergence of cell shrinkage. The strong association of DNA condensation with nisin exposure indicates that nisin interferes with chromosome replication or segregation in S. aureus.Published versio

Effects of grade, smiley, and text on Danish and Finnish consumers’ perceptions of food safety inspection reports

Publicly accessible food safety inspection reports are a standard procedure to inform consumers on restaurants' food safety levels in many countries. This study examined how different formats of food safety inspection report are associated with consumer perceptions related to food safety, as well as other perceptions about the restaurant. The study was conducted in Denmark and Finland with similar inspection grade systems but differences in the distribution of awarded grades. We conducted a population-based survey experiment with a between-subjects design on nationally representative samples of the 18-70 years old Danish (n = 978) and Finnish (n = 907) populations. Respondents received one of six food safety inspection reports with different combinations of inspection grade with a smiley and/or text. According to the results, both Danish and Finnish consumers' food safety perceptions of the same grade were more positive when the report included a smiley, and more negative when the report included a text. Finnish respondents perceived a good food safety inspection grade more positively than Danish respondents but there were no country differences when the grade was poor. In addition, food safety inspection results elicited perceptions that were not related to food safety in both countries. The results suggest that if the grade is poor, the inclusion of text is effective in eliciting perceptions of increased food safety risk. If the grade is good, a standalone smiley may be most effective in eliciting positive perceptions of a high food safety level. Moreover, these results indicate the importance of carefully evaluating how to develop public accessible inspection grades to ensure they are correctly interpreted by consumers in different countries.Peer reviewe

Antimicrobial Resistance and Virulence Gene Profiles of Methicillin-Resistant and -Susceptible <i>Staphylococcus aureus</i> From Food Products in Denmark

Foods may potentially serve as vehicles for the transmission of antimicrobial-resistant variants of Staphylococcus aureus that are important in a human clinical context. Further, retail food products can be a cause of staphylococcal food poisoning. For these reasons and to account for source attribution and risk assessment, detailed information on the population structure, resistance, and virulence profiles of S. aureus originating from retail food products is necessary. In the current study, whole-genome sequences from 88 S. aureus isolates were subjected to bioinformatics analyses in relation to sequence types, antimicrobial resistance, and virulence profiles. The sequence types (ST) identified belonged to 13 clonal complexes (CC) with CC5 and CC398 being the most common. CC398 was identified as the dominant clone (n = 31). CC5 was identified as of avian origin, with the presence of φAVβ prophage genes (n = 13). In total, 39.8% of the isolates contained multiple resistance genes, and methicillin-resistant Staphylococcus aureus (MRSA) isolates were found in CC8, CC9, and CC398. Genes conferring resistance to the antimicrobial classes of β-lactams, tetracycline, and erythromycin were detected in this study, all of which are commonly used in Danish livestock production. The tst gene encoding the toxic shock syndrome toxin was for the first time identified in ST398 isolates, probably as a result of a single acquisition of a SaPI-like element. The sushi-CC398 isolates carrying the scn gene likely originated from a human reservoir, while the other isolates originated from livestock. Taken together, our results show that both human and animal reservoirs contribute to contamination in food products and that retail foods may serve as a vehicle of S. aureus between livestock and humans.Published versio