Most commonly prescribed antibiotics these days don't interact with alcohol much (if at all). Having one or two beers even with the ones that do generally will only decrease their effectiveness a little, but there are a few that can become dangerous with alcohol consumption (they are clearly labeled as such).
In general, in cases like Chris is experiencing, the alcohol prohibition is related more to the illness and the meds other than antibiotics. Pneumonia is severely exasperated by the alcohol exhaled into the lungs from the blood stream causing increased irritation and inflammation leading to increased fluid build up in the lungs.