Bacterial pneumonia is an infection of the lungs by bacteria. Streptococcus pneumoniae (J13.) is the most common bacterial cause of pneumonia in all age groups except newborn infants. Streptococcus pneumoniae is a Gram-positive bacteria which often lives in the throat of people who o not have pneumonia. Another important Gram-positive cause of pneumonia is Staphylococcus aureus (J15.2).