Bacteriology

Question 1

Peptidoglycan

Answer 1

Thick layer present in cell envelopes.
Complex molecule of peptide and glycan providing rigid support in bacterial cell wall. Polymer of 1,4-N acetyl glucosamine and acetyl muramic acid with alternating amino acid insertion. Gram negatives have slightly different peptide bonding between them than gram positives.
Thick layer present in Gram positive bacteria, and some within inner leaflet of Gram negative membrane.

Question 2

Difference between Gram negative and positive bacteria

Answer 2

Characterised by different response to staining with crystal violet and carbol fuchsin. Gram positive have thick peptidoglycan single membrane, and negative have envelope with outer leaflet studded with LPS.

Question 3

Teichoic and lipoteichoic acids

Answer 3

Highly variable anionic polymers in Gram positive bacteria. Involved in shape, growth and division but full function unknown

Question 4

Lipopolysaccharide

Answer 4

Key component of Gram negative bacteria outer leaflet. Made up of lipid A, core polysaccharides and O antigen. Very important PAMP inducing inflammatory responses.

Question 5

O antigen

Answer 5

Outermost part of LPS. Is an oligosaccharide of 15-20 repeats of three or four sugars. Lots of variations in O antigens both intra and interspecies. Nature of repeating sugar units may help determine virulence?

Question 6

Lipid A

Answer 6

Endotoxic portion, responsible for systemic inflammatory response.

Question 7

Other antigens (potentially diagnostic)

Answer 7

K polysaccharide = prominent in heavy capsules.

H antigen = flagella protein

Question 8

Bacterial capsule

Answer 8

High molecular weight polymers on surface. Very diverse and may include structural mimics of host molecules. Important in resisting host immunity, eg Strep. pneumoniae resistant to phagocytosis.

Question 9

Flagella

Answer 9

Long thin filaments which rotate as a motor at up to 15,000 rpm. Made up of basal body, hook and filament (flagellin subunits). Flagellin is a TLR5 ligand so can be recognised, and some bacteria thus shed flagella

Question 10

Pilli

Answer 10

Shorter filamentous surface structures made of many protein subunits. Involved in adhesion, eg in uropathogenic E coli UTIs.

Question 11

Secretion systems

Answer 11

Specialised structures, (9 different) which transport proteins across bacteria membranes, and possibly also against third host membrane. = molecular battering syringe.

Question 12

Symbiont

Answer 12

Commensal in a mutually beneficial relationshi[

Question 13

Opportunistic pathogen

Answer 13

Causes disease in an immunocompromisd host (eg drugs, genetics, break in skin)

Question 14

Virulence

Answer 14

Degree of damage inflicted by pathogen.

Question 15

Bacterial cells/human cells

Answer 15

Bacterial: 10^14
Human: 10^13

Question 16

Tropism

Answer 16

Ability to find and establish a niche in the body, in or outside of cells

Question 17

Defining pathogens - Stanley Falkow

Answer 17

1. Virulence factor should be expressed by a pathogen
2. Deletion of gene encoding virulence factor leads to attenuation
3. re-addition of that gene should restore virulence

Question 18

Cholera (example 1)`

Answer 18

Vibrio cholerae present in areas where sanitation is broken down. Gram neg bacterium with single terminal flagellum and pillus. Produces A1B5 toxin which passes through pentamer pore into cell causing death. Also stimulates adenylate cyclase, more ATP cause ion channels to remain open, so loss of chloride ions, followed by water. Secretory diarrhoea followed by dehyfration and loss of electrolytes.

Question 19

Example 2: C. difficile

Answer 19

Clostridium difficile = Gram positive rods forming spores. Enters epithelial cells and then breaks down lamina propria through secreted exotoxins. ?

Question 20

Antimicrobial

Answer 20

Interferes with growth and reproduction of a microbe

Question 21

Antibacterial

Answer 21

Agents which reduce or eliminate harmful bacteria

Question 22

Antibiotic

Answer 22

Type of antimicrobial used as medicine, normally natural compounds

Question 23

Bactericidal

Answer 23

Kills bacteria

Question 24

Bacteriostatic

Answer 24

Halts bacterial growth (eg remove drug then growth will continue)

Question 25

Selective toxicity

Answer 25

Molecular target is not present in host OR system is different in host OR access of target (into cells) is different.

Question 26

Beta lactams

Answer 26

Inhibit cell wall synthesis by inhibiting peptidoglycan synthesis, eg penicillin target penicillin binding proteins.

Question 27

Macrolides

Answer 27

Inhibit membrane function. Bind to 50s ribosome subunit.

Question 28

Aminoglycosides

Answer 28

INhibitors of protein synthesis, binding to 30s ribosome subunit.

Question 29

Sulphonamides

Answer 29

Anti metabolites by inhibiting folic acid synthesis (dihydrofolate reductase)

Question 30

Quinolones

Answer 30

Inhibitors of nucleic acid synthesis, by inhibiting A subunit of DNA gyrase and topoisomerase IV.

Question 31

Tetracyclines eg doxycycline

Answer 31

Bind to 30s ribosomal subunit.

Question 32

Rifampicin

Answer 32

Targets DNA dependednt RNA polymerase always used in combination.

Question 33

Polymyxins

Answer 33

Insert cationic polypeptides into bacteria membrane (Gram neg) so permeabilising bacterua.

Question 34

Mechanisms of drug resisitance

Answer 34

Prevent access by reducing permeability of increasing efflux. Inactivate drug by modification. Or modify the target of the drug.

Question 35

Reducing permeability mechanisms

Answer 35

Gram negative bacteria are intrinsically less permeable. Increase specificity of pores/available channels. Eg E. Coli and Enterobacter have accumulated mutations in porin genes following carbapenem exposure.

Question 36

Spread of resistance via integrons

Answer 36

Integrons are specific cassettes acquired by bacteria with basic promoter, cassette and recombination site, a combine together to increase modules of resistance.

Question 37

Spread of resistance via plasmids.

Answer 37

Plasmids are small circular sections of extra-chromosomal DA which can be transferred between strains by transformation or conjugation.

Question 38

Colonisation

Answer 38

First stage in human diseases is the ability to adhere to cutaneous or mucosal surfaces. Using pili fimbriae and polysaccharide capsule. May necessitate killing commensal flora

Question 39

Invasion

Answer 39

Tissue invasion is facilitated by enzymes which degrade matrix. Eg. elastase and hyaluronidase.

Question 40

Streptococcus pyogenes structure

Answer 40

Spherical Gram positive cocci arranged in chains or pairs. Adhere to pharyngeal epithelium via pili made of lipoteichoic acid and M protein. Group A are beta haemolytic.

Question 41

Determining difference between staphylococci and streptococci

Answer 41

Streptococci are catalase negative, and then need to be identified by antigen group. Staph also have traditional yellow/gold appearance.

Question 42

Streptococci pyogenes virulence factor

Answer 42

M protein protrudes from cell and interferes with ingestion by phagocytes. Hyaluyonic acid capsule also helps prevent phagocytosis. Exotoxin production leading to widespread symptoms. Different toxins determining reaction, eg erythrogenic toxin = scarlet fever, pyrogenic toxin A = TSS, streptolysin O = haemolysin etc.

Question 43

Immunity to strep pyogenes

Answer 43

Antibodies to M protein provides type specific immunity, but as there are over 80 serotypes then multiple infections can occur. And no effective vaccine except against Strep pneumoniae.

Question 44

Streptococci diseases

Answer 44

Wide variety, pyogenes is leading cause of pharyngitis and cellulitis, also impetigo, necrotising fasciculitis and streptococcal toxic shock syndrome.

Question 45

Staphylococcus aureus structure

Answer 45

Non-spore forming, non-motice Gram positive cocci. Divides in one plane (bunches of grape appearance). Catalase positive and oxidase negative.

Question 46

Staph aureus pathogenesis

Answer 46

Yellow pigment = staphyloxanthin, inactivates microbicidal effects of superoxides and other ROS in neutrophils. Protein A binds Fc portion of IgG at complement bindinng site, so preventing alternative pathway of complement activation. Teichlkc acids mediate adherence to mucosal calls, and lipoteichoic acids induce cytokines like IL-1 and TNF from macrophages -- non-endotoxin septic shock

Question 47

Staph aureus toxins

Answer 47

Cause symptoms, and three different types. 1. Enterotoxin: causes food poisoning as superantigen stimulates release of IL-1 and 2 from macrophages and helper T. Cytokines stimulate enteric nervous system to activate vomiting centre in brain. 2. Toxic shock snydome: acts as a superantigen, similatly stimulating release of IL-1, 2 and TNF. Often occurs in tampon using women or in individuals with wound infections. 3. Exfoliatin: protease cleaving desmoglein in desmosomes leading to separation of skin = scalded skin syndrome.

Question 48

Immune interactions of staph aureus

Answer 48

Some exotoxins produced able to kill leukocytes, eg alpha toxin and PV leukocidin, causing necrosis of skin and haemolysis via pore forming in cell membranes. More than 90% of S aureus strains have plasmids encoding beta lactamases, and then additional 30% are resistant to methicillin. MRSA individuals are generally given vancomycin but some are very difficult to treat.

Question 49

E. coli structure

Answer 49

Straight Gram negative rod, most abundant facultant anaerobe in the colon and faeces. 3 identifying antigens = O antigen, H flagella antigen and J capsular antigen.

Question 50

Virulence of e coli

Answer 50

Has pili for adherence, a capsule, endotoxin and three exotoxins which act in a cell specific way. (1) heat labile toxin stimulates adenylate cyclcase (same as cholera), to then increase cAMP, stimulating ion channel opening in membranes => watery diarrhoea. (2) heat-stable toxin which stimulates guanylate cyclase. (3) Shiga toxin removes an adenine from the large rRNA so stopping protein synthesis – associated with bloody diarrhoea. Some O serotypes particularly cause UTIs as there pili have adhesin proteins binding to specific receptors on UT epithelium.

Question 51

E. coli interactions with immune response

Answer 51

Presence of capsule interferes with phagocytosis, K1 antigen specifically helping strains causing neonatal meningitis? Normally antibiotic treatment is straightforward (nitrofurantoin for UTIs), E coli sepsis more parental antibiotic, diarrhoeal not normally treated with antibiotics.