Bacterial and fungal DNA extraction from blood samples: manual protocols.

A critical point of molecular diagnosis of systemic infections is the method employed for the extraction of microbial DNA from blood. A DNA isolation method has to be able to fulfill several fundamental requirements for optimal performance of diagnostic assays. First of all, low- and high-molecular-weight substances of the blood inhibitory to downstream analytical reactions like PCR amplification have to be removed. This includes human DNA which is a known source of false-positive results and factor decreasing the analytical sensitivity of PCR assays by unspecific primer binding. At the same time, even extremely low amounts of microbial DNA need to be supplied to molecular diagnostic assays in order to detect low pathogen loads in the blood. Further, considering the variety of microbial etiologies of sepsis, a method should be capable of lysing Gram-positive, Gram-negative, and fungal organisms. Last, extraction buffers, reagents, and consumables have to be free of microbial DNA which leads to false-positive results. Here, we describe manual methods which allow the extraction of microbial DNA from small- and large-volume blood samples for the direct molecular analysis of pathogen.