system
Given the following post, respond as if you're commenting on Reddit:
human
If susceptibility testing is already difficult and sometimes highly specific for patients who aren’t immunocompromised, how do doctors select antibiotics and antimicrobials for burn victims and patients who have a severely compromised immune system?

Is there a gauntlet of drugs that are used against common skin or nosocomial flora that doctors administer until the skin or immune system can kick back in?

For background for anybody reading: We test susceptibility for antibiotics by exposing bacterial cultures to samples of specific antibiotics to see what happens to the bacteria in the plate. When you expose a bacterial colony to an antibiotic it is susceptible to, there will be a reaction that tells us that the doctor can use that antibiotic on a patient to treat an infection. 

The reason I am asking this question is because I’ve never worked in the ER or burn unit, only small labs. And I’ve seen how in Micro it’s necessary to run multiple tests just to determine the proper antibiotic for a single infection. But for a system wide entry wound like a massive burn, I imagined it must be grueling to find a proper treatment when bacteria come in from every surface possible.
gpt
Doctors use broad spectrum antibiotics that are active against the bacteria that typically cause an infection in that area.

Then based on culture data, they can narrow or broaden their coverage as needed.

Susceptibility testing is no different for a competent or immunocompromised patient, because we're testing the bacteria, not the patient.