1. Antimicrobial Medications

2. Antibiotics Antimicrobial drugs naturally produced by microorganisms
Penicillium species: Penicillins
Cephalosporium specis: cephalosporins
Streptomyces species:
lincosamides, aminoglycosides, tetracyclines, chloramphenicol

3. Features of antimicrobial drugs
Selective toxicity
Therapeutic index
Antimicrobial action
Bactericidal
Bacteristatic
Spectrum of activity
Broad spectrum
Narrow spectrum
Combination effects
Antogonistic
Synergistic
Additive

4. Pharmacokinetics: what happens to the drug in the body?
Absorption
Tissue distribution
Metabolism
Route of excretion
Rate of elimination

5. Adverse effects Adverse drug reaction Toxic effects
Suppression of normal microbiota

6. Mechanisms of antimicrobial drugs
Inhibition of cell wall synthesis
Inhibition of protein synthesis
Inhibition of nucleic acid synthesis
Inhibition of biosynthetic pathways
Disruption of cell membrane integrity

8. Targets of Cell Wall Synthesis

9. Penicillin
Inhibits formation of tetrapeptide side chains

10. How organisms degrade penicillins

11. Family of Penicillins Natural penicillins
Penicillinase-resistant penicillins
Broad-spectrum penicillins
Extended-spectrum penicillins
Penicillins plus beta-lactamase inhibitors

12. Family tree of penicillins

13. β-lactams
Penicillins
Cephalosporins
Carbapenems
Vancomycin
Bacitracin

14. Cephalosporins Derived from fungus, Acremonium cephalosporium
Chemical structure makes them resistant to beta-lactamase: low affinity for penicillin binding proteins
Grouped into first, second, third, and fourth generation cephalosporins

15. Vancomycin
Binds to the terminal amino acids of the peptide chain of NAM molecules, blocks peptidoglycan formation

16. Antibiotics that inhibit protein synthesis

17. Oxazolidinones
Reversibly bind to the 50S subunit, interfere with initiation of protein synthesis
Used for treating gram positive infections resistant to Beta-lactam drugs and Vancomycin
Ex: Linezolid

18. Aminoglycosides Bactericidal
Irreversibly bind to 30S ribosome, cause misreading of the mRNA
Transported into cells that actively respire (not effective against ananerobes, streptococci, enterococci)
Ex: streptomycin, gentamicin, tobramycin

19. Tetracyclines & Glycylcyclines
Bind reversibly to 30S, block attachment of the tRNA to ribosome
Actively transported into bacterial cells
Effective against gram positive and gram negative
Resistance: due to decrease in uptake or increase in excretion
Ex: Doxycycline

20. Macrolides
Reversibly bind to the 50S, prevent continuation of protein synthesis
Drug of choice for patients allergic to penicillins
Not good for Enterobacteriaceae
Ex: Erythromycin, Azithromycin
Resistance: enzymes that alter drug, decreased uptake

21. Inhibition of protein synthesis: Chloramphenicol
Rare side effect = irreversible bone marrow suppression
Banned in food animals
Making a come-back in companion animal medicine due to effectiveness against multi-drug resistant staphylococci

22. Antibiotics that inhibit nucleic acid synthesis
Fluoroquinolones
Interferes with function of topoisomerase
Rifamycins
Blocks prokaryotic RNA polymerase from initiating transcription

23. Antibiotics that inhibit biosynthetic pathways
Sulfonamides
Trimethoprims

24. Sulfonamides (sulfa drugs)
First synthetic drugs to treat microbial infections
Used to treat urinary tract infections (UTIs)
Combination of trimethoprim and sulfamethoxazole (TMP-SMZ) example of synergism

25. Drugs used together inhibit folic acid synthesis

26. Tests for microbial susceptibility
Kirby-Bauer (disk diffusion method)

27. Tests for microbial susceptibility
Minimum Inhibitory Concentration: MIC
Grow bacteria in a serial dilution of the antimicrobial being tested
Fixed concentration of bacterial cells
Observation of turbidity after 16 hrs-24 hrs of growth
Lowest concentration of drug that inhibits growth = MIC

28. Minimum Inhibitory Concentration
Manual broth dilution method
Automated broth dilution method
E-test

29. Determining the Minimum Inhibitory Concentration (MIC)

30. Automated MIC

31. E-test for MIC

32. Zone size & MIC values Raw data Meaningless without interpretation
Correlation of in vitro results with achievable levels of drug concentration in a live patient
Correlation with actual clinical outcome

33. What resistance looks like…

34. Mutant Prevention Concentration?

35. Mechanisms of acquired drug resistance
Destruction or inactivation of the drug: drug inactivation enzymes
Alteration of target molecule (mutation)
Decreased uptake: alteration of porins
Increased elimination: efflux pumps

36. Acquiring resistance
Spontaneous mutation
Gene transfer
R plasmids

37. Emerging antimicrobial resistance

38. Streptococcus pneumoniae
Altered penicillin binding proteins
DNA-mediated transformation

39. Enterococcus species
Gram positive enteric cocci; facultative anaerobes; formerly classified as Group D streptococcus
Common cause of nosocomial infections
Enterococcus faecalis, Enterococcus faecium
Intrinsic resistance:
Acquired resistance

40. Mycobacterium tuberculosis
Multidrug-resistant M. tuberculosis
Resistance to isoniazid & rifampin
Extensively drug-resistant M. tuberculosis
Resistance to isoniazid & rifampin + 3 or more of the 2nd line drugs

41. Enterococcus species: intrinsic resistance
Low affinity of penicillin binding proteins for many β-lactam antibiotics, esp. cephalosporins
Resistance to potentiated sulfonamides (i.e., trimethoprim-sulfa): able to utilize external sources of folate
Low permeability for aminoglycosides
Treatment with a cell-wall active drug such as ampicillin is synergistic (allows the drug to get into the cell) UNLESS high-level gentamicin resistance is present

42. Enterococcus species: acquired resistance
High-level gentamicin-resistance: plasmid-encoded inactivating enzymes
Tetracycine resistance: efflux pumps, ribosomal protection
Macrolide resistance: efflux pumps
Vancomycin resistance: altered drug binding site on cell wall

43. Enterobacteriaceae Gram negative enteric rods
Intrinsic resistance to many drugs due to outer membrane
β-lactamases: enzymatic inactivation of the lactam ring
Extended spectrum β-lactamases (ESBL+)
Carbapenem-resistance: enzymatic inactivation

44. Staphylococcus species
Staph aureus
Staph pseudintermedius
Staph schleiferi
Methicillin-resistant staph: penicillinase + altered penicillin-binding proteins with low affinity for β-lactam drugs (mecA gene on R plasmid)
Vancomycin-resistant staph

45. Methicillin-resistant staphylococci
MRSA: methicillin-resistant Staph aureus
drug resistance + increased pathgenicity
MRSP: methicillin-resistant Staph pseudintermedius
Acquisition of drug resistance is not associated with acquisition of new virulence factors
MRSS: methicillin-resistant Staph scheiferi
Drug resistance/no new virulence factors
Coagulase-negative MRS

46. Coagulase test
Tests for coagulase enzyme = virulence factor produced by Staphylococcus aureus, Staph. pseudintermedius and Staph. schleiferi subspecies coagulans
Important in differentiating potentially pathogenic from non-pathogenic species of staphylococci

47. Coagulase enzymes
Bound coagulase (“clumping factor”) – attached to bacterial cell wall
Coagulase enzyme + fibrinogen in plasma → fibrin clot surrounding bacteria: prevents antibody and complement binding, prevents phagocytosis, protects from NETs
Free coagulase – secreted enzyme
Coagulase enzyme + CRF → conversion of prothrombin to thrombin and fibrinogen to fibrin

48. Coagulase slide test
Rabbit plasma + bacteria: agglutination within 1-2 minutes = positive result
Detects only bound coagulase
False negatives or equivocal results are common
Negative or equivocal tests have to be confirmed with tube test

49. Coagulase tube test
Rabbit plasma + bacteria: coagulation of the plasma = thickening OR formation of fibrin clumps or threads
Standard practice = read at 4 hrs, if negative recheck at 24 hrs
Tests not read at 4 hrs that are negative at > 4 hrs will have to be repeated because early positive results may revert to a negative result

50. Coagulase negative staph
Common isolates from skin cultures
Non-pathogenic commensuals
Rarely part of mixed population in deep skin/ wound infections (furuncles)
Rarely cause bacteremia or other systemic infections in immune-compromised individuals
Commonly carry plasmids with mecA gene

51. MRS: colonization vs. infection
Sharing of plasmids + high antimicrobial use
Selecting for MRS population
Increasing % of staphylococcal isolates from non-lesional skin and nasal mucosa are methicillin resistant

52. Responsible drug use Use vs misuse of antimicrobial drugs
Responsibilities of health care professionals?
Responsibilities of patients?
Responsibilities of pet owners?
Public education
Over the counter antimicrobial drugs
Developing countries
US feed stores

53. Antimicrobial Stewardship
Increasing drug resistance
Fewer drugs in development
Drugs being developed don’t have novel targets
Drugs being developed are broad spectrum
“Bad Bugs, No Drugs” task force 10x20 initiative

54. Antimicrobial Stewardship
4 D’s of antimicrobial therapy
Right Drug,
Right Dose,
De-escelation to pathogen directed therapy
Right Duration of therapy
Prevent overuse, misuse and abuse
Minimize the development of resistance