16 topics with concise breakdowns: definitions, key facts, and class questions.
This section is dedicated to a vast and clinically significant group of Gram-negative facultatively anaerobic bacilli. It covers the primary culprits of intestinal, respiratory, and sexually transmitted infections: from the common Escherichia coli to the formidable Vibrio cholerae.
The material covers the morphology, properties, and diagnostic methods for key pathogens. We will analyze the Enterobacteriaceae family in detail (Escherichia, Shigella, Salmonella, Klebsiella, Proteus, and Yersinia). A separate major block is dedicated to vibrios—you will study the pathogenicity factors, epidemiology, clinical presentation, and treatment of cholera, as well as halophilic vibrios and Aeromonas. The section concludes with Haemophilus species, including Haemophilus influenzae and the causative agent of chancroid.
In tests and exams, differential diagnosis of enterobacteria (especially lactose fermentation), the mechanism of action of choleragen, and capsular antigens of Haemophilus influenzae are most frequently asked. Pay special attention to the pathogenicity factors of each pathogen—this is the key to understanding the clinical picture and choosing the laboratory diagnostic method.
Which method allows the detection of the toxin directly in the sample without inoculation?
For this purpose, an immunochemical method is used, specifically enzyme-linked immunosorbent assay (ELISA), which detects the toxin directly in the test sample.
Why can infection with ETEC lead to the development of immunity against cholera?
The heat-labile enterotoxin (LT) of Escherichia shares structural homology with cholera toxin (especially their B-subunits), leading to the development of cross-immunity.
Can a Klebsiella infection be prevented with a vaccination?
No, specific prophylaxis has not been developed, and a vaccine does not exist.
How do Shigella species fundamentally differ from Salmonella species biochemically?
Shigella species do not produce gas during glucose fermentation and do not generate hydrogen sulfide ($H_2S$), whereas Salmonella species actively produce hydrogen sulfide and gas.
Why do Salmonella colonies turn black on bismuth sulfite agar?
Salmonella species possess high biochemical activity and produce hydrogen sulfide (H2S). It reacts with bismuth sulfite in the medium, causing the colonies to turn black.
Why do Proteus infections frequently lead to kidney stone formation?
Proteus species secrete the enzyme urease, which hydrolyzes urea to form ammonia. This leads to alkalinization of the urine, causing calcium and magnesium salts to become insoluble and precipitate.
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