10 topics with brief reviews: definitions, key facts, and class questions.
This section covers pleomorphic Gram-positive non-spore-forming bacteria (excluding actinomycetes in the broad sense) often characterized by a complex cell wall structure. Here we examine corynebacteria (including the diphtheria agent), mycobacteria (tuberculosis, leprosy, and nontuberculous forms), nocardiae, as well as key representatives of normal and opportunistic microflora — bifidobacteria, propionibacteria, and Gardnerella vaginalis.
In tests and exams, you are guaranteed to be asked about the specifics of the Ziehl-Neelsen stain for mycobacteria, the cord factor, and the resistance mechanisms of the tuberculosis pathogen. The second favorite topic of examiners is diphtheria. You need to know clearly how diphtheria exotoxin works, how to detect volutin granules, and why the Elek test is needed.
For clinical practice, pay attention not only to the 'major killers' but also to opportunistic flora. Understanding the role of Gardnerella vaginalis in the development of bacterial vaginosis and the importance of bifidobacteria for the gut is essential for every future physician.
What features distinguish C. pseudodiphtheriticum from the diphtheria agent?
The cells of the false diphtheria bacillus are small, uniform in size, arranged in a 'palisade' pattern, and practically devoid of volutin granules. In addition, this species produces urease, which serves as an important biochemical marker.
Why do pseudomembranes form in diphtheria?
In response to cell destruction by dermonecrotoxin, thrombokinase is released. Under the action of hyaluronidase, fibrinogen exits the blood and clots into fibrin, forming a dense plaque.
Do mycobacteria form spores or capsules?
No, all members of this genus are non-motile rods that do not form spores and lack capsules.
Why do tuberculosis mycobacteria not stain with Gram stain?
Due to the high content of waxes and lipids in the cell wall, they poorly take up standard water-soluble stains. The Ziehl-Neelsen staining method involving heating is used to detect them.
Can the leprosy agent be cultured on nutrient media in a laboratory?
No. Mycobacterium leprae is an obligate intracellular parasite and does not grow on artificial nutrient media.
How do mycobacterioses fundamentally differ from tuberculosis in terms of epidemiology?
The main difference is the lack of contagiousness. Nontuberculous mycobacteria are acquired from the environmental source; transmission of the pathogen from a sick person to a healthy one does not occur.
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