Acute bacterial prostatitis is caused by:
Ref. Textbook of Microbiology and Immunology BY Parija. Page. 182, 261
 
Bacterial prostatitis is usually caused by E. coli or another gram-negative rod, with one of two presentations.

Acute bacterial prostatitis presents with fever and chills; prostate examination should be gentle or not performed at all, as massage may result in a wave of bacteremia.
Chronic bacterial prostatitis presents as recurrent episodes of bacterial cystitis
E. Coli
Streptococcus viridans
Staphylococcus aureus
Proteus