This chapter deals with the nightmare which ensues when a patient without an immune system encounters the angry soup of multi-resistant organisms in a large teaching hospital. It has not been updated for some number of years, and is viewed by its author as derelict. The newer summary of sepsis in the bone marrow transplant recipient covers virtually the same territory, but better.
An immunocompetent host typically suffers sepsis at the hands of known, commonly found enemies:
An immunocompromised neutropenic host has much more to offer to a bacterium with ambition. There is a list of organisms which one needs to consider in addition to the above.
So; the list of differentials is pretty broad. A broad cover is therefore deployed.
An excellent resource for this topic are the chapters in Oh's Manual dealing with severe sepsis (ch 61, by A Raffaele de Gaudio) and with the immunocomrpomised host (ch 59, by Steve Wesselingh and Martyn A H French).
An older, yet similarly respectable source is Shoemaker (2005); Chapter 155 (Infections in the immunocompromised patient) by Andrew Githaiga, Magdaline Ndirangu and David L. Paterson covers this topic with great detail.
The Surviving Sepsis Campaign has these published guidelines to peruse.
There is a particularly useful UpToDate article on this topic, available only to their customers and their friends.