Detection of microbial genes in a single leukocyte by polymerase chain reaction following laser capture microdissection

Although isolation and identification of bacteria in a clinical specimen constitute essential steps for the diagnosis of bacterial infection, positive results of the bacterial culture are not always attained, despite observing the bacteria by Gram staining. As bacteria phagocytosed by the leukocytes are considered as the causative agents of infectious diseases, this study aims to introduce a new approach for the collection of only bacteria phagocytosed by the neutrophils in an animal model using laser capture microdissection (LCM) followed by the DNA identification using polymerase chain reaction (PCR). We inoculated representative bacteria (Escherichia coli and Staphylococcus aureus) into the abdominal cavities of specific pathogen-free C57BL/6 J mice. After 6 h inoculation, we collected the fluid samples from the peritoneal cavities of mice and demonstrated peritonitis by the increase of neutrophils. Then, we smeared the neutrophils on the membrane slides and collected single-cell phagocytosing bacteria by LCM. The supernatant of the cell lysate was supplied for the PCR reaction to amplify the 16S rRNA gene, and we validated the DNA sequences specific for the inoculated bacteria. In addition, PCR using specific primers for E. coli and S. aureus identified each species of bacteria. Hence, this study suggests that the combination of LCM and PCR could be a novel approach to determine bacteria in infectious diseases. Nevertheless, further investigation is warranted to test various additional bacterial taxa to demonstrate the general applicability of this method to clinical samples.

Rapid and effective method for separation of Staphylococcus aureus from somatic cells in mastitis milk

Quantitative PCR can be an effective method for identifying the bacteria causing mastitis. However, PCR detection is hampered by the presence of inflammatory somatic cells. To eliminate this problem, we attempted to establish methods that allow the effective separation of bacterial cells from somatic cells in mastitis milk with amino-silica. Somatic cells and Staphylococcus aureus cells have different sizes, surface structures, and overall electrical charges; therefore, their adsorption and desorption behavior on amino-silica was also different. We found that although amino-silica could efficiently adsorb both somatic cells and Staph. aureus, somatic cells were adsorbed much more strongly than bacterial cells. We identified conditions under which most of the somatic cells adsorbed and only Staph. aureus desorbed from amino-silica upon addition of a desorption solution. We demonstrated that this procedure effectively eliminated somatic cells in heavily contaminated milk samples, which resulted in improved clarity of the PCR band. These results indicate that pretreatment of the samples with amino-silica made the PCR-based strategy for identifying and quantifying disease-causing bacteria applicable for all milk samples.

Application of new primer-enzyme combinations to terminal restriction fragment length polymorphism profiling of bacterial populations in human feces

New primer-enzyme combinations for terminal restriction fragment length polymorphism (T-RFLP) targeting of the 16S rRNA gene were constructed by using the T-RFLP analysis program (designated TAP T-RFLP) located at the Ribosomal Database Project website, and their performance was examined empirically. By using the fluorescently labeled 516f primer (Escherichia coli positions 516 to 532) and 1510r primer (positions 1510 to 1492), the 16S rRNA gene was amplified from human fecal DNA. The resulting amplified product was digested with RsaI plus BfaI or with BslI. When the T-RFLP was carried out with fecal DNAs from eight individuals, eight predominant operational taxonomic units (OTUs) were detected with RsaI and BfaI digestion and 14 predominant OTUs were detected with BslI digestion. The distribution of the OTUs was consistent with the results of the computer simulations with TAP T-RFLP. The T-RFLP analyses of the fecal DNAs from individuals gave characteristic profiles, while the variability of the T-RFLP profiles between duplicate DNA preparations from the same samples were minimal. This new T-RFLP method made it easy to predict what kind of intestinal bacterial group corresponded to each OTU on the basis of the terminal restriction fragment length compared with the conventional T-RFLP and, moreover, made it possible to identify the bacterial species that an OTU represents by cloning and sequencing.