Here is a list with some of the microbial organisms that are to blame in cases of pneumonia:

- bacteria: these too can be categorized by the procedure that is used to see the bacteria under a microscope. The stains that are seen under the microscope then tell us what type of bacteria is the one that cause pneumonia. There are two main types: gram positive bacteria and gram negative bacteria.

The gram positive bacteria that cause pneumonia appear colored blue on the strain. The most common gram positive bacteria that cause pneumonia are:

- streptococcus pneumonia: this is the most common of all pneumonia causes; researchers believed that this type of bacteria was the cause of almost ninety five percent of all pneumonia cases that were community- acquired; however, recent studies have shown that in fact this particular bacteria only accounts for less then thirty percent of all pneumonia cases that are community acquired;

- staphylococcus aureus is another gram- positive bacteria that is second on the most common causes of pneumonia list; however, this type of bacteria sits at the top of the list when it comes to pneumonia cases that are acquired in the hospital; this is rather uncommon in adults that are healthy, but it can develop almost five days after a person has suffered from a condition that has weakened his/ her defense system;

The gram- negative bacteria that cause pneumonia are colored pink. These type of bacteria that cause pneumonia are the most common agents of infection in hospitals. Some of the main gram- negative bacteria that cause pneumonia are:

- haemophilus influenzae: this generally affects people that suffer from chronic lung problems or they affect older people;

- klebisiella pneumoniae is the main bacteria responsible for pneumonia in people that are alcoholics or people who suffer from physical debilities;

- pseudomonas aeruginos is the most common bacteria that cause pneumonia in people that suffer from severe or chronic lung problems;

- neisseria meningitidis is another bacteria that is thought to play a very big role in pneumonia patients; this is one of the most important causes in developing meningitis, but recent studies have shown that this bacteria is also responsible for pneumonia;