International pneumococcal clones match or exceed the fitness of other strains despite the accumulation of antibiotic resistance.

A few international pneumococcal clones dominate the population of antibiotic-resistant pneumococci. Despite the scientific paradigm that a loss in fitness is the price for acquisition of resistance, these clones spread successfully. One hundred fifty-four isolates from adult patients with community-acquired pneumonia (CAP) were analyzed. Thirty percent showed a close relationship to international clones and had fitness equal to or exceeding that of other strains (P = 0.015); these factors may result in the endurance of these strains despite a reduction of antibiotic usage.