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Medical Microbiology > General Microbiology > Flashcards

General Microbiology Flashcards

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Biology 101 flashcards USMLE® Step 1 flashcards
1
Q

Describe the colour (in a Gram stained smear) of Gram positive and Gram negative bacteria, respectively!

A

Gram positives: dark blue (purple)
Gram negatives: red (pink)

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2
Q

Describe the 3 important parts of the bacterial lipopolysaccharide!

A

lipid A + core polysaccharide + O-specific (polysaccharide) side chain

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3
Q

Which 2 roles are attributed to bacterial pili (fimbriae)?

A

adhesion; conjugation (sex fimbriae)

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4
Q

Mention a bacterium and its virulence factor that is encoded by a lysogenic bacteriophage!

A

Corynebacterium diphtheriae – diphtheria toxin

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5
Q

List the possible shapes of bacteria. Write one example for each category!

A

coccus (spherical): Staphylococcus, Streptococcus
rod: Clostridium, Corynebacterium, E. coli
curved rod: Vibrio
helical: Treponema, Borrelia, Leptospira

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6
Q

What are the essential components of bacterial cells? Specify at least 3!

A

cytoplasm, nucleoid (genome), cell membrane, (cell wall)

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7
Q

Which part of the bacterial cell carries the endotoxin?

A

Outer membrane of Gram negative bacteria

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8
Q

What roles are attributed to the bacterial capsule in the infectious process?

A

antiphagocytic effect, adhesion

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9
Q

Describe the definition for obligate anaerobic bacteria!

A

They are able to replicate only in the absence of oxygen (under anaerobic conditions). Some of them are quickly killed by oxygen, some are able to survive but do not grow while oxygen is present.

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9
Q

Describe the definition for facultative anaerobic bacteria!

A

They are able to replicate both in the presence and in the absence of oxygen. In aerobic conditions, they perform respiration, in anaerobic condition, they perform fermentation.

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10
Q

What does sterilisation mean?

A

Sterilisation means the killing or removal of all microorganisms (including bacterial spores).

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11
Q

Mention 3 reliable methods of sterilization!

A

autoclaving, hot air oven, gamma-radiation, filtration (fluids), gas sterilisation
(not acceptable: boiling, pasteurisation, UV!)

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12
Q

Specify an exact set of parameters (temperature and time) necessary for effective sterilization by autoclaving!

A

121 °C for 20 - 30 min. (at 1 bar overpressure)

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13
Q

Specify an exact set of parameters (temperature and time) necessary for effective sterilization using a hot air oven (with circulation)!

A

160 °C for 60 min.

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14
Q

What does disinfection mean?

A

The purpose of disinfection is the killing of pathogenic microbes and to decrease the number of viable microbial cells. Resistant microbes and spores may survive, so disinfection does not the kill all microbes.

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15
Q

Mention 5 groups of disinfectants!

A

alcohols, phenol derivatives, detergents, chlorine, iodine, aldehydes

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16
Q

Mention 2 groups of disinfectants acting on the microbial membrane structures!

A

a., (cationic) detergents (quaternary ammonium compounds)
b., phenol compounds (cresol, hexachlorophene, chlorohexidine)
c., alcohols (ethanol, isopropanol)

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17
Q

What does active immunisation mean?

A

It means the transfer of microbes or microbial products (vaccines) to the human body to induce long-lasting immunity against a specific infectious disease

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18
Q

What does passive immunisation mean?

A

It means the administration of preformed antibody (immune globulins produced in animals or humans) to the human body resulting in short-term protection against a specific infectious disease

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19
Q

Mention 2 bacterial infectious diseases that can be prevented or treated by passive immunisation!

A

prevention: tetanus
treatment: diphtheria, botulism, tetanus

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20
Q

Describe precisely what toxoid means!

A

Inactivated bacterial exotoxin that is not toxic but immunogenic

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21
Q

Mention 2 infectious diseases that can be prevented by vaccines containing toxoids!

A

Diphtheria, tetanus, (pertussis)

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22
Q

What does the DTaP vaccine contain?

A

diphtheria and tetanus toxoids and
acellular pertussis vaccine (toxoid and other purified antigens)

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23
Q

What does the vaccine against tuberculosis (BCG) contain?

A

live attenuated Mycobacterium bovis (Bacille Calmette-Guerin)

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24
Q

What is the nature of antigen in the vaccines used to prevent infections by Streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis?

A

capsular polysaccharide (either alone or conjugated to a carrier protein)

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25
Q

Specify the 4 groups of bacterial vaccines (classified according to the nature of the bacterial component)!

A

a., live, attenuated vaccines
b., killed (inactivated) bacterial vaccines
c., toxoid vaccines
d., subunit vaccines (capsular polysaccharide or purified protein)

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26
Q

What are the main advantages and disadvantages of live attenuated vaccines as compered to killed vaccines?

A

Advantages: induce not only serum antibodies but also cellular immunity and local IgA antibodies. Some may be applied orally. Usually fewer doses are needed.

Disadvantages: Attenuated strains may revert to virulent in rare cases. They may cause disease in immunosuppressed patients. Live attenuated microbes are usually heat sensitive and must be refrigerated.

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27
Q

Mention 2 bacterial exotoxins that act by ADP ribosylation!

A

Diphtheria toxin, cholera toxin, pertussis toxin

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28
Q

Mention 2 bacterial exotoxins that are neurotoxic!

A

tetanus toxin, botulinum toxin

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29
Q

What is the mechanism of action of diphtheria toxin?

A

Inhibition of protein synthesis in eukaryotic cells by ADP-ribosylation of ribosomal EF-2 (elongation factor-2)

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30
Q

What is the mechanism of action of tetanus toxin?

A

It causes spastic paralysis by blocking the release of inhibitory neurotransmitters (glycine and GABA) in synapses

31
Q

Mention at least three of the physiological effects of septic shock (characterized by the presence of large amount of bacterial endotoxin in the blood)!

A

fever, hypotension, disseminated intravascular coagulation (DIC), complement activation, impaired organ perfusion, hypoglycaemia

32
Q

Mention 2 non-essential bacterial organelles that enhance bacterial virulence! Describe how their function contributes to pathogenicity!

A

Capsule: antiphagocytic effect, adhesion to tissues
Fimbriae: binding to tissues
Flagellae: spreading the bacteria

33
Q

Mention 4 extracellular enzymes of bacterial origin functioning as virulence factors!

A

coagulase, streptokinase (fibrinolysine), streptodornase (DNase), hyaluronidase, IgA protease, collagenase, elastase, urease

34
Q

What is a vector? Write an example!

A

Vector is an arthropod that transmits infection from human to human, or from animal to human
examples. tick – Lyme disease; louse – epidemic typhus; mosquito – malaria

35
Q

Mention 2 infectious diseases that are transmitted to humans by tick bites!

A

tick-borne encephalitis; Lyme-disease; tularemia; Rocky-Mountain spotted fever; etc.

36
Q

What is a reservoir? Write an example!

A

The normal host of a pathogen (human or animal) serving as a continuous source of infection to other hosts (such as humans).
Salmonella typhi – human; Yersinia pestis - rodents

37
Q

Mention at least 5 possible transmission ways of infections!

A

respiratory droplets (infectious aerosol)
faecal-oral
direct contact (such as sexual)
fomites (inanimate objects)
blood

38
Q

What is the difference between prevalence and incidence of an infectious disease?

A

It can be differentiated in chronic diseases. Prevalence: total number of diseases (per 100,000 people). Incidence: number of new cases in a year (per 100,000 people).

39
Q

What is the difference between mortality and case fatality rate of a disease?

A

Mortality: total number of deaths caused by the disease in a population of 100,000 people in a year. Case fatality rate: rate of death (in percent) among patients suffering from the disease.

40
Q

Describe the rules of collecting native urine for culturing bacteria! Describe also the conditions for keeping the specimen before sending it to the laboratory!

A

A midstream specimen, taken preferably in the morning, after thorough cleaning of the external genital area. Keep it up to 1 h at room temperature or up to 24 h at +4 oC.

41
Q

Describe the rules of collecting hemoculture specimens!

A

It is recommended to obtain 3 specimens (with at least 30 min. between the specimens). The specimens should be preferably taken before fever spikes (during rising fever), from fresh peripheral venipuncture sites. If possible, both aerobic and anaerobic bottles should be used (3 x 2 bottles altogether). The site of venipuncture and the plug of the bottle containing the medium must be properly disinfected. The amount of blood injected to the bottle should be about 10 % of the liquid medium. The specimen should be taken before giving antibiotics to the patient.

42
Q

Explain the purpose for using beta-lactamase inhibitors in antibacterial therapy!

A

Combination of beta-lactamase inhibitors (such as clavulanic acid or sulbactam) with beta-lactamase sensitive penicillins (such as amoxicillin or ampicillin) can overcome resistance mediated by many but not all beta-lactamases.

43
Q

Mention a group of antimicrobial drugs that acts on the bacterial ribosome and usually has bactericidal effect!

A

aminoglycosides

44
Q

Mention 2 broad-spectrum (effective against both Gram positive and Gram negative bacteria) penicillin derivatives.

A

ampicillin, amoxicillin, piperacillin, (azlocillin, mezlocillin)

45
Q

Mention an antifungal and an antibacterial drug that alter the function of the cell membrane!

A

antifungal: Amphotericin B; antibacterial: polymyxins

46
Q

What are the possible mechanisms of acquired penicillin resistance of bacteria? Mention at least 3 mechanisms!

A
  1. beta-lactamase production
  2. PBP (target) alteration
  3. reduced permeability 4. active efflux
47
Q

Mention a drug belonging to macrolides!

A

erythromycin, spiramycin, roxithromycin, clarithromycin, azithromycin

48
Q

Mention 2 drugs belonging to aminoglycosides!

A

streptomycin, gentamicin, tobramycin, amikacin…

49
Q

Mention 2 groups of antimicrobial drugs that act on the 30 S subunit of the bacterial ribosome!

A

aminoglycosides, tetracyclines, glycylcyclines

50
Q

Mention 3 antimicrobial drugs that act on the 50 S subunit of the bacterial ribosome!

A

chloramphenicol, erythromycin (and other macrolides), clindamycin, linezolid, streptogramins

51
Q

Mention 3 groups of antibacterial drugs inhibiting bacterial cell wall synthesis!

A

penicillins, cephalosporins, carbapenems, monobactams, glycopeptides

52
Q

Mention 4 groups of antibiotics which have bactericidal effects!

A

penicillins, cephalosporins, aminoglycosides, fluoroquinolons etc.

53
Q

What is the mode of action of the antibiotics sulfonamides and trimethoprim, respectively?

A

They inhibit the synthesis of folic acid. Sulfonamides: inhibit the synthesis of dihidro- folate (they are PABA analogues). Trimethoprim: inhibition of dihidrofolate-reductase.

54
Q

Which antibiotics inhibit bacterial DNA gyrase/topoisomerase enzymes?

A

nalidixic acids, fluoroquinolons

55
Q

What is the mode of action of the antibiotic vancomycin?

A

Inhibits cell wall synthesis in Gram positive bacteria by blocking transglycosylation / transpeptidation.

56
Q

Mention 4 groups of antibiotics inhibiting protein synthesis of bacteria!

A

Aminoglycosides, tetracyclines, chloramphenicol, macrolides, lincosamides

57
Q

Give an example of synergistic interaction between 2 antibiotics!

A

penicillin combined with gentamicin, or a sulfonamide combined with trimethoprim

58
Q

Give an example of antagonistic interaction between 2 antibiotics!

A

penicillin combined with tetracycline

59
Q

What does selective toxicity mean?

A

Selective inhibition of the growth of the microorganism without damage to the host.
In other words: the drug is highly toxic to the bacteria, but not toxic (or have very low toxicity) to the human host.

60
Q

What is the mechanism of action of lysozyme? Where is lysozyme found in the host?

A

It destroys bacterial cell wall peptidoglycan by hydrolysing the glycosyl bonds between NAM (N-acetylmuramicacid) and NAG (N-acetylglucosamine). It occurs in tear, saliva, respiratory secretions.

61
Q

What roles can be attributed to specific antibodies in antibacterial immunity? Specify at least 3!

A

a. neutralisation of exotoxins and enzymes
b. complement-dependent bacteriolysis (Gram-negatives)
c. opsonisation (helping phagocytosis)
d. interfering with attachment of bacteria to mucosal surfaces

62
Q

Which type of hypersensitivity is involved in the tuberculin test?

A

late type (type IV)

63
Q

Which Gram negative organelles or structures are associated with the O, H and K antigens, respectively?

A

O- LPS, H-flagella, K-capsule

64
Q

How can one identify the different surface antigens of Gram negative bacteria?

A

with slide (rarely tube) agglutination tests using antibodies of known specificity

65
Q

Starting from a fixed smear, specify the main steps of Gram stain in the appropriate order!

A
  1. Crystal violet
  2. Lugol solution (iodine)
  3. Differentiation: ethanol wash
  4. Counterstaining: safranin or fuchsin
    (wash with tap water after each step)
66
Q

Starting from a fixed smear, specify the main steps of the acid-fast stain (Ziehl-Neelsen) in the appropriate order!

A
  1. Stain with carbol-fuchsin (with heating)
  2. Differentiation: wash with acid-ethanol
  3. Counterstaining with methylene-blue (wash with tap water after each step)
67
Q

Describe the principle of acid fast staining procedures!

A

Mycobacteria contain a high amount of special lipids called mycolic acids. They can be stained by hot carbol-fuchsin, but they resist decolourisation with acid-alcohol, so they remain red. All other cells are decolorised by acid-alcohol and then stained blue by the counter stain (methylene blue).

68
Q

What is the difference between agglutination and precipitation?

A

Both are serological reactions where specific binding of the antigen with the antibody directly results in a reaction visible by the naked eye. The difference is in the nature of the antigen. Agglutination: particulate antigen (RBC, bacteria, latex particles). Precipitation: antigen is in solution before the reaction.

69
Q

What does titer mean in serological tests?

A

The highest dilution of the serum sample that still gives a positive reaction in the test.

70
Q

How can one differentiate between past and current infections in serological tests?

A

Current infections are indicated by: either IgM class specific antibodies or, in the case of paired serum specimens (taken from the same patient at least 7-10 days apart), a significant (at least 4-fold) rise in the titer of specific antibodies.

71
Q

Specify the main steps of an ELISA test in which we detect antibodies from patients’ serum samples using known antigens!

A
  1. Binding antigen to plastic surface (in wells of a 96-well plate)
  2. Adding diluted serum sample (specific antibodies bind to the antigen)
    3.Adding conjugate (secondary antibody conjugated with an enzyme)
  3. Adding the substrate of the enzyme, reading the colour reaction.
    (wash with buffer after steps 1-3.)
72
Q

How can one determine the minimal inhibitory concentration (MIC) of an antibiotic to a bacterial isolate?

A

Inoculate the bacterial isolate into a series of test tubes containing 2-fold dilution series of the drug (prepared in appropriate liquid culture medium). After overnight incubation, the lowest concentration of drug that prevents visible growth of the organism is the MIC.

73
Q

What is the mechanism of action of lysozyme? Where is lysozyme found in the host?

A

It destroys bacterial cell wall peptidoglycan by hydrolysing the glycosyl bonds between NAM (N-acetylmuramicacid) and NAG (N-acetylglucosamine). It occurs in tear, saliva, respiratory secretions.

74
Q

Describe the principle of the Kirby-Bauer (disk diffusion) method used to determine antibiotic sensitivity!

A

Disks impregnated with different antibiotics are placed on the surface of appropriate agar media that has been previously inoculated with the bacterium isolated from the patient. After overnight incubation, the antibiotics diffusing from the discs may cause zones of inhibition around the discs. The size of the zone should be compared to standard values to determine antibiotic sensitivity.

Medical Microbiology (3 decks)

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