A 27-year-old man develops acute pain and swelling of the left knee 5 days after an episode of urethritis. On physical examination, the left knee is swollen, warm, and tender to the touch. No other joints are affected. Laboratory examination of fluid aspirated from the left knee joint shows numerous neutrophils. A Gram stain of the fluid shows with gram-negative intracellular diplococci. No crystals are seen. Which of the following infectious agents is most likely responsible for his condition?