An 82 year old woman presents with fever, seizures and a history of anorexia, diarrhoea and vomiting.Following a normal CT scan of head, a lumbar puncture is performed.

The immediate results are as follows:

CSF slightly turbid in appearance. 
300 polymorphs /mm3,

240 monocytes/mm3 
Glucose           2.5mmol/l
Protein             0.6 g/l (0.2-0.4 g/l).

a) What is the likely diagnosis?

b) The microbiologist rings to inform you that the gram stain demonstrates numerous small non-branching Gram-positive bacilli. What is the likely diagnosis?

c) What are the appropriate antibiotics for this organism?

d) You discover the patient is allergic to your choice of antibiotic. Suggest an alternative antibiotic.