Appendicitis. Illustration of a human appendix, showing it red and inflamed in appendicitis and a close-up view of bacteria, the causative agents of appendicitis. The most common microorganisms recovered from appendix with acute appendicitis are Escherichia coli and Bacteroides sp., the less commonly found are, Klebsiella pneumoniae, Streptococcus sp. Enterococcus sp. Pseudomonas aeruginosa and other bacteria.