Which of the following is a gram negative rod known for causing infections in humans?

Klebsiella is recognized as a gram-negative rod that is significant in human infections. This bacterium belongs to the Enterobacteriaceae family and is commonly associated with various healthcare-associated infections, including pneumonia, urinary tract infections, and bloodstream infections. Its gram-negative status is characterized by a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharides (endotoxins), which can contribute to its virulence and resistance to certain antibiotics.

In contrast, Clostridium is a gram-positive anaerobic bacillus, which is not classified as a gram-negative rod. Streptococcus pneumoniae, while a considerable human pathogen, is a gram-positive cocci, not a rod. Diphtheria refers to the disease caused by Corynebacterium diphtheriae, which is also a gram-positive bacillus. Therefore, Klebsiella stands out as the only option that correctly aligns with the criteria of being a gram-negative rod causing infections in humans.