Tolerance And Mechanism Of Antibiotic-resistant Staphylococcus Aureus To Food-associated Stresses

Staphylococcus aureus(S.aureus)is one of the most common food-borne pathogens.It is widely found in nutrient-rich foods such as meat,poultry and eggs,and can produce enterotoxin under appropriate conditions.Staphylococcal foodborne disease can lead to vomiting and diarrhea.Clinically,S.aureus infection can cause various inflammations such as pneumonia and pericarditis,and even cause sepsis.In order to treat diseases caused by bacterial infections,various antibiotics have been invented and widely used in humans,animals and even plants.With the large-scale and abuse use of antibiotics,more and more bacteria have acquired resistance to antibiotics.Antibiotic-resistant bacteria in turn put tremendous pressure on clinical treatment and pose a huge threat to human health.Antibiotic-resistant S.aureus is one of the most important antibiotic-resistant bacteria,which has been reported to be detected in different foods.Some studies have found that antibiotic-resistant bacteria could be higher tolerance to some common food-associated stresses compared with the susceptible counterparts,resulting in incompleted inactivation during sterilization of food.Therefore,the following studies investigations will be conducted in this study.1.Antibiotic resistance of food-borne S.aureusSix strains of S.aureus isolated from chicken were tested for the profile of antibiotic resistance by Kirby-Bauer disk diffusion and micro-broth dilution method.Three of them were antibiotic-sensitive bacteria,which were susceptible to the 10 antibiotics used in the experiment,while the other 3 strains were found to be resistant to antibiotics.The results of antibiotic resistance genes showed that the results of antibiotic resistance phenotypes and antibiotic resistance genes did not match completely.Antibiotic-resistant bacteria may not carry resistance genes corresponding to the antibiotics,and even phenotypic sensitive bacteria may be detected the presence of antibiotic resistance genes.2.Comparison of the tolerance of antibiotic-resistance and-sensitive S.aureus to food-associated stressesAfter obtaining basic information of the antibiotic-resistance of S.aureus,we compared the resistance of antibiotic-sensitive and-resistant S.aureus against acid(HCl),heat and osmotic pressure(Na Cl)to reveal the effect of antibiotic resistance on the stress tolerance of S.aureus.The experimental results showed that the survival rate of antibiotic-resistant S.aureus under acid and osmotic stress was higher than that of antibiotic-sensitive bacteria,and the difference in survival rate was most obvious under the condition of p H=2 and sodium chloride concentration of 30%.However,the antibiotic-sensitive and-resistant S.aureus showed insignificant resistance towards heat stress.3.Preliminary study on the tolerance mechanism of antibiotic-resistant S.aureus under food-associated stressesFrom the aspects of cell morphology,cell membrane damage and intracellular enzyme activity,the mechanism of antibiotic-resistant S.aureus to gain higher resistance to acid and osmotic pressure was explored.We found that under acid and osmotic stress,the relative activity of TTC-dehydrogenase in antibiotic-resistant bacteria was significantly higher than that of sensitive bacteria.The surface morphology and internal structure of antibiotic-resistant bacteria were less damaged under scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The antibiotic-sensitive bacteria showed more severe disruption in cell membrane integrity and esterase activtiy,which was indicated by the propidium Iodide(PI)and 5-(and 6)-carboxyfluorescein diacetate,succinimidyl ester(CFDA)flourscent probes combined with confocallaser scanning microscopy(CLSM).Under heat stress,there was no significant difference between the above indicators of antibiotic-resistant bacteria and antibiotic-sensitive bacteria.The study found that after acid or osmotic treatment,the bacterial morphology and internal structure of antibiotic-resistant S.aureus were more complete,the metabolic activity in cells were more active than that of antibiotic-sensitive bacteria,and the cell membrane damage is not as severe as that of antibiotic-sensitive bacteria,so it can help antibiotic-resistant bacteria resist the adverse environments.In general,this thesis takes Staphylococcus aureus isolated from food as the research object to detect its antibiotic-resistant phenotype and antibiotic-resistant genotype.Based on the basic information of Staphylococcus aureus,the tolerance of sensitive and antibiotic-resistant Staphylococcus aureus to three kinds of common processing stresses in the processing:acid,heat and osmotic pressure was studied.Furthermore,the mechanism of tolerance of Staphylococcus aureus to food-associated stresses was further explored.