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PBP inhibitor

• Blocks cell-wall cross-linking
• Bactericidal against actively dividing organisms — the beta-lactam mechanism
• Resistance via altered PBPs (MRSA) or beta-lactamases

PBPs and beta-lactam action

One enzyme family, one covalent trick

• Peptidoglycan is a mesh: sugar strands cross-linked by short peptides. The cross-link is made by a transpeptidase that grabs the peptide’s terminal D-Ala-D-Ala and joins it to a neighboring strand. Those transpeptidases are the penicillin-binding proteins.
• The beta-lactam ring is a structural mimic of D-Ala-D-Ala. The PBP attacks it as if it were substrate, its active-site serine opens the ring, and the enzyme is left covalently acylated — a dead end that hydrolyzes only very slowly.
• Cross-linking stops while the cell keeps remodeling its own wall with autolysins. The wall weakens from within and the cell lyses: beta-lactams are bactericidal, and only against organisms that are actively building wall.
• TIME-dependent killing: cure tracks the fraction of the dosing interval with free drug above the MIC. Hence frequent dosing and extended infusions; higher peaks add little.
• A bacterium carries several PBPs with different jobs: PBP1 builds the side wall, PBP2 keeps the rod shape, PBP3 makes the septum for division. Which ones a drug prefers shows up down the microscope — lysis, round forms, or long filaments.

So why are there thirty-four of them?

Three distinctions among beta-lactams — same mechanism

• ONE — beta-lactamase stability. Bacteria make enzymes that open the ring before it reaches a PBP. Staphylococcal penicillinase, the plasmid TEM/SHV enzymes, the extended-spectrum beta-lactamases, chromosomal AmpC and the carbapenemases are successive answers to successive drugs; each beta-lactam is defined by which of them it survives.
• TWO — PBP preference. Aztreonam binds only Gram-negative PBP3, so it has no Gram-positive or anaerobic activity at all. Ceftaroline is the one cephalosporin that binds staphylococcal PBP2a. Mecillinam-type agents prefer PBP2. Preference, not potency, sets the spectrum.
• THREE — access. A Gram-negative PBP sits behind an outer membrane; the drug must cross through a porin and survive efflux. Bulky, lipophilic antistaphylococcal penicillins cannot — which is why nafcillin has no Gram-negative activity despite being a fine transpeptidase inhibitor.
• Everything clinicians call “generation” or “spectrum” is a summary of those three. The peg is the same mechanism in every case; the variant art marks the scaffold.
• Vancomycin, glycopeptides and lipoglycopeptides also stop cross-linking, but bind the D-Ala-D-Ala SUBSTRATE, not the enzyme. They are not PBP inhibitors and carry a different peg.

One mechanism, eleven pegs

Penicillins — four clinical groups on one scaffold

Natural · amino · penicillinase-stable · antipseudomonal

• NATURAL (penicillin G, penicillin V). Streptococci, Treponema pallidum, Neisseria meningitidis, oral anaerobes, Actinomyces. Destroyed by staphylococcal penicillinase, so they are useless against almost all S. aureus. Penicillin G is parenteral; the benzathine depot gives weeks of low levels for syphilis and rheumatic-fever prophylaxis.
• AMINOPENICILLINS (amoxicillin, ampicillin). An added amino group lets them cross Gram-negative porins: Enterococcus, Listeria, Haemophilus influenzae and some E. coli and Proteus join the list. Still penicillinase-labile — which is why they are so often paired with clavulanate or sulbactam.
• PENICILLINASE-STABLE / ANTI-STAPHYLOCOCCAL (nafcillin, oxacillin, dicloxacillin, cloxacillin, flucloxacillin, and historically methicillin). A bulky side chain shields the ring from staphylococcal penicillinase. They are the drugs of choice for methicillin-susceptible S. aureus and have essentially no Gram-negative or enterococcal activity.
• ANTIPSEUDOMONAL (piperacillin, ticarcillin). The broadest penicillins — Pseudomonas aeruginosa and most Enterobacterales — but readily hydrolyzed, so they are given with tazobactam or clavulanate.
• All of them are renally cleared except nafcillin and oxacillin, which are hepatic — the reason nafcillin keeps its dose in renal failure.

Cephalosporins — what the generations actually track

Gram-negative reach and beta-lactamase stability rise; Gram-positive potency drifts down, then returns

• FIRST (cefazolin, cephalexin, cefadroxil, cephradine). Best Gram-positive cover of the class — streptococci and MSSA — plus Proteus, E. coli and Klebsiella. Cefazolin is the standard surgical prophylaxis; none of them cross into the CSF.
• SECOND (cefuroxime, cefaclor, cefprozil; and the cephamycins cefoxitin and cefotetan). More Haemophilus and Moraxella. The cephamycins are the anaerobe members — Bacteroides fragilis — used for intra-abdominal and gynecologic cover.
• THIRD (ceftriaxone, cefotaxime, ceftazidime, cefdinir, cefixime, cefpodoxime, ceftizoxime). Broad Gram-negative activity and the first of the class to reach the CSF reliably. Ceftriaxone and cefotaxime are the meningitis and gonorrhea agents; ceftazidime is the one with antipseudomonal activity, at the cost of Gram-positive potency.
• FOURTH (cefepime). A zwitterion that crosses porins quickly and is a poor AmpC substrate: Gram-negative reach like ceftazidime, Gram-positive cover back up near first-generation, plus Pseudomonas. Neurotoxicity — encephalopathy, myoclonus, non-convulsive status — is dose- and renal-function-related.
• FIFTH (ceftaroline). The anti-MRSA cephalosporin: it binds PBP2a, the enzyme that makes MRSA MRSA. It is not antipseudomonal.
• No cephalosporin covers Enterococcus.

Carbapenems and the monobactam

The widest beta-lactam, and the narrowest

• CARBAPENEMS (meropenem, imipenem, ertapenem, doripenem). The broadest beta-lactams: Gram-positive, Gram-negative and anaerobic cover, and stable against ESBLs and AmpC — which is what makes them the standard answer to an ESBL-producing organism.
• Not all are equal: ertapenem has a long half-life and once-daily dosing but NO activity against Pseudomonas or Acinetobacter; meropenem, imipenem and doripenem do cover Pseudomonas.
• Imipenem is given with cilastatin, which blocks renal dehydropeptidase-1 from degrading it. Imipenem carries the highest seizure risk of the group — worse with renal impairment, high dose or prior CNS disease; meropenem is preferred for CNS infection.
• Every carbapenem drops valproate concentrations sharply and unpredictably, within a day or two, and the interaction cannot be managed by raising the valproate dose — choose another antibiotic or another anticonvulsant.
• MONOBACTAM (aztreonam). A single ring, binding only Gram-negative PBP3 — so aerobic Gram-negatives including Pseudomonas, and literally nothing else. Its value is in penicillin allergy: it shares no side chain with the penicillins and is not cross-reactive. The one exception is ceftazidime, with which it shares an identical side chain.

Resistance — one: the beta-lactamases

Beta-lactamases and the drugs developed against them

• A beta-lactamase opens the ring in solution, before it ever reaches a PBP. The enzymes are grouped either by sequence (Ambler A, B, C, D) or by what they destroy — the second is what matters at the bedside.
• STAPHYLOCOCCAL PENICILLINASE: in the great majority of S. aureus. Answered by the penicillinase-stable penicillins and by the cephalosporins.
• PLASMID BROAD-SPECTRUM (TEM, SHV) in Enterobacterales: answered by adding clavulanate, sulbactam or tazobactam, which sacrifice themselves on the enzyme.
• EXTENDED-SPECTRUM (ESBL): destroys the third-generation cephalosporins as well. Carbapenems are the reliable answer.
• AmpC: chromosomal and inducible in Enterobacter, Serratia, Citrobacter and others — an organism can test susceptible to a third-generation cephalosporin and then derepress on treatment. Cefepime and the carbapenems are stable to it; classical inhibitors are not.
• CARBAPENEMASES (KPC, OXA-48, and the metallo-enzymes NDM/VIM/IMP): the current frontier, answered by newer inhibitor combinations rather than by any older beta-lactam. The metallo-enzymes use zinc rather than a serine and are untouched by the serine-enzyme inhibitors.

Resistance — two: changing the PBP itself

MRSA, penicillin-resistant pneumococcus, and why the mechanism peg still fits

• If the enzyme will not bind the drug, no amount of beta-lactamase stability helps. This is the second, independent route to resistance.
• MRSA acquires mecA, which encodes PBP2a — a transpeptidase with such low affinity for beta-lactams that it keeps building wall while every native PBP is acylated. That one gene confers resistance to ALL penicillins, cephalosporins and carbapenems at once. “Methicillin-resistant” is shorthand for the whole class.
• Ceftaroline is the exception: it binds allosterically opened PBP2a and is active against MRSA.
• Streptococcus pneumoniae does not make a beta-lactamase at all. It takes up DNA from relatives and builds mosaic PBP2x and PBP2b genes with stepwise lower affinity — which is why pneumococcal penicillin resistance is graded rather than all-or-none, and why raising the dose still works for pneumonia but not for meningitis.
• Gram-negatives add a third route that needs no enzyme change at all: lose a porin, or upregulate an efflux pump, and the drug never reaches a PBP it binds perfectly well.

What it costs

Shared by the class, then member by member

• HYPERSENSITIVITY is the class limitation — the whole range from maculopapular rash to IgE-mediated anaphylaxis. Most people labeled “penicillin-allergic” are not, and the label itself leads to worse antibiotic choices; formal evaluation is worth doing.
• Cross-reactivity between penicillins and cephalosporins is far lower than the figure taught for decades, and it tracks SIDE-CHAIN similarity rather than the shared ring: amoxicillin and cefadroxil, or ampicillin and cephalexin, share an R1 group, while most third- and fourth-generation cephalosporins share none. Aztreonam is the safe outlier except with ceftazidime.
• Ampicillin or amoxicillin given during Epstein-Barr infection produces a near-universal rash that is NOT an allergy.
• SEIZURES at high concentration are a class property — beta-lactams antagonize GABA-A — and surface in renal impairment or with CNS disease. Imipenem is the worst offender; penicillin G at very high dose and cefepime also do it.
• MEMBER-SPECIFIC: ceftriaxone precipitates in bile (pseudolithiasis) and must not be co-administered with calcium-containing solutions in neonates; cefepime causes encephalopathy when dosed for better renal function than the patient has; nafcillin extravasation causes tissue injury and it is a CYP3A4 inducer; the carbapenem-valproate interaction above.
• Every broad beta-lactam selects for C. difficile and resistant flora. Choose the narrowest effective agent — the rationale for thirty-four class members.

Drugs with this mechanism (34)

Self-check

• Why do beta-lactams kill actively dividing organisms?
  They block cell-wall cross-linking.
• What exposure measure predicts beta-lactam cure in this deck?
  The fraction of the dosing interval with free drug above the MIC.

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