Microbiological results do not become available for 24 to 72 hours so in serious infections to prevent further complications or death we need to give antibiotics without knowing the exact type of bacteria (gram + gram - or anaerobes) . The idea is to give a combination of antibiotics that cover all three types of bacteria so at least one of them will have effect.Combination of antibiotics are also useful if the infections is caused by more than one type of organism.(ex: an infection with gram + and gram - ). Every class of antibiotics has a spectrum ( some type of bacteria that they have effect on). Penicilins are effective against gram + bacteria. Aminoglycosides (ex: Gentamicin ) are effective against gram - bacteria including pseudomonas. Metronidazole is effective on Anaerobes. By giving all 3 we make sure at least one will have effect on the bacteria of the pacient. There are some things to keep in mind:some people are allergic to penicilins , we might need to adjust the dose of aminoglycoside (gentamicin) if they have renal problems, the bacteria might be resistant to the type of antibiotic we give etc. Antibiotic Sensitivity Charts: A simple one : https://i1.wp.com/teachim.org/wp-content/uploads/2017/11/Final-board-2-1.jpg?resize=900%2C516u0026ssl=1 A detailed one: https://therapeuticseducation.org/sites/therapeuticseducation.org/files/Antibiotic_Sensitivity_December_2015.pdf
