Mechanism Of Multidrug Resistance And Biofilm Formation By Salmonella

Salmonella spp.are health-threatening food-borne pathogens and millions of human cases were reported every year.The cure with antibiotics once solved the problem for humans but the abuse of these drugs brought about antibiotic-resistant strains ceaselessly.The increasingly common spread of antibiotic-resistant Salmonella is a major public-healthcare issue worldwide.Moreover,biofilm is another case threatening people's health.Salmonella could forming biofilm to defense adverse conditions in the environment,which make the strains ineradicable.In food processing industry,salmonella in biofilm could mainly be responsible for contamination and cross-contamination of equipment,causing food borne illness.In the present study,we investigated the effect of selected antibiotic combinationagainst salmonella and the mechanism of cross-resistance and collateral sensitivity in that strain.The role that luxS gene played in biofilm formation was also covered in this study as well as the environment-related factors in food processing industry that influenced biofilm formation of salmonella.The results were shown as follows:In this study,we wished to explore:(i)antibiotic or polypeptide combinations toinhibit multidrug-resistant SalmonellaBredeney;(ii)the regulation of cross-resistance and collateral sensitivity of antibiotics and polypeptides.We undertook a study to select antibiotic combinations.Then,we promoted drug-resistant strains of S.Bredeney after 15 types of antibiotic treatment.From each evolving population,the S.Bredeney strain was exposed to a particular single drug.Then,we analysed how the evolved S.Bredeney strains acquired resistance or susceptibility to other drugs.A total of 105 combinations were tested against S.Bredeney following the protocols of CLSI-2016 and EUCAST-2017.The synergistic interactions between drug pairings were diverse.Notably,polypeptides were more likely to be linked to synergistic combinations:56%(19/34)of the synergistic pairings were relevant to polypeptides.Simultaneously,macrolides demonstrated antagonism towards polypeptides.The latter were more frequently related to collateral sensitivity than the other drugs because the other 13 drugs sensitized S.Bredeney to polypeptides.In an experimental evolution involving 15 drugs,single drag-evolved strains were examined against the other 14 drugs and the results compared with the minimal inhibitory concentration of the ancestral strain.Single drug-evolved S.Bredeney strains could alter the sensitivity to other drugs,and S.Bredeney evolution against antibiotics could sensitize it to polypeptides.Biofilms are communities of bacterial cells that serve to protect them from external adverse influences and enhance bacterial resistance to antibiotics and sanitizers.Here,we studied the regulatory effects of glucose and sodium chloride on biofilm formation in Salmonella serovar Dublin(S.Dublin).To analyze expression levels of the quorum sensing gene luxS,we created a luxS knockout mutant.Also,antimicrobial resistance,hydrophobicity,autoinducer-2(AI-2)activity and relative gene expression of both the wild-type(WT)and the mutant strain were investigated.Our results revealed that glucose is not essential for S.Dublin biofilm formation but had an inhibitory effect on biofilm formation when the concentration was over 0.1%.NaCl was found to be indispensable in forming biofilm,and it also exerted an inhibitory effect at high concentrations(>1.0%).Both the WT and the mutant strains displayed significant MIC growth after biofilm formation.An increase of up to 32,768 times in the resistance of S.Dublin in biofilm phonotype against antibiotic(ampicillin)compared to its planktonic phonotype was observed.However,S.Dublin luxS knockout mutant only showed slight differences compared to the WT strain in the antimicrobial tests although it displayed better biofilm-forming capacity than the WT strain.The mutant strain also exhibited higher hydrophobicity than the WT strain,which is a feature related to biofilm formation.The production of the quorum sensing autoinducer-2(AI-2)was significantly lower in the mutant strain than in the WT strain since the LuxS enzyme,encoded by the ltxS gene,plays an essential role in AI-2 synthesis.However,the limited biofilm-forming ability in the WT strain indicated AI-2 is not directly related to S.Dublin biofilm formation.Furthermore,gene expression analysis of the WT and mutant strains revealed upregulation of genes related to biofilm stress response and enhanced resistance in the luxS mutant strain,which may provide evidence for the regulatory role of the luxS gene in biofilm formation.Biofilm is the commonest state for microorganisms to maintain in the environment.As one of the most distributed bacterial food borne pathogens,salmonella forming biofilm could brought about serious public health problems to humans.In this study,we investigated some microbial and environmental factors as:temperature,nutrients,glucose,Sodium chloride and pH in the food processing industry.The results demonstrated that salmonella from different origins showed different biofilm-forming abilities.The temperature most suitable for strain growth(37?)is not the most favorable for biofilm formation(25-30?).And biofilm adhesion is not entirely dependent on high concentration of the culture.Moreover,biofilm could be formed under the condition of pH 4-9,indicating that biofilm formation could be adhered in a wide range of pH value.Also,salmonella tends to form biofilm in low nutrient-based condition without sugar or salt and the high value of glucose and sodium chloride will inhibit the biofilm formation.