Mecillinam is used in the treatment of infections due to susceptible gram-negative bacteria, especially urinary tract infections which are most commonly caused by Escherichia coli. Mecillinam is active against most pathogenic Gram-negative bacteria, except Pseudomonas aeruginosa and some species of Proteus. Several studies have also found it to be as effective as other antibiotics for treating Staphylococcus saprophyticus infection, though it is Gram-positive, possibly because mecillinam reaches very high concentrations in urine.
Worldwide resistance to mecillinam in bacteria causing urinary tract infection has remained very low since its introduction; a 2003 study conducted in 16 European countries and Canada found resistance to range from 1.2% (Escherichia coli) to 5.2% (Proteus mirabilis). Another large study conducted in Europe and Brazil obtained similar results — 95.9% of E. coli strains, for instance, were sensitive to mecillinam.