**Core Concept**
Klebsiella species are a type of gram-negative, non-motile bacteria that belong to the family Enterobacteriaceae. They are typically found in the human gut and can cause a range of infections, including pneumonia and urinary tract infections. The lack of motility in Klebsiella species is due to the absence of flagella, which are the whip-like structures used by many bacteria to move.

**Why the Correct Answer is Right**
Klebsiella species do not possess flagella, which are necessary for bacterial motility. Flagella are composed of flagellin proteins that are encoded by the flg genes. The absence of these genes in Klebsiella species leads to a lack of flagella formation, resulting in non-motility. This characteristic distinguishes Klebsiella from other Enterobacteriaceae species, such as Escherichia coli and Salmonella, which are motile.

**Why Each Wrong Option is Incorrect**
**Option B:** Citrobacter species are also gram-negative bacteria, but they are capable of motility due to the presence of flagella. They are often found in the human gut and can cause infections similar to Klebsiella.
**Option C:** Escherichia species, such as E. coli, are motile due to the presence of flagella. They are also gram-negative bacteria and are commonly found in the human gut.
**Option D:** Salmonella species are also gram-negative bacteria and are capable of motility due to the presence of flagella. They are often associated with food poisoning and are found in a range of environments.

**Clinical Pearl / High-Yield Fact**
Klebsiella pneumoniae is a type of Klebsiella species that is commonly associated with hospital-acquired infections, particularly pneumonia. This species is often resistant to multiple antibiotics, making it a challenging organism to treat.

**✓ Correct Answer: A. Klebsiella spp**