what are the components of peptidoglycan and why is it an important cell structure?

N-acetylmuramic acid (NAM)

N-acetylglucosamine (NAG)

the sugar components are cross-linked to ensure cell integrity; this layer is essential for maintanence of cell shape and to resist cell lysis due to changes in osmotic pressure

• cell wall is constantly being remodeled with autolysin enzymes that break old linkages to allow addition of new dimers
• glycosyltransferase enzymes catalyze the addition of new dimers to the polymer

catalyze cross-links between D-alanine of one chain and the L-lysine or L-glycine in another chain

the terminal D-alanine is removed

aka: penicillin binding proteins

what are some notable features of penicillin structure?

• 4-membered ß-lactam ring
• resembles histidine + valine
• structure mimics the terminal D-alanine-D-alanine of peptidoglycan
  • react w/ and form covalent bonds with transpeptidase enzymes involved in forming peptidoglycan cross-links
  • this action earns the enzyme's name: penicillin binding protein (transpeptidase)

irreversible inhibitor of transpeptidase by covalently binding to the enzyme active site

the bacteria is then unable to cross link, and the cell wall loses integrity, leading to cell lysis

what are some general positive characteristics of all penicillins?

• very high therapeutic index
• good tissue penetration (even enter CSF for treatment of meningitis)
• bacteriocidal (usually)
• many have short half-lives

• active tubular secretion

• dosage adjustment is essential for patients with renal insufficiency

what are the most common adverse effects of penicillins?

• hypersensitivity reactions
  • rashes
  • serum sickness
  • immune-mediated cytopenias
  • immediate-type reactions (hives, anaphylaxis)
• neurotoxicity @ HIGH CONCENTRATIONS

• destroyed by acid, so only IV or IM
• IV half life is very short (30mins)
• concentrations in most tissues equivalent to serum
• cleared by kidney & dose must be adjusted for renal function
• gram positive cocci, gram negative cocci, (most gram positive) anaerobes, spirochetes
  • only active in the above if they lack B-lactamases
    

• streptococcus
  • prophylaxis
• treponema pallidum
  • syphilis
• clostridium

• many gram negative
  • doesn't penetrate cell wall
• staph aureus
  • destroyed by B-lactamases
• strep pneumoniae
  • altered target
• neisseria gonorrhoeae
  • destroyed by B-lactamases

• amino group attached that makes it polar enough to gain entry to some gram negative organisms through porins (E. coli, H.influenzae)
• still susceptible to B-lactamases
• ampicillin: IV, PO
  
• amoxicillin: ONLY PO;excellent oral bioavailability
  

what are β-lactamases and how may they be acquired by bacteria?

• hydrolyze β-lactam ring
• may be encoded on plasmid or on chromosome (TEM-1 type); could be constitutive or induced by exposure to a β-lactam antibiotic (AmpC-type)
• can have narrow or broad spectrum:
  • penicillinases
  • cephalosporinases
  • ESBLs

they have mutations that enable them to degrade some antibiotics that were designed to resist β-lactamase cleavage

they cleave essentially all classes of β-lactams

what are some examples of penicillinase-resistant penicillins?

• NAFCILLIN
• DICLOXACILLIN

these penicillins have BULKY SIDE groups that prevent their entry to the active side of β-lactamases so they are essentially resistant to these enzymes

methicillin is the prototype, but it is no longer used

what is the spectrum of activity of nafcillin and dicloxacillin and how are they administered?

• similar spectrum to penicillin G + bacteria that employ β-lactamases, like:
  • high activity against gram +
  • does not penetrate outer membrane of gram negatives
  • Staphylococcus aureus
    • new penicillin-binding protein (transpeptidase) in S. aureus' MecA gene has made it nafcillin & generally β-lactam antimicrobial-resistant
• nafcillin: IV
• dicloxacillin: PO QID

what is unique about piperacillin's spectrum of activity?

• more polar side chain (carboxylic acid or ureidic acid) than ampicillin and so gains entry to the porins of highly-resistant Gram negative bacilli
  • Pseudomonas aeruginosa
    • an important opportunistic and nosocomial (hospital-acquired) pathogen
• "anti-pseudomonal" penicillin (piperacillin is the most widely used of this category)
  

• resemble β-lactam antibiotics, but have little or no antimicrobial activity on their own
• available in fixed combinations with β-lactam antimicrobials
• purpose is to extend the spectrum of activity to include many β-lactamase-producing bacteria
• VERY BROAD SPECTRUM AGENTS
• USEFUL IN POLYMICROBIAL INFECTIONS OR IF CAUSATIVE AGENT UNKNOWN

what are common β-lactamase inhibitor/β-lactam antimicrobial combinations used?

1. AMPICILLIN-SULBACTAM
   • adds Staphylococcus and β-lactamase-producing anaerobes
     
2. AMOXICILLIN-CLAVULANIC ACID
   • similar to above, but taken PO
     
3. PIPERACILLIN-TAZOBACTAM
   • adds Staphylococcus and β-lactamase-producing anaerobes

• used against many β-lactamase-producing bacteria, like S.aureus
• current generation of cephalosporins have no activity against MRSA and enterococci
• resistance against cephalosporins due to:
  • porins: pseudomonas
  • β-lactamases: AmpC, ESBLs
  • altered PBPs: MRSA, enterococcus
    

what are the first generation cephalosporins and what is their spectrum of activity?

resistant to penicillinases (just like nafcillin) and they penetrate the outer membrane of many gram negative bacilli (just like ampicillin); NO ENTEROCOCCUS; often used in skin and soft-tissue infections, generally caused by streptococci and staphylococci [MRSA is altering this prescribing pattern now]

• Cefazolin
  • long half life
  • prophylaxis during surgery
• Cephalexin
  • oral cephalosporins
    

what is a main second generation cephalosporin and what is its spectrum of activity?

increased activity against gram negative bacilli; excellent activity against gram negative anaerobes and so useful in intraabdominal infections and for prophylaxis in intraab surgery with risk of bowel content spillage

cefoxitin is IV admin

what are some hallmarks of 3rd generation cephalosporins and what is its main drug?

• excellent activity against GRAM NEGATIVE COCCI and BACILLI
• great penetration of CSF (meningitis tx)
  
• CEFTRIAXONE (main 3rd gen cephalosporin)
  
  • extremely long half life
  • used in long term outpatient IV therapy of serious infections (endocarditis, osteomyelitis), even those due to non-MRSA S. aureus
    
  • excretion:biliary tract & so dose adjustments not req'd for patients with renal insufficiency (unless liver failure present)
    

what are some hallmarks of 4th generation cephalosporins and what is its main drug?

• great anti-Pseudomonas drug
• resistant to most β-lactamases and many ESBLs
• retain gram positive activity
• Cefepime ( IV)
  • resistricted use @ UMMS to limit resistance
    

• gram negative only
  • used strictly in infections caused by aerobic and facultative anaerobic gram negative bacteria
    
• use restricted @ UMMS
• no cross reactivity between aztreonam and penicillins are very rare
  • safe to give to a person who has reacted to penicillin or to cephalosporins
    

• β-lactamase resistant
• VERY BROAD SPECTRUM (broadest of all antimicrobial agents)
  • highly active against gram negative bacilli and anaerobes and gram positives
    
• imipenem is oldest member of carbepenems, but doripenem is used at UMMS due to price and availability

• inhibits transpeptidase and glycosyltransferase activity by binding to D-alanine-D-alanine
• cannot penetrate gram negative outer membrane --> GRAM + ONLY
  
• inferior to ß-lactams
• renal clearance
  

• infusion reactions
  • "red man syndrome" (flushing and erythematous rash on face, neck, upper torso): non IgE mediated release of histamine causing vasodilation, itching; must infuse diluted drug very slowly...
• ototoxicity
• nephrotoxicity

• does not cross GI tract, crosses CSF only when meninges are inflamed, and poorly penetrates into respiratory secretions
• IV, except for C. dificile colitis tx
• used for:
  • MRSA infections ***
  • ß-lactamase producing Enterococci ***
  • penicillin-resistant S. pneumoniae
    
  • gram + infections in pts allergic to penicillin/cephalosporins
    

how has vancomycin resistance been achieved by enterococcus (VRE) and S. aureus (VRSA)?

• VanA
  • at this locus there is substitution of D-alanine-D-lactic acid for D-alanine-D-alanine in the peptidoglycan side chain
  • found on plasmid
  • recent reports on its transfer to S. aureus
• VanB, VanC

• cyclic lipopeptide that inserts its lipophilic tail into cell membrane, oligomerizes, and creates a pore that leads to rapid K+ efflux --> membrane depolarization --> cessation of vital cellular processes
• activity not dependent on actively growing cells
• activity:
  • gram + bacteria (including MRSA, VRSA, VRE) *alternative to vancomycin*
  • gram + anaerobes
  • NO gram negative activity

• IV; long half life
• complicated skin and soft tissue infections, bacteremia, right sided endocarditis
• does NOT penetrate well the lung or BBB
• kidney excretion

daptomycin side effects & treatment failure associations

• side effects:
  • mild GI disturbances
  • mild rash
  • mild headache
  • creatinine phosphokinase (CPK) elevations that indicate rhabdomyolysis (lysis of skeletal muscle) might have occured with bid dosing
• treatment failures:
  • associated increased MICs (minimum inhibitory complexes), perhaps mediated by increased cell wall thickness and decreased target penetration

• topical
• gram positive
• inhibits synthesis of cell wall, LPS, capsule
• active against S. aureus and Strep pyogenes (Group A strep)
• used for superficial skin infections, but effectiveness is questionable

• bind to LPS and disrupt outer membrane
• active ONLY against gram negative bacilli
• mostly in topical preparations nowadays due to nephrotoxicity when used systemically
  
• exceptions to IV use in these resistant strains:
  • acinetobacter
  • pseudomonas
    

• oral agent used exclusively for urinary tract infections
• reaches high and prolonged concentrations in urinary tract allowing single dose therapy
• resistance can quickly develop due to mutations in the transporter used to import drug into bacterial cell wall; not to be used in difficult to treat infections!