An attempt at an epidemiological explanation - Epib.nl

Introductionization of N. meningitidis, immunotyping is not regularly performed and the immunotypeis often omitted from the phenotype description.Phenotyping has proved to be a valuable tool for studying the spread ofMD!' 19 22 2$.31 For the future prevention of MD by vaccination, monitoring of thechanges in the distribution of serotypes and subtypes will become of utmost importance(see below) .GenotypingIn order to assess the genetic relationship of different meningococcal isolates,various genotyping methods are available. Common methods are muItilocus enzymeelectrophoresis and restriction fragment length polymorphism." 39In multilocus enzyme electrophoresis, also called electrophoretic typing, thepattern of isoenzyme variants of several common cytoplasmic enzymes is used toestimate the genetic content of meningococcal isolates. Isoenzyme variants aredetermined by using specific enzyme stains after starch gel electrophoresis." Based onthe results of the electrophoretic migration of each enzyme, each isolate is thenassigned a multidigit score. Each multidigit score thus obtained forms an electrophoretictype (ET), designated by an arabic number." The various ETs are labelled from1 to the total number of different ETs detected among the isolates tested. Thenumber of different ETs, therefore, depends on the number of isolates. Thus, thedesignation of ETs is nominal and varies from survey to survey, and ET numbersallocated in different surveys usually are not similar or exchangeable, unless the ETnumbers of one survey are adapted to those of another. Each separate ET representsa clone with a particular genetic background. On the basis of the isoenzyme variantsof the various enzymes, the genetic relationship of the ETs of the tested isolates isassessed by the use of multivariate statistical methods," and clusters (or lineages) ofclones that are genetically closely related can he distinguished." 2S '053740·43 Electrophoretictyping has been applied for the determination of the genotype of meningococcalisolates in the Netherlands"Restriction fragment length polymorphism (RFLP) is another method for theassessment of genetic relationship of meningococci. 39 The procedure involves extractingcellular DNA and fragmentating it with restriction endonucleases. The fragmentsare subsequently separated by electrophoresis. Following denaturation of the DNA,the fragment bands are blotted. After hybridization with a DNA probe, the fragmentswhich contain the sequence to which the probe is directed are visua lized. By the useof the probe, a reduction in the number of DNA bands is obtained, which facilitatesthe interpretation of the DNA patterns. The choice of the probe is based on repeatedDNA sequences found in various parts of the chromosome. In order to be able tocharacterize all possible meningococcal strains, it is essential to select a probe withbroad specificity. RFLP types are designated according to the various RFLP patternsfound in the study under consideration, and the number of different RFLP types alsodepends on the number of isolates tested. RFLP results are similar to those obtained7