| In recent years,many scholars have found the existence of Aeromonas in a variety of agricultural products,even in animals and clinical patients.Aeromonas is a sort of opportunistic pathogen.The widespread distribution of Aeromonas poses a threat to the survival of humans and animals.Diarrhea and bacteremia caused by Aeromonas are worldwide diseases.In order to figure out the distribution,genetic diversity,antibiotic resistance and pathogenicity of Aeromonas among food in Pudong New district,Shanghai,Aeromonas isolated from agricultural products in 6 supermarkets of Pudong New district in Shanghai were used as test strains in this study,79 food isolates were sequenced by concatenated cyrB-cpn60 sequencing method,and the genetic diversity of the isolates was analyzed by multi-locus sequence typing(MLST)using six housekeeping genes gltA,groL,gyrB,MetG,ppsA and recA.Secondly,PCR amplification was adopted to detect the distribution of virulence and drug resistance genes in Aeromonas.Finally,the antibiotic resistance of the isolates was assessed by broth dilution method,and the pathogenicity of Aeromonas was evaluated by β-hemolysis and extracellular proteolysis assay,caenorhabditis elegans liquid toxicity assay,cytotoxicity assay and laboratory animal toxicology assay.The results were as follows:1.The 79 Aeromonas strains were identified as 8 species by the molecular identification of gyrB-cpn60,which were A.veronii(78.5%),A.caviae(7.6%),A.dhakensis(3.8%),A.salmonicida(3.8%),A.bivalvium(2.5%),A.hydrophila(1.3%),A.jandaei(1.3%)and A.media(1.3%).Among them,A.veronii not only has the highest detection rate,but also has a wide range of distribution,widely existing in 8 sorts of foodstuffs.The species of Aeromonas from fish were the largest,including A.veronii,A.caviae,A.dhakensis,A.hydrophila,A.bivalvium;A.veronii,A.caviae,A.salmonicida and A.media were identified from pork;A.veronii,A.caviae,A.salmonicida and A.bivalvium were selected from clams.However,fewer Aeromonas species were isolated from the samples of shrimp,spiral shell,chicken,beef and mutton.It is worth noting that the pathogenic bacteria A.dhakensis and A.hydrophila mainly came from fish and shrimp samples.2.A total of 1255 food samples were collected from six supermarkets in Pudong New District,Shanghai.The detection rates of Aeromonas in six supermarkets were 7%(14/200),7.5%(15/200),7%(14/200),6%(12/200),5.4%(13/240)and 5.1%(11/215),respectively,all of which were less than 10%.The data showed that animal-derived food in these six vegetable markets was contaminated,suggesting that we should strengthen the control and hygiene monitoring of agricultural products.3.MLST genotyping divided 79 Aeromonas strains into 73 sequence types(STs),65 of which were newly discovered,indicating high genetic diversity of Aeromonas strains among food in Pudong New Area,Shanghai.The STs of Aeromonas in the minimum spanning tree was characterized by weak clustering and sparse distribution,suggesting that there was no dominant ST type of Aeromonas among foodstuffs in Pudong New district of Shanghai,and the epidemic trend was not obvious.In order to better study the genetic characteristics and epidemic trend of Aeromonas,we need to analyze more Aeromonas isolates to achieve the purpose of disease prevention.4.The results of β-hemolysis and extracellular proteolysis assay,C.elegans LT assay,cytotoxicity assay and animal toxicology assay showed that the virulence of A.dhakensis and A.hydrophila was higher than that of A.veronii,A.caviae and A.bivalvium.Among all isolates,the most common virulence genes were cytotoxic enterotoxin gene(act,73.4%),polar flagellum gene(fla,69.6%),type Ⅲ secretory system(TTSS)related genes aex T(35.4%)and asc V(30.4%),while the hemolysin gene(hlyA)and aerolysin gene(aer A)were mainly distributed in A.dhakensis and A.hydrophila strains.5.Among all the tested strains,the resistance rate of first-generation cephalosporin(cefazolin)was 93.7%.The drug resistance rates of the third generation cephalosporins were all lower than 10%.Tetracycline resistance gene tet E accounted for 49.4%.10.1% of Aeromonas were multidrug-resistant strains.This study showed that these isolates collected from foodstuffs had a high degree of genetic diversity.Aeromonas was mainly distributed in fish and other seafood,but also sporadically distributed in livestock meat.The distribution,MLST typing,drug resistance and pathogenicity of different Aeromonas species were various.The detection rate of drug resistance gene mcr-3 in Aeromonas was up to 19.0%.Act was the virulence gene with the highest detection rate among Aeromonas,and hlyA may be one of the indispensable factors in virulent strains of Aeromonas.A few isolates,such as A.dhakensis and A.hydrophila,are highly pathogenic and can cause severe infections,indicating a potential risk to food safety and public health from foodborne Aeromonas.The detection data of this study will provide better prevention and treatment strategies for the control of diarrheal diseases caused by Aeromonas in Shanghai. |
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