one response to your question, and welcome others .
How to get at this depending on your background? Much has already been posted on the board.
The answer depends a lot on what you know and when you know it.
To start: someone feels ill, usu develops a fever as evidence of infection, and toughs it out at home, or ends up seeing doctor somewhere. Initial question is- does the person have an infection, or an adverse drug reaction, or fever related to a tumor, an inflammatory condition(lupus, for example). Sometimes this is easy and sometimes not.
Let's say it is decided that this pt(patient) has an infectious cause of a fever. Then what is the source? Maybe easy to tell- bad cough as a symptom, or diarrhea, or skin infection/cellulitis/abscess. Maybe not so easy- nothing tells you the source of infection and you have to work and search and ponder.
Who is the pt? Immunosuppressed on steroids or chemo or immune therapy(rheumatoid arthritis etc)
Diabetic? Elderly? It goes on and on, but comes down to how much you need to worry about any given pt- do they fit into categories where they are likely to do well, or likely to get into trouble.
If you know the source of infection then you target your therapy to most likely causes. Not so sick pt with a cough and runny nose most likely viral- wait it out, symptomatic treatment(Acetominophen, cough med etc). But take a sicker pt- older diabetic smoker with decreased oxygen level, high temperature with a cough, and the x ray shows pneumonia.You are left to choose an antibiotic based on the most likely pathogens, and there is a lot of data about pathogens, and there are consensus recommendations about treatment of pneumonia for outpatients, for inpatients(those needing hospitalization), inpts who came from facilities with likely resistant bugs, and for inpts on ventilators, etc. Current recommendations say to always treat for Strep pneumonia and then to add treatment for MRSA sometimes, double cover for PsA(Pseudomonas) sometimes, cover for Legionella, etc.
In other words it depends on who the pt is and how sick they are and what the source of infection is. if it is diverticulitis you cover for gm- and for anaerobes especially, similar for cholecystitis(gall bladder/gallstone blockage). Or for cellulitis must cover for Staph and Strep. And on it goes, with many guides out there to tell you most likely pathogen for any given infection-UTI, sore throat, meningitis....
Once you have defined the infection- grown the bug in the lab-you will have the data on sensitivity and know exactly what drug will work and what will not. But this takes time and very often you never know. You treat based on probabilities.
So for a sick pt with a known source you cover with antibiotics for likely bacterial sources until you know the exact bug, and then you narrow coverage.
Or you do not know a source, you just know someone is quite ill with a fever, and blood pressure is low, and the stakes are quite high, and your loved one is in danger. Then you cover broadly. You give multiple big gun drugs to try to cover all the bad bases.
A good gm- drug would kill the most likely pathogens for any given pt, then, until or if you ever know exactly what the bacterial infection is actually caused by. Broad gm- coverage includes EColi, and Klebsiella and PsA as examples and that is why the charts always tell you how well the drugs do against these bacteria. Because PsA is a feared pathogen and a real killer ,it tends to be covered at the start for many types of febrile illness, until you can narrow things down. It is not always a likely pathogen, though, for example, in diverticulitis, or other GI problems,or UTI, less likely to be a problem.
To switch to gm+:
This is why Brilacidin is such a beauty. A sick person who develops pneumonia while in the hospital (or at a nursing center or rehab center) may well have MRSA and must have MRSA coverage when admitted. The hospital team will absolutely give a drug for MRSA in those situations and Brilacidin looks to be much nicer than Vancomycin, the usual go to drug. It would be given right away( increased sales) while also saving lives.
A gm- drug with broader coverage, including PsA, would be given more frequently, up front, when gm- coverage indicated by likely source of infection. This does not mean a drug has to cover everything gm-, not any any means, to be successful. But a better broader drug would be given much more frequently while attempting to narrow down antibiotic coverage, by determining the source of infection and exact bacterial source if possible.
That is a start of an explanation, anyway.
