IDSA have guidelines. To summarize: drain the pus, kill the bugs. How hard is it?
CT. Enhanced is best.
Trauma, cancer or foreign body like a bone. Something needs to cause a perforation of gut. It might be diverticula, a surgical misadventure or a perforated appendix. The weirdest cause I ever saw was a bread bag square tie that was (I hope) accidently eaten and perforated the colon.
Candida is often part of duodenal perforations, rarely seen in this era of H2 blockers and PPI's.
There is a curious literature, that may not pan out, suggesting that adenovirus may play a role: military recruits who get the adenovirus vaccine have less appendicitis and those that do are more like to be adenovirus positive on PCR (Link)
I prefer a third generation cephalosporins PLUS metronidazole It is every bit as good as a carbapenam (Pubmed). Alternatives include a carbapenem OR penicillin/beta-lactamase inhibitors OR a quinolone PLUS metronidazole. Clindamycin can be used in place of metronidazole.
If you are going to opt for the non-operative approach, then ertapenem (1 g/d) for 3 days followed by 7 days of oral levofloxacin 500 mg qd and metronidazole 500 mg tid is non-inferior to taking out the appendix (PubMed).
The traditional clindamycin/aminoglycoside is probably inferior (PubMed) due to both increasing resistance and lousy pharmacokinetics.
Duration? 4–7 days unless it is difficult to achieve adequate source control (PubMed). I am a huge fan of the STOP IT trial. For intrabdominal infection, 4 days was fine as long as there was good source control and longer courses of antibiotics only delayed the needed source control (PubMed). This was true in the subset of patient who were septic (Pubmed). It's all about the source control. And here is a little secret that most people do no know: broadening antibiotic coverage when the patient is looking infected is not a substitute for draining pus/source control.
While taking it out is the usual treatment of acute apendicitis (a first choice in all the guidelines), a trial of antibiotics will prevent most patients from needing surgery. Not unreasonalbe, especially if prior surgeries make a trip to the OR problematic. But, "It remains to be determined whether the benefits of potentially avoiding an operation with the antibiotics-first approach are outweighed by the burden to the patient related to future appendicitis episodes, more days of antibiotic therapy, lingering symptoms, and uncertainty that may affect quality of life (PubMed)."
The change of relapsing after medical therapy for appendicitis is about 15% (PubMed).
Drain drain drain. You have to control the infection for the antibiotics to work.
In both kids (PubMed) and adults antibiotics can cure acute appendicitis without surgery (PubMed). BUT. Primary appendicectomy has a lower rate of post-operative complications (PubMed). So the best bet is take it out.
Where I live, E. coli is 30% resistant to all forms of penicillin, so avoid penicillins if local resistance is > 10% or so.
I tend not to worry about the enterococcus, it is other beasts that will kill your patient, and it will go away of you drain the pus. Usually. The enterococcus does not increase mortality or need for re-operation, but may increase the rate of abscess formation (PubMed).