Although there are well over 100 antibiotics, the majority come from only a few types of drugs. These are the main classes of antibiotics.Penicillins such as penicillin and amoxicillinCephalosporins such as cephalexin (Keflex)Macrolides such as erythromycin (E-Mycin), clarithromycin (Biaxin), and azithromycin (Zithromax)Fluoroquinolones such as ciprofolxacin (Cipro), levofloxacin (Levaquin), and ofloxacin (Floxin)Sulfonamides such as co-trimoxazole (Bactrim) and trimethoprim (Proloprim)Tetracyclines such as tetracycline (Sumycin, Panmycin) and doxycycline (Vibramycin)Aminoglycosides such as gentamicin (Garamycin) and tobramycin (Tobrex)Most antibiotics have two names, the trade or brand name, created by the drug company that manufactures the drug, and a generic name, based on the antibiotic's chemical structure or chemical class. Trade names such as Keflex and Zithromax are capitalized. Generics such as cephalexin and azithromycin are not capitalized.Each antibiotic is effective only for certain types of infections, and your doctor is best able to compare your needs with the available medicines. Also, a person may have allergies that eliminate a class of antibiotic from consideration, such as a penicillin allergy preventing your doctor from prescribing amoxicillin.In most cases of antibiotic use, a doctor must choose an antibiotic based on the most likely cause of the infection. For example, if you have an earache, the doctor knows what kinds of bacteria cause most ear infections. He or she will choose the antibiotic that best combats those kinds of bacteria. In another example, a few bacteria cause most pneumonias in previously healthy people. If you are diagnosed with pneumonia, the doctor will choose an antibiotic that will kill these bacteria.Other factors may be considered when choosing an antibiotic. Medication cost, dosing schedule, and common side effects are often taken into account. Patterns of infection in your community may be considered also.In some cases, laboratory tests may be used to help a doctor make an antibiotic choice. Special strains of the bacteria such as Gram stains, can be used to identify bacteria under the microscope and may help narrow down which species of bacteria is causing infection. Certain bacterial species will take a stain, and others will not. Cultures may also be obtained. In this technique, a bacterial sample from your infection is allowed to grow in a laboratory. The way bacteria grow or what they look like when they grow can help to identify the bacterial species. Cultures may also be tested to determine antibiotic sensitivities. A sensitivity list is the roster of antibiotics that kill a particular bacterial type. This list can be used to double check that you are taking the right antibiotic.Only your doctor can choose the best class and the best antibiotic from that class for your individual needs.