Clinical Microbiology Made Ridiculously Simple 6th Edition / Клинична Микробиология направена изключително лесна: CHAPTER 1. BACTERIAL TAXONOMY

3)

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Interior to the core polysaccharide is the third

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component, lipid A, which is a disaccharide with mul­ tiple fatty acid tails reaching into the membrane. Lipid A is toxic to humans and is known as the gram-negative endotoxin. When bacterial cells are lysed by our efficiently working immune system, fragments of mem­ brane containing lipid A are released into the circula­ tion, causing fever, diarrhea, and possibly fatal endotoxic shock (also called septic shock).

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Embedded in the gram-negative outer membrane are porin proteins, which allow passage of nutrients. These are also unique to gram-negative organisms.

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What does this mean clinically?

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The differences between gram-positive and gram­

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negative organisms result in varied interactions with the environment. The gram-positive thickly meshed peptidoglycan layer does not block diffusion oflow mol­ ecular weight compounds, so substances that damage the cytoplasmic membrane (such as antibiotics, dyes, and detergents) can pass through. However, the gram­ negative outer lipopolysaccharide-containing cell mem­ brane blocks the passage of these substances to the peptidoglycan layer and sensitive inner cytoplasmic membrane. Therefore, antibiotics and chemicals that attempt to attack the peptidoglycan cell wall (such as penicillins and lysozyme) are unable to pass through.

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Interestingly, the crystal violet stain used for Gram staining is a large dye complex that is trapped in the thick, cross-linked gram-positive cell wall, resulting in the gram-positive blue stain. The outer lipid-contain­ ing cell membrane of the gram-negative organisms is partially dissolved by alcohol, thus washing out the crystal violet and allowing the safranin counterstain to take.

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Fig. 1-7. Summary of differences between gram­ positive and gram-negative bacteria.

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BACTERIAL MORPHOLOGY Bacteria have 4 major shapes:

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1)

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Cocci: spherical.

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2)

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Bacilli: rods. Short bacilli are called coccobacilli.

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3)

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Spiral forms: comma-shaped, S-shaped, or spiral­

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shaped.

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4)

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Pleomorphic: lacking a distinct shape (likejello).

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The different shaped creatures organize together into

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more complex patterns, such as pairs (diplococci), clus­ ters, strips, and single bacteria with flagella.

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Fig. 1-8. Bacterial morphology.

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SO, WHAT ARE THE NAMES?!!!! Gram-Positive

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Start by remembering that there are 7 classic gram­ positive bugs that cause disease in humans, and basically every other organism is gram-negative.

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Of the gram-positives, 3 are cocci, and the other 4 are rod-shaped (bacilli).

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The 3 gram-positive cocci both have the word coccus in their names:

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1)

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Streptococcus and 2) Enterococcus form strips of

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COCCI.

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3)

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Staphylococcus forms clusters of cocci.

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Two of the 4 gram-positive rods produce spores (spheres that protect a dormant bacterium from the

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harsh environment). They are:

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4)

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Bacillus

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5) Clostridium

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The last 2 gram-positive rods do not form spores:

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6)

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Corynebacterium

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7)

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Listeria

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Gram-Negative

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Of the gram-negative organisms, there are only

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two groups of gram-negative cocci. Both are actually diplococci (look like 2 coffee beans kissing): Neisseria and Moraxella.

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There is also just 1 group of spiral-shaped organisms: the Spirochetes. This group includes the bacterium Treponema pallidum, which causes syphilis.

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The rest are gram-negative rods or pleomorphic.

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Exceptions:

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1)

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Mycobacteria are weakly gram-positive but

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stain better with a special stain called the acid-fast stain (See Chapter 15). This special group includes or­ ganisms that cause tuberculosis and leprosy.

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