Comparison of Beta-lactams and Other Antibiotic Families

Compare the carbapenem above to the carbacephem and cephems below. Note the structure the heterocyclic cores. 

So we see that penicillins, cephalosphorins, and carbapenems all have a heterocyclic core with a beta-lactam ring. There are also beta-lactam antibiotics that have only beta-lactam at the core. An example is given below.

Below: a ureidopenicillin

In contrast, for bacteria with resistance to beta-lactam antibiotics, medicines like colistin must be used. Colistin is a member of the polymyxin family. 

It's interesting to note that beta-lactam antibiotics were derived from fungi and polymyxins came from gram-positive bacteria. This implies that a new class of antibiotics could be derived from gram-negative bacteria. Microorganisms wage constant chemical war on each other. 

Quinolones are another interesting group. Like the other antibiotics mentioned here, it too has a nitrogen ring. 

Aminoglycosides do not have nitrogen rings, but they do have nitrogen atoms with double bonds to carbon (an amino bond). 

All these antibiotics work by acting the cell walls of bacteria. Those walls come in two basic varieties shown below. Since both walls contain peptidoglycan, it seems the use of antibiotic from both beta-lactam and another family is necessary. The proper blend of antibiotics is at least an important question as discovering new ones. 

Indeed, triple antibiotic ointment contains a blend of aminoglycoside, a polymyxin, and a polypeptide antibiotics. 

Polypeptide antibiotics like bacitracin have high molecular weights.