Genetic evidence of clonal descent of Escherichia coli O157:H7 associated with hemorrhagic colitis and hemolytic uremic syndrome

Genetic relatedness of 100 strains of Escherichia coli, isolated mostly from patients with hemorrhagic colitis or hemolytic uremic syndrome, was determined for chromosomal genotypes on the basis of allelic variation at 17 enzyme-encoding loci detected by multilocus enzyme electrophoresis. Fifteen of the 17 loci were polymorphic, with an average of 3.5 alleles per locus. Comparison of the observed combinations of alleles among strains revealed 25 distinct multilocus genotypes, which were used to define naturally occurring cell lineages or clones. Cluster analysis of the genotypic data revealed that isolates of serotype O157:H7 fall into a well-defined group of clonal genotypes that share alleles, on average, at 90% of their enzyme loci. The O157:H7 clonal group is only distantly related to other Verotoxin-producing strains belonging to other serotypes of E. coli. The results strongly support the hypothesis that isolates of E. coli O157:H7 obtained from geographically separate outbreaks and sporadic cases of hemorrhagic colitis and hemolytic uremic syndrome belong to a pathogenic clone that occurs throughout North America.     

Evolutionary genetics of the encapsulated strains of Haemophilus influenzae.

Genetic relationships among 2209 isolates of Haemophilus influenzae of polysaccharide capsule serotypes a, b, c, d, e, and f were determined by analyzing electrophoretically demonstrable allelic variation at 17 chromosomal enzyme loci. We distinguished 280 electrophoretic types (ETs), representing distinctive multilocus genotypes. Genetic diversity among ETs of isolates of the same serotype was, on average, only 67% of that in the total sample, and no ETs were shared among isolates of different serotypes. Cluster analysis of the ETs revealed 2 primary phylogenetic divisions at a genetic distance of 0.66 and 12 major lineages diverging from one another at distances greater than 0.42. In general, strains of different phylogenetic lines or groups of allied lineages have characteristic cap region restriction fragment length polymorphism patterns obtained by digestion of genomic DNA with EcoRI. Strains producing serotype c, e, and f capsules have no close relationships to those of other encapsulated strains. Lineages of both serotype a and b strains occur in each primary phylogenetic division, most probably as a result of the transfer of serotype-specific sequences of the cap region between clonal lineages. Serotype a strains allied in division I with a group of abundant serotype b clones and the serotype d strains apparently are more virulent than the serotype a strains in division II, which are related to serotype b and f strains that rarely cause invasive disease.     

Global genetic structure and molecular epidemiology of encapsulated Haemophilus influenzae

A collection of 2,209 isolates of six polysaccharide capsule types of Haemophilus influenzae, including 1,975 serotype b isolates recovered in 30 countries was characterized for electrophoretically demonstrable allele profiles at 17 metabolic enzyme loci. Two hundred eighty distinct multilocus genotypes were distinguished, and cluster analysis revealed two primary phylogenetic divisions. The population structure of encapsulated H. influenzae is clonal. Currently, most of the invasive disease worldwide is caused by serotype b strains of nine clones. Strains producing serotype c, e, and f capsules belong to single divisions and have no close genetic relationships to strains of other serotypes. Serotype a and b strains occur in both primary phylogenetic divisions, probably as a result of transfer and recombination of serotype-specific sequences of the cap region between clonal lineages. A close genetic relatedness between serotype d isolates and some strains of serotypes a and b was identified. There are strong patterns of geographic variation, on an intercontinental scale, in both the extent of genetic diversity and the clonal composition of populations of encapsulated strains. The analysis suggests that the present distribution of clones is, in part, related to patterns of racial or ethnic differentiation and historical demographic movements of the human host populations.     

Clones of serogroup B Neisseria meningitidis causing systemic disease in The Netherlands, 1958-1986

Serogroup B isolates of Neisseria meningitidis recovered from 278 patients with systemic disease in the Netherlands between 1958 and 1986 were analyzed with respect to serotype and multilocus enzyme genotype. Of the isolates, 28% were serotype 2b and 53% were neither serotypeable nor serosubtypeable. There were 145 distinct multilocus genotypes (electrophoretic types, ETs), with up to 31 isolates belonging to the same ET. Temporal changes in the genotypic composition of meningococcal populations in the Netherlands were demonstrated by the recent occurrence of disease caused by three clone lineages, I, III, and VI, that were not found before 1975. The epidemic of 1966-1967 and the hyperendemic wave of 1972 were caused, in large part, by two closely related but distinct clones of serotype 2b isolates, ET-11 and ET-17, respectively. Deviations in male-to-female ratio and age distribution of patients were observed for disease caused by isolates of individual clone lineages.     

Identification of a high-virulence clone of type III Streptococcus agalactiae (group B Streptococcus) causing invasive neonatal disease

Chromosomal genotypes of 128 isolates of six serotypes (Ia, Ib, Ic, II, Ic/II, and III) of Streptococcus agalactiae (group B Streptococcus) recovered predominantly from human infants in the United States were characterized by an analysis of electrophoretically demonstrable allelic profiles at 11 metabolic enzyme loci. Nineteen distinctive electrophoretic types (ETs), representing multilocus clonal genotypes, were identified. Mean genetic diversity per locus among ETs of isolates of the same serotype was, on average, nearly equal to that in all 19 ETs. Cluster analysis of the ETs revealed two primary phylogenetic divisions at a genetic distance of 0.65. A single clone (ET 1) represented by 40 isolates expressing type III antigen formed division I. Division II was composed of 18 ETs in three major lineages diverging from one another at distances greater than 0.35 and included strains of all six antigenic classes. The type III organisms in division I produce more extracellular neuraminidase and apparently are more virulent than the type III strains in division II, which are related to strains of other serotypes that cause disease much less frequently. The existence of this unusually virulent clone accounts, in major part, for the high morbidity and mortality associated with infection by type III organisms.     

A single clone of Staphylococcus aureus causes the majority of cases of toxic shock syndrome.

Genetic relationships among 315 isolates of the bacterium Staphylococcus aureus expressing toxic shock syndrome toxin-1 (TSST-1) recovered primarily from humans with toxic shock syndrome (TSS) in five countries on two continents were determined by analyzing electrophoretically demonstrable allelic variation at 20 chromosomal enzyme loci. Forty-nine distinctive electrophoretic types (ETs), representing multilocus enzyme genotypes, were identified. Cluster analysis of the ETs revealed two major phylogenetic divisions separated at a genetic distance of 0.35 and seven branches diverging from one another at distances greater than or equal to 0.20. A single clone (ET 41) accounted for 88% of cases of TSS with a female urogenital focus and 53% of TSS cases involving nonurogenital (predominantly wound) infections. With few exceptions, strains representing different phylogenetic lines had characteristic TSST-1 gene (tst) restriction fragment length polymorphism patterns obtained by digestion of genomic DNA with Cla I. Strains recovered from ovine and bovine hosts with mastitis were genotypically distinct from the major human TSS clone. The expression of TSST-1 in cell lineages representing the total breadth of multilocus genotypic diversity in the species S. aureus as a whole is interpreted as evidence that the TSST-1 gene is evolutionarily old. The recovery of a single clone from the majority of individuals afflicted with TSS having a urogenital focus and from the genital tract of a large proportion of asymptomatic female carriers strongly suggests that this clone is especially well adapted for colonization of these anatomic sites.     

Streptococcus pyogenes causing toxic-shock-like syndrome and other invasive diseases: clonal diversity and pyrogenic exotoxin expression.

Genetic diversity and relationships among 108 isolates of the bacterium Streptococcus pyogenes recently recovered from patients in the United States with toxic-shock-like syndrome or other invasive diseases were estimated by multilocus enzyme electrophoresis. Thirty-three electrophoretic types (ETs), representing distinctive multilocus clonal genotypes, were identified, but nearly half the disease episodes, including more than two-thirds of the cases of toxic-shock-like syndrome, were caused by strains of two related clones (ET 1 and ET 2). These two clones were also represented by recent pathogenic European isolates. A previous report of a relatively high frequency of expression of exotoxin A among isolates recovered from toxic-shock-like syndrome patients in the United States was confirmed; and the demonstration of this association both within clones and among distantly related clones supports the hypothesis that exotoxin A is a causal factor in pathogenesis of this disease. Near identity of the nucleotide sequences of the exotoxin A structural gene of six isolates of five ETs in diverse phylogenetic lineages was interpreted as evidence that the gene has been horizontally distributed among clones, presumably by bacteriophage-mediated transfer.     

动物源沙门菌的耐药表型、基因型及流行特征研究

目的 研究动物源沙门菌耐药及流行特征。方法 对116株动物源沙门菌进行血清学分型、药物敏感试验、耐药基因(簇)检测和多位点序列(MLST)分析。结果 鸡源沙门菌多为ST11克隆肠炎沙门菌和ST17克隆印第安那沙门菌,猪源沙门菌多属于ST40克隆德尔卑沙门菌。ST11克隆主要表现为对氨苄青霉素、萘啶酸、四环素、头孢哌酮耐药,且多数为多重耐药(MDR);ST17克隆多数对9种以上的药物特别是头孢类及氟喹诺酮类耐药,为超级耐药(Super-MDR)克隆。ST11克隆中blaTEM-1-like携带率最高,ST17克隆中blaOXA-1-like、blaCTX-M、blaTEM-1-like及floR、aadA2、sul1及aac(6')-1b高携带率是其成为超级耐药克隆的主要原因。结论 抗菌药物特别是头孢类、氟喹诺酮类药物在动物生产中应当控制使用,对控制耐药菌的产生及传播有重要意义。     

Evolution of erythromycin-resistant Streptococcus pneumoniae from Asian countries that contains erm(B) and mef(A) genes

To investigate the genetic characteristics of erythromycin-resistant Streptococcus pneumoniae in Asian countries, 110 pneumococcal isolates were analyzed by multilocus sequence typing (MLST). MLST analysis showed that 2 clonal complexes--CC236 (Taiwan19F-14 clone) and CC81 (Spain23F-1 clone)--are the major lineages of erythromycin-resistant S. pneumoniae in the Asian region. Pneumococcal isolates containing both the erm(B) and the mef(A) genes are thought to have originated from the Taiwan19F-14 clone containing the mef(A) gene, after introduction of the erm(B) gene. Further evolution of this variant clone has generated resistant strains with different sequence types. Dissemination of these variant clones of the Taiwan19F-14 could be the main reason for the high frequency of pneumococcal isolates containing both erm(B) and mef(A) in some Asian countries. Data suggest that the high prevalence of erythromycin-resistant S. pneumoniae in the Asian region is partly due to the clonal spread of a few multidrug-resistant clones.     

The evolutionary history of methicillin-resistant Staphylococcus aureus (MRSA)

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of hospital-acquired infections that are becoming increasingly difficult to combat because of emerging resistance to all current antibiotic classes. The evolutionary origins of MRSA are poorly understood, no rational nomenclature exists, and there is no consensus on the number of major MRSA clones or the relatedness of clones described from different countries. We resolve all of these issues and provide a more thorough and precise analysis of the evolution of MRSA clones than has previously been possible. Using multilocus sequence typing and an algorithm, BURST, we analyzed an international collection of 912 MRSA and methicillin-susceptible S. aureus (MSSA) isolates. We identified 11 major MRSA clones within five groups of related genotypes. The putative ancestral genotype of each group and the most parsimonious patterns of descent of isolates from each ancestor were inferred by using BURST, which, together with analysis of the methicillin resistance genes, established the likely evolutionary origins of each major MRSA clone, the genotype of the original MRSA clone and its MSSA progenitor, and the extent of acquisition and horizontal movement of the methicillin resistance genes. Major MRSA clones have arisen repeatedly from successful epidemic MSSA strains, and isolates with decreased susceptibility to vancomycin, the antibiotic of last resort, are arising from some of these major MRSA clones, highlighting a depressing progression of increasing drug resistance within a small number of ecologically successful S. aureus genotypes.