The most common bacteria found in conjunctivitis are Staphylococcus aureus, Streptococcus pneumoniae and Haemophilus influenzae.  In chronic conjunctivitis, certain Staphylococcus species, Moraxella and opportunistic bacteria are found.

In children, H.influenzae and S.pneumoniae are the most commonly isolated pathogens.

In adults, elderly and immunocompromised patients, Staphylococcus species are the most common isolates as well as species isolated from the gut, including Proteus mirabilis, Pseudomonas species, Klebsiella pneumoniae and Serratia species.