Mecillinam is an extended-spectrum penicillin antibiotic that binds specifically to penicillin binding protein 2 (PBP2), and is only considered to be active against Gram-negative bacteria. It is used primarily in the treatment of urinary tract infections, and has also been used to treat typhoid and paratyphoid fever. 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, even though it is Gram-positive, possibly because mecillinam reaches very high concentrations in urine. The adverse effect profile of mecillinam is similar to that of other penicillins. The most common side effects of mecillinam use are rash and gastrointestinal upset, including nausea and vomiting.