Microbiology Exam 1 Short Answer

Microbiology Exam 1 Short Answer Responses

Following below are the short answer questions with responses. Good for practice reviews.

[01-04] Compare kingdoms Protozoa, Plantae, Fungi, and Animalia by describing their specific properties – Lecture 1, Slide 9 – All four kingdoms belong to Eukarya domain which consist of eukaryotic organisms that are made of eukaryotic cells that have nucleus, membrane-bound organelles (mitochondria, endoplasmic reticulum, Golgi apparatus) and are dividing by mitosis or meiosis All kingdoms, except Plantae, also include human pathogens. Kingdom Protozoa – Uni- or di-cellular organisms. Some of them can do photosynthesis and have the cell wall made of cellulose. Kingdom Plantae consist of multicellular photosynthetic organisms that have cell wall made of cellulose. Kingdom Fungi consist of multi- and uni-cellular organism that are not photosynthetic but have cell wall made of chitin. It is also a major source of antibiotics. Kingdom Animalia consist of multicellular organisms that cannot do photosynthesis and have no cell wall.

22. [03-05] Why are Gram-positive bacteria generally more sensitive to antibiotics than Gram-negative and Acid-fast bacteria? – Lecture 1, Slide 28 – Gram-negative bacteria have second membrane, called outer membrane and acid-fast bacteria have highly hydrophobic cell wall, which makes the entry of antibiotics in to bacterial cells harder than in to Gram-positive bacteria that have only single membrane and hydrophilic cell wall. 

23. [03-09] Compare prokaryotic and eukaryotic cells by describing their similarities and differences – Lecture 1, Slide 38 – Similarities: Both Prokaryotic and eukaryotic cells are built of proteins, lipids, nucleic acids (RNA and DNA) and have plasma (cytoplasmic) membrane as natural semi-permeable boundary. Both types of cells have 70S ribosomes. Differences: Eukaryotic cells are much bigger than prokaryotic cells. Eukaryotic cells have multiple linear chromosomes arranged in pairs, while prokaryotic cells have single circular chromosome. Eukaryotic cells have nucleus, membrane-bound organelles and 80S ribosomes, while prokaryotic cells have none of them. Prokaryotic cells are going via simple division (binary fission), while eukaryotic cells are going via mitosis or meiosis.

24. [06-01] What are metabolism and its two branches? – Lecture 3, Slides 2, 5 – Metabolism is a sum of all chemical reactions that take place in the cell. It has two branches – catabolism and anabolism. Catabolic reactions (also called exergonic) break down energy-rich and complex molecules and harvest energy in form of ATP. Anabolic reactions (also called endergonic) consume energy and build up complex molecules out of simpler ones. Therefore, both branches are coupled or interconnected via ATP molecules which allows the flow of energy from catabolic reactions to anabolic reactions. 

25. [06-12] Describe the role of electron transport chain and chemiosmosis in cellular respiration – Lecture 3, Slide 17 – Electron transport chain is a set of membrane proteins. The energy-rich electrons are damped by electron carriers (NADH and FADH₂) on ETC. As these electrons travel along ETC, protons are pumped across the membrane, creating proton gradient. Chemiosmosis is the movement of protons across membrane along the proton gradient formed with help of energy-rich electrons and electron transport chain. Chemiosmosis is used for synthesis of ATP during cellular respiration and photosynthesis. 

26. [10-01] Explain taxonomic hierarchy. Give examples – Lecture 3, Slides 24, 25 –Taxonomic hierarchy is a system of taxa (classification units) of various ranks. The high ranked taxon may include multiple taxa of lower ranks. Taxa from highest to lowest: Domain à Kingdomà Phylum à Class à Order à Family à Genus à Species. These taxa are used to classify biological species. The closer two species related to each other, the lower taxon they will share.

27. [11-10] Compare the properties of bacterial genera Mycoplasma and Mycobacterium. Name human pathogens from these genera.– Lecture 4, Slides 10, 11, 13, 14, 20, 22 – Similarities: both genera, Mycoplasma and Mycobacterium, belongs to domain Bacteria that includes prokaryotic organisms that are lacking nucleus and membrane-bound organelles. Species from both genera are obligate aerobes, do not produce endospores, do not produce any toxins and can cause infections in humans. Differences: genus Mycoplasma belongs to phylum Firmicutes, consisting of Gram-positive bacteria with relatively low (23-50%) GC content. Mycoplasmas are lacking cell wall and form punctate (tiny) colonies. Some species exhibit gliding motility. Principal human pathogens are: M. pneumoniae is causing mild form (walking) pneumonia; M. hominis is causing pelvic inflammatory disease, bacterial vaginosis in women, infertility in men. Genus Mycobacterium belongs to phylum Actinobacteria that consist of Gram-positive bacteria with high (~70%) GC content. Mycobacteria have acid-fast cell wall that is very hydrophobic and contains compounds that enable bacterial survival inside macrophages by inhibiting digestion. As a result, mycobacteria have very long generation time from 24 hours to 12 days. Principal human pathogens are: M. tuberculosis is causing tuberculosis; M. leprae is causing leprosy (Hansen disease).

28. [13-02] What defines the virus’ host specificity? – Lecture, Slide 24 – The host specificity of the virus is defined by 1) specific interaction of surface antigens (viral adhesin) and host cell surface protein (cell receptor). 2) Restriction-Modification system of the host cell that is consisting of two enzymes restrictase that can cleave DNA into fragments at restriction sites and DNA modifying enzyme (DNA methylase) that can modify these restriction sites in host DNA.

29. [13-03] What is lysogenic conversion? Give two examples naming bacterial species, virus involved, toxin produced, and disease caused in humans – Lecture 4, Slide 28 – Lysogenic conversion is the change of bacterial phenotype due to viral infection, when viral genome is inserted into bacterial chromosome and virus become provirus. Usually it results in increased pathogenicity as bacterial cell acquires ability to produce toxins. Examples: Corynebacterium diphtheriae lysogenic for β (beta) virus produces secretory diphtheria toxin and causes diphtheria in humans; Escherichia coli lysogenic for λ (lambda) virus produces shiga toxin and causing hemolytic uremic syndrome; Streptococcus pyogenes lysogenic for T12 phage producing pyrogenic exotoxins Spe A and Spe C and causing scarlet fever in humans; Vibrio cholerae lysogenic for CTXϕ virus produces cholera toxin and causes cholera in humans.

30. [14-06] Describe fever development caused by infection of Gram-negative bacteria. What are benefits of fever development? – Lecture 5, Slide 14 – Fever development due to Gram-negative bacteria infection. Gram-negative bacterium is ingested by macrophage. Formed phagosome is fused with lysosome and ingested bacterium is destroyed. Endotoxin released from the cell wall of Gram-negative bacterium triggers production of interleukin 1 by macrophage. Interleukin 1 secreted by macrophage enters blood stream and is delivered to hypothalamus. In response to appearance of interleukin 1, hypothalamus produces prostaglandin that causes development of fever. Benefits of fever development: 1) reduction of bacterial growth rate; 2) increased activity of T helper and T cytotoxic cells in the host; 3) increased production of antimicrobial substances by the host.

31. [15-01] Distinguish Antigen, Epitope, Hapten, Antibody. – Lecture 5, slide 18 – Antigen is any compound that can induce the antibody production in the host. Various macromolecules, particularly proteins, may act as antigens. Epitope is a part of antigen that can be recognized by a specific antibody. In proteins it is a stretch of 6 amino acids in proteins. The same epitope may be present in different antigens. Hapten is small molecule that is not immunogenic (it does not cause immune response). Hapten becomes immunogenic only if it forms a complex with a carrier molecule. For example, penicillin is not causing immune response in 95% of human population. However, penicillin forms complex with albumin in 5% of human population due to mutation and causes allergic reaction. Antibody is a protein produced by immune system in response to appearance of foreign antigens in the host. Each antibody recognizes and binds to a particular epitope of the antigen. Antibody would have cross-reactivity, if the same epitope is present in different antigens.

32. [15-04] Describe antibody-dependent cellular toxicity as protective mechanism resulting from antibody-antigen binding – Lecture 5, Slide 20 – If antibody binds to a large pathogen, cancerous or infected cell that is too big to be ingested by phagocytic cell, the sticking out Fc (antibody tail) can be recognized by Fc receptors of Natural killers. Natural killers would attack such antibody-labeled cell eventually killing it by delivering to the cell various toxic substances that either lyse the targeted cell or trigger apoptosis (programmed cell death) in it. 

33. [16-03] Describe four exceptions from Koch’s postulates. Give microbial examples for each exception – Lecture 6, Slide 10 – 1) Some pathogens cannot be grown in pure cultures, like Treponema pallidum causing syphilis; 2) Some pathogens have humans as the only host, like HIV or Shigella; 3) Some pathogens can cause multiply disease, like S. pyogenes can cause impetigo, Scarlet fever, Necrotizing fasciitis; 4) Same disease is caused by various microbes, like pneumonia can be caused by M. pneumoniae, S. pneumoniae, N. pneumoniae

34. [16-04] Describe the breaching and infection of mucous membrane by Shigella spp. Name virulence factors involved in this mechanism – Lecture 6, Slides 13, 14 – It occurs in two distinct steps: Step 1. Exploiting antigen sampling function of microfold (M) cells. M (microfold) cells are localized in the small intestines. They transport the samples of gut content across epithelial cells and present it to macrophages. Along with gut content sample, Shigella is taken in by M cell at apical membrane. The microbe is moved across M cell. At basolateral membrane Shigella cell is presented to macrophage and then bacterium is ingested. Due to specific properties of its cell wall, microbe inhibits the formation of phagolysosome and survives inside the macrophage. Shigella produces secretory exotoxin that is killing macrophage & freeing microbe. Step 2. Directed uptake by host cell. Freed from macrophage Shigella cell binds to basolateral membrane of epithelial cell via adhesins. Using type III secretion system microbe injects effector molecules that are disturbing cytoskeleton of the host cell and are causing membrane ruffling. Shigella sinks into the ruffles and forces the host cell to take it in via endocytosis. Virulence factors are in bold.

35. [16-07] Describe cholesterol-dependent cytolysin action. Give examples – Lecture 6, Slides 18, 19 – Cholesterol-dependent cytolysins belong to type II exotoxins that are membrane damaging as they bind to plasma (cytoplasmic) membrane of host cells and compromise its integrity. It leads to cell lysis and development of tissue necrosis, which in turn increases the invasiveness of a microbe. Cholesterol-dependent cytolysins are produced and secreted by bacteria as non-active monomers. They bind only to cholesterol-containing membranes (cholesterol is not found in bacteria, with Mollicutes as exception). In the presence of cholesterol, monomers are assembled into complexes with large pores causing cell lysis. Examples: perfringolysin O of Clostridium perfringens; streptolysin O (oxygen-sensitive) and streptolysin S (oxygen-stable) of S. pyogenes;  toxin (also called -hemolysin) and leukocidin of S. aureus; pneumolysin of Streptococcus pneumoniae.

PART 5. Bonus Questions.

BQ1. Describe endosymbiotic theory and the data supporting it. – Slide 39, Lecture 1 – The theory suggests that chloroplasts and mitochondria have prokaryotic origin. Eukaryotes had originated from the ancestor that is common to prokaryotes as well. Metabolically active intracellular parasites like rickettsia gave rise to mitochondria. While blue-green photosynthetic bacteria gave rise to chloroplasts. Data supporting this theory: 1) DNA of mitochondria and chloroplasts are similar to the prokaryotic one. 2) Ribosomes of mitochondria and chloroplasts are 70S (prokaryotic type). 3) Composition of inner membrane of mitochondria & chloroplasts are similar to that of prokaryotes

BQ2. Human pathogens can be divided into four groups, according to their cell wall. Explain how the type of cell wall or absence of it affects the treatment of infection caused by corresponding bacteria: – Lectures 1-6 – 1) Gram positive bacteria – they have only plasma membrane and no endotoxins associated with their cell wall. Therefore, this group of bacteria generally are more sensitive to antibiotic treatment. Any type of antibiotic, bactericidal or bacteriostatic, can be used in treatment of infections caused by Gram-positive bacteria; 2) Gram negative bacteria – they have two membranes, plasma membrane and outer membrane. Therefore, this group of bacteria is generally less sensitive to antibiotic treatment. In addition, cell wall of Gram-negative bacteria contain endotoxin, which is released upon bacterial cell death. As a result, only bacteriostatic antibiotics are used in treatment of infections caused by Gram-negative bacteria; 3) Acid-fast bacteria – they have highly hydrophobic cell wall due to presence of mycolic acids. As a result, these bacteria are the most resistant to antibiotic treatment as it is very hard for drugs to get inside the cell of acid-fast bacteria. Therefore, only bactericidal antibiotics are used in treatment of infections caused by acid-fast bacteria; 4) Mycoplasmas – these bacteria are lacking the cell wall. As a result, they are naturally resistant to antibiotics affecting synthesis of bacterial cell wall, like penicillins