Why is it important to identify microorganisms

Microbiological studies


In order to reveal a pathogen as the cause of a disease, a microbiological examination is necessary. A distinction is made between direct detection methods and indirect detection methods. Bacteria can be stained directly (e.g. Gram stain) displayed under the microscope or grown in a culture. Some viruses can be detected directly by detecting nucleic acids in the polymerase chain reaction. If direct detection methods are unsuccessful, an indirect detection method can be considered with the determination of specific antibodies. The disadvantage is that the antibody reaction only occurs after a time delay (approx. 1 week) and this only proves that the infection has passed through.

Direct evidence

Bacteria detection

Virus detection

  • Virus culture or virus cultivation
    • In general, viruses can also be grown; Viruses are quickly isolated by the host, however, so that this is only possible a few days after the onset of an illness. In addition, viruses do not have their own metabolism, so that cultivation alone is not possible. However, virus-containing body materials can be placed in a cell culture, where they can then multiply. This becomes clear under the light microscope through the so-called cytopathogenic effect. The infestation of the cells by the virus has a cell morphological effect. However, this is often not virus-specific, so that further investigations must be added to identify the virus. The immunofluorescence test is suitable for this
  • Antigen detection
    • E.g. by direct immunofluorescence
      • Objective: Identification of proteins based on their specific antigen structure
      • Marking: addition of antibodies (against the antigen sought) that are coupled with fluorochromes
      • Representation: The treated sample is viewed with the aid of a fluorescence microscope, which shows whether the antigen sought is present
  • Detection of nucleic acids
    • Polymerase Chain Reaction (PCR): Method of amplifying DNA in vitro
      • In contrast to virus cultivation, specific primers must be added to the reaction mixture for each suspected virus, so that a pathogen can only be detected by means of PCR if it is specifically searched for. The primers are then lengthened in the course of the polymerase chain reaction and the corresponding nucleic acid segment of the pathogen is amplified until the resulting gene sequence can be assigned to a pathogen on the basis of gene databases

Indirect evidence

  • Antibody detection / serology
    • If the direct detection of pathogens remains unsuccessful (pathogen has already disappeared or cannot be cultivated), antibodies can still be detected in the serum / liquor
    • IgM production: only increased shortly after an acute infection
    • IgG production: increased later but longer after infection
      • Activity parameters in chronic infections
      • Increasing titer with acute infection
    • Problems
      • Antibody reaction only after a time delay (approx. 1 week)
      • The presence of specific antibodies only proves that an infection has been passed through


  1. Hof, Dörries: Dual series medical microbiology. 3rd edition Thieme 2004, ISBN: 3-131-25313-4.