'MLEE, multilocus enzyme electrophoresis.

'MLEE, multilocus enzyme electrophoresis.

do not provide enough unrelated parameters to obtain a good reflection of genotype. For example, serotyping of Streptococcus pneumoniae discriminates among only a limited number of groups. In addition, some virus species, such as human cytomegalovirus and measles virus, cannot be divided into different types or subtypes by serology, because significant antigenic differences do not exist. Second, the expression of many genes is affected by spontaneous mutations, environmental conditions, and by developmental programs or reversible phenotypic changes, such as high-frequency phenotypic switching. Because of this, many of the properties measured by phenotypic methods have a tendency to vary, and for the most part they have been replaced by genotypic methods. The one major exception is multilocus enzyme electrophoresis (MLEE),19,20 which is a robust phenotypic method that performs comparably with many of the most effective DNA-based methods.21,22 Characteristics of selected phenotypic methods are presented in Table 8.6. These methods have been characterized by: typeability, which is the ability of the technique to assign an unambiguous result (i.e., type) to each isolate; reproducibility, which is when a method yields the same results upon repeat testing of a bacterial strain; discriminatory power, which is the ability of the method to differentiate among epidemiologically unrelated isolates; ease of interpretation, which refers to the effort and experience required to obtain useful, reliable typing information using a particular method; and ease of performance, which reflects the cost of specialized reagents and equipment, technical complexity of the method, and the effort required to learn and implement the method.

Extremely sensitive and specific molecular techniques have recently been developed to facilitate epidemiologic studies. Our ability to use these

TABLE 8.7 Molecular characterization of genetic diversity at different hierarchical levels (modified from ref. 24)



Regions of DNA

Discrimination above


Highly conserved coding

level of species

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