[Molecular diagnosis of infectious disease]

With the development of molecular biology over the decades, molecular technologies have become available for clinical diagnosis of infectious disease. The principle of the technique is to detect specific genes of microorganisms. Compared with the traditional culturing techniques, molecular technologies have several advantages: 1) direct detection and rapid identification of organisms that are slow growing or those currently lacking a system for in vitro cultivation; 2) nucleic acid-based methods for epidemiologic typing of microorganisms; and 3) identification of genotypic markers of microbial resistance to specific antibiotics. When sufficient amounts of nucleic acid can be obtained, direct probe tests (simple molecular hybridization-based techniques) may be performed. In many instances with small amounts of sample nucleic acids, amplification techniques(e.g., polymerase chain reaction[PCR]) are extremely useful. However, the application of PCR to clinical specimens has potential pitfalls due to the susceptibility of PCR to inhibitors, contamination, and experimental conditions. In some cases, DNA fingerprinting or restriction fragment-length polymorphism analysis is used and it has become the method of choice for epidemiologic analysis. The major disadvantage of the molecular method is that we cannot differentiate whether the microorganisms identified are alive or dead and chemosensitivity testing cannot be performed.