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Pati,   One way of classifying bacteria is by the appearance of their are outer cell membrane (or peptidoglycan layer). The peptidoglycan layer is visible under the microscope through staining techniques (a.k.a. dying). Serattia, psuedomonas, and strep are all classified as gram-negative bacterias due to their thin peptidoglycan layer. Some antibiotics that are effective in killing gram-negative bacteria include Cipro and erythromycin. Your father's doctor should be able to recommend the best option.   Check out this link, http://library.thinkquest.org/25462/list.html, for a complete list of common antibiotics and the bacterias they treat.