Antimicrobial Medications
Antibiotics
• Antimicrobial drugs naturally
produced by microorganisms
– Penicillium species: Penicillins
– Cephalosporium specis:
cephalosporins
– Streptomyces species:
• lincosamides, aminoglycosides,
tetracyclines, chloramphenicol
Features of antimicrobial drugs
• Selective toxicity
– Therapeutic index
• Antimicrobial action
– Bactericidal
– Bacteristatic
• Spectrum of activity
– Broad spectrum
– Narrow spectrum
• Combination effects
– Antogonistic
– Synergistic
– Additive
Pharmacokinetics: what happens to the
drug in the body?
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Absorption
Tissue distribution
Metabolism
Route of excretion
Rate of elimination
Adverse effects
• Adverse drug reaction
• Toxic effects
• Suppression of
normal microbiota
Mechanisms of antimicrobial drugs
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Inhibition of cell wall synthesis
Inhibition of protein synthesis
Inhibition of nucleic acid synthesis
Inhibition of biosynthetic pathways
Disruption of cell membrane integrity
Targets of Cell Wall Synthesis
Penicillin
• Inhibits formation of tetrapeptide side chains
How organisms degrade penicillins
Family of Penicillins
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Natural penicillins
Penicillinase-resistant penicillins
Broad-spectrum penicillins
Extended-spectrum penicillins
Penicillins plus beta-lactamase inhibitors
Family tree of penicillins
β-lactams
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Penicillins
Cephalosporins
Carbapenems
Vancomycin
Bacitracin
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
Vancomycin
• Binds to the terminal amino acids of the
peptide chain of NAM molecules, blocks
peptidoglycan formation
Antibiotics that inhibit protein
synthesis
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
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
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
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
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 multidrug resistant staphylococci
Antibiotics that inhibit nucleic acid
synthesis
• Fluoroquinolones
– Interferes with function of topoisomerase
• Rifamycins
– Blocks prokaryotic RNA polymerase from initiating
transcription
Antibiotics that inhibit
biosynthetic pathways
• Sulfonamides
• Trimethoprims
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
Drugs used together inhibit folic acid
synthesis
Tests for microbial susceptibility
• Kirby-Bauer (disk diffusion method)
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
Minimum Inhibitory Concentration
• Manual broth dilution method
• Automated broth dilution method
• E-test
Determining the Minimum Inhibitory
Concentration (MIC)
Automated MIC
E-test for MIC
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
What resistance looks like…
Mutant Prevention Concentration?
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
Acquiring resistance
• Spontaneous mutation
• Gene transfer
– R plasmids
Emerging antimicrobial resistance
Streptococcus pneumoniae
• Altered penicillin binding proteins
– DNA-mediated transformation
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
Mycobacterium tuberculosis
• Multidrug-resistant
M. tuberculosis
– Resistance to isoniazid
& rifampin
• Extensively drugresistant M.
tuberculosis
– Resistance to isoniazid
& rifampin + 3 or more
of the 2nd line drugs
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
Enterococcus species:
acquired resistance
• High-level gentamicin-resistance: plasmidencoded inactivating enzymes
• Tetracycine resistance: efflux pumps,
ribosomal protection
• Macrolide resistance: efflux pumps
• Vancomycin resistance: altered drug binding
site on cell wall
Enterobacteriaceae
• Gram negative enteric
rods
• Intrinsic resistance to
many drugs due to
outer membrane
• β-lactamases:
enzymatic inactivation
of the lactam ring
• Extended spectrum βlactamases (ESBL+)
• Carbapenemresistance: enzymatic
inactivation
Staphylococcus species
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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
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
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
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
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
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
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
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
Responsible drug use
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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
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
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