Background: The possibility of an infectious etiology for Alzheimer's disease (AD) has been repeatedly postulated over the past three decades, with the roles of both viruses and bacteria having been investigated. Chlamydophila (formerly Chlamydia) pneumoniae (Cpn) and spirochetal bacteria have been two of the most frequently implicated bacterial groups in AD pathogenesis.

Objective: A meta-analysis was performed where data were combined from 25 studies examining the association between AD and spirochetal bacteria or Cpn.

Methods: Comprehensive search of several electronic databases. Data were extracted from published studies and a random-effects model was used to analyze the data.

Results: A statistically significant association between AD and detectable evidence of infection of either bacterial group was demonstrated. Over a ten-fold increased occurrence of AD was noted when there is detectable evidence of spirochetal infection (OR: 10.61; 95% CI: 3.38–33.29), with a more conservative risk estimate demonstrating over a four-fold increased occurrence of AD (OR 4.45; 95% CI: 2.33–8.52). Over a five-fold increased occurrence of AD was noted with Cpn infection (OR 5.66; 95% CI: 1.83–17.51).

Discussion: There appears to be a strongly positive association between bacterial infection and AD.