 The carboxypenicillins feature the beta-lactam backbone of all penicillins but also feature a carboxylic acid or carboxylic acid ester group in the variable side-chain.The carboxypenicillins exhibit activity against Gram negative organisms including "Pseudomonas aeruginosa" and "Proteus" species.They are inactive against certain Gram positive pathogens such as "Staphylococcus aureus", "Enterococcus faecalis", and "L. monocytogenes".