Patozone

Antibiotics

 

General Mechanisms of Action

1. Block cell wall synthesis by inhibiting peptidoglycan cross-linking

- penicillin, methicillin, ampicillin, piperacillin, cephalosporins, aztreonam, imipenem

 

2. Block peptidogltcan synthesis

- bacitracin, vancomycin

 

3. Block nucleotide synthesis

- sulfonamides, trimethoprim

 

4. Block DNA topoisomerases

- fluoroquinolones

 

5. Block mRNA synthesis

- Rifampin

 

6. Damage DNA

- metronidazole

 

7. Block protein synthesis at 50S ribosomal subunit

- Cloramphenicol, macrolides, clindamycin, streptogramins (quinupristin, dalfopristin), linezolid

 

8. Block protein snth at 30S ribosomal subunit

- aminoglycosides, tetracyclines

 

Penicillin

 

Penicillin G (IV formn), Penicillin V (oral), Prototype B-lactam antibiotics

 

Mech: 1) Binds penicillin-binding proteins (PBPs)

- PBPs are transpeptidases that are involved in the final stages of the synth of peptidoglycan (a major component of cell walls)

2) Blocks transpeptidase cross-linking of cell wall

3) Activates autolytic enzymes

 

Clinical use: Bactericidal for gram-pos cocci (grp A and B Streptococcus), gram-pos rods (Claustridium and Listeria), gram-neg cocci (Neisseria), spirochetes.

- Not penicillinase resistant

 

Tox: Hypersensitivity rxns, hemolytic anemia

 

 

Methicillin, nafcillin, dicloxacillin (penicillinase- resistant penicillins)

 

Mech: same as penicillin; narrow spectrum; penicillinase resistant bc of bulkier R group

*** use Naf (nafcillin) for staph ***

 

Clinical use: S aureus (except MRSA; resistant bc of altered PBP target site)

 

Tox: hypersensitivity rxns, methicillin can cause interstitial nephritis

 

Ampicillin, amoxicillin (aminopenicillins)

 

Mech: same as penicillin; wider-spectrum; penicillinase sensitive (also combined c clavulanic acid)

*** amOxicillin has great Oral bioavailabilitty than ampicillin ***

 

Clinical use: Extended-spectrum penicillin - certain gram-pos bacteria and gram-neg rods (Haemophilus influenzae, E coli, Listeria monocytogenes, Proteus mirabilis, Salmonella, enterococci) -- good for UTIs

 

** Coverage: ampicillin/amoxicillin HELPS get rid enterococci ***

 

Tox: HS rxns, ampicillin rash (esp occurs when ampicillin given during infectious mono [EBV]), and pseudomembranous colitis (esp ampicillin)

 

Ticarcillin, carbenicillin, piperacillin (antipseudomonals, aka the carboxypenicillins)

 

Mech: Same as penicillin; extended spectrum

 

Clinical use: Pseudomonas spp and gram-neg rods; susceptible to penicillinases -- used with clavulinic acid B-lactamase inhibitor

 

Penicillinase (B-lactamase) inhibitors: Clavulanic Acid, Avibactam, Sulbactam, Tazobactam

*** CAST ***

- have little activity on their own

- inhibit B-lactamases - used in combo c B-lactams to inc activity

-- Amoxicillin + clavulanic acid = Augmentin

-- Ampicillin + sulbactam = Unasyn

-- Piperacillin + tazobactam = Zosyn

-- Ceftazidime + avibactam = Avycaz

 

*** TCP: Takes Care of Pseudomonas ***

 

Tox: HS

 

 

Cephalosporins

 

Mech: B-lactam drugs that inhibit cell wall synthesis but are less susceptible to penicillinases. Bactericidal

 

Cefazolin, Cephalexin, Cephalothin (!st generation, narrow spectrum)

- used for gram-pos cocci (Staph and Strep, except MRSA), Proteus mirabilis, E coli, Klebsiella pneumoniae

*** 1st generation = PEcK ***

 

Cefoxitin, Cefotetan, Cefaclor, Cefuroxime (2nd generation, Expanded spectrum)

- adds some gram-neg and anaerobic coverage; used for gram-pos cocci, Haemophilus influenzae, Enterobacter aerogenes, Neisseria spp, Proteus mirabilis, E coli, Klebsiella pneumoniae

*** 2nd generation = HEN PEcKS ***

 

Ceftriaxone, Cefixime, Cefotaxime, Ceftazidime (3rd generation, Broad spectrum)

-- more Gram-neg activity (but less Gram-pos and anaerobic activity); serious gram-neg infx resistant to other B-lactams; meningitis (most penetrate blood-brain barrier)

-- Ceftrixone -- meningitis and gonorrhea

-- Ceftazidime -- Pseudomonas

 

Cefepime (4th generation, Extended spectrum)

Very good Gram neg activity, better B-lactamase activity; Inc activity against Pseudomonas and gram-pos bugs

 

Ceftaroline (4+ generation)

Spectrum activity Gram-pos (including MRSA) and Gram-neg bacteria (not ESBL)

 

Tox: HS, vit K deficiency

- cross HS c penicillins in <10% of pts; inc nephrotoxicity of aminoglycosides

- disulfiram-like rxn c alcohol use (in cephalosporins c a methylthiotetrazole group like cefamandole)

 

Resistance: B-lactamases, including extended spectrum B-lactamases; altered penicillin binding protein (PBP)

 

Aztreonam

 

A monobactam resistant to B-lactamases

- inhibits cell wall synth (binds to PBP3)

- synergic c aminoglycosides

- no cross allergenicity c penicillins

 

Clinical use: Gram-neg rods only -- no activity against gram-pos or anaerobes

- for penicillin allergy pts, and pts c renal insufficiency who cannot tolerate aminoglycosides

 

Tox: usually non-toxic; maybe GI upset

 

 

Imipenem/cilastatin, meropenem, Ertapenem (Carbapenems)

 

Broadest spectrum activity Gram-pos (except MRSA) and Gram-neg bacteria including anaerobes

 

Mech: Imipenem is a broad-spectrum B-lactamase resistant carbapenem; always given with cilastatin (inhibitor of renal dehydropeptidase I) to dec inactivation of drug in renal tubules

 

Clinical use: gram-pos cocci, gram-neg rods, and anaerobes; wide-spectrum, but the side effects limit use to life-threatening infx or after other drugs fail

- meropenem, though, has a dec risk of seizures and is stable c dihydropeptidase I

 

Tox: GI distress, skin rash, CNS toxicity (seizures) at high plasma levels

 

 

Vancomycin

 

Mehc: inhibits cell wall mucopeptide formation by binding D-ala D-ala portion of cell wall precursors. Bactericidal

 

Clinical use: Gram-pos only - for serious multidrug -resistant organisms, such as S aureus, enterococci and Clostridium difficile (oral dose for seuodmembranous colitis)

 

Tox: Nephrotoxic, Ototoxic, Thrombophlebitis, flushing (red man syndrome, can be prevented by slow infusion and antihistamines) -- generally does NOT have many problems

 

Resistance: Amino acid change of D-ala D-ala to D-ala D-lac

*** Pay back 2 D-ala (dollars) for vandalizing ***

 

- may inc cell-wall thickness

 

Polymyxin B and E (aka Colistin)

 

Mech: bind to lipopolysaccharide in outer membrane of Gram-neg bacteria, disrupting the inner and outer membranes, possibly c a detergent-like mech of action

- produced in nature by gram-pos bacteria such as Paenibacillus polymyxa

 

Clinical Use: multiple-drug resistant Pseudomonas aerugenosa or carbapenemase-producing Enterobactericeae

- are not absorbed through GI tract, so only given orally to treat gastrointestinal infx

- can be given as drops for otitis media

- can be given parenteral

 

Tox: neurotoxic, nephrotoxic, used as a last resort

 

Protein Synthesis Inhibitors

 

*** Buy AT 30; CCEL (sell) at 50 ***

- target small bacterial ribosome (70S made of 30S and 50S subunits), leaving human chromosome (80S) unaffected

 

30S inhibitors

A = Aminoglycosides (bactericidal)

T = Tetracclines (bacteriostatic)

 

50S inhibitors

C = Chloramphenicol and Clindamycin (bacteristatic)

E = Erythromycin (macrolides) [bacteriostatic]

L = Linezolid (variable)

 

Gentamicin, Neomycin, Amikacin, Tobramycin (Aminoglycosides)

 

*** "Mean" GNATS canNOT kill anaerobes ***

 

Mech: bactericidal; inhibit formation of initiation complex and cause misreading of mRNA

- need O2 for uptake, thus are not effective against anaerobes

 

Clinical use: severe gram-neg rod infx; synergistic c B-lactamse antibiotics against S aureus and Enterococcus

- neomycin for bowel surgery

 

Tox: Nephrotoxic (esp if used c cephalosporins), Ototoxic (esp if used c loop diuretics), Teratogen

 

Resistance: Transferase enzymes inactivate the drug by acetylation, phosphorylation or adenylation

- also may alter membrane permeability

- modification (methylation) of ribosomal target

 

Tetracycline, Doxycycline, Demeclocycline, Minocycline (Tetracyclines)

 

*** Demeclocycline -- ADH antoagonist; acts as Diuretic in SIADH ***

 

Glycylcyclines (Tigecycline) - broader-spectrum gram-neg and anti-staphylococcal activity than other tetracyclines

 

Mech: Bacteriostatic. Binds to 30S and prevents attachment of aminoacyl-tRNA; limited CNS penetration

- doxyxyxline fecally eliminated and can be used in pts c renal failure

-- must NOT take c milk, antacids, or iron-containing preparations bc divalent cations inhibit absorption in gut

 

Clinical use: Borrelia burgdorferi, M pneumoniae. Drugs ability to accumulate intracellularly makes it very effective against Rickettsia and Chlamydia

 

Tox: GI distress, discoloration of teeth and inhibition of bone growth in kiddos, photosensitivity.

- Contraindicated in pregnancy

 

Resistance: Dec uptake into cell or inc efflux out of cell by plasmid-encoded transport pumps

- modification (methylation) of ribosomal target

- tetracycline-modifying enzymes

 

Erythromycin, Azithromycin, Clarithromycin (Macrolides)

 

Inhibit protein synth by blocking translocation (macroSlides); bind to 23S rRNA of the 50S ribosomal subunit. Bacteriostatic

 

Clinical use: Atypical pneumonias (Mycoplasma, Chlamydia, Legionella), URIs, STDs, gram-pos cocci (streptococcal infx in pts allergic to peniciilin) and Neisseria

 

Tox: prolonged QT interval (esp erythromycin), GI problems, acute cholestatic hepatitis, eosinophilia, skin rash,

 

Resistance: Methylaion of 23S rRNA binding site

- efflux pumps (macrolides and ketolides)

 

 

Chloramphenicol

 

Mech: blocks eptide bond formation at 50S ribosomal subunit. Bacteriostatic

 

Clinical use: Meningitis (Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumonae); conservative use 2/2 tox but often still used in developing countries 2/2 low cost

 

Tox: Anemia (dose-dependent), aplastic anemia (dose dependent), gray baby syndrome (in premature infants bc lack liver UDP-glucoronyl transferase)

 

Resistance: Plasmid-encoded acetyltransferase inactivates drug

 

 

Clindamycin

 

Mech: Blocks peptide bond formation at 50S ribosomal subunit. Bacteriostatic

 

Clinical use: anaerobe infx (Bacteroides fragilis, Clostridium perfringens) in aspiration pneumonia or lung abscesses

 

Tox: Pseudomembranous colitis

 

 

Sulfamethoxazole (SMX), Sulfisoxazole, Sulfadiazine (Sulfonamides)

 

Mech: PABA antimetabolites inhibit dihydropteroate synthetase. Bacteriostatic

 

Clinical use: Gram-pos, gram-neg, Nocardia, Chlamydia. Triple sulfas or SMX for simple UTI

 

Tox; HS, hemolysis if G6PD deficient, nephrotoxic (tubulointerstitial nephritis), photosensitivity, kernicterus in infants, displaces other drugs from albumin (warfarin)

 

Resistance: Altered enzyme (bacterial dihydropteroate synthetase), dec uptake or inc PABA synth

 

 

Trimethoprim

 

Mech: Inhibits bacterial dihydrofolate reductase. Bacteriostatic

 

Clinical use: used in combo c sulonamides (TMP-SMX) causing sequential block of folate synth. Combo for UTIs, Shigella, Salmonella, Pneumocystic jiroveci pneumonia, Burkholderia cepacia and Stenotrophomonas maltophilia

 

Tox: Megaloblastic anemia, leukopenia, granulocytopenia (can be alleviated c supplemental folinic acid; the leucovorin rescue)

 

 

Ciprofloxacin, Norfloxacin, Levofloxacin, Ofloxacin, Sparfloxacin, Moxifloxacin, Gatifloxacin, Enoxacin  (Fluoroquinolones) and Nalidixic acid (a quinolone)

 

Mech: inhibits DNA gyrase (topoisomerase II). Bactericidal. Must not be taken c antacids

 

Clinical use: gram-neg rods or urinary and GI tracts (including Pseudomonas), Neisseria, some gram-pos organisms

 

Tox: Tendonitis and tendon rupture in adults, leg cramps and myalgias in kiddos; contraindicated in pregs and kiddos bc damages cartilage

 

Resistance: chromosome-encoded mutation in DNA gyrase and/or DNA topoisomerase

 

 

Metronidazole

 

Mech: forms free radical toxic metabolites in bacterial cell that damage DNA. Bactericidal, and antiprotozoal

 

Clinical use: treats Giardia, Entamoeba, Trichomonas, Gardnerella vaginalis, Anaerobes (Bacteroides and C difficile)

- used c bismuth and amoxicillin (or tetracycline) for "triple therapy" against H pylori

 

Tox: disulfiram-like rxn c alcohol, headache, metallic taste

Antimicrobial Susceptibility Testing

 

Qualitative

Kirby Bauer disc diffusion -- results interpreted as susceptible, intermediate, or resistant

 

Quantitative

Broth or Agar dilution -- results interpreted as lowest concentration of antibiotic that inhibits bacterial growth (MIC)

 

Gradient Diffusion

Set up like a qualitative test, read like a quantitative test

 

Not everything that is tested is reported -- report based on clinical efficacy, prevalence of resistance, minimizing emergence of resistance and cost

 

Reported as determined by cutoffs set by the CLSI using epidemiological and pharmacodynamic data:

Susceptible (S) -- susceptible to usual doses in accessible sites; does not mean in the middle of an abscess

 

Intermediate (I) -- susceptible at inc doses or where the drug is concentrated; usually means a drug to use in urine or if no better choice is available

 

Resistant (R) -- resistant to the usual doses

 

Disk Diffusion

Bacteria spread in a confluent lawn on plate

- antibiotic disk added to lawn and diffuses from disk into agar

- a zone of inhibition is formed around the disk

- size of the zone is correlated c the MIC

 

Minimum Inhibitory Concentration (MIC) Testing

Liquid or solid media c varying dilutions of antibiotic

- lowest concentration that inhibits growth

 

Gradient Diffusion (Etest)

Bacteria spread in confluent lawn on plate

- antibiotic gradient strip added to lawn and antibiotic diffuses into agar

- an ellipse of inhibition is formed around the strip

- where organism growth intersects the plastic strip = MIC

 

B-lactamase Detection

- filter paper impregnated c chromogenic cephalosporin (nitrocefin)

- results in minutes

- limited applications

-- detects resistance to ampicillin/penicillin (not cephalosporins) in Haemophilus, Neisseria gonorrhoeae, Moraxella catarrhalis, Enterococcus, and Staphylococcus

 

Resistance Detection Issues

 

S. pneumoniae

- overall resistance increasing

- Penicillin resistance -- altered PBPs

-- Intermediate resistance - 30-40%

-- High level resistance -- 5-15%

 

Testing isolates

-- oxacillin screen - susceptible >20 mm

-- confirm results <19 mm c MIC testing

- E-test or broth microdilution

 

Separate interpretive standards for penicillin and cephalosporins for:
- meningitis (S<0.06, R>0.12)

- Non-meningitis parenteral (S<2, I=4, R>8)

- Oral therapy (S<0.06, I=0.12-1, R>2)

 

S. aureus

 

Methicillin-Resistant S Aureus (MRSA)

S aureus is a major healthcare and community-acquired organism, causes ~1/5 cases if bacteremia in the US/Canada

- MRSA first described in 1961; first penicillinase- resistant semi-synthetic (methicillin) introduced in 1960

-- oxacillin resistance mediated by acquisition of mecA gene -- codes for altered PBP --> PBP2a

 

MRSA should be considered resistant to ALL penicillins, cephalosporins, B-lactam / B-lactamase inhibitor combinations, carbapenems

 

Heterogeneous resistance -- MRSA grows more slowly, MRSA growth enhanced by NaCl

 

Detection -- oxacillin screening agar, cefoxitin disk test, PBP2a antigen, molecular testing, chromogenic media

 

Oxacillin Screening Agar

- 6 ug/mL oxacillin, Mueller-Hinton agar supplemented c 4% NaCl, incubated 24 hrs before reading; requires separate piece of media, 24h reporting delay if not performed on all isolates

 

Cefoxitin Disk Screen

30 ug cefoxitin disk; Report zone ≤19mm as oxacillin resistant; Also works for coagulase(-) staph, zone ≤24mm; Relatively inexpensive; Requires a separate plate, but may be combined with D-test; Now incorporated into automated testing systems

 

PBP 2a Antigen

Rapid antigen test for PBP2a; gene product of mecA

– Rapid but expensive; May require overnight induction for best sensitivity

 

Molecular Testing

Used both for screening and culture confirmation

– False-positives due to gene deletions outside the detection amplicon

 

Chromogenic Media

Selective / differential media specific for MRSA; no AST required

 

Tx for MRSA: Vancomycin

 

Vancomycin Intermediate S. aureus (VISA)

Relatively rare; Difficult to detect

– Cannot use disk diffusion testing for vancomycin

• Decreased fitness; assoc c poor clinical response

 

Vancomycin Resistant S aureus (VRSA)

• Vancomycin MIC >8

• Acquisition of vanA cluster from Enterococcus

• Typically very high MIC, no loss of fitness

• May be missed by automated systems

• Vancomycin Screening Agar required for sensitive detection

 

Other vancomycin issues -- recent data suggests that S. aureus c MIC of 2 vs vancomycin (still S by CLSI breakpoints) have poorer outcomes when tx'd c vancomycin

- perform and report MIC or E-test on systemic MRSA isolates

 

 

Inducible clindamycin resistance

- macrolide resistance -- methylation of ribosome (erm gene - constitutive or inducible)

- efflux (msrA gene)

- staph resistant to both erythromycin and clindamycin = constitutive erm gene

- staph with an inducible erm gene that's treated c clindamycin can be easily induced to become resistant due to cross-resistance c erythromycin, leading to treatment failure

 

Until recently conventional automated AST did not detect inducible resistance well

• Test all Staph resistant to erythromycin and susceptible to clindamycin to determine if

– Inducible erm expression – clindamycin will become

resistant, cannot be used for therapy

– Efflux (msrA expression) – clindamycin can be used for therapy

• Perform D test

• 2 ug clindamycin disk; 15 ug erythromycin disk

• Place 15 - 26 mm apart

• Should see round zone of inhibition, but if zone for

clindamycin is blunted (capital D)…

– Isolate contains inducible erm gene

– Report: Erythromycin – R and Clindamycin – R

 

• If you see the expected round zone of inhibition for

clindamycin…

– Isolate has efflux-mediated erythromycin resistance (msrA gene)

– Report: Erythromycin – R and Clindamycin – S

• Detection of inducible clindamycin resistance now

incorporated into automated testing systems

 

Vancomycin-Resistant Enterococcus (VRE)

Enterococci naturally resistant to lots of antibiotics

– Cephalosporins, SXT, clindamycin

– Low level resistance to aminoglycosides (aminoglycoside modifying enzymes)

• Therapy for serious infections often requires combination therapy – cell wall-active agent + aminoglycoside

• But some enterococci can modify their ribosome resulting in…

High level aminoglycoside resistance: Detected by testing the isolate in 500 ug/mL gentamicin or 1000 ug/mL streptomycin

– If susceptible – combination therapy with cell wall agent (if susceptible) WILL BE effective

– If resistant – combination therapy with cell wall agent (if susceptible) WILL NOT BE effective

 

• And now there’s vancomycin resistance too

• VRE – nosocomial transmission

– Van A (E. faecium [most VRE isolates] and E. faecalis); High level R to Vanc (> 64 μg/ml)

– Van B (E. faecium and E. faecalis); Low level R to Vanc (4-32 μg/ml); S to teicoplanin

– Van C (E. gallinarum and E. casseliflavus)

• Low level R to Van (usually Vanc=I) and S to teicoplanin; Not nosocomial transmission

• Resistance mechanism – modified peptidoglycan precursor

 

Tx for VRE: linezolid and streoptogramins (quinupristin/dalfopristin)

 

Gram-neg bacilli

 

Extended-spectrum B-lactamase (ESBL)

• Point mutations of common (TEM and SHV) B-lactamases responsible for ampicillin resistance in E. coli, Klebsiella and Proteus

• The bad thing…

– Capable of hydrolyzing extended spectrum cephalosporins

– Outcome data suggests that they do cause treatment failures in vivo

• The good thing…

– They are inhibited by clavulanic acid

• DISK: ≥ 5mm zone increase = ESBL

• Confirmed ESBL producers should be reported as resistant to ALL penicillins and cephalosporins (regardless of in vitro result)

• Exceptions:

– Cephamycins – not hydrolyzed by ESBLs (Report as tested)

• Cefoxitin, cefotetan, cefmetazole

 

– B-lactam/ B-lactamase inhibitor combinations

• The inhibitors inhibit the ESBLs

• Amixicillin-clavulanate, piperacillin-tazobactam

 

Carbapenemases

Assoc c outbreaks of multi-resistant Klebsiella

pneumoniae, other Enterobacteriaceae, Pseudomonas and Acinetobacter

• Most common – KPC (K. pneumoniae carbapenemase)

-- other carbapenemase, like metallo B lactamase (MBL) and the SME-1 in Serratia marcescens can also make a positive Modified Hodge Test, but are not frequently seen in the US

– Plasmid-borne – Confers resistance to all B-lactams

– Easily spread

– Difficult to detect – Low level expression: Ertapenem is most sensitive screening drug

• Screening – Look for elevated carbapenem MICs (may still be ‘S’) AND resistance to at least one 3rd generation cephalosporin

• Confirmation – Modified Hodge test

• Drugs used to treat these isolates include tigecycline and colistin

 

Modified Hodge Test (MHT)

– Disk diffusion

– Isolate = QC organism (E. coli ATCC 25922)

– Disk = meropenem or ertapenem

– Patient isolate (spoke in the wheel): Streak from disk to outer edge of plate

– Incubate like disk diffusion

 

• Other important carbapenemases

– New Delhi metallo-B-lactamase (NDM-1) in E. coli, K. pneumoniae

– Verona Integron-Encoded metallo-B-lactamase (VIM) - K. pneumoniae and P. aeruginosa

– Imipenem-resistant metallo-beta-lactamase (IMP) – most often seen in P. aeruginosa and Acinetobacter

– Carbapenem-hydrolyzing class D beta-lactamases or oxacillinases (OXA) – do not have extended spectrum beta lactamase activity - Acinetobacter