The radiograph reveals right upper lobe consolidation, typical for a lobar pattern of pneumonia. The gram stain shows gram positive cocci in pairs and short chains. The individual cocci have a bullet, or lancet, shape. These findings are most characteristic for Streptococcus pneumoniae (pneumococcus) infection. There are dozens of serotypes, so one infection does impart immunity against subsequent infections. The polysaccharide capsule of pneumococcus determines its virulence, because this capsule prevents activation of the alternate pathway of complement that generates the C3b which opsonizes the bacteria to promote phagocytosis. Virtually all pneumococci used to be very sensitive to antibiotic therapy, particularly with the penicillins. However, widespread antibiotic use has selected for and promoted antibiotic resistant strains that have acquired new capsular genes so that now at least a fourth of pneumococci are resistant to penicillins.