A 62-year-old male underwent dental extraction 1 week ago. For the past 3 days, he has presented with cold intolerance, fever, cough with purulent sputum, and slightly foul-smelling sputum. X-ray shows a left lower lobe lung abscess. Gram stain of sputum smear reveals gram-negative bacilli, but there is no growth on routine culture. The most likely pathogen is: