What are the likely causative organisms in a patient with COPD and pneumonia?

Any patients with chronic obstructive pulmonary disease (COPD) are predisposed to pneumonia, and because of the underlying damage to their lungs, a pneumonia in these patients takes longer to go away, and carries a higher mortality rate than average.

Any patient with a pneumonia in the context of COPD needs to have their sputum cultured, so that you can tailor your antibiotics specifically to their requirements. However, before such results come back, you need to start empiric therapy, which implies that you have to know which organisms are common in this situation.

And the answer is... Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis. The first two are common enough causes of pneumonia, but perhaps I should introduce you to the third one quickly.

M. catarrhalis was first discovered 112 years ago, and is a Gram negative aerobic diplococcus. (A diplococcus is a coccus that generally goes around with a partner, so that if you look down a microscope, you usually see two little spheres clinging tightly to each other.) It is fundamentally a 'respiratory tract organism', causing infections in both the upper and lower tracts. When it causes pneumonia, it often gives an 'atypical' picture, with a tendency to produce a more patchy and bilateral picture than with, say S. pneumoniae.

It is often resistant to penicillins, and so a macrolide, like erythromycin, usually has to do the trick instead.