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Terms in this set (70)
Describe the colour (in a gram stained smear) of gram positive and gram negative bacteria, respectively!
gram positives: dark blue/purple
gram negatives: red/pink
Describe the 3 important parts of the bacterial lipopolysaccharide!
lipid A +
core polysaccharide + o-specific (polysaccharide) side chain
Which 2 roles are attributed to bacterial pilli (fimbriae)?
Adhesion; conjugation (sex fimbriae)
Mention a bacterium and its virulence factor that is encoded by a lysogenic bacteriophage!
Corynebacterium diphtheriae-diphtheria toxin. Streptococcus pyogenes - erythrogenic toxin
Specify 2 genera containing obligate intracellular pathogens!
Mention at least 5 possible transmission ways of infections!
1. Respiratory droplets (infectious aerosol)
3. Direct contact (such as sexual)
4. Fomites (inanimate objects)
6. Vectors (blood-sucking arthropods)
List the possible shapes of bacteria. Write one example per category.
1. Coccus (spherical): staphylococcus, streptococcus
2. Rod: clostridium, corynebacterium, gram negative rods
3. Curved rod: vibrio
4. Helical: spirochetes (treponema, borrelia, leptospiro)
What are the essential components of bacterial cells? Specify at least 3!
Cytoplasm, nucleoid (genome),
cell membrane (cell wall)
What is a vector? Write an example!
Vector is an arthropod that transmits infection from human to human, or from animal to human. Examples: tick-lyme disease; louse-epidemic typhus;
What is a reservoir? Write an example!
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
Mention 3 reliable methods of sterilization!
hot air oven, gamma-radiation, filtration (fluids), gas sterilisation (not acceptable; boiling, pasteurisation, UV!)
Mention 5 groups of disinfectants!
Alcohols; aldehydes (alkylating agents);
phenol derivatives; detergents; chlorine + iodine = oxidising agents
Mention a group of antimicrobial drugs that acts on the bacterial ribosome and usually has bactericidal effect!
Describe precisely what toxoid means!
Inactivated bacterial exotoxin that is not toxic but immunogenic
Mention 2 broad-spectrum (effective against both gram positive and gram negative bacteria) penicillin derivatives.
Ampicillin, amoxicillin, piperacillin, azlocillin, mezlocillin
Mention an antifungal and an antibacterial drug that alter the function of the cell membrane!
Antifungal: amphotericin B, azoles; Antibacterial: polymyxins
What are the possible mechanisms of acquired penicillin resistance of bacteria? Mention at least 3 mechanisms!
1. Beta-lactamase production
2. PBP (target) alteration
3. Reduced permability
4. Active efflux
Mention a drug belonging to microlides!
Erythromycin, spiramycin, roxithromycin, clarithromycin, azithromycin
Mention 2 drug belonging to aminoglycosides!
Streptomycin, gentamycin, tobramycin, amikacin
Mention 2 groups of antimicrobial drugs that act on the 30 S subunit of the bacterial ribosome!
Mention 3 groups of antibacterial drugs inhibiting bacterial cell wall synthesis!
Penicillins, cephalosporins, carbapenems, monobactams, glycopeptides
Which part of the bacterial cell carries the endotoxin?
Outer membrane of gram negative bacteria
What role is attributed to the bacterial capsule in the infectious process?
Mention 2 infectious diseases that are transmitted to humans by tick bites!
Tick-borne encephalitis; lyme-disease; tularemia;
rocky-mountain spotted fever; recurrent fever (borrelia recurrentis) etc.
Which type of hypersensitivity is involved in the tuberculin test?
Late type(type 4)
Mention 2 bacterial infectious diseases that can be prevented or treated by passive transfer of specific immunoglobulins!
Treatment: diphtheria, botulism, tetanus
What do the vaccine(s) against diphteria and tetanus contain?
Diphtheria and tetanus toxoid
What does the vaccine against pertussis contain?
Previously: killed bacteria
Currently: acellular vaccine (toxoid + other purified proteins)
What does the vaccine against tuberculosis (BCG) contain?
Live attenuated mycobacterium bovis (bacille calmette-guerin)
What is the nature of antigen in the vaccines used to prevent infections by streptococcus pneumoniae, Haemophilus influenzae and Neisseria meningitidis?
Capsular polysaccharide (either alone or conjugated to a carrier protein)
Mention 2 bacterial exotoxins that are neurotoxic!
Tetanus toxin, botulinum toxin
Specify the 4 groups of bacterial vaccines according to the nature of the antigen!
1. Live, attenuated vaccines
2. Killed bacterial vaccines
3. Toxoid vaccines
4. Subunit vaccines (capsular polysaccharide or purified protein)
What is the mechanism of action of lysozyme? Where is lysozyme found in the host?
It destroyes bacterial cell wall peptidoglycan by hydrolysing the glycosyl bonds between NAM (N-acetylmuramicacid) and NAG (N-acetylglucosamine). It occurs in tear, saliva, respiratory secretions.
Which gram negative organelles or structures are associated with the O, H and K antigents, respectively?
O-LPS, H-flagella, K-capsule
Mention 2 bacterial exotoxins that act by ADP ribosylation!
Diphtheria toxin, cholera toxin, pertussis toxin
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 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*C.
Describe the rules of collecting hemoculture specimens!
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.
Mention two clinical soecimens that are heavily contaminated with bacteria belonging to the normal body flora!
Throat and nasal secretions, faeces, vaginal secretions.
Mention atleast three of the physiological effects of septic shock (characterized by the presence of large amount of bacterial endotoxin in the blood!)
Fever, hypotension, disseminated intravascular coagulation (DIC), complement activation, impaired organ perfusion, hypoglycaemia
What pathogenic role is attributed to lysogenic conversion?
Lysogenic conversion: transfer transfer of genes from one bacterium to another by lysogenic bacteriophages. The bacteria may gain exotoxins (diphtheria toxin, streptococcus pyogenes erythrogenic toxin), or the structure of LPS epitopes may be altered (Shigella, Salmonella).
Describe the principle of acid fast staining procedures!
Mycobacteria contain a high amount of special lipids called mycolic acids. They can be stained by hot carbol-fuchsin, but they resist decilourisation with acid-alcohol, so they remain red. All other cells are stained blue by the counter stain (methylene blue).
Mention 2 non-essential bacterial organelles that enhance bacterial virulence! Describe how their function contributes to pathogenicity!
Capsule: antiphagocytic effect, adhesion to tissues.
Fimbriae: binding to tissiues. Flagellae: spreading the bacteria.
Mention a differentiating culture medium and describe which groups of bacteria are distinguished in the mentioned medium!
Eosin-methylene blue (EMB) agar: inhibits gram positive bacteria and inhibits the swarming of Proteus. Differentiates between lactose fermenters and non-fermenters. Lactose fermenters produce high amounts of acids, which is indicated as dark blue colonies by eosin and methylene blue.
McConkey agar: inhibits gram positive bacteria by bile salts and crystal violet. It differentiates between lactose fermenters and non-fermenters. Lactose fermentation is indicated by neutral red. Lactose fermenters form pink to red colonies, while non-fermenters form colourless colonies.
Describe the definition for facultative anaerobic bacteria!
They are able to grow and metabolize both in the presence and absence of oxygen. In aerobic conditions they perform respiration, in anaerobic condition, they perform fermentation.
Mention 4 extracellular enzymes of bacterial origion functioning as virulence factors!
Coagulase, streptokinase (fibrinolysine), streptodornase (DNase), hyaluronidase, IgA protease, collagenase, elastase, urease
What is the difference between prevalence and incidence of an infectious disease?
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).
What is the difference between morality and lethality of a disease?
Morality: total number of deaths caused by the disease in a population (usually 100,000 people). Lethality: rate of death (in percent) among patients suffering from the disease.
Mention 2 groups of disinfectants acting on the microbial membrane structures!
1. (Cationic) detergents (quaternary ammonium-compounds);
2. Phenol compunds (cresol, hexachlorophene, chlorohexidine)
3. Alcohols (ethanol, isopropanol)
Describe the principle of the Kirby-Bauer (disk diffusion) method used to determine antibiotic sensitivity!
Disks impregnated with different antibiotics are placed on the surface of appropiate agar media that has been 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 standards to determine antibiotic sensitivity.
Explain the purpose for using beta-lactamase inhibitors in antibacterial therapy!
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.
What are the main advantages and disadvantages of live attenuated vaccines as compared to killed vaccines?
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 refrigerted.
Mention 4 groups of antibiotics which have bactericidal effects!
Penicillins, cephalosporins, aminoglycosides, fluoroquinolons etc.
Which antibiotics inhibit bacterial DNA gyrase enzyme?
Nalidixic acids, fluoroquinolons
What is the mode of action of the antibiotics sulfonamides and thrimethoprim, respectively?
They inhibit the synthesis of folic acid. Sulfonamides: inhibit the synthesis of dihidrofolate (they are PABA analogues). Trimethoprim: inhibit of dihidrofolate-reductase.
What is the mode of action of the antibiotic vancomycin?
Inhibits cell wall synthesis in Gram positive bacteria by blocking transpeptidation.
Mention 4 groups of antibiotics inhibiting protein synthesis of bacteria!
Aminoglycosides, tetracyclines, chloramphenicol, macrolides, lincosamides
Regarding the joint effects of 2 antibiotics, what does synergism mean?
The effect of the two drugs together is significantly higher than the sum of the effects of the two drugs acting separately.
Regarding the joint effects of 2 antibiotics, what does antagonism mean?
The effect of the two drug together is significantly lower than the effect of the more effective drug alone.
What does selective toxicity mean?
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.
How can one identify the different surface antigens of Gram negative bacteria?
With slide agglutination tests using antibodies of known specificity
What is the mechanism of action of diphtheria toxin?
Inhibition of protein synthesis in eukaryotic cells by ADP-ribosomal EF-2 (elongation factor-2)
What is the mechanism of action of tetanus toxin?
It causes spastic paralysis by blocking the release of inhibitory neurotransmitters(glycine and GABA) in synapses
What roles can be attributed to specific antibodies in bacterial immunity? Specify at least 3!
1. Neutralisation of exotoxins and enzymes
2. Complement-dependent bacteriolysis (gram-negatives)
3. Opsonisation (helping phagocytosis)
4. Interfering with attachment of bacteria to mucousal surfaces
Starting from a fixed smear, specify the main steps of gram stain in appropiate order!
1. Crystal violet
2. Lugol solution (iodine)
3. Differentiation: ethanol wash
4. Counterstaining: safranin or fuchsin (wash with water after each step)
Starting from a fixed smear, specify the main steps of the acid-fast stain (Ziehl-Neelsen) in the appropiate order!
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)
What is the difference between agglutination and precipitation?
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.
What does titer mean in serological tests?
The highest dilution of the serum sample that gives a positive reaction in the test.
How can one differentiate between past and current infections in serological tests?
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.
Specify the main steps of an ELISA test in which we detect antibodies from patients' serum samples using known antigens!
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)
4. Adding the subatrate of the enzyme, reading the colour reaction. (Wash with buffer after steps 1-3.)
How can one determine the minimal inhibitory concentration of an antibiotic to a bacterial isolate?
Inoculate the bacterial isolate into a series of test tubes containing 2-fold dilution series of the drug (prepared in appropiate liquid culture medium). After overnight incubation, the lowest concentration of drug that prevents visible growth of the organism is the MIC.
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