Retrospective Screening And Analysis Of Mcr-1 And Bla_NDMin Gram-Negative Bacteria In China

ObjectivesCurrently,Gram-negative bacteria have developed multi-drug and broad-spectrum drug resistant,and the numbers of species and strains carrying mcr-1 and blaNDM genes are increasing.In this study,mcr-1 and blaNDM distribution of 12858 Gram-nagetive bacteria isolated from wildlife,patients,livestock,poultry and environment in 14 provinces of China from 2010 to 2019 and the antibiotics resistance in regard to polymyxins,carbapenems and other clinically antibiotics of positive strains were investigated.MethodsA total of 12,858 Gram-negative bacteria were isolated from patients,livestock and poultry,environmental specimens and wild animals in 14 provinces of China from 2010 to 2019.All strains were identified using VITEK ? Compact system(bioMerieux,France)or API 20E strips(bioMerieux,France).These strains were systematically biochemically identified and screened for mcr-1 and blaNDM genes.The positive strain determined by sequencing was further subjected to PCR and sequencing to obtain an open reading frame of the gene.The minimum inhibitory concentration(MICs)of positive strains against polymyxins,carbapenems and other clinically antibiotics were determined by broth microdilution method.ResultsA total of 70 strains of 10 species carried the mcr-1 gene were detected from>118 species of 12,858 Gram-negative bacteria.Thirty-six strains were isolated from patients(0.62%,36/5,828),and 29 strains were isolated from livestock and poultry(4.07%,29/712),5 strains were isolated from the environment(5.43%,5/92);six strains of 3 species carrying blaNDM genes,and all isolates were from patients,the positive rate was 0.10%(6/5,828);among 6,226 strains isolated from wild animal specimens,none of the strains were mcr-1 or blaNDM positive.Two new mcr-1 gene variants were identified in this study:compared with mcr-1.1,mcr-1.21(genbank:MK965883)changed from C to T at the 1234 nt of the open reading frame,the codon changed from valine to serine,and the polymyxins resistant phenotype did not change;another variant(genbank:MK965884)was open reading at the 1344 nt of the box,G changes to A,the codon changes from tryptophan to a stop codon,and the resistant phenotype is sensitive to polymyxins.The drug susceptibility results in polymyxins and carbapenem showed that 66 strains carrying the mcr-1 gene were resistant to polymyxins and 4 strains were sensitive to polymyxins,all 70 strains were sensitive to carbapenem;the 6 strains carrying blaNDM gene had different degrees of resistance to carbapenem and were sensitive to polymyxins.The results of drug resistance to other common antibiotics showed that the strain carrying the mcr-1 gene also had a high degree of drug resistance to tetracyclines,penicillins,quinolones and fluorquinolones,folate pathway antagonists,come certain cephems and aminoglycosides,and only susceptible to some certain types of cephems.The strain carrying the blaNDM gene was resistant to all these antibiotics except polymyxins.ConclusionsThe findings that none of the strains isolated from wildlife in China carried the mcr-1 or blaNDM genes and strains carrying either of the genes were limited and included mostly normal flora of opportunistic or low pathogenic organisms indicated that the emergence of and increase in polymyxin and carbapenem-resistant strains was mainly related to the selective pressure of antibiotics,and transfer of mcr-1 or blaNDM genes between bacteria was relatively limited.