Bacteriology (from the Greek βακτήριον “bacterium” e λόγος “study”) is the branch of biology that studies morphology, ecology, genetics, and biochemistry of bacteria as well as many other aspects related to them (medicine, industry, and agriculture). Bacteria were discovered in 1676 by Anton von Leeuwenhoek. Modern techniques of study originate from about 1870 with the use of stains and the discovery of culture methods using plates of nutrient agar media. Much pioneering work was done by Louis Pasteur, Robert Koch, and Ferdinand Cohn.

Medical bacteriology is a branch of medicine that studies bacteria in relation to disease in the human species. The study of bacteria in the laboratory relies on a few fundamental tools:

  • culture media used to isolate and preserve pure cell lines;
  • generic and specific stains, to be observed under a light microscope (generally);
  • staining with specific labeled antibodies, to be observed under a fluorescence microscope;
  • biochemical tests;
  • serological tests;

The bacteriological analysis aims to highlight pathogenic or potentially pathogenic bacteria and to perform, when necessary, the antibiogram to know which antibiotics have a favorable effect on the elimination of the isolated microorganism. The report of the analysis then allows the doctor to decide whether and which antibiotic to prescribe, depending on the symptoms, the severity of infection and / or any other concomitant diseases. Clinical materials are divided into departments depending on the type of collection:

  • Blood cultures
  • Various infections
  • Respiratory infections
  • Coprocultures
  • Urinary infections
  • Genital infections

Bacteriological investigation tools

  • Bacterial culture media: a large amount of culture media is used for isolation, culture and maintenance of bacterial cells. In order for the bacterium to grow and divide one must also consider the physical agents such as pH, temperature, necessary gases (oxygen, carbon dioxide) that are optimal for the growth of the desired bacterial species. After inoculation of the bacterium, the inoculated medium is incubated for the time necessary for the development of a studious amount of cells. Morphological, biochemical, and serological study should be performed on pure colonies.
  • Bacterial staining: to facilitate observation and recognition, bacteria can be stained or modified with chemical agents. Bacterial staining can be simple or differential. In the first case we proceed with the application of a single solution of dye (methylene blue, fuchsin, eosin …), on a glass slide on which a small amount of bacterial cells had been stretched and fixed. After staining, excess dye is removed by rapid rinsing. In the case of differential staining, two dyes are made to act in succession, of which the first is carefully removed so as to highlight the various bacterial populations and the second is used to stain the cells that have not stained with the first dye. To this category belong Gram Staining and Ziehl-Neelsen Staining. Once the staining is completed, the slide is observed under a light microscope. To recognize the main morphological traits of bacteria it is necessary to observe at 1000 magnifications.
  • Microscopy: for microbiological studies are obviously used special investigation tools such as the light microscope, useful for observing cells and their microstructure, or the electron microscope for the fine study of ultrastructure.
  • Biochemical tests: biochemical tests are used to highlight the presence of certain biochemical characteristics in the bacterium under investigation. These tests can be performed already in the culture medium itself (differential medium). For example, growth on a medium containing only lactose and a red pH indicator that turns yellow in an acidic environment, allows me to easily distinguish colonies of bacteria that fement lactose from those of bacteria that are unable to do so. Sometimes biochemical tests are combined with each other as in the Kligler test that allows to determine lactose fermentation, gas production and H2S production. Other tests are performed by adding a drop of appropriate reagent to a small amount of pure bacterial colony placed on a slide (e.g. oxidase and catalase).
  • Serological tests: with serological tests it is possible to obtain information regarding the presumed contact of the organism with viruses, bacteria, toxins or any antigen that causes the formation of antibodies.

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