Pharmacology · Antibacterial Agents

Penicillins

Beta-lactam chemistry, mechanism of action, spectrum, and resistance

All penicillins share a four-membered beta-lactam ring that covalently inactivates penicillin-binding proteins — the transpeptidases responsible for peptidoglycan cross-linking — producing bactericidal, time-dependent killing.
The four subclasses (natural, aminopenicillins, antistaphylococcal, extended-spectrum) each occupy a distinct clinical niche defined by spectrum, beta-lactamase stability, and route of administration.
Nafcillin is eliminated hepatically and does not require renal dose adjustment — an important pharmacokinetic distinction from all other penicillins.
Piperacillin-tazobactam should not be used as definitive therapy for extended-spectrum beta-lactamase bacteremia even when in vitro susceptibility is reported; carbapenems are preferred.
Resistance arises through four mechanisms — beta-lactamase production, extended-spectrum beta-lactamases, altered penicillin-binding proteins (mecA), and reduced outer membrane permeability — each with distinct clinical implications.

Abbreviations: PBP = penicillin-binding protein · MRSA = methicillin-resistant Staphylococcus aureus · MSSA = methicillin-susceptible Staphylococcus aureus · ESBL = extended-spectrum beta-lactamase · MIC = minimum inhibitory concentration · fT>MIC = free drug time above MIC · IV = intravenous · IM = intramuscular · KPC = Klebsiella pneumoniae carbapenemase · NDM = New Delhi metallo-beta-lactamase · GFR = glomerular filtration rate

Mechanism of Action

How Beta-Lactams Kill Bacteria

Time-Dependent Bactericidal Activity

Step 1

Drug Enters

Crosses outer membrane via porins (gram-negative) or directly (gram-positive)

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Step 2

Binds PBP

Covalently acylates active-site serine of PBP transpeptidase — permanent inactivation

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Step 3

Cross-Linking Blocked

Peptidoglycan synthesis halted; autolysins continue degrading existing wall

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Result

Cell Lysis

Bactericidal; efficacy depends on fT>MIC — extended or continuous infusion maximizes effect

Penicillin Subclasses

Natural Penicillins

Penicillin G & V

Streptococci, T. pallidum, oral anaerobes, Listeria
Drug of choice for syphilis — no resistance ever emerged
Penicillin G: IV / IM only (acid-labile)
Penicillin V: oral (acid-stable)
No activity vs. gram-negative enteric bacteria or staphylococci

Aminopenicillins

Ampicillin / Amoxicillin

+ H. influenzae, E. coli, Enterococcus faecalis, Salmonella
Amoxicillin: better oral bioavailability — preferred orally
Ampicillin: preferred IV (Listeria meningitis, enterococcal endocarditis)
Beta-lactamase susceptible — add inhibitor for broader coverage
No activity vs. MRSA or Pseudomonas

Antistaphylococcal

Nafcillin / Oxacillin / Dicloxacillin

MSSA only — preferred over vancomycin for serious MSSA infections
Bulky side chain provides steric protection from staphylococcal beta-lactamase
Nafcillin: hepatic elimination — no renal dose adjustment needed
Dicloxacillin: oral; take on empty stomach (food reduces absorption)
No activity vs. MRSA (mecA/PBP2a resistance)

Extended-Spectrum

Piperacillin-Tazobactam

Pseudomonas aeruginosa, Enterobacteriaceae, anaerobes, streptococci
Tazobactam inhibits most beta-lactamases — broadens coverage
Do not use as definitive therapy for ESBL bacteremia
Carbapenem preferred for serious ESBL infections (inoculum effect)
Renal dose adjustment required

Key Pharmacokinetics

Agent	Route	Elimination	Renal Adjustment	Key Notes
Penicillin G	IV / IM	Renal (tubular secretion)	Yes — risk of neurotoxicity if accumulated	Short half-life; continuous infusion used for serious infections
Penicillin V	Oral	Renal	Yes	Acid-stable; lower peak levels than IV penicillin G
Amoxicillin	Oral	Renal	Yes	Better oral bioavailability than ampicillin; food does not impair absorption
Ampicillin	IV / oral	Renal	Yes	Preferred parenterally; poor oral bioavailability
Nafcillin	IV	Hepatic (biliary)	No	Preferred antistaphylococcal agent in renal failure
Oxacillin	IV	Hepatic + renal	Minor	Alternative to nafcillin; similar spectrum
Dicloxacillin	Oral	Hepatic + renal	Minor	Take fasted; food reduces absorption by ~50%
Pip-Tazo	IV	Renal	Yes	Extended infusion (3–4 hr) improves fT>MIC for resistant organisms

Resistance Mechanisms

Mechanism	Example	Key Organisms	Clinical Response
Beta-lactamase (Class A)	TEM, SHV penicillinases	Staphylococci, gram-negative enteric bacteria	Beta-lactamase inhibitor combinations (clavulanate, tazobactam, sulbactam)
ESBL (Class A)	CTX-M-15	E. coli, Klebsiella pneumoniae	Carbapenem for serious infections — pip-tazo unreliable despite in vitro susceptibility
Carbapenemase (Class A/B/D)	KPC, NDM	Klebsiella, Acinetobacter, Pseudomonas	Ceftazidime-avibactam (KPC); meropenem-vaborbactam (KPC); NDM requires ceftazidime-avibactam + aztreonam
Altered PBP (mecA)	PBP2a — low beta-lactam affinity	MRSA	Vancomycin, daptomycin, linezolid; ceftaroline (only beta-lactam with PBP2a affinity)
Porin loss + efflux	OprD loss, MexAB-OprM	Pseudomonas aeruginosa	Higher doses; extended infusion; combination therapy; consider newer beta-lactam–inhibitor combinations

Clinical Rules: ESBL Bacteremia and MRSA

Piperacillin-tazobactam is unreliable for bloodstream infections caused by ESBL-producing organisms. Even when the isolate tests susceptible in vitro, high bacterial burden overwhelms tazobactam (the inoculum effect). Use a carbapenem as definitive therapy for serious ESBL infections — the MERINO trial confirmed 30-day mortality was significantly higher with pip-tazo versus meropenem in this setting.

MRSA requires vancomycin, daptomycin, or linezolid. No conventional penicillin or cephalosporin is effective — the mecA-encoded PBP2a has such low affinity for all standard beta-lactams that the organism continues cross-linking peptidoglycan at clinically achievable drug concentrations. Ceftaroline is the sole beta-lactam exception.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 43: Beta-Lactam Antibiotics	McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 50: Penicillins, Cephalosporins, and Other Beta-Lactam Antibiotics	McGraw-Hill, 2023
Tipper DJ, Strominger JL	Mechanism of action of penicillins: a proposal based on their structural similarity to acyl-D-alanyl-D-alanine	Proc Natl Acad Sci USA. 1965;54(4):1133–1141
Spratt BG	Penicillin-binding proteins and the future of beta-lactam antibiotics	J Gen Microbiol. 1983;129(5):1247–1260
Harris PNA, Tambyah PA, Lye DC, et al.	Effect of piperacillin-tazobactam vs meropenem on 30-day mortality for patients with E. coli or Klebsiella pneumoniae bloodstream infection and ceftriaxone resistance: a randomized clinical trial	JAMA. 2018;320(10):984–994
Bush K, Bradford PA	Beta-lactams and beta-lactamase inhibitors: an overview	Cold Spring Harb Perspect Med. 2016;6(8):a025247
Lakhundi S, Zhang K	Methicillin-resistant Staphylococcus aureus: molecular characterization, evolution, and epidemiology	Clin Microbiol Rev. 2018;31(4):e00020-18
Craig WA	Pharmacokinetic/pharmacodynamic parameters: rationale for antibacterial dosing of mice and men	Clin Infect Dis. 1998;26(1):1–12
Pitout JD, Laupland KB	Extended-spectrum beta-lactamase-producing Enterobacteriaceae: an emerging public-health concern	Lancet Infect Dis. 2008;8(3):159–166
Wright AJ	The penicillins	Mayo Clin Proc. 1999;74(3):290–307