Micro for Step 1 - quick hitters

57 quick hitting "to know" for the USMLE Step 1. There are some details here that will enable you to maximize your exam.

Remember! This is a thinking exam. They want you to understand the "Big Picture", then be able to relate details over a spectrum of presentations.
One: Read the stem
Two: Predict what they are asking and what an answer may entail.
Three: Look for clues within the vignette.
Four: Eliminate choices
Five: Answer
Six: Ask yourself. Does this answer their question?


1. Mycoplasmas lack peptidoglycan - are "cell wall-less" and live in association with eukaryotic cells

2. UDP is attached to the precursor but does NOT end up in peptidoglycan cell wall

3. Acid-fast and Gram negative both LACK sterols in their memb.'s, and both have 2 memb's.

4. Spirochetes and Gram neg. have an outer memb. containing porins- are needed to transport hydrophilic molec.'s

5. The end product of sporulation is a spore which is resistant to heat, detergents, dryness and DNA damaging radiation

6. Definition of species is arbitrary - b/c they reproduce asexually

7. Group Translocation: sugar uptake by bacteria where sugar is modified by phosphorylation - the phosphate in PEP ends up on the sugar. THE ONLY transport process where the substrate is modified during uptake.

8. Triclosan reduces the # of bacteria but does not sterilize

9. Quinolone - inhibits DNA gyrase, therefore, inhibiting DNA synthesis

10. Vancomycin - inhibits cell wall synthesis causing cell lysis WITHOUT attaching to PBP's. Vanco binds to the D-ala-D-ala of peptide side chain.

11. Gram pos.' do NOT use permeability for B-lactam resistance, b/c the PBP's (targets) are already outside the cell

12. Macrolide - bacteriostatic - reversibly blocks protein synthesis (the 50S)

13. Type III derived from flagella. Type IV derived from pili.

14. Sulfonamides - work by inhibiting bacterial folic acid synthesis

15. There is only a single RNA polymerase in bacteria

16. mRNAs are polycistronic

17. Transcription of genes for anabolic pathways can be regulated by BOTH positive and negative regulation

18. Transcription of most bacterial genes is controlled at the level of initiation

19. In the trp operon of E. coli - the translation of the upstream ORF controls formation of the attenuator, but the attenuator does NOT control translation
1. The trp attenuator system is a sensor of level of charged trp tRNA, but does not regulate the level of trp tRNA
2. attenuator system regulates transcription of the trp operon by premature termination of transcriptional elongation.
3. attenuator is a single-stem loop - tRNA is a cloverleaf structure.

20. DNA rep. in E. coli - bidirectional starting at oriC. dnaA binds initially to 9mer causing local denaturation, dnaC helps load dnaB helicase onto the origin

21. DNA polymerase III replicates both leading and lagging strands of DNA in E. coli

22. Competence factors are utilized in transformation in Gram POSITIVE bacteria, not gram negative. (pn-"competence is a positive thing"…)

23. Antibiotic treatment does NOT cause mutations

24. F+ cell's F pilus attaches to F- cell causing localized fusion, F plasmid DNA is nicked, allowing transfer of pre-existing strand of F plasmid DNA but not chromosomal DNA to recipient, converting it to an F+ cell.

25. Specialized Transduction - results in 2 copies of any bacterial genes in recipient cell. Uses a phage integrase to place entire transducing phage genome into the chromosome at an att site. Therefore, have one copy of some genes at both normal location in chromosome and at att site.

26. No gain in copy number of incoming gene - Transformation, Conjugation b/w Hfr and F- cell, & generalized transduction. These use homologous recombination.

27. IS elements on F plasmid allow it to become integrated into the host chromosome.
1. OriT is origin of transfer during conjugation
2. tra genes encode apparatus allowing transfer of DNA
3. OriV used for vegetative replication of plasmid
4. IS elements are small transposons homologous to IS elements on the chromosome.
5. Integration is by homologous recombination

28. Antibiotic resistance genes are usually found w/in transposons; transposons can transpose w/in a cell, but not b/w cells. Abx resistance genes are usually transferred b/w cells on plasmids by conjugation.

29. Antibiotic resistance acquired by MUTATION usually involves modification of the target of the antibiotic

30. Antibiotic resistance conferred by plasmid-encoded genes usually involves modification of the antibiotic itself.

31. IS elements are small transposons that encode only a transposase

32. Composite transposons have IS or IS-like elements at ends, NOT integrons
33. Tn3 type transposons encode their own transposase


34. TnA type transposons encode both a transposase and an Abx resistance gene

35. Integrons - build up arrays of Abx resistance genes on plasmids
- can provide promoters for Abx resistance genes

36. Composite transposons, Tn3 type transposons, integrons, and IS elements can all be found on R plasmids

37. Iterons are small repeated DNA sequences that occur at the OriV of some plasmids

38. Operons are the transcriptional units of bacteria

39. Food-borne cholera most commonly caused by under-cooked shellfish from Vibrio-infested waters
a. Note: If you are in restaurant where the water is not safe, drink alcohol w/ no ice.

40. V cholerae incubation range- 12 hrs. - 3 days.

41. Liver and Immune dysfunction predispose to V. vulnificus infection, often from Gulf Coast oysters. Aggressive in infection of skin and soft tissues

42. Cholera cause moderate fever, NOT high fever.

43. Most common commensal organisms in anterior urethra are lactobacilli, streptococci, and coagulase-negative staphylococci.
a. Lactobacilli colonize the vagina at birth
b. " are common in the stomach since they are acid-tolerant

44. Siderophores solubilize and sequester Fe3+
a. Most pathogens use iron:
i. as essential cofactor in e'n transport
ii. in RNA synthesis
iii. for resistance to ROS

45. Cholera toxin is an A-B toxin
a. A = active
b. B = binding

46. In Gram staining, if iodine mordant is not used effectively, Gram+ cells will take up safranin and appear red (like Gm-)

47. PCR reaction - DNA fragments are extended in a 5'-->3' direction.

48. Salmonella and Shigella are both lactose-neg., but Salmonella is motile while Shigella is non-motile

49. ETEC LT-1 is an A-B toxin structurally and functionally similar to cholera toxin: it binds to same receptor, has ADP-ribosylating activity which leads to increased cAMP levels. Fluid loss manifests as watery diarrhea.

50. Yersinia enterocolitica affects the terminal ileum - can mimic acute appendicitis if mesenteric lymph nodes are involved (pseudoappendicitis).

51. Shigella dysenteriae causes dysentery by invasion and replication in enterocytes lining colonic mucosa, cell-to-cell spread of bacteria, and by action of Shiga toxin

52. Campylobacter jejuni is the most common agent of gastroenteritis in U.S. - more cases than from Salmonella and Shigella combined.

53. Salmonella typhi and Salmonella paratyphi are strict HUMAN pathogens, no animal reservoir.
a. Maintained by human colonization, mainly of the gallbladder, sometimes w/o symptoms.

54. Salmonella colerasuis and Salmonella typhimurium have animal reservoirs, but can cause severe disease in humans

55. H pylori mediated damage to gastric mucosa is primarily caused by immune response- secretion of inflammatory cytokines such as- . . . ultimately leading to gastric epithelial cell death.
a. IL-8 - recruitment of neutrophils
b. PAF - stimulates gastric acid secretion
c. NOS - mediates tissue damage
56. EPEC are the ONLY group of E. coli expressing a Type III Secretion System (TTSS).
a. inject numerous effector molecules into intestinal epithelial cells by TTSS.
b. leads to close attachment of bacteria to epithelial cells and effacement of microvilli.

57. EHEC is associated w/hemolytic uremic syndrome (HUS).
a. HUS is associated with Shiga toxin (pref. Stx-2) expressed by EHEC, which destroys glomerular endothelial cells, leading to renal failure.
b. Note: Shigella dysenteriae also exrpresses Shiga toxin similar to Stx-1 of EHEC and can cause renal failure, though not as common.