Serratia marcescens is a gram-negative enterobacteria. It is not involved in respiratory tract infections except in critically ill patients or patients that have a chronic pulmonary condition that affects the lung anathomy, like bronchiectasis. Lungs of patients with bronchiectasis are chronically colonized by several bacteria that can t be found in a lung without disease. Most of the time these bacterias will coexist with host immune system in a balance. If by any means this balance is broken, there may be an exacerbation of symptoms. It is important to know that not every symptom that you feel has to be caused by the bacteria and it really can be just colonizing your lungs. The use of antibiotics against a colonizing bacteria has the potential to induce resistance to the antibiotics, making it hard to treat a real infection when it occurs. It is important to discuss with your physicians why they consider this bacteria a colonizing agent and if all your symtoms really correlate with it or not.