Provided herein is a protein, referred to as a Pn3Pase protein, that degrades the capsular polysaccharide of serotype 3 Streptococcus pneumoniae. The disclosure includes a genetically modified cell that includes a Pn3Pase protein, and compositions that include the protein, the polynucleotide encoding the protein, the genetically modified cell, or a combination thereof. Also provided are methods for using a Pn3Pase protein, including methods for contacting a S. pneumoniae having a type III capsular polysaccharide with a Pn3Pase protein, increasing deposition of at least one complement component on the surface of a S. pneumoniae, treating an infection in a subject, treating a symptom in a subject, decreasing colonization of a subject by S. pneumoniae, or a combination thereof.