Antimicrobials 3

Describe features of a bacterial cell

Components - plasmid, ribosomes, cytoplasm, flagella, chromosomes, cell membrane, cell wall. More simple than eukaryotic cells. Unicellular (no protection from external stresses). Ribosomes for protein synthesis

1 of 69

What are the four overall strategies for antibiotics?

Disrupt bacterial cell walls. Inhibit protein synthesis. Inhibit nucleic acid synthesis. Antimetabolite

2 of 69

Why target bacterial cell walls? (1)

A defective or no cell wall compromises both growth and ability of bacteria to divide. Implies less fit cells or dead cells. Bacterial cell walls are made of peptidoglycan in similar ways. Give rigidity yet flexibility

3 of 69

Why target bacterial cell walls? (2)

Without a cell wall, bacteria are very osmotically fragile

4 of 69

What is the starting point for bacterial identification?

Gram staining. Used to classify cells as either Gram-positive or Gram-negative based on differential dye retention. Helps to recognise a structural difference

5 of 69

Describe features of bacterial cell walls

Gram positive cells have no outer membrane and more peptidoglycan than Gram negative cells. Wall itself is made of peptidoglycan (also murein or muropeptide)

6 of 69

Describe features of bacterial cell wall synthesis (1)

When dividing for growth, bacteria need a new cell wall (fresh peptidoglycan). Peptidoglycan is unique to bacteria and so not in eukaryotic cells (good target for selective toxicity)

7 of 69

Describe features of bacterial cell wall synthesis (2)

Structurally, peptidoglycan is a chain of alternating molecules of N- acetylglucosamine and N-acetylmuramic acid of 10-65 disaccharide residues

8 of 69

Describe features of bacterial cell wall synthesis (3)

On the N-acetylmuramic acid there is a short peptide chain (3 to 5 aa). In E. coli this is often L-Ala-D-Glu-L-Lys*-D-Ala-D-Ala. Differs from species to species, though D-Ala D-Ala very common

9 of 69

Describe features of bacterial cell wall synthesis (4)

Peptide bridges formed by transpeptidases between strands of polymers create a rigid yet flexible coating for the cell. Linear strands cross-linked by transpeptidation. Peptidoglycan monomers attache to grow linear strand

10 of 69

Describe features of bacterial cell wall synthesis (5)

Peptidoglycan subunits cross membrane attached to bactoprenol - a large lipid carrier (joined by a pyrophosphate bridge). Peptidoglycan (muropeptide) subunits are assemble in the cytoplasm

11 of 69

Give examples of two antibiotics that disrupt cell walls

Beta lactams and glycopeptides (most commonly used). Other antibiotics such as polypeptides (e.g. bacitracin) also target cell walls

12 of 69

Describe features of beta lactam antibiotics (1)

Chains of peptidoglycan are cross linked by a range of bacterial enzymes including transpeptidases and transglycosylases. Bacterial transpeptidases can be bound by beta lactam antibiotics

13 of 69

Describe features of beta lactam antibiotics (2)

When given to a growing cell, beta lactams then bind to transpeptidases. This blocks the formation of cross-links between the peptidoglycan chains. Leads to increased internal osmotic pressure. Also activates autolysins that degrade the cell wall

14 of 69

Describe features of beta lactam antibiotics (3)

Beta lactams inhibit transpeptidation by binding to PBPs on maturing peptidoglycan strands. Penetration of beta lactams into cell wall. Leads to death of bacterial cell. Beta lactams are bactericidal

15 of 69

What are the three subgroups within beta lactams?

Penicillins, cephalosporins, carbapenems

16 of 69

Describe features of penicillins (1)

Four different structures of penicillins were originally isolated – these were labelled F, G, K and X. Of these penicillin G seemed to have the best properties and was easiest to derive. Penicillin G is now commonly known as benzylpenicillin

17 of 69

Describe structure of benzylpenicillin

Beta lactam ring. Thiazolidine ring. Side chain place of modification. E.g. contrast between penicillin V (phenoxymethylpenicillin)

18 of 69

Describe features of benzylpenicillin (1)

Immediate impact in treating bacterial endocarditis. Useful only for relatively narrow range of pathogens (gets broken down by acid). Penicillin V though acid resistant - oral active (side chain giving physical property difference)

19 of 69

Describe features of benzylpenicillin (2)

Biggest problem though was penicillinase hydrolysis at the beta lactam ring of these natural penicillin antibiotics → abolished activity

20 of 69

What were the two strategies which emerged into order to deal with the penicillinase producing strains? (1)

Make penicillinase-tolerant penicillins e.g. meticillin (now discontinued) and flucloxacillin

21 of 69

What were the two strategies which emerged into order to deal with the penicillinase producing strains? (2)

Make penicillinase inhibitors e.g. clavulanic acid, sulbactam, tazobactam. Little/no antibacterial activity in themselves but bind irreversibly and inactivate specific penicillinases/beta-lactamases

22 of 69

What were the two strategies which emerged into order to deal with the penicillinase producing strains? (3)

Allow combined approach - penicillin (antibiotic) and tazobactam (inhibitor) - more effective

23 of 69

Give examples of penicillins which might be considered for clinical use (1)

Penicillin G - narrow spectrum with good activity against streptococci and some other Gram positives. Poor against staphylococci and Gram-negatives including P. aeruginosa

24 of 69

Give examples of penicillins which might be considered for clinical use (2)

first broad spectrum penicillin derived to treat certain Gram negatives e.g. H. influenzae in some middle ear infections. Resistance hampers “blind” treatment though

25 of 69

Give examples of penicillins which might be considered for clinical use (3)

Amoxicillin - similar, and often combined with clavulanic acid (co-amoxiclav) – fairly good spectrum but ineffective still with most pseudomonad or staphylococcal infections

26 of 69

Give examples of penicillins which might be considered for clinical use (4)

Penicillinase-resistant penicillins e.g. flucloxacillin – narrow spectrum (as penicillin G, though better against staphylococci except MRSA)

27 of 69

Give examples of penicillins which might be considered for clinical use (5)

Extended-spectrum/ antipseudomonal penicillins e.g. piperacillin often with tazobactam are reserved for P. aeruginosa infections

28 of 69

Describe the general structure of cephalosporin C

Beta lactam ring, thiazolidone ring, dihydrothiazine ring. Also have a modifiable side chain

29 of 69

Describe features of cephalosporins (1)

Two modifiable regions means more cephalosporin structures than for penicillins. Modifications at this position give more influence on its pharmaceutical properties. Still beta lactams, bactericidal. More broad spectrum than penicillins

30 of 69

Describe features of cephalosporins (2)

Four “generations” of cephalosporins have been recognised with another generation proposed. Not an ideal way of categorisation though gives some idea of what changes were valued

31 of 69

Describe features of the first generation of cephalosporins

Narrow spectrum antibiotics good against streptococci and some other Gram positives e.g. cephalexin - original cephalosporins derived

32 of 69

Describe features of the second generation of cephalosporins

Expanded spectrum cephalosporins more effective vs some Gram negatives e.g. Neisseria and Haemophilus e.g. cefuroxime – more stable vs beta lactamases

33 of 69

Describe features of the third generation of cephalosporins

Broad spectrum further increased action against Gram negatives Also ceftazidime which has good antipseudomonal activity e.g. cefotaxime combined beta lactamase stability with better overall activity

34 of 69

Describe features of the fourth generation of cephalosporins

Extended spectrum – used to treat Pseudomonas infection e.g. cefepime slightly improved general activity against Gram-negative bacteria

35 of 69

Describe features of the fifth generation of cephalosporins

E.g. ceftobiprole again targeting P. aeruginosa. E.g. ceftaroline which is also active against MRSA.

36 of 69

Describe features of the antibiotic evolution (1)

New generations of cephalosporins are being derived. Reflected in the names used. Narrow spectrum – 1st generation. Expanded spectrum – 2nd generation. Broad spectrum cephalosporins – 3rd generation. Extended spectrum cephalosporins – 4th generation

37 of 69

Describe features of the antibiotic evolution (2)

Start with an active compound and modify it to direct its functionality. Make the original more broad spectrum

38 of 69

Describe features of the antibiotic evolution (3)

Intuitively, broad spectrum might “feel” better because the coverage of species impacted is wider

39 of 69

Describe features of the high numbers of bacterial cells in the human body

Every body contains 10^14 cells. 1.3 kg of microbes. Main locations - oral cavity and upper respiratory tract, skin, GI tract (>90% of human associated microbes)

40 of 69

What are the issues with the use of broad spectrum antibiotics? (1)

Doses of broad spectrum bactericidal antibiotics here will kill sensitive commensal species as well as the pathogens targeted e.g. predisposition to Clostridium difficile infection after antibiotic use

41 of 69

What are the issues with the use of broad spectrum antibiotics? (2)

Wider health implications too as this could remove a percentage of beneficial microbiota. Whilst broad spectrum antibiotics are clinically very important, it’s therefore important to consider all the likely impacts they’ll have

42 of 69

Describe features of carbapenems (1)

Beta lactam antibiotics (same MOA). Carbapenems were particularly resistant to beta lactamase action. They also show very high activity to most aerobic and anaerobic Gram- negative and Gram-positive species (ultra broad spectrum)

43 of 69

Describe features of carbapenems (2)

Exceptions to this include MRSA and intracellular chlamydiae. Examples of carbapenems have included imipenem and more recently meropenem.

44 of 69

Describe features of carbapenems (3)

As they are so typically effective, antibiotics like meropenem have in the UK been restricted in their use e.g. for Pseudomonas infections. Carbapens (contain beta lactam ring), ertapenem considered for use in community acquired infections

45 of 69

Describe features of carbapenems (4)

Main use may be targeting extended spectrum beta lactamase (ESBL) producing Gram negatives

46 of 69

Describe features of carbapenems and resistance (1)

Resistance to carbapenems – often the last antibiotic effective vs. multi-resistant strains of Klebsiella pneumoniae and Escherichia coli – has been found. Carbapenemases can destroy most penicillin-like antibiotics

47 of 69

Describe features of carbapenems and resistance (2)

First found in Klebsiella pneumoniae in 1996. Mortality for patients with a KPC producing strain is high

48 of 69

Describe features of carbapenems and resistance (3)

More recently, in December 2009 a new type of carbapenemase was reported. New Delhi Metallo-beta-lactamase-1 (great concern)

49 of 69

Describe features of carbapenems and resistance (4)

Reason for this? “Most bacteria with the NDM-1 enzyme remain susceptible to 2 antibiotics, neither of which is ideal for general use

50 of 69

Describe features of carbapenems and resistance (5)

These are colistin (old and rather toxic antibiotic) and tigecycline (a newer antibiotic than can only be used in some, not all types of infection). A few isolates with NDM are completely resistant to antibiotics, including colistin and tigecycline”

51 of 69

Describe features of carbapenems and resistance (6)

Use of powerful antibiotic (last resort) on healthy chickens to boost weight

52 of 69

Describe features of glycopeptide antibiotics (1)

Teichoplanin and here vancomycin are the most commonly used. These are complex molecules consisting of sugars and peptides.

53 of 69

Describe features of glycopeptide antibiotics (2)

It prevents peptidoglycan subunits from adding and crosslinking in the growing cell wall by binding to the Dala – Dala of the short peptide chain on N-acetylmuramic acid

54 of 69

Describe features of glycopeptide antibiotics (3)

Glycopeptides are too bulky to penetrate the outer membrane of Gram- negative species so are only effective against Gram-positive species (both anaerobes and aerobes).

55 of 69

Describe features of glycopeptide antibiotics (4)

Reserved for those that are seriously ill with infections by Gram positive species. Particularly for infections by MRSA and Clostridium difficile. This is partly for reasons of toxicity, but more to avoid resistance emerging in these infections which

56 of 69

Describe features of vancomycin resistance (1)

Despite the high efficacy of this antibiotic, resistance to it has arisen in Enterococcus faecium and Enterococcus faecalis (increasing clinical concern). Vancomycin-Intermediate Staphylococcus aureus have also now been identified

57 of 69

Describe features of vancomycin resistance (2)

With rare reports of fully resistant forms. In one such case, “Genetic analyses suggest that the long-anticipated transfer of vancomycin resistance to a methicillin-resistant S. aureus occurred in vivo

58 of 69

Describe features of vancomycin resistance (3)

By interspecies transfer of Tn1546 from a co-isolate of Enterococcus faecalis”. The emergence of resistant isolates is significant as these have often been reserved as drugs of “last resort” for certain infections.

59 of 69

Describe features of inhibiting protein synthesis (1)

If protein synthesis can be blocked or at least badly affected, this will result in either no protein (cells are exposed). This would make the cell less fit (less able to divide) or prevent the cell from functioning properly

60 of 69

Describe features of inhibiting protein synthesis (2)

This in turn makes that cell less able to persist

61 of 69

Describe features of ribosomes (1)

Ribosome is the site of protein synthesis. In Bacteria, the ribosome has two subunits, the 30S and 50S subunits which come together to form the overall 70S subunit (S - estimation for the size of components)

62 of 69

Describe features of ribosomes (2)

This is a complex set of proteins and rRNA molecules. 23S(50S)-basedpeptidyltransferase - forms peptide links. Complexity gives us antibiotic targets

63 of 69

State features of translation - initiation (1)

Three sites. A (aminoacyl) site – this is where after initiation each “new” aminoacyl- tRNA attaches. P (peptidyl) site – where the peptide bonds are formed. E (exit) site – where tRNA leaves the ribosome (protein synthesis - sequence of amino acids)

64 of 69

State features of translation - initiation (2)

Peptide elongation occurs (tRNA/anticodon loop). Dynamic process. With the polypeptide being derived by the successive addition of amino acids

65 of 69

State features of translation - initiation (3)

For an average protein synthesised from a bacterial gene, this needs 330 amino acids formed in the right order into a chain.

66 of 69

Describe features of ribosomes in bacterial cells (1)

E. coli has between 3,000-20,000 ribosomes/ cell – this number is highly regulated. 20,000 ribosomes represents c. 25% of the mass of an individual bacterial cell

67 of 69

Describe features of ribosomes in bacterial cells (2)

This makes bacterial cells normally very efficient at making proteins. This needed as some bacterial species double in number every 20 minutes. Also, bacterial cells need to exploit sources of nutrition. Need to respond to changing conditions

68 of 69

Give examples of antibiotics which inhibit protein synthesis

Aminoglycosides, tetracyclines, chloramphenicol, macrolides, clindamycin

69 of 69

Get Revising

Antimicrobials 3

Other cards in this set

Card 2

Front

Back

Card 3

Front

Back

Card 4

Front

Back

Card 5

Front

Back

Comments

Similar Pharmacy resources:

Other cards in this set

Card 2

Front

Back

Card 3

Front

Back

Card 4

Front

Back

Card 5

Front

Back

Comments

Related discussions on The Student Room

Similar Pharmacy resources: