Tolerance of Salmonella enterica serovar Typhimurium to nisin combined with EDTA is accompanied by changes in cellular composition

Sensitivity of Salmonella Typhimurium to nisin in vitro and in orange juice under refrigeration

The influence of environmental factors on Salmonella sensitivity to nisin in vitro and in refrigerated orange juice were investigated. Nisin activity was observed in the different conditions, but the highest efficiency was achieved at lower pH (4.0) and with higher bacteriocin concentration (174 µM). Moreover, the bactericidal action was directly proportional to the incubation period. When tested in orange juice, nisin caused a reduction of up to 4.05 logarithm cycles in the Salmonella population. So, environmental factors such as low pH and low temperature favored the sensitization of Salmonella cells to the bactericidal action of nisin. Therefore, this may represent an alternative to control Salmonella in refrigerated foods.

In-depth in vitro Evaluation of the Activity and Mechanisms of Action of Organic Acids and Essential Oils Against Swine Enteropathogenic Bacteria

Alternative antimicrobials require a deep understanding of their action mechanisms by in vitro assays which support science-based field use. This study focuses on the characterization of bactericidal mechanisms of potential antimicrobial compounds, two organic acids and three single essential oil (EO) compounds against swine enteropathogenic bacteria Escherichia coli, Salmonella enterica subsp. enterica serovar Typhimurium, and Clostridium perfringens. Target concentrations of the compounds were evaluated using the inhibitory potential of the vapor phase and bacterial viability after short-term exposure, while cell targets were disclosed using flow cytometry (FC), Fourier-transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). All tested compounds exhibited vapor phase activity against the three bacterial species, except sodium salt of coconut fatty acid distillates against C. perfringens. Survival test results evidenced that effects on bacterial viability were concentration dependent and higher in single EO compounds than in organic acids. In detail, thymol and its isomer carvacrol were the most effective compounds. Further characterization of thymol and cinnamaldehyde activity revealed that thymol main target was the cell membrane, since it caused striking damages in the membrane permeability, integrity and composition evidenced by FC and FTIR in the three enteric pathogens. In contrast, cinnamaldehyde was more effective against enterobacteria than against C. perfringens and only caused slightly damages at the highest concentration tested. Its target at the molecular level differed between enterobacteria and C. perfringens isolates. The SEM micrographs allowed us to confirm the results previously obtained for both EO compounds by other techniques. Altogether, the study showed the straight effect of these antimicrobials, which could constitute relevant information to optimize their feed inclusion rates in field studies or field use.

Carnobacterium maltaromaticum as bioprotective culture in vitro and in cooked ham

The bioprotective effects of Carnobacterium maltaromaticum (CM) strains were assessed in vitro and in sliced cooked ham. CM strains were tested in vitro against Listeria monocytogenes (LM), Escherichia coli O157:H7 (EC) and Salmonella Typhimurium (ST). In vitro effect was evaluated using co-culture (with and without EDTA) and cell-free supernatant (CFS). CFS was tested by agar well diffusion and minimum inhibitory concentration. In cooked ham, the inhibitory effect of CM on L. innocua (LI) and on the physicochemical parameters were evaluated for 7 days at 4 °C. In co-cultures at -1 °C and 4 °C, all CM isolates inhibited LM. A slight inhibition was observed against the Gram-negative bacteria with the addition of EDTA. CFS did not show inhibitory effect under the studied conditions. In cooked ham, CM inhibited LI growth and did not affect the physicochemical parameters of the product during storage. CM strains show potential to be used as bioprotective cultures in cold-stored cooked ham and improve its safety.

In vitro Susceptibility and Evaluation of Techniques for Understanding the Mode of Action of a Promising Non-antibiotic Citrus Fruit Extract Against Several Pathogens

The screening for alternatives to antibiotics is an urgent need for the pharmaceutical industry. One of these alternatives seems to be the citrus fruit extracts, which are showing a significant antibacterial activity against Gram-negative and Gram-positive bacteria. One of these citrus extracts, named BIOCITRO®, is assessed in this study to elucidate its bacteriostatic and bactericidal effect and its mode of action on the important pathogens Campylobacter coli, C. jejuni, Escherichia coli, Salmonella enterica ssp. enterica, Clostridium difficile, C. perfringens, and Staphylococcus aureus. For most of the strains tested of these bacteria the product was bactericidal as well as bacteriostatic at the same concentration, and the minimum bactericidal concentrations ranged from 16 to 256 μg/mL. Regarding the mode of action, important changes in the permeability, structure, composition and morphology of the bacterial envelope were evidenced using flow cytometry, Fourier transform infrared spectroscopy and scanning electron microscopy. The main effect of the product was found over carbohydrates and polysaccharides, inducing the release of microvesicles by the cells in addition to other specific effects. During the study, the techniques used were evaluated to clarify their contribution to the knowledge of the mode of action of the product. The survival test elucidated whether the modifications displayed using other techniques affected the viability of the cells or on the contrary, the cells remained viable even with evident changes in their structure, composition or morphology. Flow cytometry showed that for some strains the proportion of cells detected with altered membrane permeability were higher than the number of non-viable cells, and therefore the damage did not affect the viability of some cells. On the contrary, some cells observed using scanning electron microscopy with no apparent damage, were demonstrated non-viable using the survival test, making this technique indispensable in studies of the mode of action of antimicrobials to make a correct interpretation of the data from other techniques.

In vitro susceptibility of Brachyspira hyodysenteriae to a commercial citrus fruit extract

Brachyspira hyodysenteriae is the main etiological agent of swine dysentery (SD). Nowadays, treatment and control of SD is increasingly difficult due to the emergence of antimicrobial resistance together with the restrictions on the use of antibiotics in veterinary practice. The aim of this study was to evaluate, as an alternative in the control of this disease, the antimicrobial activity and the main mechanism of action of BIOCITRO, a citrus extract commercialized as raw material and used as feed additive, against B. hyodysenteriae. Ten isolates of B. hyodysenteriae were used to assess the minimum inhibitory and minimum bactericidal concentrations (MIC and MBC) of BIOCITRO by broth microdilution method. Moreover, stationary phase cultures of two B. hyodysenteriae strains were subjected for 90min to four different concentrations of BIOCITRO and compared with the untreated controls by flow cytometry (FC), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results showed that BIOCITRO has a relevant bacteriostatic and bactericidal effect against B. hyodysenteriae with MIC and MBC values ranging from 32 to 128partspermillion (ppm). It induces damage in at least 35% and 76% of the bacterial cells when exposed to 128 and 256ppm of BIOCITRO respectively as revealed by the intake of propidium iodide by FC. Relevant changes in the structure of the bacterial cells were observed by SEM and confirmed by FTIR. According to these results, BIOCITRO seems to be a satisfactory alternative to the use of antibiotics in the control of SD.

Stress enhances the sensitivity of Salmonella enterica serovar Typhimurium to bacteriocins

AIMS

The aim of this study was to evaluate the potential application of bacteriocins against Gram-negative bacteria when associated with others food preservation methods.

METHODS AND RESULTS

Salmonella was subjected to heat, cold, acid and chemical (with ethylenediaminetetracetate and trisodium phosphate) stresses. Then, the cells were recovered and subjected to treatment with bacteriocins (500 AU ml(-1) ) for 6 h. Heat and cold stress were those that promoted more sensitization to bactericidal activity of nisin. Under the same conditions, bovicin HC5 acted more rapidly than nisin reducing the number of viable cells to undetectable levels after 20 min of treatment. Similar results with use of nisin only were observed after 6 h of treatment.

CONCLUSIONS

Stress conditions used in food industry, such as temperature and pH, and use of chelating agents or membrane disruptors, sensitized Salmonella Typhimurium cells to bacteriocins produced by lactic acid bacteria, such as nisin and bovicin HC5.

SIGNIFICANCE AND IMPACT OF THE STUDY

Food preservation methods sensitized Gram-negative bacteria to bacteriocins activity, which demonstrate the potential of nisin and bovicin HC5 to inhibit the growth of Salmonella.