Klebsiella pneumoniae is a Gram-negative, rod-shaped bacterium that is part of the Enterobacteriaceae family. While it is normally found in the human intestines and does not typically cause harm in healthy individuals, it can become pathogenic under certain conditions. This bacterium is a common cause of healthcare-associated infections, particularly in hospitals, where it can lead to serious infections, including pneumonia, urinary tract infections (UTIs), bloodstream infections, and wound or surgical site infections [10].

K. pneumoniae is rod-shaped and encapsulated, with a characteristic mucoid (slimy) appearance due to a thick polysaccharide capsule that protects it from immune defenses. It is facultatively anaerobic, meaning it can survive in both oxygen-rich and oxygen-poor environments. K. pneumoniae is known for its ability to acquire antibiotic resistance. Certain strains, particularly carbapenem-resistant Klebsiella pneumoniae (CRKP), are highly resistant to multiple antibiotics and pose a significant public health risk. Klebsiella pneumoniae infections are particularly dangerous in immunocompromised individuals or those with chronic illnesses, such as diabetes, liver disease, or lung disease [11].

K. pneumoniae capsule and other virulence factors, including lipopolysaccharide (LPS) and siderophores (molecules that scavenge iron), contribute to its ability to evade immune responses and establish infections. In cases of pneumonia, K. pneumoniae often leads to symptoms such as high fever, chills, productive cough with thick, blood-tinged sputum (“currant jelly” sputum), and chest pain. Its ability to cause severe, necrotizing lung infections means it can lead to complications like lung abscesses, cavitation, and empyema [12]. In the gut, K. pneumoniae, especially with antibiotic treatment, can be associated with dysbiosis.

Gut dysbiosis can involve the vagus nerve in Alzheimer’s disease. The vagus nerve is a key component of the gut-brain axis, a bidirectional communication system between the gut and the brain. Gut dysbiosis can lead to the production of inflammatory mediators and metabolites that can travel through the vagus nerve to the brain, contributing to neuroinflammation and the progression of Alzheimer’s disease [13].

Klebsiella pneumoniae infection could theoretically contribute to increased risk of Alzheimer’s disease through mechanisms involving chronic inflammation, immune system activation, and potential neuroinvasion. Here are several key pathways that might link K. pneumoniae to Alzheimer’s disease risk:

1. Chronic Systemic Inflammation: K. pneumoniae infections can lead to persistent or recurrent inflammation in the body, especially in immuno-compromised or elderly individuals. Chronic systemic inflammation is a known risk factor for Alzheimer’s disease, as inflammatory molecules (like cytokines) can cross the blood-brain barrier, potentially leading to neuroinflammation. This neuroinflammation is implicated in the progression of Alzheimer’s pathology, including amyloid-beta (Aβ) plaque deposition and tau protein hyperphosphorylation [1].
2. Blood-Brain Barrier (BBB) Disruption: Some studies suggest that infections can compromise the integrity of the BBB, allowing pathogens or inflammatory mediators to reach brain tissue. K. pneumoniae, particularly in cases of severe pneumonia or bloodstream infection (sepsis), could contribute to BBB disruption. This weakened barrier can expose the brain to systemic pathogens and inflammatory molecules that may promote Alzheimer’s pathology [1].
3. Direct Neuroinvasion: While rare, certain strains of K. pneumoniae, especially hypervirulent strains, have been implicated in meningitis and brain abscesses, indicating their potential to invade the central nervous system (CNS) [14]. If K. pneumoniae were able to reach brain tissue, it could trigger localized inflammation and oxidative stress, further contributing to the neurodegenerative processes involved in Alzheimer’s.
4. Molecular Mimicry and Immune System Activation: K. pneumoniae, like many pathogens, can produce antigens that resemble human proteins, leading to an autoimmune response [15]. This molecular mimicry could result in the immune system mistakenly attacking brain tissue or proteins like amyloid precursor protein (APP) or tau, accelerating Alzheimer’s-related damage.
5. Microbiome Interactions: K. pneumoniae is part of the human gut microbiome, and recent studies suggest that gut bacteria may influence brain health through the gut-brain axis [1]. Disruptions in gut microbiota composition such as an overgrowth of K. pneumoniae could contribute to systemic inflammation and alter immune responses in ways that affect brain function and increase Alzheimer’s risk. Elevated levels of certain gut bacteria have been associated with increased Aβ accumulation and cognitive decline, suggesting that K. pneumoniae might contribute in a similar way.
6. Oxidative Stress and Amyloid-Beta Aggregation: Some bacterial infections have been associated with increased oxidative stress, which can exacerbate the aggregation of Aβ and tau, key pathological features in AD [16]. By increasing oxidative stress, a K. pneumoniae infection could indirectly encourage the accumulation of these proteins in the brain, contributing to neurodegeneration.

In sum, while direct evidence linking K. pneumoniae to Alzheimer’s disease is still under investigation, the pathways of inflammation, immune activation, BBB disruption, and potential neuroinvasion provide a plausible basis for how recurrent or chronic K. pneumoniae infections could increase Alzheimer’s risk, especially in older adults or individuals with compromised immune systems.