General Microbiology

What the USMLE Requires You To Know

•Differences among viruses, fungi, bacteria, and parasites

•Differences between eukaryotic and prokaryotic cells

•Important normal flora

•Major mechanisms of pathogenicity

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MAJOR MICROBIAL GROUPS

Table ll-1-1. Comparison of Medically Important Microbial Groups

*Besides viruses, two other aceltular forms exist:

• Viroids: obligate intracellular but acellular parasites of plants; naked RNA; no human diseases.

•Prions: acellular particles associated with Kuru, etc.; insensitive to nucleases.

Abnormal prion proteins (PrP) modify folding of normal prion-like proteins found in the body (coded for by human genes).

**If the diameter of a cell described in a clinical case is >2 µ, then it is probably a eukaryotic cell.

***Polycistronic mRNA carries the genetic code for several proteins. (It has multiple Shine-Dalgarno sites.)

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15. mutans secretes a biofilm that glues it and other oral flora to teeth, producing dental plaque. 2Nontypeable for Haemophi/us means no capsule.

3Group B streptococci colonizevagina of 15-20% ofwomen and may infect the infant during labor or delivery, causing septicemia and/or meningitis (as may E. coli from fecal flora).

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Type Ill Secretion Systems

•Tunnel from the bacteria to the host cell (macrophage) that delivers bacterial toxins directlyto the host cell

•Have been demonstrated in many pathogens: E. coli, Salmonella species,

Yersinia species, P aeruginosa, and Chlamydia

Inflammation or Immune-Mediated Damage

Examples

•Cross-reaction ofbacteria-induced antibodies with tissue antigens causes disease. Rheumatic fever is one example.

•Delayed hypersensitivityandthe granulomatous response stimulated by the presence ofintracellular bacteria is responsible for neurological damage in leprosy, cavitation in tuberculosis, and fallopian tube blockage resulting in infertilityfrom Chlamydia PID (pelvic inflammatorydisease).

•Immune complexes damage the kidneyin post streptococcal acute glomeru­ lonephritis.

•Peptidoglycan-teichoic acid (large fragments) ofgram-positive cells:

-Serves as a structural toxin released when cells die.

-Chemotactic for neutrophils.

Physical Damage

•Swelling from infection in a fixed space damagestissues; examples: meningitis and cysticercosis.

•Large physical size oforganism may cause problems; example: Ascaris lumbricoides blocking bile duct.

•Aggressive tissue invasion from Entamoeba histolytica causes intestinal ulceration and releases intestinal bacteria, compounding problems.

TOXINS

Toxins mayaidininvasiveness, damage cells, inhibit cellularprocesses, or triggerim­ mune response and damage.

Structural Toxins

•Endotoxin (Lipopolysaccharide = LPS)

-LPS is part ofthe gram-negative outer membrane.

-Toxic portion is lipidA: generally not released (and toxic) until death of cell. Exception: N. meningitidis, which over-produces outer membrane fragments.

-LPS is heat stable and not stronglyimmunogenic so it cannot be con­ verted to a toxoid.

-Mechanism

0 LPS activates macrophages, leading to release ofTNF-alpha, IL-1, and IL-6.

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0Damage to the endothelium from bradykinin-induced vasodilation leads to shock.

°Coagulation (DIC) is mediated through the activation ofHageman factor.

•Peptidoglycan, TeichoicAcids

Exotoxins

•Are protein toxins, generallyquite toxic and secretedby bacterial cells

(some gram +, some gram -)

•Can be modified by chemicals or heat to produce atoxoid that still is immunogenic,but no longer toxic so can be used as a vaccine

•A-B (or"two") component protein toxins

B component binds to specific cell receptors to facilitate the internaliza­ tion ofA.

A component i.s the active (toxic) component (often an enzyme such as an ADP ribosyl transferase).

Exotoxins may be subclassed as enterotoxins, neurotoxins, or cytotoxins.

•Cytolysins: lyse cells from outside bydamaging membrane.

C. perfringens alphatoxin is a lecithinase.

-Staphylococcus aureus alpha toxin inserts itselfto form pores in the membrane.

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6.A cancer chemotherapy patient has to have her intravenous port revised after it becomes blocked and the catheter is found to contain bacterial contaminants. Which ofthe following attributes is most likelyto be a factor in this pathogenesis?

(A)Biofilm production

(B)Ergosterol containing membrane

(C) Peptidoglycan layer

(D)Possession of IgA protease

(E)Possession of pili

7.A 45-year-old female executive goes to a cosmetic surgeon with the com­ plaint of frown lines on her forehead which she feels are negatively affecting her appearance. Rather than undergoing surgery, she opts to try injection of BOTOX. What is the mechanism of action of this toxin?

(A)It blocks release of acetylcholine.

(B)It inhibits glycine and GABA.

(C) It is a lecithinase.

(D)It is a superantigen.

(E)It ribosylates eukaryotic elongation factor-2.

(F)It ribosylates Gs.

Answers and Explanations

1.Answer: C. Gram-positive cocci (alpha hemolytic streptococci) and gram­ negative cocci (Neisseriae) are normally present in the throat. There is no way to differentiate pathogens from non-pathogens by the Gram stain.

2.Answer: E. The cephalosporin that inhibits prokaryotic cell peptidoglycan cross linkage willnot likely be effective against the complex carbohydrate cell wall. Isoniazid, which appears to inhibit mycolic acid synthesis, also would not likelywork. Metronidazole would not work on an aerobic organism. Shiga toxin is only effective against eukaryotic ribosomes. Tetracycline (the correct answer) would have the greatest chance of success. However, it may not be taken up by the cell, or the cell could have an effective pump mechanism to get rid of it quickly.

3.Answer: C. Mitochondria are found only in eukaryotic organisms so both vi­ ruses and bacteria lack them.

5.Answer: E. The attribute of microorganisms which associates most strongly with the causation of granulomas is the fact that they live intracellularly. This causes stimulation ofthe THl arm ofthe immune response, and the production

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ofthe cytokines ofcell-mediated immunity, with the net result ofthe formation of granulomas in the infected tissues. Some organisms which are extracellular willalso produce granulomas, but in those cases it is generally the chronic per­ sistence and indigestibility of the pathogen which cause that result. Lipopoly­ saccharide (choice A) is a synonym for endotoxin, which causes gram negative shock, but not granuloma formation. Pili (choice B) are surface structures of some bacteria which mediate attachment to cellular surfaces. Exotoxins (choice C) are secreted toxins which may cause cell damage in a number ofways, and superantigens (choice D) cause stimulation of large numbers of clones of T lymphocytes and macrophages to cause symptoms similar to endotoxin shock.

6.Answer: A. Catheters, shunts and prosthetic devices which are left in the body long-term, are almost always coated with Teflon which is extremely slippery. Organisms which are capable of adherence to Teflon (or the enamel of teeth), do so by creation of a biofilm, which allows them to change the surface tension of the liquid around them and thereby "glue" themselves to the material. Er­ gosterol (choice B) is the major sterol in the cell wall offungi, and is important in membrane integrity, but not adherence. Peptidoglycan (choice C) is the cell wall material of bacteria, and is responsible for the shape of bacteria, but not their adherence. IgA proteases (choice D) can assist in the adherence of bacte­ ria to mucosal surfaces, but would not be important in adherence to an intra­ venous catheter, and although pill (choice E) mediate attachment ofbacteria to human cells, they would not be important in adherence to Teflon.

7.Answer: A. Botulinum toxin (in BOTOX) inhibits release of acetylcholine and results in a flaccid paralysis. Inhibition of glycine and GABA (choice B) de­ scribes the action of Tetanus toxin which causes a rigid paralysis. The toxin of Clostridium perfringens is a lecithinase (choice C) which directly disrupts cell membranes. Toxic shock syndrome toxin- 1 and the pyrogenic exotoxins of Streptococcuspyogenes actas superantigens (choiceD) which cause systemic in­ flammatory response syndrome. Ribosylation ofeukaryotic elongation factor-2 (choice E) is the mechanism ofaction ofthe diphtheria toxin and Pseudomonas exotoxin A. Ribosylation of Gs (choice F) is the mechanism of action of the cholera toxin and the labile toxin of Enterotoxigenic Escherichia coli.

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