Monobactams are β-lactam

Beta-lactam

A β-lactam ring, is a four-membered lactam. It is named as such, because the nitrogen atom is attached to the β-carbon relative to the carbonyl...

 compounds wherein the β-lactam ring is alone and not fused to another ring (in contrast to most other β-lactams, which have at least two rings). They work only against Gram-negative bacteria.

The only commercially available monobactam antibiotic is aztreonam

Aztreonam

Aztreonam is a synthetic monocyclic beta-lactam antibiotic , with the nucleus based on a simpler monobactam isolated from Chromobacterium violaceum. It was approved by the U.S. Food and Drug Administration in 1986...

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Other examples of monobactams are tigemonam

Tigemonam

Tigemonam is a monobactam antibiotic....

, nocardicin A

Nocardicin A

Nocardicin A is a β-lactam antibiotic included in the monobactam subclass. It is secreted by the bacterium Nocardia uniformis subsp. tsuyamenensis as a metabolic product catalyzed by the enzyme nocardicin-A epimerase....

, and tabtoxin

Tabtoxin

Tabtoxin, also known as wildfire toxin, is a simple monobactam biotoxin produced by Pseudomonas syringae. It is the precursor to the antibiotic tabtoxinine β-lactam....

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Adverse effects to monobactams can include skin rash and occasional abnormal liver functions.

They have no cross-hypersensitivity reactions with penicillin

Penicillin

Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

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