| BackgroundMicrobial food safety is a dominating issue not only in developed countries but also in developing countries including China.Hundreds of publications on foodborne pathogens area accessible globally in the major databases.However,the research results differ greatly because of substantial variations in the sample size,test methodologies,samples used and interpretations used in the published reports.In order to calculate a robust estimate with a higher statistical power,we applied the meta-analytic approach for assessing the prevalence of foodborne pathogens in common food items in China,and Africa.During the analysis of the data on foodborne bacterial pathogens in China,it was observed that the incidences of infections caused by Salmonella Dublin(S.Dublin)are the most fatal type of infection.S.Dublin,a cattle adapted serovar causes enteritis,and systemic disease in bovines.Phenotypic genotypic comparison of some basic adaptive properties,virulence assessment in Caenorhabditis elegans(C.elegans),a nematode animal model and massive parallel whole-genome sequencing approach were implemented to assess and evaluate the genomic variations among the cattle and human isolates of S.Dublin.Results1.Meta-analysis offoodborne pathogens in China a,nd selected African countries The estimated overall prevalence of pathogens in the foods was 8.5%(95%CI:8.2-8.7).The highest prevalence of pathogen was in the aquatic produce at 12.8%(12.0-13.5)while the least was in the vegetables at 3.0%(2.6-3.4).The most prevalent pathogen was Vibrio at 21.3%(19.6-23.1)while the least was pathogenic E.coli at 4.3%(3.3-5.2).The major pathogens in Chinese foods in decreasing order of prevalence were Vibrio parahaemolyticus,Campylobacter,Bacillus cereus,Staphylococcus aureus,Salmonella,Enterobacter,Listeria monocytogenes and pathogenic E.coli.A distinct pattern of pathogen distribution-with a higher intensity in the eastern coastal provinces;and availability of minimal data from the western provinces was revealed.In the African context,Enterobacteriaceae,Escherichia coli,Salmonella,Staphylococcus aureus and Listeria monocytogenes was the frequently reported organisms.The average prevalence of foodborne pathogens was 34.2%(29.0-39.3%).A striking feature was a high and approximately parallel prevalence of the major pathogens in ready to eat(RTE)foods and raw foods.E.coli averaged 37.6%in raw foods and 31.6%in RTE foods.The corresponding prevalence for Salmonella was 19.9%vs 21.7%;S.aureus,27.8%vs 25.1%and L.monocytogenes,19.5%vs 6.7%.2.Phenotypic and genotypic analyses of S.DublinMultilocus sequence typing(MLST)analysis showed that the isolates of S.Dublin in our collection were a homogeneous population belonging to the sequence type(ST)10.In spite of the differences in their source of origin(for example humans and bovines),the strains behaved similarly and uniformly under different incubation environments.There was no significant difference in the motility under differing incubation conditions except in anaerobic incubation of bovine isolates.Only a few strains formed trace amount biofilm in aerobic or anaerobic incubation.The morphotype was uniform among the strains of both origins,with only two animal(2.08%)and two humans(20%)strains forming the typical red,dry and rough(rdar)morphotype in aerobic incubation.All the strains(100%)formed smooth and white(saw)morphotype in anaerobic incubation.There were high rates of resistance to tetracycline(92%in animals vs 50%in humans),sulfamethoxazole(95%in animals vs 70%in humans),and chloramphenicol(89%in animals vs 50%in humans)in aerobic incubation.In anaerobic incubation,resistance was high toward streptomycin(96%in animals' vs 70%in humans),ceftiofur(94%in animals' vs 100%in humans)and chloramphenicol and sulfamethoxazole(87%in animals)vs 100%to sulfamethoxazole in human isolates.The genotype-phenotype concordance in terms of antimicrobial resistance was>80%in all classes of antimicrobials except quinolones.In quinolones,the concordance was high under aerobic incubation but it was discordant in anaerobic incubation.The major type of discordance was the presence of phenotypic resistance but the absence of acquired resistance genes.We deduce that,in anaerobic incubation,the cellular mechanisms and biochemical reactions conferring the resistance to quinolones becomes active thus causing this discrepancy despite the absence of acquired resistance genes.The animal model virulence assessment using C.elegans revealed that human strains were more virulent(median survival time of 7 days)as compared to the animal isolates(median survival time of 9 days).3.Comparative genomics of S.Dublin isolates from bovines and humansA genomic comparison was made using the set of our collection of 105 strains and 34 genome sequences downloaded from the global databases.At the genomic level,the bovine isolates clustered distinctly and away from the other human and animal isolates.The bovine isolates contained more than 30 acquired antibiotic resistance determinants whereas the human isolates contained less than 25 antibiotic resistance determinants.The number of mutations responsible for antibiotic resistance was also higher in bovine isolates(four to five)as compared to human isolates(only three or less).The bovine isolates harboured>3 different types of plasmids while the human isolates harboured<2 different types of plasmid.The presence of the IncA/C2 plasmid was a distinct feature in bovine isolates and was typically not seen in human or other animals' isolates.The genomic comparison showed a very high degree of uniformity among the isolates irrespective of the origin.Massively parallel whole-genome sequencing and the subsequent sequence analysis could not show any typical animal or human isolates dependent features.Even the isolates,that showed some phenotypic differences,did not show any genotypic alterations that could be attributed to this phenotype.It could be deduced from the core and pan-genomic analysis that the bovine isolates likely accumulated the pseudogenes during the course of evolution and transition from the human hosts to the other animals and bovines host.This enhanced the adaptation to the new niche and finally become host adapted serovars.4.Conclusion and implicationsThe results of meta-analysis warrants the need for renewed perspectives in formulating policies on microbiological food safety and risk mitigation.Our results support the basis of the selection of Vibrio and Salmonella as the sentinels of food safety in China but contradict the idea of their localization at the coastal and inward regions.Application of uniform surveillance programs and harmonized laboratory protocols could yield homogeneous outputs.On the pathogen level,the few differential behaviour of a highly homogeneous population of S.Dublin of bovine or human origin,therefore,can be correlated to the source-sink dynamics where the isolates behave differently upon different environmental cues for better adaptation rather than retain to the permanent mutations.Similarly,the variation of resistance in anaerobic and anaerobic incubation,especially regarding the quinolones,could significantly affect the choice,regimen and the dosage of the antibiotics used for animal or human therapeutics.Formation of less biofilm which allows the bacteria to remain in the planktonic state and thus move freely,could facilitate the motility in anaerobic or aerobic incubation as seen in our assays and serve to be a beneficial strategy of survival by the pathogen.Increased resistance to antimicrobials in the anaerobic condition positively influences the growth of bacteria under the effects of antimicrobial therapy of the host.This again coupled with the ability to modify the cellular machinery according to the signals from the environment and the host-pathogen interaction escalates the survival fitness of S.Dublin. |
PDF Full Text Request